Metallurgy – Corrosion

Chapter 22

Metallurgy e Corrosion INTRODUCTION Metallurgy is generally dealt with by material scientists and material engineers, who study the physical and chemical behavior of metallic elements, their intermetallic compounds and their mixtures, which are referred to as alloys. Metallurgy can also be described as the technology of metals, the way in which science is applied to the production of metals and the engineering of metal components for use in products for manufacturers and consumers. The production of metals involves the processing of ores to extract the metal they contain, and the mixing of metals or other elements to produce alloys. The use of metals often impacts the environment throughout the life cycle of the material. The supply involves the mining of the ores or minerals through processing into metals or alloys having specific composition,

and manufacturing products. Lack of maintenance practice during use may result in the manufactured products causing environmental harm. Further, at the end of its active life, a metallic component may enter the solid waste stream or require energy for recycling into new material. The determined metric for environmental impact is referred to as embodied energy. The life cycle approach (LCA) is often employed to determine the energy consumption from mining, processing, etc. This involves the material from when mined to when it is ready, to be shipped to customers in the form of bulk metal. The recycle material method accounts for a certain percentage of metal being recycled, and Table 22-1 provides embodied energy estimates for some materials. Figure 22-1 shows the periodic table of the elements as an interactive periodic table including properties, compounds, isotopes.

TABLE 22-1 Gross Energy Requirement to Mine, Process, Smelt and Refine the Listed Metals, or Manufacture Resin and Form into the Listed Plastics Embodied Energy using Recycling Method Metal

MJ/kg

Notes

Aluminum

155

Assumes 25.6% extrusions, 55.7% rolled, and 18.7% castings with 33% recycled content

Copper

42

Assumes 37% recycled content (virgin copper tube & sheet 57 MJ/kg; recycled copper 16.5 MJ/kg)

Lead

25

Assumes 61.5% recycled content

Nickel

164

Cited source no longer online

Polyethylene, HDPE pipe

84.4

Feedstock energy (55.1 MJ/kg) included

Polyvinyl Chloride (PVC) pipe

67.5

Feedstock energy (24.4 MJ/kg) included

Stainless Steel

56.7

Highly conflicting data. Based on Type 304 SS

Steel

24.4

Worldwide recycled content 42.7%

Steel Pipe

19.8

Assumes recycled content 59.% (virgin steel pipe 45.4 MJ/kg)

Titanium

361 to 745

Large range of data, small sample size

Zinc

53.1

Assumes 30% recycled content (virgin zinc 72 MJ/kg; recycled zinc 9 MJ/kg)

Ludwig’s Applied Process Design for Chemical and Petrochemical Plants. http://dx.doi.org/10.1016/B978-0-7506-8524-5.00022-7 Copyright © 2015 Elsevier Inc. All rights reserved.

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1210 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

FIGURE 22-1 The periodic table of the elements. (Used by permission: www.ptable.com)

MATERIAL SELECTION Various factors need to be considered in the selection of materials [1]: Uniformity: The plant may have been standardized on certain materials. Maintainability: Exotic materials (meaning, materials that are new and unfamiliar to the plant) may be difficult to maintain due to the need for additional training, parts or equipment. Flexibility: A higher grade material may be justified upon the basis that the unit could be used for more chemistries than initially planned. Cost. Environmental impact: Mining, processing, transportation and disposal affect the environment. Counterfeit materials: The true nature of the materials should be ascertained, as for example, Type 304 stainless steel (SS) is marked and sold as Type 316 SS. Equipment specifications should require that the seller

provides Material Certifications for all critical materials in the fabrication. Physical spot checks should be conducted, and these should confirm that critical materials meet their specifications. Table 22-2 shows an application orientation for various metals in wet processes, and www.matweb.com provides a useful database of over 8,000 metals, plastics, ceramics and composites. The compilations are the datasheets supplied by manufacturers and distributors.

EMBRITTLEMENT This is the loss of a material’s ductility due to chemical or physical change, resulting in crack propagation without appreciable plastic deformation. The most common modes are: Cryogenic Embrittlement: where certain metals transform to a body-centered cubic structure when chilled. Carbon steel (American Society for Testing and Materials

Metallurgy e Corrosion Chapter | 22

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TABLE 22-2 Wet Corrosion Performance Guide [13] Environment

Not Suggested

Good

Better

Best

Chlorides (pitting crevice corrosion)

304L

Alloy 20, 316L, LDX 2101, 600

400, 2205, 317L

AL-6XN, 625, C-276, Titanium, C22, 686, ZERON 100

Chloride Stress

304L, 316L

LDX 2101, 904L

AL-6XN, Alloy 20,

400, 600, 625, 686, C-276, C22

2205, 317L

ZERON 100

Corrosion Cracking Hydrochloric acid

Titanium (b), 600, Alloy 20, 2205 LDX 2101, 317L

200(a), 400(a), 625, ZERON 100

C22, C-276, 686

Zirconium (a), Hastelloy B-2(a), Tantalum, Titanium (b)

Hydrofluoric acid

200, 600, 316L, 317L

C-276, C22, 686, 400 (N2 purged)

400 (a), Silver (a)

Gold, Platinum

Sulfuric acid

Titanium, 600

316L, 317L, LDX 2101, 2205

AL-6XN, 625

Alloy 20, C-276, Tantalum, ZERON 100

Phosphoric acid (commercial)

200, 400, 316L, 317L

904L, 2205

AL-6XN, Alloy 20, ZERON 100

G-30, 625

Nitric acid

904L, Al-6XN, 200, 400, 600

304L, Alloy 20, 2205, ZERON 100

625

Zirconium, Tantalum

Caustic

304L, 316L, 317L, Tantalum

Alloy 20, 2205, LDX 2101, ZERON 100

600, 625, 400, 686, C22, C-276

200(a)

(a) Presence of oxygen or oxidizing salts may greatly increase corrosion. (b) Titanium has excellent resistance to hydrochloric acid containing oxidizers such as FeCl3, HNO3, etc. However, titanium has very poor resistance to pure, reducing, HCl. This chart is intended as guidance for what alloys might be tested in a given environment. It must NOT be used as the major basis for alloy selection, or as a substitute for competent corrosion engineering work.

[ASTM] A442, A516, A517) loses ductility below about
45 C. Austenitic stainless steels are often specified for cryogenic service, but suitable carbon steel grades are also available (ASTM A537, A203, A533, A543). Sulfur stress cracking: This is a type of hydrogen embrittlement in which steels react with hydrogen sulfide to form metal sulfides and hydrogen.

titanium, zirconium and niobium-based alloys, and high temperature hydrogen attacks on steels, as this is evidenced by surface decarburization and the formation of internal voids. High strength steels (e.g. tensile strength over 1,100 MPa or 150 kpsi,) nickel and titanium alloys crack when hydrogen molecules assemble within the metal structure.

ENVIRONMENTAL CRACKING

Stress Corrosion Cracking (SCC)

The brittle cracking of a metal due to the tensile stresses that it is under, combined with any number of environmental factors is referred to as environmentally assisted cracking (EAC). Examples of EAC include hydrogen embrittlement (HE), stress corrosion cracking (SCC) and liquid-metal embrittlement (LME). Each case of EAC has its own corrosive combinations of specific metals and environmental triggers [2].

Where there is a combination of a corrosion and tensile stress (either externally applied or internally applied by residual stress), stress corrosion cracking will occur. This may be either intergranular or transgranular depending upon the alloy and the type of corrosion. Austenitic stainless steels are particularly susceptible to stress corrosion cracking. Chlorides in the process stream cause this type of attack. If the chlorides are removed, stress corrosion cracking will be eliminated. Caution must be exercised when installing insulation, as it must be specified as chloride-free, or a special coating must be used before painting a stainless steel pipe. Another potential cause of stress cracking is the reaction of sodium hydroxide with stainless Grade 2. A further cause of SCC is concrete as there are reports of SCC in stainless steel vessels that rested

Hydrogen Embrittlement (HE) This occurs in the presence of hydrogen, and is characterized by the formation of internal cracks, blisters or voids in steels; embrittlement of steels or high strength nickel-based alloys; formation of embrittling hydrides in

1212 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

on concrete pads. Austenitic stainless steels have failed during downtime because the piping or tubes were not adequately protected from chlorides. A possible measure is to blanket austenitic stainless steel piping and tubing during downtime with an inert gas (nitrogen). In cases where water solutions contain hydrogen sulfide, austenitic steels fail by SCC when they are quenched and tempered to high strength and hardness (above about Rockwell C24). Caution is required when using cleaning solutions and solvents that contain chlorinated hydrocarbons (e.g. sodium hypochlorite, chloroethene, methylene chloride and trichlorethane). Where furnace tubes become sensitized and fail by stress corrosion cracking, the remaining tubes can be stabilized by a heat treatment, as this removes the stress. Chemically stabilized steels such as Type 304L have been successfully used in a sulfidic corrosion environment. Straight chromium ferritic stainless steels are less sensitive to stress corrosion cracking than austenitic steels (18 Cr e 8 Ni). SCC occurs with brasses in solutions containing ammonia, steel in caustic or amine solutions, stainless steel and aluminum alloys in solutions containing chlorides and titanium alloys in nitric acids and methanol. In many cases, the conditions that produce SCC are limited to a specific range of electrochemical potential, pH and temperature. Therefore, SCC can be alleviated by modifying these conditions or by eliminating specific embrittling agents, such as chlorides, caustics, amines, hydrogen or liquid metals. For example, the electrochemical potential (EP) can be controlled by adding chemical agents to shift its value out of SCC range. Furthermore, the SCC of steel in the presence of carbonates and nitrates are two cases where control of the environment is used on a practical basis to minimize failures [3]. Since nitrates are commonly found with nitrites, which act as cracking inhibitors, the nitrate-to-nitrate ratio is often optimized to control CSS. In chloride-containing waters, the use of stainless alloys higher in Cr, Mo, W, and Nb content not only minimizes pitting, but usually increases resistance to SCC as well. Materials that are often susceptible to SCC in chloride environments are austenitic stainless steels containing chromium and nickel and a number of aluminum alloys. The susceptibility to SCC increases with increasing temperature, and for a number of alloys/environment combinations, a safe temperature can be indicated, below which the susceptibility to SCC is negligible. In the offshore industry, stainless steel tubes are blasted e cleaned with a fine non-metallic abrasive e and then an epoxy coating is applied. This is carried out to minimize the tendency of SCC, especially on insulated stainless steels with an operating temperature of 60 C or higher. In a corrosive environment, an applied stress need not necessarily be cyclic in nature to bring about premature

failure. The combined effects of corrosion and an applied tensile stress lower than the yield stress may also cause rapid failure by stress corrosion cracking. This phenomenon is often referred to as season cracking, although this term should only be used for stress corrosion in brasses in an ammonia-containing environment. Other specific names are gasworks cracking, which refers to steel in nitrate solution and caustic cracking of steel in alkaline solution. Stress corrosion cracking occurs with particular specific combinations of metal/alloy and electrolyte under a fairly narrow range of conditions, and is commonly found in alloys that form a protective oxide. The composition and pH of the environment are particularly important variables. Table 22-3 shows combinations which may result in stress corrosion cracking. The nature of the failure is not necessarily the same for the same metal or alloy in different media. A second important variable is applied stress. A threshold stress may have to be exceeded before stress corrosion cracking can occur. Temperature is another factor that influences the rate of stress corrosion cracking. Metallurgical factors also have an important role, as these include alloy composition and residual stresses resulting from heat treatment or working operations. Methods of preventing stress corrosion cracking depend on the recognition and control of these variables in practice [4]. Figure 22-2 illustrates the sequence of events in failures by SCC, and Figure 22-3 shows stress corrosion cracking of a heat exchanger brass tube after exposure to ammonia vapor. The consequence is brittle failure caused by tensile stress and the environment. Prevention involves selecting suitable materials, and keeping the stress level low. A case study is reviewed later in the chapter in which SCC is suspected to have caused a vessel to explode and resulted in a fatality.

TABLE 22-3 Examples of Environments that May Cause Stress Corrosion Cracking (SCC) Metal/Alloy

Environment

Aluminum alloys

Aqueous chloride solutions

Copper alloys

Ammonia

Lead

Lead acetate solution

Magnesium alloys

Aqueous chloride solutions

Mild steel

Hot and concentrated alkaline solutions

Stainless steels

Aqueous chloride solutions

Titanium alloys

Aqueous chloride solutions

Metallurgy e Corrosion Chapter | 22

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SEQUENCE OF FAILURE BY STRESS CORROSION CRACKING Process Fluid Protective Coating Material

Steps

I

II

III

IV

I. Localized breakdown of oxide film II. Corrosion pit formation III. Initiation & growth of Stress Corrosion Cracking IV. Terminal failure

FIGURE 22-2 Sequence of failure by stress corrosion cracking. (Used by permission: Toyin Ashiru [4])

FIGURE 22-3 Stress corrosion cracking (SCC). (A) SCC is cracking induced on metal from combined influence of tensile stress and corrosive medium. (B) SCC of brass tube exposed to ammonia vapor, and (C) mercurous nitrate solution. (Used by permission: Toyin Ashiru [4])

Liquid Metal Embrittlement (LME) This involves the embrittlement of metals in the presence of certain other liquid metals, including mercury (Hg), zinc (Zn), lead (Pb) and cadmium (Cd). This form of corrosion may involve metal to metal alloying (i.e. amalgamation) in addition to embrittlement and cracking. Common sources of liquid metals are overspray of zinc-based coatings into

areas that are subsequently welded, galvanized steel in high temperature systems, lead in thread lubricants, and parts per billion mercury levels in hydrocarbon plant feed stocks. Environmentally assisted cracking may be prevented by lowering either the applied or residual tensile stresses in the material or manufactured component. Stress relief is essential when the residual forces of welding approach the material’s yield strength.

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CASE STUDY 1 High-Pressure Vessel Rupture at Nihon Dempa Kogyo (NDK) Crystal Manufacturing Company in Belvidere, Illinois, USA [5] On December 7, 2009 at approximately 2:30pm, a violent rupture occurred in the Number 2 vessel which was growing synthetic quartz crystals under conditions of extremely high pressure and temperature. One piece of steel from the building was blown 650 feet, striking the driver who was walking back to his vehicle. Another piece e a vessel fragment weighing over 8,000 pounds e tore through a wall at the facility, skimmed across a neighboring car park and struck the wall of an automotive supply company where seventy people were working. Facility Description Nihon Dempa Kogyo (NDK) Co. was founded in 1948 in Tokyo, Japan and began producing synthetic crystal products. Over time it expanded its production and sales facilities to the US, Europe and Asia. NDK produces and sells crystal-related products such as oscillators, ultrasonic transducers and synthetic quartz, and these products are commonly used in cellphones and wireless internet devices. The facility in Belvidere, Illinois is a synthetic quartz manufacturing facility, which is five stories tall and houses eight vertical pressure vessels referred to as autoclaves. The vessels are 50 ft. tall and made of steel that is over 8 inches thick. NDK Crystals shares the property with NDK America Inc., the sales and marketing portion of the company (see Figure 22-4). The Belvidere facility is the only NDK production facility in the US. Vessel Description The eight NDK vessels were designed and built for crystal growing operations that use high operating pressures, so the vessel are constructed from materials in a thickness

which can withstand these extreme conditions. The vessels consist of a 48 ft. long cylindrical shell and a 2 ft. thick closure head that operators clamp to the top. The cylindrical wall is 8.1 in. thick and the top and bottom of the vessels are significantly thicker than the vessel walls, at 181/4 inches near the lid and 161/4 inches at the base (Figure 22-5). Engineered Pressure Systems, Inc. (EPSI) designed and engineered the vessels, and Sheffield Forgemasters (Sheffield, UK), a large steel casting and forging supplier in the UK, forged the vessels in 2002 and 2003. Sheffield constructed the vessels from alloy steel to meet the material specifications of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code SA e 723 Grade 2 steel. The 140,000 pound vessels have a maximum allowable working pressure (MAWP) of about 30,000 psig (2069 barg) and a maximum operating temperature of 750 F (399 C). Operators replace the pressure relief device after each crystal growing cycle because of the high operating pressure. Each vessel went through about three such cycles per annum. Process Description The NDK quartz manufacturing vessel simulated natural geological crystal growth through heat and high pressure, in a process known as hydrothermal synthesis. The process uses raw mined quartz or lasca, which is lowered in baskets into the bottom of the vessel using an overhead crane. Operators then add 800 gals of 4% sodium hydroxide and water solution, and a small amount of lithium nitrate into the vessels. Next, operators hang ‘seed crystals’ at the top of the vessel. The seed crystals are wafer-thin, pure quartz crystals with the desired grain structure on which the new crystals grow. Operators then seal the vessel with 10,000 pound lids clamped at the top. External electric heaters slowly increase the vessel temperature to 700 F (371 C), which boils the

FIGURE 22-4 Overhead view of NDK facilities. (Used by permission: www.csb.gov)

Metallurgy e Corrosion Chapter | 22

Pressure Senior and Rupture Disc

18 1/4 inch head thickness

Lid Seal Ring

Seed Crystal Racks

Vessel Wall Heaters

Sodium Hydroxide Solution Insulation

Lasca (mined quartz)

16 1/4 inch bottom thickness 8 inch minimum wall thickness

FIGURE 22-5 NDK autoclave cross section. (Used by permission: www.csb.gov)

caustic liquid and increases the pressure inside the vessel to about 29,000 psig (2000 barg) (Figure 22-5). The natural quartz crystals or lasca dissolve in the liquid solution. Natural circulation causes the supersaturated solutions to rise to the top of the vessel, where the temperature is a few degrees lower. High-purity quartz crystals grow on the seed crystals over a period of 100 to 150 days and form a high-purity, synthetic quartz product. When the vessel run is complete and it has returned to ambient temperature, operators remove the finished quartz product and the racks from the vessel. NDK laboratory technicians examine the crystals for quality control purposes and ship the product to the NDK facility in Japan for further processing. Operators then clean the empty vessel interior with a pressure washer and vacuum the water out of the vessel. All excess caustic and water solutions are transferred to a holding tank where they are treated and neutralized prior to disposal. There is another crystal manufacturer in the US, located in Eastlake, Ohio, which has been in operation for 50 years.

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The process in this facility uses a sodium carbonate in smaller, thinner-walled vessels at a lower temperature and pressure than the sodium hydroxide process operated by NDK in Belvidere. Acmite Coating The caustic sodium hydroxide solution and silica react with the iron in the steel vessel wall and form a coating known as acmite (also known as aegirine, a greenish-gray rockforming mineral that is stable at high pressures and temperatures). The presence of acmite, or sodium iron silicate is significant in the manufacturing of synthetic quartz, and its use in lower-strength steels was documented as early as 1954. Acmite serves a dual purpose for crystal manufacturing; to protect the surface of the vessel from corrosion and to avoid iron contamination in the final product. A coating was applied by the operators when the vessel was first received, to act as a protective boundary between the liquid and the wall of the steel vessel. During this preparative run, operators added lasca, seed crystal, the caustic solution and a small amount of lithium nitrate to the vessel and heated it to 650 F and 17,000 psig (343 C and 1172 barg) for 2 to 3 weeks. During vessel operation, the acmite coating formed rapidly on the inner surface and coated the vessel wall. However, the high-strength alloy steel vessel material was susceptible to corrosion; NDK relied on the acmite layer to prevent the alkaline process environment from corroding and thus weakening the vessel wall. Incident Description On December 7, 2009, at approximately 2:30pm, vessel No. 2, under an operating pressure of 29,000 psig (2000 barg), suddenly and violently ruptured, 120 days into its 150 day operating cycle. A white cloud of steam and debris rapidly expanded outward from the facility, traveled onto the interstate, and dissipated within seconds. The sudden release of superheated liquid caused an 8 ft. tall by 4 ft. wide vessel fragment, weighing approximately 8,000 lbs., to travel through two concrete walls and finally land about 435 ft. from the NDK building. The fragment skimmed across a neighboring facility’s car park and slammed into the wall of an adjacent business office. The force of the impact pushed the wall inward causing furniture to shift and ceiling tiles to fall. One person working near the wall was injured. Further, the thrust from the escaping liquid caused the base of the vessel to violently shear away from its foundation and blew pieces of structural steel out of the building into the car park of a nearby rest stop gas station. A piece of structural steel struck and killed a truck driver at the rest stop. After shearing from its base and throwing shrapnel out of the facility, the vessel swung from the building and landed on the ground outside (Figure 22-6). The NDK production facility sustained major damage, as the exterior insulating panels were completely blown off and the force from the explosion and the displaced vessel destroyed steel framing, stairwells and floor grating near vessel No. 2. The office and laboratory areas sustained varying

1216 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

FIGURE 22-6 NDK Crystal damage after the incident; Vessel No. 2 rests on ground outside structure. The force of the explosion blew out most of the exterior wall panels. (Used by permission: www.csb.gov)

degrees of damage, and much of the final product was destroyed. The product storage area, laboratory and production offices were heavily damaged. The NDK America facility attached to the NDK Crystals production facility also received major damage due to the vessel rupture. Analysis of Failure US Chemical Safety and Hazard Investigation Board (CSB) investigators reviewed the process data for the vessel over the 120 days prior to the incident and found no evidence of a process deviation that might have caused the vessel to fail. The rupture disk on the lid of the vessel No. 2 did not activate, indicating that there was no excessive pressure buildup in the vessel prior to rupture. Vessel No. 2 pressure and temperature readings remained constant until the vessel ruptured. As a result, CSB undertook a joint metallurgical examination and testing agreement with OSHA, NDK, the insurance company and other interested parties to ascertain the cause of the vessel failure, in particular to determine whether the failure was caused by characteristics of the crystal-growing process or a problem with the vessel originating from its design and fabrication. CSB investigators observed non-destructive testing evaluations; destructive testing was performed on the 8,600 lbs. vessel fragment. After consultation with engineers and metallurgists, several samples were cut from the vessel fragment’s inner diameter, outer diameter, thickness and fracture surfaces for microscopic examination and testing. The CSB commissioned an independent review of the data by expert metallurgists at the National Institute of Standards and Technology (NIST) Materials Reliability Division, to assist in identifying the failure mechanism.

Stress Corrosion Cracking After a review of the metallurgical testing data, the CSB found strong evidence of cracking on and near the inner diameter of the vessel fragment. The cracks reduced the vessel material’s toughness (toughness is the ability of a metal to resist fracture by absorbing the energy of applied stresses), which eventually led to large flaws that in turn resulted in the catastrophic failure. SCC was the likely failure mechanism that caused the cracks, as SCC is the formation of cracks through the simultaneous action of applied stresses and a corrosive environment. Microscopic examination of the steel revealed strong evidence that SCC existed in many regions of the vessel fragment (Figure 22-7). The vessel at NDK contained an alkaline, or basic, sodium hydroxide and water solution that is generally known to damage some steels. The caustic liquid created a degrading environment inside the vessel, particularly in small surface scratches, and possibly resulted in the development of SCC. The fracture initiated at an existing, surface-breaking crack in the inner diameter of the lower portion of the vessel at its base. EPSI designed the vessels using the material NDK selected under the implied assumption that the acmite layer would prevent SCC in the vessel wall. However, the evidence of SCC in the fragment suggested that the acmite did not adequately protect the vessel, or that it was removed from this region of the vessel by some type of mechanical process, such as scratching or abrasion, during product removal or vessel cleaning between the runs. During the fragment testing, metallurgists performed energy dispersive spectroscopy (EDS) (an analytical technique that characterizes chemical elements of a sample surface) to characterize

Metallurgy e Corrosion Chapter | 22

FIGURE 22-7 Micrograph of cracking found on the fracture surface about 0.75 inches from inner diameter surface of vessel fragment. According to NIST, the branching of the crack is a telltale sign of SCC. (Used by permission: www.csb.gov)

the coating on the vessel’s inner surface. The test results reported the presence of silicon, titanium and aluminum in significant quantities, and sulfur dioxide and chloride in smaller quantities. These are known impurities in the lasca that is used as the feedstock for crystal growth. This evidence further suggested that the process fluid could penetrate the surface cracks in the vessel, indicating that the acmite coating was inadequate. Temper Embrittlement Another mechanism that could have contributed to the formation of the critical cracks, or accelerated the SCC, is temper embrittlement. This is a phenomenon inherently present in heat-treated steels. Heat treatment is a process that alters the hardness and ductility of a material’s

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microstructure. The control of time and temperature is critical for ensuring that a material has the optimum mechanical properties for its intended use. Tempering is a heat treatment technique to decrease the steel’s hardness and increase its ductility (decreasing brittleness), allowing the material to absorb more energy prior to failure. The metallurgists found possible evidence of temper embrittlement in the vessel sample from a review of Charpy impact test data. The Charpy impact test (also referred to as the Charpy V-notch test) is a standardized test that determines the amount of energy absorbed by a material during fracture. In the testing procedure, a hammer on a pendulum arm strikes the sample specimen opposite the pre-machined notch. The energy absorbed by the specimen is determined by measuring the decrease in motion of the pendulum arm. This absorbed energy is a measure of a given material’s notch toughness, and along with the appearance on the fracture surface can determine the minimum service temperature of a material. The CSB made comparisons between the manufacturing certification and post-failure testing, and found that there were considerable variations in toughness in different regions of the vessel. This general loss of toughness was identified in all orientations, which according to NIST indicates a general damage mechanism, such as temper embrittlement was acting in addition to SCC. The reduction in Charpy energy shows that the vessel’s ability to absorb energy before failure had diminished over time, which may be attributed to SCC (Figure 22-8). Key Findings The investigations by CSB resulted in the following key findings, and further details of CSB observations are in its reports [3]: 1. Stress corrosion cracking likely caused the catastrophic rupture of a high-pressure crystal production vessel at NDK Crystals Inc. that caused the death of a member of the public 650 ft away at a highway rest stop.

FIGURE 22-8 Comparison of Charpy impact test results after manufacture and post failure. Manufacturer’s Charpy data were taken from a different orientation. Post failure data points are the average of three tests in four orientations. All tests measured at 212 F. (Used by permission: www.csb.gov)

1218 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

2. NDK relied upon the in-process formation of an acmite coating inside the production vessel to protect the low alloy, high-strength steel from caustic sodium hydroxide used in the manufacturing process. However, NDK did not verify the integrity or effectiveness of this coating, and the caustic chemicals promoted stress corrosion cracking that weakened the vessel. 3. The Illinois Board of Boiler and Pressure Vessel Safety did not conduct internal inspections of the NDK vessels as required under state regulations for pressure vessels subject to internal corrosion. Instead, the state says it relied on the company to perform internal inspections, but it did not verify whether these were actually occurring. The state conducted three certificate inspections of the vessel that failed in 2003, 2006, and 2009 (less than three months prior to the incident), but these inspections focused only on accessible external surfaces. 4. In 2007, NDK learned that stress corrosion cracking was occurring in four of eight pressure vessel lids at the facility. A consultant to NDK’s insurance company warned NDK of “serious reservations” about returning the vessels to service after this discovery and specifically cited the possible danger to members of the public at the nearby rest stop in case of a catastrophic vessel failure. 5. Despite the insurance company warning, NDK did not perform the recommended non-destructive examinations of all vessels prior to returning the vessels to service. 6. NDK did not perform annual internal inspections as recommended by the vessel designer when the vessels were initially constructed. 7. Temper embrittlement, or some other form of heat treatment embrittlement, cannot be ruled out as a contributing faction in addition to SCC. The vessels exceeded the ASME wall thickness recommendations for closed-end forgings, which may have resulted in improper heat treatment during the manufacturing process. 8. The ASME Boiler Pressure Vessel Code does not have specific wall thickness limitations for pressure containing components.

CREEP AND CREEP RUPTURE LIFE [3] Creep is a phenomenon in which the material elongates over time under constant applied stress, usually at elevated temperatures. For example, a material such as tar will creep on a hot day under its own weight; however, for steels, creep occurs at temperatures above 650 F (350 C). Depending upon the stress load, time and temperature, the extension of a metal associated with creep finally ends in failure. Creep rupture and stress rupture are the terms used to indicate the stress level needed to produce failure in a

material at a given temperature for a particular period of time. For example: The tensile strength of carbon steel at 900 F (480 C) is 54,000 psi (3,700 bar), whereas the stress to cause rupture in 10,000 hours is only 11,500 psi (800 bar). The stress required to produce rupture of carbon steel in 10,000 hours (1.14 years) at a temperature of 900 F (480 C) is substantially less than the ultimate tensile strength of the steel at the corresponding temperature. The factors that influence creep are: 1. For any given alloy, a coarse grain size possesses the greatest creep strength at the highest temperatures, while at lower temperatures, a small grain size is better. 2. Creep becomes an important factor for different metals and alloys at different temperatures. For example, lead at room temperature behaves similarly to carbon steel at 1,000 F (540 C) and to a certain extent, stainless steels and super alloys at 1,200 F (650 C) or higher. 3. Relatively small changes in composition often alter the creep strength appreciably, with the carbide-forming elements being the most effective for improving strength.

Creep Rupture Life If creep is allowed to persist, the material may fail, and therefore, the designer must not only consider design stress values such that the creep deformation will not exceed a limiting amount during the required service life, but also ensure that fracture does not occur. In refinery practice, the design stress is usually based on the average stress required to produce rupture in 100,000 hours or the average stress needed to produce a creep rate of 0.01% per 1,000 hours with appropriate factors of safety. A criterion based on rupture strength is considered, because rupture life is easier to determine than low creep rates.

Thermal Fatigue Thermal fatigue results from temperature cycles in service. An alloy can still fail even though it is correctly selected and operated within its normal design limits for creep strength and hot gas corrosion resistance. Thermal fatigue is not restricted to complex structures and assemblies, as it can occur on the surface of simple shapes and appear as a network of cracks. Thermal fatigue is expected where the stresses from the expansion and contraction of temperature changes exceed the elastic limit or yield strength of a material that is not quite brittle. If the material is as brittle as glass and its elastic limit is exceeded, prompt failure by cracking can be expected. The term thermal shock can be applied to a treatment that induces prompt failure of a brittle material.

Metallurgy e Corrosion Chapter | 22

A tubular material would not crack because its elastic limit must be exceeded considerably before it fails. However, repetitive thermal stress, in which some flow occurs in both heating and cooling cycles, can result in either cracking or deformation to a degree that makes a part unserviceable. The tolerable temperature gradient for keeping deformation within arbitrary limits can be determined from the following equation: S ¼ aMTKð1
yÞ

(22-1)

where: S ¼ Stress or elastic limit or yield strength, psi. a ¼ Coefficient of thermal expansion in microinches, per inch per  F. M ¼ Modulus of elasticity (Young’s Modulus), psi or modulus of plasticity. T ¼ Temperature difference,  F. K ¼ Restraint coefficient. n ¼ Poisson’s ratio. Note: When the elastic modulus is used, the elastic limit or proportional limit should be used with it in the formula. When the plastic modulus or Secant Modulus is used, it should be used with the corresponding yield strength. Use of Equation 22-1 is restricted by the difficulty of obtaining good values. The stress value S, reflects an engineer’s judgment in the selection of an elastic limit or some arbitrary yield strength. The modulus value must match this. The restraint coefficient, K, is seldom known with any precision [1]. Readers should consult Hall [2] for further details on thermal fatigue.

Abrasive Wear Abrasive wear can be classified as: Gouging abrasion: A high-stress phenomenon that is likely to be accomplished by high comprehensive stress and impact. Most of the wearing parts in gouging abrasion service are made of some grade of austenitic manganese steel because of its outstanding toughness coupled with good wear resistance. Grinding abrasion: A high-stress abrasion that pulverizes fragments of the abrasive that become sandwiched between metal faces. The suitable alloys range from austenitic manganese steel (which dominate the field) through hardenable carbon and medium alloy steels to the abrasion-resistant cast irons. Erosion: A low-stress scratching abrasion. The abrasive is likely to be gas borne (as in catalytic cracking units), liquid borne (as in abrasive slurries), or gravity pulled (as in catalytic transfer lines). Because of the association of velocity and kinetic energy, the severity of the erosion may increase in proportion to some power

1219

(usually up to the third) of the velocity. The angle of impingement also affects the severity, as at supersonic speeds even water droplets can be seriously erosive.

MARTENSITIC STAINLESS STEELS IN REFINING AND PETROLEUM PRODUCTION An important characteristic of steel is the ability to alter its microstructure through heat treatment. Each constituent imparts a particular set of properties to the final product. For example, quenching a steel in water makes it very hard but brittle through the formation of martensite. By quenching the quenched steel, some ductility can be restored with some sacrifice in hardness and strength. Martensitic stainless steels are iron-chromium alloys with over 10.5% chromium, and they can be hardened by suitable cooling at room temperature following a high temperature heat treatment. The corrosion-resistant properties of these alloys have been less well developed than other types of stainless steel. This is due to the requirement of restricting their chromium contents to relatively low levels and their often high carbon contents, which limit their corrosion resistance compared with other stainless steels. Martensitic stainless steels are hardenable and possess high strengths and hardness while offering low cost. In order for an alloy to be a martensitic stainless steel, it must be an iron-based material that contains at least 10.5% (by weight) of chromium and must be capable of being substantially transformed through heat treatment to the hard, metastable phase called martensite. For this transformation to occur, the alloy must first be transformed by thermal treatment to the stable high temperature austenite phase. The temperature range in which austenite can form depends on the amount of chromium present in ironchromium alloys. Between 11 and w12% chromium, binary FeeCr alloys go from being capable of forming 100% austenite to being fully ferritic and incapable of forming any austenite. Other elements, notably carbon and nickel, enlarge the so-called gamma loop, thus allowing austenite to be produced in higher chromium steels. However, this phenomenon places an upper limit on the chromium content of an austenitic stainless steel of given carbon and nickel content. Once steel has been austenitized, it can be transformed to martensite by rapid cooling. Water quenching is not required, as the high alloy content of martensitic stainless steels gives them great hardenability so the rapid cooling can be performed by air cooling. Low carbon martensite exhibits reasonable ductility and toughness, but most freshly-formed martensite with higher carbon levels is often too hard and brittle for use. Therefore, most martensitic stainless steels are usually given a

1220 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

secondary heat treatment, known as tempering. The effect of tempering is controlled by the tempering temperature and time. The formation of austenite and its subsequent transformation to martensite has a profound grain refining effect, which can substantially improve the toughness of the martensitic stainless, especially in heavy sections. Some martensitic stainless steels have been used in oil or natural gas production and in oil refining. The use of metallic materials in oil production applications is regulated by the NACE International Standard MR01-75, where the concern is the potential for HE cracking (referred to as sulfide stress cracking) in aqueous service environments containing dissolved H2S (a common impurity in these applications). The materials have provided satisfactory field service in some sour environments but may, however, exhibit threshold stress levels for sulfide stress cracking in NACE Standard TM0177 that are lower than those of other materials included in the Standards. All stainless and martensitic stainless steels exhibit good or excellent resistance to atmospheric corrosion. However, the higher corrosiveness of marine atmospheres does exhibit significant differences. The high strengths attainable with martensitic and precipitation hardenable stainless steels greatly increases their susceptibility to SCC, particularly in the presence of chloride ions. The chromium content of the martensitic stainless steels gives them good oxidation resistance and allows their use at temperatures of up to 1300 F (w700 C). However, tempering of the martensitic structures and consequent loss of strength usually limits the application of these alloys to lower temperatures [7].

Heat Treatment of Steel The common types of heat treatment are [8]: Annealing (Full Annealing): One of the most common heat treatments for steel is annealing. It is used to soften steel and to improve ductility. In this process, the steel is heated into the lower regions of the austenite phase field and slowly cooled to room temperature. The resulting microstructure consists of coarse ferrite or coarse ferrite plus pearlite, depending upon the carbon and alloy content of the steel. The process gets rid of stresses in the metal and makes the grain structure large and soft-edged,so that when the metal is hit or stressed it dents or perhaps bends, rather than breaking; it is also easier to sand, grind or cut annealed metal. Normalizing: Steel is normalized by heating into the austenite phase field at temperatures somewhat higher than those used by annealing followed by air cooling. Many steels are normalized to establish a uniform ferrite plus pearlite microstructure and a uniform grain size.

Process Annealing (Recrystallization Annealing): Process annealing takes place at temperatures just below the eutectoid temperature of 1341 F (727 C). This treatment is applied to low-carbon, cold-rolled sheet steels to restore ductility. In aluminum-killed steels, the recrystallized ferrite will have an ideal crystallographic texture (preferred orientation) for deep drawing into complex shapes such as oil filter cans and compressor housings. Crystallographic texture is produced by developing a preferred orientation of the ferrite grains, i.e.; the crystal axes of the ferrite grains are oriented in a preferred rather than a random orientation. Spheriodizing: To produce steel in its softest possible condition, it is usually spheriodized by heating just above or just below the eutectoid temperature of 1341 F (727 C) and holding at that temperature for an extended period of time. This process breaks down lamellar pearlite into small spheroids of cementite in a continuous matrix of ferrite. To obtain a very uniform dispersion of cementite spheroids, the starting microstructure is usually martensite. This is because carbon is more uniformly distributed in martensite than in lamellar pearlite. The cementite lamella must first dissolve and then redistribute the carbon as spheroids whereas the cementite spheroids can form directly from martensite. Stress Relieving: Steel products with residual stress can be heated to temperature approaching the eutectoid transformation temperature of 1341 F (727 C) to relieve the stress. Quenching: Is the process of cooling a high-carbon steel very quickly after heating, thus “freezing” the steel’s molecules in the very hard martensite form, which makes the metal harder. There is a balance between hardness and toughness in any steel, in which the harder it is, the less tough or impact-resistant it becomes, and the more impact-resistant it is, the softer it becomes. To produce the higher strength constituents of bainite and martensite, the steel must be heated into the austenite phase field and rapidly cooled by quenching in oil or water. High-strength, low alloy (HSLA) steels are produced by this process followed by tempering. It must be noted that micro-alloying additions such as Nb, V, and Ti can also produce HSLA steels. These micro-alloyed steels obtain their strength by thermomechanical treatment rather than heat treatment. Tempering: When quenched steels (martensitic steel) are tempered by heat treatment to temperatures approaching the eutectoid temperature of 1341 F (727 C), the dissolved carbon in the martensite forms cementite particles, and the steels become more ductile. Tempering relieves stresses in the metals that were caused by the hardening processes and makes the metal less hard while making it better able to sustain impacts without breaking. Quenching and tempering are used in

Metallurgy e Corrosion Chapter | 22

a variety of steel products to obtain desired combinations of strength and toughness. Often, mechanical and thermal treatments are combined in what is referred to as thermo-mechanical treatments for better properties and more efficient processing of materials. These processes are common to high alloy special steels, super alloys and titanium alloys.

Plating Electroplating is a common surface treatment technique that involves bonding a thin layer of another metal such as gold, silver, chromium or zinc to the surface of the product. It is used to reduce corrosion as well as to improve the product’s aesthetic appearance.

Thermal Spraying Thermal spraying techniques are another popular finishing option, and often have better high temperature properties than electroplated coatings.

Use of Duplex 2205 for Revamps in Hydropressing Units; the Reaction Effluent Cooler (REAC) [9] In the hydropressing industry, the REAC is one of the most important components in the high pressure recycled gas loop. It provides the final cooling solution before separating the vapor (recycle gas) from the oil effluent and the sour water. The outlet temperature directly impacts the recycle gas molecular weight, as larger hydrocarbon molecules drop out of the vapor phase. The same mechanism also affects the hydrogen partial pressure, which directly impacts the life of the reactor catalyst. Operating under high pressure and low temperatures can result in ammonium bisulfide (NH4HS) and ammonium chloride (NH4Cl) precipitation, leading to pressure drop and corrosion and/or erosion-corrosion. Some refiners have experienced cracking in REAC welds, leading to fires and loss of both time and money. Tube metallurgy has a huge impact on a REAC’s life expectancy and the materials currently used in REAC systems include carbon steel, Types 300 and 400 series stainless steels, duplex stainless steel Alloys 3RE60 and 2205, Alloy 800, Alloy 825, Alloy 625 and Alloy C-276. Early refiners used carbon steel, which was found to be effective with ammonium hydrosulfide (NH4HS) concentrations up to 3 wt%. However, as sour opportunity crudes became available, the resulting NH4HS levels rose into the double digits. In cases where the resulting NH4HS concentration was between 3e8 wt%, polysulfide injection was used as a cost-effective solution for the continued use of carbon steel, and especially for revamps. However, over

1221

time, the persistent plugging and operation problems and frequency of inspection and the foul smell of the polysulfide greatly diminished its use. The increasingly sour feeds demanded more corrosion-resistant materials, such as the nickel-based Alloys 625, 800 and 825. However, as the cost of material increased, a more economic alternative with comparable protection against corrosion was sought. Duplex stainless steels are alternatives, as they offer advantages of both the ferritic and austenitic stainless families. Furthermore, they are cost effective due to their high strength and reduced alloy element content compared to other higher alloys. However, since these materials consist of dual phase microstructure, heat treating, fabrication and welding techniques require careful monitoring and reviewing to ensure that the balanced microstructure is not compromised. The most commonly used grade is Duplex 2205. Early implementations of duplex REACs failed to show significant reliability improvements, and a few units failed by hydrogen embrittlement cracking or sulfide stress cracking (SSC). Improvements in steel manufacturing have minimized microstructural deterioration during fabrication. Many of the reported incidents involving Duplex 2205 REAC failure have a common factor: pressures greater than 1000 psig (69 barg) and a NH4HS concentration of 6 wt% or greater. Case study 2 highlights these conditions. CASE STUDY 2 Quality-Controlled Replacement of Carbon Steel with Duplex 2205 for Revamps in Hydropressing Units, the Reactor Effluent Cooler (REAC) [9] A company in Asia had a carbon steel REAC running for a number of years, which required an expansion of the unit using REAC metallurgy upgrade to Duplex 2205. The design pressure of the REAC was w2400 psig (165.5 barg) and the NH4HS concentration was expected to be 5 wt % during the modeling phase. A new Duplex 2205 REAC was installed and operated without problems for about two years. Suddenly fire erupted due to REAC failure; nearby equipment and piping were damaged. Cracks were observed on the weld joints between the top plate and tube sheet, as well as the bottom plate and the tube sheet of the floating header. Fin tubes were deformed, and the walkway was nearly unusable. An investigation to find the root cause of the REAC failure was commissioned. The investigation discovered that during fabrication of the REAC, Charpy impact testing was conducted at 32 F (0 C) instead of
40 F (
40 C) (as specified by Chevron Lummus Global, [CLG]). This oversight led to inadequate toughness and low ductility of the welds as well as less than favorable microstructural phase balance. Hardness values in the heat-affected zones (HAZ), and at the weld of failed specimens were higher than those recommended (in the range of 313e359 HV10 vs. 310 HV10 maximum). As a result, CLG concluded that REAC failure was due to SSC.

1222 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

REAC Safety Through Design Failures of REACs can be mitigated or eliminated by considering safety through design. Design considerations that are recommended are [9]: l Using balanced symmetrical piping. l Limiting tube velocities based on metallurgy, NH4HS concentration and H2S partial pressure. l Specifying fabrication and welding guidelines for Duplex 2205 above and beyond those specified in API TR 938C. l Using single point water injection instead of multi-point water injection. Symmetrical piping for REAC inlet tubes is essential for reactor effluent as it is impossible to implement any type of flow control for two phase flow, therefore, the use of symmetrical piping will provide even distribution of gas and liquid phases as it enters the air cooler. During basic engineering, the following tube velocity limitations are implemented, based on the tube metallurgy and the NH4HS concentration: Material carbon steel tubes and piping with less than 3 wt% NH4HS 2205 Duplex stainless steel tubes and piping with 3e12 wt % NH4HS Alloy 825 tubes and piping with up to 15 wt %, NH4HS

Allowed velocity, ft/s 10e20

Preferred velocity, ft/s 15

10e30

25

10e40

35

In all cases, tube velocities falling below 10 ft/s become hazardous, as phase separation and corrosion can result. The service life of a REAC can be improved by using balanced symmetrical inlet piping, limiting tube velocities based on metallurgy and NH4HS concentration, singlepoint water injection and working with suppliers and welders who have good quality control with Duplex 2205.

CORROSION The worldwide cost of corrosion runs into billions of dollars annually, and failures due to corrosion in the chemical process industry result in catastrophic effects (e.g., explosion, environmental, property and loss of life) and countries are finding ways of preventing this from occurring. In refinery operation, a combination of aging plants, greater fluid corrosiveness and health, safety and environmental requirements have made corrosion management an essential consideration. Steel pipework and vessels are always susceptible to corrosion or erosion, and unless monitored there are risks of failure with catastrophic consequences, and the financial costs of operational interruption, repairs and reputational damage must be considered. As oil and gas operators produce and process ever

more corrosive or erosive hydrocarbon streams, the demands on plant metallurgy greatly increase. Permanently installed sensor systems can deliver a continuous picture of asset condition over time at a comparable cost to that of a single manual inspection. This can be correlated with process conditions that may be causing corrosion or erosion, and ways can be devised to minimize corrosion or erosion, such as using inhibitors. The asset manager can thus move beyond merely knowing whether corrosion or erosion is occurring to understanding why and at what rate. This understanding allows operators to make better informed decisions. Corrosion can be defined as the destruction of a metal by chemical or electrochemical reaction with its environment or as the undesirable deterioration of a metal or alloy, i.e., an interaction of the metal with its environment that adversely affects those properties of the metal that are to be preserved. For initiation and propagation of corrosion attack, a “cell” must be established. This is an electrical circuit in which transport of electrons and ions takes place between differently charged electrodes. Corrosion occurs at the anode, where metal dissolves. The anode is often physically separated from the cathode, where an oxygen reduction reaction takes place. An electrical potential difference exists between these sites, and current flows through the solution from the anode to the cathode. This is accompanied by the flow of electrons from the anode to the cathode through the metal, as illustrated in Figure 22-9. For example, when steel is exposed to an industrial atmosphere, it reacts to form the reaction product rust, of approximate composition Fe2O3$H2O, which being loosely adherent does not form a protective barrier that isolates the metal from the environment; the reaction thus proceeds at an approximately linear rate until the metal is completely consumed. Copper, on the other hand, forms an adherent green patina, corresponding approximately with bronchantite, CuSO4$3Cu(OH)2, which is protective and isolates the metal from the atmosphere. Copper roofs installed several hundred years ago are still performing satisfactorily, and it is apparent that the formation of brochantite is not deleterious to the function of copper as roofing material [10]. For steel, the typical anodic oxidation reaction is: Fe/Fe2þ þ 2e


(22-2)

This reaction is accompanied by: Fe2þ þ 2OH
/FeðOHÞ2

(22-3)

The ferrous hydroxide then combines with oxygen and water to produce ferric hydroxide, FeðOHÞ3 . The primary cathodic reaction in the cooling system is: 1 O2 þ H2 O þ 2e
/2OH
2

(22-4)

Metallurgy e Corrosion Chapter | 22

1223

Water (Electrolyte) Fe2+

O2

OH-

O2

Fe(OH)3 H2O Fe(OH)2

Cathode

Anode Electron flow

Anodic Reactions

Cathodic Reactions

Chemical oxidation

Chemical Reduction

Fe → Fe o

2+

+ 2e

1 2 O 2 + H 2 O + 2e → 2OH −

2Fe ( OH )2 + 1 2 O2 + H 2 O → 2Fe ( OH )3

FIGURE 22-9 Corrosion occurs when anodic and cathodic reactions take place on a metal surface.

The production of hydroxide ions creates a localized high pH at the cathode, approximately 1e2 pH units above that of the bulk water. Dissolved oxygen reaches the surface by diffusion, as indicated by the wavy lines in Figure 22-9. The oxygen reduction reaction controls the rate of corrosion in cooling systems; the rate of oxygen diffusion is usually the limiting factor [12]. Another important cathodic reaction is: 2Hþ þ 2e
/H2

(22-5)

At neutral or higher pH, the concentration of Hþ ions is too low for this reaction to contribute significantly to the overall corrosion rate. However, as the pH decreases, this reaction becomes more important, until a pH of about 4, when it becomes the predominant cathodic reaction [4].

Corrosion as a Chemical Reaction at a Metal/Environment Interface Consider corrosion as a heterogeneous chemical reaction which occurs at a metal/non-metal interface and which involves the metal itself as one of the reactants. Corrosion can be expressed by a simple chemical reaction: aA þ bB ¼ cC þ dD

(22-6)

where A is the metal and B the non-metal reactant (or reactants), and C and D are the products of the reaction. The non-metallic reactants are frequently referred to as the environment, although it should be observed that in a complex environment, the major constituents may play a very subsidiary role in the reaction. Therefore, in the “atmospheric” corrosion of steel, although nitrogen constitutes approximately 75% of the atmosphere, its effect

compared with that of moisture, oxygen and sulfur dioxide, solid particles, etc. can be disregarded (in the high temperature reaction of titanium with the atmosphere, however, nitrogen is a significant factor). One of the reaction products (e.g. C) will be an oxidized form of the metal, and D will be a reduced form of the nonmetal. C is usually referred to as the corrosion product, although the term could equally apply to D, and in its simplest form Equation, 22-1 becomes: aA þ bB ¼ cC e.g., 4Fe þ 3O2 ¼ 2Fe2 O3

(22-7)

where the reaction product can be regarded either as an oxidized form of the metal or as the reduced form of the nonmetal. Reactions of this type, which do not involve water or aqueous solutions, are referred to as “dry” corrosion reactions. The corresponding reaction in aqueous solution is referred to as a “wet” corrosion reaction, and the overall reaction (involving a series of intermediate steps) can be expressed by: 4Fe þ 2H2 O þ 3O2 ¼ 2Fe2 O3 $H2 O

(22-8)

Therefore, in all corrosion reactions one (or more) of the reaction products will be an oxidized form of the metal, aqueous cations (e.g. Fe2þ (aq), Fe3þ (aq)), aqueous anions 2
(e.g. HFeO
2 ðaqÞ; FeO4 ðaqÞ, or solid compounds (e.g. Fe(OH)2, Fe3O4, Fe3O4$H2O or Fe2O3$H2O), while the other reaction product (or products) will be the reduced form of the non-metal. Corrosion may be regarded, therefore, as a heterogeneous redox reaction at a metal/ non-metal interface in which the metal is oxidized and

1224 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

the non-metal is reduced. In the interaction of a metal with a specific non-metal (or non-metals) under specific environmental conditions, the chemical nature of the non-metal, the chemical and physical properties of the reaction products and the environmental conditions (temperature, pressure, velocity, viscosity, etc.) will be essential in determining the form, extent and rate of the reaction. Figure 22-10 shows practical environments in corrosion, and also serves to emphasize the relationship between the detailed structures of the metal, the environment and external forces such as stress, fatigue, velocity and impingement.

Corrosion of Materials Some materials will corrode faster than others when exposed to a particular environment. For example, carbon steel will corrode rapidly in seawater, while gold is inert. Why do they behave so differently? This can be explained by the energy levels of the various materials. All materials have a natural desire to be at the lowest possible energy level. For most metallic materials, this low energy level can be obtained when the metals occur as mineral, oxide or similar compounds and not in the form of the metal or alloy that is used for

construction purposes. In nature, only a few pure metals are found. These metals are referred to as “noble metals” and do not need to go through energy-consuming purification or refining processes before being taken into use. Thus, the energy level of the noble metals is almost the same as the level of the most stable form found in nature. For magnesium, zinc, aluminum, iron and steel, the situation is different. These metals (or their alloys) are not found free in nature. For example, iron and steel are extracted from iron ore in a blast furnace or electro-oven where the iron ore together with coal or coke is heated up to a very high temperature. Large amounts of energy are required to produce iron or steel. Thus, the energy levels of the materials in use are much higher than the natural energy levels of the metals found in nature. Nature does not look upon the new energy levels as very favorable and starts a process of bringing the metals back to their origins. The metals will decompose (corrode), and energy will be released; a corrosion process has started. In a very simplified model, the more energy required producing a metal or alloy, the higher is the driving force to start a corrosion process (Figure 22-11) [4]. Figure 22-12 shows a corrosion process in steel. The material dissolves when corrosion occurs. A large portion of the resulting corrosion products will remain on the metal surface, even though

FIGURE 22-10 Environments in corrosion. (Used by permission: Shreir, L. L., Jarman, R. A., and G. T. Burstein, Corrosion Volume 1, 3rd. Ed., Butterworth e Heinemann)

Metallurgy e Corrosion Chapter | 22

1225

FIGURE 22-11 Energy levels of materials. (Used by permission: Toyin Ashiru [4])

FIGURE 22-12 Corrosion process on steel. (Used by permission: Toyin Ashiru [4])

some may be transported away from the electrolytes with high flow velocities. In most cases, corrosion processes cause the formation of voluminous corrosion products (e.g., iron oxide, Fe2O3) on the metal surface. The bestknown corrosion product is rust. Materials can be made more stable, usually by one of two methods, or their combination:

expected. There are many reasons for these negative surprises, and the most common ones are [4]:

1. Addition of alloying elements to give certain properties. For example, stainless steels are alloyed with chromium, nickel and molybdenum. These materials will make the steel more stable (resistant to environmental attack) and will promote the formation of a strong, dense protective oxide film on the metal surface. 2. Formation of protective films on the metal surface that are more resistive than the base material (usually oxide films, as is the case for stainless steel and aluminum).

l

Furthermore, the opposite effect is frequently experienced, if materials suffer corrosion where inertness is

l

l l

l

Exposure to the environment is unexpectedly more severe than specified in the design, e.g., due to contamination. Unfavorable alloying elements have been used. Impurities or contaminants introduced into the alloy. Welding or machining has changed the microstructure of the alloy or the heat-affected zone (HAZ). An external energy source alters the electrochemical potential (such as stray current, welding, etc.) Parameters affecting general corrosion are [4]: pH Lower pH stimulates cathodic reaction. pH affects the stability of passive films. Dissolved gas l Dissolved oxygen l Dissolved carbon dioxide l l

1226 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

Velocity l Increased velocity enhances the mass transfer rate. l Enhances mechanical erosion. Temperature l Increased temperature increases the reaction rate. l But increased temperature decreases the concentration of oxygen. l Difficult to interpret. Corrosion is measured as depth of attack in mm or in mm depth of localized attack (pitting of crevice) or in terms of loss weight (g/cm3) or as a rate (mm/years).

Types of Corrosion The anodic and cathodic sites necessary to produce corrosion can be formed due to a variety of reasons: impurities on the metal, localized stresses, differences in metal grain size or composition, discontinuities across the surface and differences in the local environment (e.g., temperature, oxygen or salt concentration). When these local differences are not large, and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching or dealloying (e.g., de-zincification in brass), galvanic corrosion, crevice or under-deposit corrosion, intergranular corrosion, stress corrosion cracking (SCC), and microbiologically influenced corrosion (MIC). Another form of corrosion, which cannot be accurately categorized as either uniform or localized, is erosion corrosion, which is sometimes referred to as flowaccelerated corrosion (FAC).

TABLE 22-4 Avoid Galvanic Corrosion by Selecting Materials that are Near Each Other in a Galvanic Series Corroded End (Anodic or Least Noble) Magnesium Magnesium Alloys Zinc Aluminum Cadmium Aluminum Alloys Steel or Iron Cast Iron 304 Stainless Steel (Active) 316 Stainless Steel (Active) Lead e tin Solders Lead Tin Nickel (Active) Inconel (Active) Brasses Copper Bronze Alloys Copper e Nickel Alloys Monel Nickel (Passive) Inconel (Passive)

Galvanic Corrosion Galvanic corrosion occurs when two dissimilar metals are in contact in a solution. The contact must be good enough to conduct electricity, and both metals must be exposed to the solution. The driving force for galvanic corrosion is the electric potential difference that develops between two metals. The consequence is that the more active metal or alloy becomes anodic (corrodes), while the more noble metal or alloy remains cathodic and is protected. Tables 22-4 and 22-5 show galvanic series of some commercial metals and alloys in seawater and soil. Tables 22-4a and 22-5a show the corrosion potential (voltage) range of metals and alloys vs. a saturated calomel electrode, and the potential of metals in soil vs. a copper/copper sulfate electrode, respectively. The relative ranking indicates the degree to which two metals e when joined together in a conductive, ionic solution (such as seawater) e will set up an anode-cathode relationship, in which the anodic (less noble) metal will preferentially corrode at a high rate. In theory, the

304 Stainless Steel (Passive) 316 Stainless Steel (Passive) Hastelloy C-276 Silver Titanium Graphite Gold Platinum Protected End (Cathodic, or Most Noble)

farther apart the two metals are in the galvanic series, the larger is the driving potential for corrosion, the larger the driving potential for galvanic current flow and the more accelerated the damage in the anodic material. From these tables, it can be seen that iron is more reactive (base) than

Metallurgy e Corrosion Chapter | 22

1227

TABLE 22-4A Corrosion potential (voltage) range of metals and alloys (vs. Saturated Calomel Electrode). Corrosion potential (voltage) range of metals and alloys (vs. Saturated Calomel Electrode) +0.2

0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

Magnesium Zinc Aluminum Alloys Cadmium Mild Steel, Cast Iron Low Alloy Steel Aluminum Bronze Naval Brass, Yellow Brass, Red Brass Tin Copper Lead-Tin Solder (50/50) Admiralty, Aluminum Brass Manganese Bronze Silicon Bronze Stainless Steels – Type 410, 416 90-10 Copper-Nickel Lead 70-30 Copper-Nickel Nickel Silver Monel Stainless Steels – Type 302, 304, 321, 347 Stainless Steels – Type 316, 317 Titanium Platinum Graphite +0.2

0

-0.2

-0.4

-0.6

Cathodic or Noble End

-0.8

-1.0

-1.2

-1.4

-1.6

Anodic or Active End

(Source: Courtesy of Toyin Ashiru [4]).

copper and therefore, has a stronger tendency to ionize and dissolve than the more inert copper (noble). The galvanic series can also be used to avoid galvanic corrosion. When making connections in a cooling water system, select materials that appear close together, e.g., use brass fittings with a bronze or cupro-nickel alloy assembly, avoid small anode against a large cathode, insulate dissimilar metals, cautious coating application may be

considered, deaeration of water, e.g. oxygen scavenger and use a suitable inhibitor. One way of preventing galvanic corrosion is to select alloys that are electrochemically similar (i.e. exhibit an open circuit corrosion potential difference of < 200 mV) or have comparably alloy constants. Examples of similar alloy classes include carbon steel and low-alloy steels, stainless steel (SS), and nickel-based and titanium alloys. Cathodic

1228 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

TABLE 22-5 Galvanic Series in Soil Base, Active Or Anodic End Magnesium

copper generated via corrosion and to complex any soluble copper to the cooling water. Figure 22-13 shows a galvanic corrosion inside a stabilizer, which occurs because dissimilar metals are in contact in a corrosive medium. A remedial solution is to insulate the metals with epoxy coating.

Zinc Aluminum Clean mild steel Rusted mild steel Cast iron Lead Mild steel (in concrete) Copper, brasses and bronzes High silicon cast iron Carbon, coke graphite Noble Or Cathodic End

protection can be effected by connecting a structure to a sacrificial anode made from a more active metal, such as magnesium, zinc or aluminum. Generally, graphite, platinum, SS, bronze and copper are protected at the expense of iron, aluminum, zinc and magnesium (the most corrosionprone common metal). Sacrificial anodes are generally limited to units requiring currents of less than 3e4 A (the ideal voltage for cathodic protection varies depending upon the metals and environment involved, but is commonly around 1 V). Optimizing the relative areas of the two metals is another important preventative measure. The area of the more actively corroding material (anode) should be large relative to the more corrosion-resistant material (cathode). For example, if a large piece of copper (cathode) is attached to a small piece of steel (anode), the steel will suffer rapid galvanic corrosion [6]. Other preventative measures involve the application of a coating to reduce the cathodic area, or the use of a corrosion inhibitor. Galvanic corrosion can be controlled by sacrificial anodes. This method is commonly used in heat exchangers with admiralty tube bundles, carbon steel tube sheets and channel heads. The sacrificial anodes are bolted directly to the steel and protect a limited area of the anode. Proper placement and sizing of sacrificial anodes is required to ensure protection. The most serious form of galvanic corrosion occurs in a cooling system that contains both copper and steel alloys. This occurs when soluble copper ions plate out of solution by a reduction reaction onto a steel surface and induce rapid micro galvanic attack on the steel. The amount of dissolved copper required to produce this effect is very small, and the resulting steel corrosion is very difficult to inhibit once it begins. A copper corrosion inhibitor is often needed to reduce the amount of soluble

Pitting Pitting corrosion is a localized form of corrosion by which cavities or ‘holes’ are produced in the material. It is the most dangerous forms of corrosion because it is more difficult to detect. Pitting occurs when anodic and cathodic sites become stationary due to large differences in surface conditions. It is generally promoted by low velocity or stagnant conditions (such as those commonly found in heat exchangers with shell side cooling) and by the presence of chloride ions. Once a pit is formed, the liquid inside it is isolated from the bulk environment and becomes increasingly corrosive over time. The high corrosion rate in the pit produces an excess of positively charged metal cations, which attract chloride anions. In addition, hydrolysis within the pit produces Hþ ions. The increase in acidity and concentration within the pit promote even higher corrosion rates, and the process becomes self-sustaining. Inhibitors can be used to control pitting. Prevention of pitting can be attained by: l l

l l l

Selecting materials resistant to particular environment. Avoiding designs with stagnation and alternating wetting/drying. Increasing process stream velocity. Considering the cautious use of inhibitors. Considering lowering the service temperature.

Chlorides and other halogens can often enhance pitting, especially in stainless steel. The localized anodic attack that ignites pitting can become autocatalytic in nature. When this happens, the solution acidifies and local metal loss is exacerbated by the presence of a small anode (i.e. pit) and a large cathode (i.e. the surrounding non-corroding material). A commonly employed mitigation technique is to increase the velocity of the fluid medium, thus preventing the solution from stagnating at the exposed metal surfaces. Minimum flow velocity requirements vary with both the alloy and the service. For example, condenser tube metals such as Alloy 400, Alloy 825 and Type 316 SS (which is often used in seawater applications) normally require a minimum flow velocity of 5 ft/s to avoid the formation of deposits and resultant pitting problems. Higher alloys materials such as Alloys 625 and C276 are more resistant to pitting and do not have minimum flow velocity limits [6]. Selecting a material with a higher pitting resistance index (PRI), as defined by the following expression, can prevent pitting corrosion. PRI ¼ Cr þ 3:3ðMoÞ þ 1:5ðNb þ WÞ þ XðNÞ

(22-9)

Metallurgy e Corrosion Chapter | 22

1229

TABLE 22-5A Potential of metals in soil (vs. Copper/Copper Sulfate Electrode)

+0. 2

0

Potential of metals in Soil (vs. Copper/Copper Sulfate Electrode) -1.0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 Pure Magnesium Magnesium Alloys Zinc Aluminum Alloy Pure Aluminum Mild Steel (clean and shiny) Mild Steel (rusted) Cast Iron Lead Mid Steel in Concrete Copper, Brass, Bronze Mill Scale on Steel High Silicon Cast Iron

Graphite Cathodic or Noble End Active End

Anodic or

Metal

Potential (*Cu/CuSO4)

Pure magnesium

-1.75

Magnesium Alloy

-1.60

Zinc

-1.10

Aluminum Alloy

-1.05

Pure aluminum

-0.8

Mild Steel (Clean & Shiny)

-0.50 to -0.80

Mild Steel (Rusted)

-0.20 to -0.50

Cast Iron

-0.50

Lead

-0.50

Mild Steel in concrete

-0.20

Copper, Brass, Bronze

-0.20

Mill Scale on steel

-0.20

High silicon cast iron

-0.20

Carbon, Graphite, Coke

+0.30

*Cu/CuSO4 stands for Copper sulfate electrode (Source: Courtesy of Toyin Ashiru [4]).

where: Cr, Mo, Nb, W and N are the concentrations by weight of chromium, molybdenum, niobium, tungsten and nitrogen respectively in the stainless alloy material, and x ¼ 0 for ferritic SS.

x ¼ 16 for duplex SS, which contains both austenitic and ferritic phases. x ¼ 30 for austenitic SS. For example, the higher pitting resistance of Alloy 2205 Duplex SS (22% Cr; 3% Mo; 0.15% N) versus Type 316SS (17% Cr; 2.5% Mo) can be ascertained by comparing their

1230 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

are resistant to particular environments, increasing the process stream velocity and possibly lowering the service temperature.

Crevice Corrosion

FIGURE 22-13 Galvanic corrosion inside a horizontal stabilizer.

respective PRI values. The PRI of Alloy 2205 is 34.3, and that of Type 316 SS is 25.3. Inert gas blanketing of aerated systems or the removal of oxidizing agents are other methods for controlling pitting. The presence of oxidizing agents or a high concentration of oxygen in a solution renders some materials more susceptible to localized corrosion. Titanium and zirconium alloys exhibit high pitting resistance in severe oxidizing environments due to their ability to form stable oxide films, which can withstand chemical attack. However, these alloys are vulnerable to other forms of corrosion, particularly in strong acids and alkalis. Examples of pitting corrosion are shown in Figure 22-14. Very localized corrosion attack results in small holes in plants. The consequence is that penetration is generally rapid, and prevention is done by selecting materials that

Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the microenvironmental level. Intensive localized corrosion frequently occurs in shielded areas of exposed metal surfaces. Solutions within a crevice are similar to solutions within a pit, in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (e.g. stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also true of protective films induced by anodic inhibitors. Joints, gaskets, laps (i.e. areas where one material overlaps another), even debris and bio films provide superficial sites where the solution can stagnate, and catalyze localized attack known as crevice corrosion. Biofilms or layers that form on the surfaces of material prevent oxygen from reaching the underlying substrate and can have the same effect as a crevice. Equipment redesign or replacement of bolted components with welded elements can eliminate areas

FIGURE 22-14 Pitting corrosion in a chemical plant.

Metallurgy e Corrosion Chapter | 22

where solution may become trapped. Openings can be sealed with non-absorbent materials to inhibit moisture penetration. Elastomeric and non-porous plastics, such as fiberglass-reinforced plastic (FRP) and Polytetrafluoroethylene (PTFE), a chemical term for the polymer CF2 and having a trademark known as Teflon, which have a high degree of chemical resistance, are often materials of choice, and scale or debris may be removed by inducing turbulent flow. The best way to prevent crevice corrosion is to prevent crevices. For cooling water systems, this requires the removal of deposits from the metal surface. Deposits may be formed by suspended solids (e.g. silt, silica) or by precipitating species, such as calcium salts. Figure 22-15 shows crevice and pitting corrosion. Remedial measures are [4]: l l l

Regular removal of deposits and drainage. Use of non-absorbent gaskets. Avoid ingress of oxygen containing water during pressure testing.

In this situation, the flow from the fluid creates a turbulence that removes the protective layers of corrosion products from the metal surface. Then the bare base material will be continuously exposed to the corrosive environment, giving very high localized corrosion rates. Soft metals are particularly vulnerable to this form of attack; e.g. copper, brass, pure aluminum and lead, but most metals are susceptible to erosion corrosion in particular flow situations. Erosion corrosion is increased by high water velocities and suspended solids. It is often localized in areas where water changes direction. Cavitation damage e due to the formation and collapse of bubbles in highvelocity turbines, pump impellers, propellers, etc. e is a form of erosion corrosion. Some factors, which are likely to cause erosion corrosion are [4]: l l

l

Erosion (Velocity Accelerated) Corrosion Inducing turbulent flow in process fluids is sometimes a means of reducing corrosion. However, when the motion of the corrosive environment is significant relative to the metal’s surface, erosion e or velocity accelerated corrosion (VAC) can occur. This condition is a direct result of the shear stress created by the medium on the walls of the vessel. The magnitude of the stress depends upon a combination of factors, including the system’s geometry, the flow regime and the fluid’s velocity, density and viscosity. Thus, erosion corrosion is a result of a too high flow velocity of a corrosive flowing fluid that causes material failure due to the combined effect of erosion and corrosion.

1231

Flow velocities exceeding the specified velocity. Sudden change in the bore or joint, which introduces a discontinuity in the otherwise smooth metal surface causing localized flow turbulence (vortices). The presence of a corrosion product or other deposits, which may disturb the laminar flow and consequently, cause turbulence.

When an electrolyte is moving relative to a metal, the rate of attack often increases because of the combined action of corrosive solution and the eroding action of movement. Pipelines carrying slimes, e.g. liquids which also contain solid particles, are particularly susceptible to erosion corrosion but most corrosive solutions can produce this effect. The nature of the damage has a characteristic appearance (Figure 22-16). Figure 22-17 shows erosion corrosion of an impeller, where deterioration was due to corrosive fluid flow over the surface. This is characterized by the grooves showing the

Pitting corrosion on free surface

Crevice corrosion under washers.

FIGURE 22-15 Crevice and pitting corrosion process.

1232 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

FIGURE 22-16 Pitting corrosion.

flow pattern. Mitigating this erosion corrosion attack involves selecting suitable materials, possibly reducing the fluid velocity, filtering out solids and applying wearresistant coatings. Erosion/velocity accelerated corrosion is commonly characterized by surface features with a directional pattern, and is most prevalent in alloys such as copper and aluminum. Surface films, which occasionally form in situ as corrosion products, protect the metal, thus reducing the corrosion rate. However, in the presence of a high enough velocity, called the limiting flow velocity, a surface film can be removed, thus continually exposing the fresh metal to the medium, which accelerates the rate of corrosion. Increases in temperature, which generally add to the severity of corrosion, may improve the stability of surface films as this is particularly true for conditions where steels are exposed to environments containing hydrogen sulfide

FIGURE 22-17 Erosion corrosion of an impeller.

(H2S) and carbon dioxide (CO2), or both. Carbonate films on steel are most stable, but are protective at temperatures above 212 F (100 C) as opposed to lower temperatures. In piping, replacing copper with cupronickels will increase the material’s resistance to high flow velocities. Cupronickels typically have a nickel content of 10 to 35%. More severe flow conditions require metals with higher nickel content. Preventative measures against erosion (VAC) include selecting alloys with greater resistance to the effects of flow velocity. In addition to using cupronickels, sometimes a system can be redesigned to reduce local turbulence, cavitation or fluid impingement of the liquid media. The most severe conditions are these of slug flow and mist flow, where it is possible to develop very high levels of shear stress on the walls surrounding the flowing media. In slug flow, the advancing slugs of liquid in the piping create conditions of high local turbulence, which can elevate the shear stress to over ten times the nominal levels. Mist flow can lead to droplet impingement in elbows and tees (see Chapters 4 and 15 of this volume series).

Dealloying Corrosion Dealloying corrosion or selective corrosion is a galvanic corrosion process. In this instance, one element (generally the most active one) is selectively removed from a solid alloy. As a result, the components of the alloy react in various proportions that differ from a solid alloy. Apart from the general term, in specific cases the process is often named after the removed element, e.g. de-zincification of brass or de-aluminification of certain aluminum/bronzes. However, this is not so in the case of graphitization of gray irons (here, the removed element is iron). In the dealloying or de-zincification of brass, it is recognized that the alloy assumes a red copper alloy color (i.e. in contrast to the original yellow). De-zincification zones are porous and mechanically weak and may result in a sudden break when the alloy is exposed to mechanical loading. When brass containing 70% copper, 30% zinc is exposed to aqueous corrosion, particularly in the presence of chloride ions (Cl
), zinc dissolves selectively from the alloy. Plug-type de-zincification produces no major change in overall dimensions but is the most potentially damaging of this type of process. The residual copper has no inherent strength, and because it is porous offers no resistance to corrosion. The solution to the problem is to choose more resistant materials. Admiralty brass, containing tin and significant traces of arsenic, is resistant to pitting, and hence to dealloying by seawater. Cupronickels (e.g. 70% Cu e 30% Ni) are even more resistant. Further examples of this process are selective corrosion of aluminum copper alloys “dealuminification” and of gray cast iron “graphitization.” In the latter case, the metal

Metallurgy e Corrosion Chapter | 22

1233

Intergranular Corrosion

FIGURE 22-18A De-zincification selectively removes zinc from the alloy, leaving behind a porous, copper-rich structure that has little mechanical strength.

All commonly-used metals and alloys are polycrystalline. The zones (‘grain boundaries’) between the tiny crystals and grains have different atomic structures and chemical compositions compared with the interior of the grains. Consequently, the electrochemical properties and corrosion resistance of grain boundaries are very different from those of the grains themselves. Under suitable aqueous conditions, grain boundaries become anodic to the surrounding (cathodic) grains, and the rate of attack is intensified by the small anode to the large cathode area ratio. Intergranular corrosion varies from light etching of grain boundaries, which may be useful in metallographic investigations of metal failures, to rapid attack with resultant embrittlement. An important form of intergranular corrosion known as weld decay may occur either by welding certain types of stainless steel or by slow stainless steel casting. Welding 303 stainless steel metal near the weld (called the heat-affected zone e HAZ) will enter the range 500e800 C and is ‘sensitized.’ That is, chromium carbide forms at the grain boundaries, leaving a deficiency of chromium for forming the protective chromium oxide film and allowing corrosion. The following ways to overcome weld decay are: l

FIGURE 22-18B Dezincification of brass. (Used by permission: www. hghouston.com) l

l

Heat treating the welded component to 110 C to redissolve the carbides followed by quenching to prevent carbide precipitation. This may cause residual stresses and distortion. Use a low carbon stainless steel (e.g. 304 stainless steel) to reduce carbide formation. Use a stabilized stainless steel containing titanium or niobium.

Stabilized steels may suffer intergranular corrosion called knife-line attack caused by temperatures greater than 1100 C in the heat-affected zone, which dissolve titanium or niobium carbide. On cooling, it is again chromium carbide, which forms with the same effects as before. This is cured by heating to 1050 C, when chromium carbides redissolve and titanium or niobium carbides reform. The material does not require quenching, which avoids distortion problems. FIGURE 22-18C Dezincification of brass. (Used by permission: www. hghouston.com)

around the cathodic graphite flakes dissolves to leave a mechanically weak network of graphite with rust and pits, while cast irons, which lack free graphite, do not suffer graphitization. Figures 22-18A, B and C show the dezincification of brass involving the preferential removal of zinc from brass (Zn/Cu alloy). This can be mitigated or prevented by reducing the oxygen content of the environment.

Hydrogen Damage The atomic hydrogen produced on carbon steel by corrosion processes in acid solution, e.g. pickling (also by electroplating, electro-cleaning and electro-polishing) does not all form hydrogen gas bubbles in the electrolyte. Some hydrogen atoms diffuse through the material and recombine at a suitable point to form hydrogen gas, generating high internal pressures, blistering (Figure 22-19) and a loss of mechanical properties (embrittlement). Hydrogen blistering may be overcome by using a surface coating, modifying the environment with inhibitors or baking out the hydrogen.

1234 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

H2

H2

Electrolyte

H+

H+

H

H

H+

H+

H

H

H2

e

e

e

e Carbon steel

FIGURE 22-19

High strength steels of tensile strength greater than 1100 Nmm
2 are particularly susceptible to hydrogen embrittlement. Where this effect is a potential problem, steels containing nickel and/or molybdenum should be used.

Microbiologically Influenced Corrosion (MIC) Microorganisms in cooling water form biofilms on cooling system surfaces. Biofilms consist of populations of sessile (i.e., immobile) organisms and their hydrated polymeric secretions. Many types of organisms may exist in any particular biofilm, ranging from strictly aerobic bacteria at the water interface to anaerobic bacteria such as sulfate-reducing (SRB) at the oxygen-depleted metal surface. The presence of a biofilm can contribute to corrosion in three ways: l l l

complex, however this corrosion type occurs in oil tanks, ballast tanks and on structural object in marine muds (e.g. pipeline). The conditions that promote these corrosion types are: l l l l

Stagnant water. Adequate nutrition for bacteria. Sulfate (for sulfate-reducing bacteria e SRB). Suitable temperatures (2e40 C).

Figure 22-20 shows a typical result of microbial corrosion, where the surface exhibits scattered areas of localized corrosion that are unrelated to flow patterns. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. This pattern of localized corrosion is sometimes referred to as pit-within-a-pit morphology [12].

Physical deposition. Production of corrosive byproducts. Depolarization of the corrosion cell caused by chemical reactions.

Deposits can cause accelerated localized corrosion by creating differential aeration cells. This same phenomenon occurs with a biofilm. The non-uniform nature of biofilm formation creates an inherent differential, which is enhanced by the consumption of oxygen by organisms in the biofilm. Many of the byproducts of microbial metabolism, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials during metabolism, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolarization, stimulates further corrosion. The mechanisms of MIC are

FIGURE 22-20 In microbiologically influenced corrosion (MIC), as a biofilm became established on a carbon steel surface, the sulfatereducing bacteria (SBRs) created the pit-within-a-pit areas of corrosion. (Used by permission: Esmacher, Mel J., and Gary Geiger, Controlling Corrosion in Cooling Water System e Part 1, Identifying Corrosion and Its Causes, CEP, pp 36e41, (2012))

Metallurgy e Corrosion Chapter | 22

Factors Affecting the Rate of Corrosion Generally, different materials show different behaviors in a given environment, and a given material can be virtually inert in one type of liquid but suffer severe corrosion in another. For example, the corrosion rate of carbon steel is very slow in pure, fresh water, while very high corrosion rates will be experienced in an acid. Therefore, predicting the corrosion rate of a material is very difficult in most cases. This is because the number of influencing parameters and variables is large. Furthermore, the rate of corrosion may increase or diminish depending on the buildup of corrosion products, and the way that the environmental and operating conditions change with time. Furthermore, variable corrosivity may be experienced for one type of electrolyte. For example, the corrosivity of seawater varies to some degree from one location to another in the ocean. Important factors like salinity, temperature and microbiological activity/growth will vary both with geographical location and season. In general, the corrosivity in an environment increases or varies with: l l l l l l l

l

Temperature Salinity Oxygen content Electrolyte (e.g. water) velocity Acidity Type of electrolyte (e.g. cargo or chemicals) Content of contaminants/pollution that promotes corrosion Microorganisms

The salinity of seawater is one of the factors that determines corrosivity. The scale is based upon the concentration of hydrogen ions in a solution. A value of 1 refers to the highest acidity, while 14 is the most basic (alkaline) and 7 is neutral. In general, corrosion will increase with increasing acidity (low pH values), but a degree of alkalinity may also promote corrosion. For example, aluminum is only passive in solutions that are close to neutral (pH 5e8). The corrosivity of the marine atmosphere increases with elevating: l l l l

Humidity Temperature Salt content Air pollution content (including soot and dust particles)

Exposure conditions are essential factors in the corrosion rate. Rapid corrosion is promoted by the following: l

Conditions that prevent wet metal surfaces from drying (poor ventilation, salts that attract humidity, etc.).

l

l

l

l

l

1235

The increased conductivity of damp films on metal surfaces (from salts and acid contaminants). Severe environmental stress or loading (e.g. high pressure and temperature). Rapid changes in environmental conditions (e.g. temperature: high/low temperature giving thermal stresses of base material and protective system and condensation on surfaces). The breakdown of passive, protective oxide layers on metal surfaces (e.g. oxide films on stainless steel and aluminum and removal of copper oxides/hydroxides at high flow rates). Galvanic coupling to a more noble material (bimetallic corrosion).

Corrosion can affect the metal in a variety of ways, which depend upon its nature and the precise environmental conditions prevailing, and a broad classification of the various forms of corrosion in which five major types are presented in Table 22-6. Table 22-7 shows examples of types of corrosion of materials in various environments.

Corrosion Control Generally, most failures of process heat transfer equipment are a result of localized corrosion. The majority of failures in carbon steel are due to deposition (under-deposit/crevice corrosion), copper plating (galvanic corrosion), or biological fouling (MIC). Corrosion of copper alloys is a result of localized deposition (under-deposit/crevice corrosion), high residual concentrations of halogens used for microbiological control or the combination of halogen and high salinity conditions. Crevice and pitting corrosion are the most prominent failure mechanisms for stainless steel [4]. The speed and severity of corrosion penetration depend on the operating environment and the material selected for the application. For example, carbon steel may be selected for an equipment with an understanding that general corrosion will progress gradually and must be monitored with corrosion coupons and corrosion probes. The designer may select a typical stainless steel for a heat exchanger, without realizing that low flow conditions can cause deposition, which can result in under-deposit pitting corrosion. Therefore, the rate of pitting corrosion is difficult to estimate because the rate of fouling is unknown and it is especially difficult to monitor with corrosion coupons. This is because the coupons may not be exposed to the elevated temperatures, low flow rates and fouling actually occurring in the heat exchanger bundle. Most heat transfer equipment is fabricated from low carbon steel because of its low cost and copper alloy because of its high heat transfer efficiency. Cooling water systems are typically fabricated from more than one metal alloy, each with different corrosion related properties. When iron corrodes, it forms iron oxide

1236 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

TABLE 22-6 Types of Corrosion [10] Type

Characteristic

Examples

1. Uniform (or almost uniform)

All areas of the metal corrode at the same (or similar) rate.

Oxidation and tarnishing; active dissolution in acids; anodic oxidation and passivity; chemical and electrochemical polishing; atmospheric and immersed corrosion in certain cases.

2. Localized

Certain areas of the metal surface corrode at higher rates than others due to heterogeneities in the metal, the environment or in the geometry of the structure as a whole. Attack can range from being slightly localized to pitting.

Crevice corrosion; filiform corrosion; deposit attack; bimetallic corrosion; intergranular corrosion; weld decay.

3. Pitting

Highly localized attack at specific areas resulting in small pits that penetrate into the metal and may lead to perforation.

Pitting of passive metals such as the stainless steels, aluminum alloys, etc. in the presence of specific ions, e.g. Cl
ions.

4. Selective dissolution

One component of an alloy (usually the most active) is selectively removed from an alloy.

De-zincification; de-aluminification; graphitization.

5. Conjoint action of corrosion and mechanical factor

Localized attack or fracture due to the synergistic action of a mechanical factor and corrosion.

Erosion-corrosion, fretting corrosion, impingement attack, cavitation damage; stress corrosion cracking, hydrogen cracking, corrosion fatigue.

TABLE 22-7 Types of Corrosion Materials in Various Environments [4] Materials

Environment and Condition

Carbon steel e Uniform corrosion

In most corrosive environments where electrolyte is stagnant.

e Uneven corrosion (deep pits)

In most corrosive environments as high flow velocities.

Galvanic corrosion

In corrosive environments when coupled to a more noble material.

Stress Corrosion cracking

For high tensile steels. Combination of mechanical loading and a corrosive environment.

Stainless steels e Crevice corrosion

Particularly in aqueous environments containing aggressive ions like chlorides in confined areas (e.g. seawater).

e Pitting corrosion

Particularly in aqueous environments containing aggressive ions like chlorides, e.g. seawater.

e Stress corrosion cracking

Combination of mechanical loading and a corrosive environment.

Aluminum e Pitting corrosion

Particularly in aqueous environments containing aggressive ions like chlorides, e.g. seawater.

e Galvanic corrosion

In corrosive environments when coupled to a more noble material.

Copper based alloys e Erosion corrosion

At flow velocities exceeding 2e2.5 m/s. Further aggravated in the presence of pollution, particularly sulfides.

Titanium e Hydrogen embrittlement

Certain high tensile types exposed close to hydrogen source.

e Fatigue

Generally good resistance. Occurs in heat exchangers.

Metallurgy e Corrosion Chapter | 22

closed systems with long retention times, because even good precipitation inhibitors cannot remain soluble. Examples of these inhibitor types are zinc, orthophosphate, polyphosphate, organic phosphorus compounds (phosphonates) and calcium carbonate. Zinc is the most effective inhibitor as it can precipitate as a hydroxide, phosphate and/or silicate salt. Mixed inhibitors absorb onto the metal surface and form chemisorbed films that simultaneously block the anodic and cathodic corrosion sites. The corrosion rate increases with increasing water conductivity (Figure 22-21). Although all ionic species contribute to conductivity, chloride ðCl
Þ and sulfate, ðSO2
4 Þ anions are the most detrimental. Both of these are responsible for accelerated corrosion in treated and untreated water. Figure 22-22 shows the relationship between pH and the corrosion rate of iron. In the highly acidic pH range (pH < 4), the iron oxide film is continually dissolved, and corrosion is accelerated by the hydrogen reduction reaction, which is the dominant cathodic reaction. Corrosion can be mitigated by altering either the metallurgy or the environment. Selecting alloys that are naturally resistant to general corrosion, e.g. stainless steel, can be cost-prohibitive. Highly alloyed materials are also more prone to failure by localized corrosion mechanisms. Changing the environment by adjusting the water chemistry or by adding corrosion inhibitors is often the most widely practiced corrosion control method. 2Hþ þ 2e
/H2

(22-10)

As the pH increases from 4 to 10, the hydrogen ion concentration diminishes and the carbonate alkalinity increases. This increases the potential for a calcium carbonate film to form at the high pH cathodic sites and to suppress the corrosion reaction. In the absence of calcium

Corrosion rate

Corrosion rate

that impedes the flow and releases soluble iron (Fe2þ) into the cooling water. The soluble iron then oxidizes and forms insoluble compounds that foul heat transfer surfaces and reduce process throughput. If copper corrosion is not adequately controlled, the soluble copper released into the cooling water will plate onto steel surfaces. Copper/iron (Cu/Fe) galvanic cells are formed, which cause pitting corrosion and result in equipment failure. For these materials of construction, corrosion control and monitoring require an integrated management approach that may include adjusting the cooling water chemistry and/or operating pH, adding biological disinfectants and selecting the appropriate corrosion inhibitors. A corrosion inhibitor is any substance that effectively decreases the corrosion rate when added to the environment. Anodic, cathodic and mixed inhibitors are three types of corrosion inhibitors. Anodic inhibitors (known as passivators) block the anodic corrosion reaction by forming a protective oxide (iron oxide) layer on the metal surface. This renders the metal surface passive to corrosion. The most common passivators for steel are nitrite, molybdenum and orthophosphate. These materials are ineffective corrosion inhibitors for copper and copper alloys. Cathodic inhibitors retard corrosion through a precipitation mechanism triggered by the localized high pH at the cathode; the precipitated film acts as a microscopic barrier that prevents electron transfer from the metal surface to the dissolved oxygen. Cathodic or precipitation inhibitors form compounds that are insoluble at this high pH (2e3 pH units above that of the bulk water), but whose precipitation can be prevented at the bulk water pH (typically 6.5e9). An appropriate precipitation/deposition inhibitor is necessary to ensure solubility in the bulk water and prevent scaling of heat transfer surfaces. Precipitation inhibitors are unsuitable for

1237

4 10 Conductivity FIGURE 22-21 Corrosion rate vs. conductivity.

pH FIGURE 22-22 Iron corrosion rates are relatively constant.

1238 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

carbonate, the corrosion rate over the pH range of 4e10 is relatively constant, for any given water chemistry. Above pH 10, iron becomes increasingly passive. Oxygen can be the main driving force for steel corrosion in cooling water. The increase in corrosion with increased temperature at a given oxygen concentration is due to more rapid oxygen diffusion. The corrosion tendency of water can be reduced by mechanically or chemically removing oxygen. Although vacuum deaeration is a suitable means of reducing the oxygen level, chemical scavengers are more frequently used for closed systems that have little or no air ingress. Commonly used oxygen scavengers include catalyzed sodium sulfite (Na2SO3), hydroxylamine and ascorbic acid. An adsorption inhibitor is typically used with the scavenger to ensure complete corrosion protection [6].

Corrosion Monitoring (CM) In the refining industry, 90% of crude unit overhead corrosion occurs during 10% of the operating time, and these periods of unstable operation may occur during crude tank switches, slop oil processing or processing of crudes. Traditional methods used to monitor and control corrosion in the overhead condensing systems of an atmospheric crude distillation (CDU) may include installation of corrosion monitoring equipment, use of caustic in the crude oil and a variety of other chemical corrosion control solutions. Some plants have chosen to upgrade the overhead condensing metallurgy and associated piping at high costs. However, these traditional approaches may provide acceptable control during the 90% of operating time when the unit is functioning normally, but they may not detect or permit adequate or timely responses to the upsets that occur or the damage they can cause during 10% of unit operating time. New techniques have been developed to detect and to capture significant changes in the corrosive environment in real time, to measure the changes accurately and to correct those changes before significant corrosion has occurred. One example is the development of an overhead analyzer that continuously measures pH, chlorides and iron in refinery process water and provides the accurate, realtime data required for effective and timely corrosion control [6]. Metal corrosion coupons, instantaneous corrosion rate meters and heated surfaces such as test heat exchangers are commonly used to monitor corrosion quantitatively. Data from these devices can be employed to optimize a treatment program to maintain plant equipment. When heat transfer data cannot be obtained for operating exchangers, monitoring devices can be useful for evaluating treatment program success without a plant shutdown. Corrosion coupons (pre-weighed metal coupons) are a

reliable method for monitoring cooling system corrosion. Coupon weight loss provides a quantitative measure of the corrosion rate, and the coupon’s visual appearance provides an indication of the type of corrosion and the amount of deposition. Measuring the depths of pits on the coupon provides an indication of pitting severity. Corrosion rate meters can measure corrosion rate at any given time. The instrument can be either electrical resistance or linear polarization. Using either technique, corrosion measurements can be prompt without removing the sensing device. The electrical resistance measures the increase in the electrical resistance of a test electrode as it becomes thinner due to corrosion. An advantage of this method is that the probes can be installed in either aqueous or non-aqueous streams. However, its disadvantages are that conductive deposits on the probe can create misleading results; temperature fluctuations must be compensated for; and pitting characteristics cannot be determined accurately. The technique based on linear polarization at low applied potentials provides instantaneous corrosion rate data that can be read directly from the instrument display in actual corrosion rate units (mils per annum). Systems using two or three electrodes are available. This method maximizes performance, simplicity and reliability. Corrosion rate meters can determine changes in corrosion rate as a function of time. They can respond to sudden changes in system conditions, including acid spills, chlorine levels and inhibitor treatment levels.

Feed Overhead

Bottoms Monitoring point FIGURE 22-23A Three point monitoring for a distillation column.

Metallurgy e Corrosion Chapter | 22

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Corrosion monitoring programs are essential in process plants, and their main objectives are: l l

Top enriching

l

Center enriching Lower enriching

Overhead Feed Top stripping

A diagnostic tool. Monitor the effectiveness of corrosion control. Provide operations and management data. l Maintenance planning and scheduling. l Reduce shutdown inspections. l Prevent unforeseen shutdowns. l Asset management. l Operations optimization. l Process improvement.

Figures 22-23a, b, c and d show monitoring points of distillation columns and a vessel with an agitator.

Center stripping Bottom stripping

Common Corrosion Mistakes Bottoms

Kirby provides sound advice for designers who are not experts in metallurgy. His seven pitfalls to avoid [11] are:

Monitoring point

FIGURE 22-23B Preferred multi-locations for corrosion monitoring in a distillation column.

1. Not understanding the details of the corrosion service. Stating only the predominant acid without the other details (such as presence of chloride ion) is an example. Kirby explains how the acronym SPORTSFAN is used.

Block valve and disconnection point Tapped blind installed here incase of a leak

Internal tubing run between chamber and tray level

Corrosion rate probe

Tapped blind installed here incase of a leak.

Tray Level N

Valve closed

Sample valve

Tray Level N - 5

Sample bomb

To sample discharge point Using side -stream in of a distillation column 1. Install bypass loops to withdraw liquid from the column or vessel. 2. Pass it over probes, and re-inject at lower point in the column or vessel. 3. Use of such a loop provides opportunity for sampling.

FIGURE 22-23C

1240 Ludwig’s Applied Process Design for Chemical and Petrochemical Plants

T.L

Vapor +6400

MAX. L. L

Liquid / vapor interface +4400

NOR L. L

I.D. 4700 Liquid +700

MIN. L. L

T.L 0.00

Monitoring point FIGURE 22-23D Vessel with agitator.

S ¼ Solvent P ¼ pH O ¼ Oxidizing potential R ¼ Reducing potential T ¼ Temperature S ¼ Salts in solution F ¼ Fluid flow conditions A ¼ Agitation N ¼ New aspects or changes to a chemical process. 2. Concentrating on overall or general corrosion, and ignoring pitting, crevice corrosion, stress corrosion, cracking, etc. 3. Ignoring alkaline service. Just because strong alkalis do not cause severe overall corrosion in carbon steel or stainless steel, do not overlook stress corrosion, cracking or effects on other materials. 4. Not considering water or dilute aqueous solutions. This can be overlooked if the other side of the tube or coils has strong chemicals, such as sulfuric acid.

5. Confusion about the L-grade of stainless steels. The L-grades, such as 304L, have lower carbon (0.03% vs. 0.08%) than the standard grades (e.g. 304). The L-grades are used to prevent sensitization from carbide precipitation during welding. This minimizes strong acid attack of the chromium-depleted areas along the welds. Do not forget to specify the L-grade for the filler metal as well as the base plate. Some consider that the purpose of the L-grade is to handle chloride stress corrosion cracking at percent or multiple ppm levels. 6. Not accounting for the oxidizing or reducing potential of acidic solutions. For non-chromium containing alloys that are capable of withstanding reducing acids, a small amount (ppm level) of oxidizing chemical can have devastating effects. 7. Neglecting trace chemicals. Watch for ppm levels of chloride with stainless steels or ppm levels of ammonia with copper-based alloys, for example.

Metallurgy e Corrosion Chapter | 22

REFERENCES [1] Hall S. Rules of thumb for chemical engineers, 5th ed. Butterworth e Heinemann; 2012. [2] Kane Russel D, Maldonado Julio G, William G. Ashbaugh. Cracking Down On CorrosionePart 1. Chem Eng March 2014:74e7. [3] Metals Handbook. Corrosion, Vol. 13. Ohio: ASM, International, Materials Park; 1988. [4] Ashiru OA. Private Communications; 2014. [5] US Chemical Safety Hazard Investigation Board (CSB) Report: NDK Crystal, Inc., Belvidere, IL. Highepressure vessel rupture, no. 2010-04-1 IL; December 7, 2009. [6] Hilton NP. Mitigate corrosion in cour crude unit. Hydro Proc September 2014:1e6. www.hydrocarbonprocessing.com. [7] Grubb JF. Martensitic stainless steel. In: Wiston RR, editor. Uhlig’s Corrosion Handbook, Chapter 39. John Wiley & Sons, Inc; 2000.

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[8] Kutz Myer, editor. 2002. Handbook of materials selection. Wiley & Sons, Inc.; 2002. [9] Lin Eric, Risse Peter. Reactor effluent air cooler through design. Revamps 2013, Petrol Technol Quart 2013:45e7. [10] Shreir LL, Jarman RA, Burstein GT. Corrosion. 3rd ed, vol. 1. ButterwortheHeinemann; 2000. [11] Kirby GN. Avoid common corrosion mistakes for performance. Chem Eng Prog 1997. [12] Esmacher Mel J, Gary Geige. Controlling corrosion in cooling water systemsepart 1: identifying corrosion and its causes. CEP; April 2011. p. 36e41. [13] Alloys Rolled, 2009. Alloy performance guide. Bulletin no. 128 use 08/100. www.rolledlloys.com, 2014.