Corrosion in Hydrogen Halides and Hydrohalic Acids

2.22 Corrosion in Hydrogen Halides and Hydrohalic Acids J. A. Richardson Anticorrosion Consulting, 5 Redhills Lane, Durham DH1 4AL, UK

ß 2010 Elsevier B.V. All rights reserved.

2.22.1 2.22.2 2.22.3 2.22.4 2.22.5 2.22.5.1 2.22.5.2 2.22.5.3 2.22.5.4 2.22.5.5 2.22.6 2.22.6.1 2.22.6.2 2.22.6.3 References

Introduction Steels and Cast Irons Stainless Steels Nickel and Nickel Alloys Other Metals and Alloys Copper and Copper Alloys Titanium and Titanium Alloys Zirconium and Zirconium Alloys Tantalum and Tantalum Alloys Noble Metals Nonmetallic Materials Thermoplastic and Reinforced Thermosetting Materials Elastomers Inorganic Materials

Abbreviations AHF Anhydrous hydrogen fluoride AES Auger electron spectroscopy ASTM American Society for Testing Materials CPVC Chlorinated polyvinyl chloride ECTFE Ethylene chlorotrifluoroethylene EIS Electrochemical impedance spectroscopy EPDM Ethylene propylene diene terpolymer ETFE Ethylene tetrafluoroethylene FEP Fluorinated ethylene propylene GRP Glass reinforced plastic HAZ Heat affected zone HIC Hydrogen-induced cracking HSC Hydrogen stress cracking PE Polyethylene PFA Perfluoroalkoxy PP Polypropylene ppm Parts per million PTFE Polytetrafluoroethylene PVC Polyvinyl chloride PVDF Polyvinylidene fluoride PWHT Postweld heat treatment SCC Stress corrosion cracking SCE Saturated calomel electrode SHE Standard hydrogen electrode SIMS Secondary ion mass spectrometry

1207 1209 1211 1212 1220 1220 1220 1221 1222 1223 1223 1223 1224 1224 1224

SOHIC Stress-oriented hydrogen-induced cracking TOF Time of flight XPS X-ray photoelectron spectroscopy

Symbols Ecorr Corrosion potential (V) icorr Corrosion current density (mA cm
2)

2.22.1 Introduction The hydrohalic acids comprise the group of monobasic acids that are formed when the hydrogen halide gases are dissolved in water, with strengths in descending order: hydroiodic (HI) > hydrobromic (HBr) > hydrochloric (HCl) > hydrofluoric (HF) The differences in strength arise from the differing sizes of the relevant halide ion. Thus, the largest iodide anion interacts relatively weakly with its complementary proton, leaving it relatively free to dissociate. In practice, the differences amongst HI, HBr, and HCl acids are not great, and all are strong acids that are highly dissociated in water at all concentrations to

1207

1208

Liquid Corrosion Environments

release protons and the corresponding halide anion as follows: HX ¼ Hþ þ X
The result, in relation to the pHs of hydrohalic acids, is that at the higher end of the pH range the solutions are effectively acidified waters. Thus, the pHs 2, 3, and 4 are equivalent to concentrations of 360, 36, and 3.6 ppm (w/w) hydrochloric acid, respectively at 25  C. However, HF acid is an exception to this behavior. There is very strong hydrogen bonding between the nonionized HF and water molecules, and the small fluoride anion imposes a high degree of order on the surrounding protons and water molecules as a result of which HF acid is a very weak acid, the pH of which varies in a complicated manner with concentration as shown in Figure 1.1 Hydrogen chloride, bromide, and iodide are all gases at ambient temperatures and available commercially in nominally anhydrous specifications, containing trace quantities of water. Anhydrous hydrogen fluoride (AHF) is a liquid that boils at 20  C, and is normally specified as containing < 400 ppm water. The significant commercial grades of the acids are as follows: 1. HI acid, the least significant acid commercially, is supplied at various concentrations, typically 47% or 55% (w/w), but concentrated grades in the range 90–98% (w/w) are also available. 2. Hydrobromic acid is manufactured in significant quantities and supplied at various concentrations, most commonly at 47–49% (w/w). 3. Hydrochloric acid is manufactured in the greatest quantities at concentrations up to 38% (w/w), above which the fuming nature of the acid

4.6 4.4 4.2 pH

4 3.8 3.6 3.4 3.2 3 0

5 10 15 20 25 30 35 40 45 Hydrofluoric acid concentration % (w/w)

Figure 1 pH of hydrofluoric acid solutions. Adapted from Hydrofluoric Acid Properties; Honeywell Specialty Materials Products, 2002.

50

introduces problems in relation to storage and transport. Most bulk commercial grades have concentrations in the range 30–35% (w/w). 4. HF acid is also significant commercially, and is available in bulk at concentrations of 49% and 70% (w/w). From the corrosion standpoint, the hydrohalic acids are commonly classified as inherently ‘reducing’ acids because the only cathodic process that the pure acids can deliver is the reduction of protons to evolve hydrogen: 2Hþ þ 2e
¼ H2 This is in contrast to ‘oxidizing’ acids such as sulfuric and nitric acid in which reductions of the acid itself, or its constituent species, occur at potentials more noble than hydrogen evolution, and to an extent that can determine the behaviors of materials exposed to the acid, as described in the separate chapters devoted to the two acids in this book. In the case of the hydrohalic acids, their ‘reducing’ characteristics are compounded by the aggressive properties of the halide anions that inhibit the formation of, and attack preexisting, protective passive layers on metals and alloys. Indeed, both HCl and HF acids find significant commercial applications in the cleaning and pickling of metals and alloys because of their ability to dissolve metal oxides. In practice, alternative cathodic processes to hydrogen evolution may be available due to the presence of specific contaminants in the acid, in particular oxidants such as dissolved oxygen and metal cations in a higher oxidation state such as ferric, Fe3þ or cupric, Cu2þ ions. The presence of the corresponding halogen gas, for example chlorine in the case of hydrochloric acid, can also have a significant effect. The effects of oxidizing agents that raise the potentials of metals and alloys are almost invariably detrimental to corrosion performance in the hydrohalic acids, in which metals and alloys are mostly unable or struggle to form protective, passive films, as will emerge below. The corrosion performances of the various classes of material in hydrohalic acids have been reviewed extensively elsewhere.2–8 Much of the data relate to hydrochloric and HF acids, reflecting their commercial significance. There is relatively little data for hydrobromic, and virtually none for HI acid in the public domain. Readers whose main interest is to identify ‘what works where’ are referred to these sources and the relevant chapters on specific materials in this book. In this chapter, the corrosion

Corrosion in Hydrogen Halides and Hydrohalic Acids

1209

700

Temperature (⬚C)

Tubes/internals

Vessels/pipe

500 00 and

Alloy 2

300

00

alloy 6

inless r–Ni sta 18–8 C steels

Alloy 400

teel

Carbon s

100

0.025

0.25 Corrosion rate (mm year−1)

2.5

Figure 2 Proposed design limits for various alloys in anhydrous hydrogen chloride. Adapted from Schillmoller, C. M. Chem. Eng. 1980, 87, 161–163.

performances of materials are reviewed with an emphasis on the principles and mechanisms that underpin their corrosion performances in the acids.

0

2.22.2 Steels and Cast Irons Steels react with anhydrous hydrogen halide gases to form the corresponding iron halide and hydrogen. In AHF, the fluoride films are protective and steels have acceptable corrosion rates at temperatures up to 200  C and velocities up to 10 m s
1, and arguably to higher temperatures at lower velocities.3,6 The velocity limitations arise because the fluoride films are vulnerable to detachment which not only increases the corrosion rates, but also adversely affects the operability of equipment due to accumulations of solid iron fluoride in seals and joints in valves, pumps, and elsewhere. In anhydrous hydrogen chloride gas, the growth of a film of FeCl2 obeys parabolic kinetics up to temperatures of 500–600  C, above which the vaporization of FeCl2 results in a switch to linear, nonprotective kinetics. As in the case of the analogous fluoride films, the chloride films on steels are protective within specific temperature and velocity constraints,5,9,10 and some widely used design temperature limits for carbon steel, austenitic stainless steels, and specific nickel alloys in anhydrous hydrogen chloride are shown in Figure 2.10 Aqueous hydrohalic acids are very aggressive toward steels and grey or ductile cast irons. A typical polarization curve for carbon steel in stagnant, aqueous 3M (10%, w/w) hydrochloric acid is shown in Figure 3.

Potential (mV) (SHE)

–100

–200 Ecorr

–300

–400 icorr 10–2

10–1

1 10 Current density (mA cm–2)

102

Figure 3 Potentiodynamic polarization curve for carbon steel in stagnant, 3 M (10%, w/w) hydrochloric acid open to air at ambient temperature. Adapted from Poorqasemi, E.; Abootalemi, O.; Peikari, M.; Haqdar, F.Corros. Sci 2009, 51, 1043–1054.

The anodic curve shows classic Tafel behavior, characteristic of active dissolution of iron: Fe ¼ Fe2þ þ 2e
The cathodic curve also shows classic Tafel behavior, characteristic of hydrogen evolution. In more dilute acids, curves are obtained that are time dependent to a small extent, and electrochemical impedance spectroscopic (EIS) measurements suggest the presence of highly nonprotective films that present limited

1210

Liquid Corrosion Environments

barriers to anodic dissolution.11 Such films that do exist are likely to consist of iron chloride, which is very soluble in hydrochloric acid and the degree of protection that they can provide is therefore very marginal. The general experience of steels in aqueous HCl, HBr, and HI acids is of active corrosion across the full concentration range at rates that preclude practical application. In aqueous HF acid, corrosion rates are generally lower, but acceptable only at concentrations above 70% (w/w), as shown in Figure 4. The corrosion resistance of steels in concentrated aqueous and liquid AHF arises from the spontaneous formation of protective iron fluoride films on steel surfaces that are much less soluble than the corresponding chloride films. Studies of the growth of fluoride films in vapors over dilute HF acids at ambient temperature have shown that the kinetics is linear, and controlled by reaction at the metal–film interface, resulting in the formation of porous, nonprotective films. However, when the HF acid concentration rises to 40% (w/w), the initial film that forms in the vapor phase has been identified as FeF24H2O, and it grows with a parabolic dependence on time. With increasing film thickness, a product identified as nonstoichiometric Fe2F57H2O occurs. The change to parabolic kinetics indicates a switch to diffusion control of reaction rate through a thickening, potentially protective film.12 The significance of such films is evident in the polarization curves for two steels in 90% (w/w) acid at 90  C, shown in Figure 5.

In both cases, the anodic kinetics is clearly influenced strongly by diffusion due to the presence of the fluoride film, while the cathodic reactions obey Tafeltype kinetics, characteristic of hydrogen evolution beneath the film.13 The adherence and protection afforded by the fluoride film has been shown to depend upon several factors. Although the laboratory test in Figure 5 demonstrates the existence of a film at 90  C, temperatures and limiting velocities are typically restricted to 65  C and 1.6 m s
1 in liquid AHF, reducing to 30  C and 0.6 m s
1 in 70% (w/w) acid.6 The carbon and residual element content of the steel, in particular the copper and nickel contents, also appear to influence the persistence of the fluoride films that form and the corresponding corrosion rates that are experienced,13 and recommendations have been formulated relating to the control of such elements.14 Corrosion rates have also been shown to be higher in the presence of oxygen in both the laboratory10 and in the field.6 Steels are vulnerable to damage due to hydrogen uptake in HF acid service that is similar to the types of damage that can be experienced in wet H2S such as hydrogen stress cracking (HSC) of stronger materials and hard welds and heat affected zones (HAZ), and hydrogen-induced cracking (HIC), stressoriented hydrogen-induced cracking (SOHIC), and blistering of plate materials. General approaches to mitigating these risks are available7,15 based on the 800 600 Potential (mV) (Ag ref)

Corrosion rate (mm year–1)

25

2.5

0.26 C +residuals 400

200 0.04 C 0

0.25 Uncontaminated HF Low or no-flow conditions Laboratory data and service experience >70%

0

10 20 30 40 50 60 70 80 90 100 Hydrofluoric acid concentration % (w/w)

Figure 4 Corrosion rate of carbon steel in hydrofluoric acid at 21–38  C as a function of concentration. Adapted from NACE International Publication 5A–171 Materials for Storing and Handling Commercial Grades of Aqueous Hydrofluoric Acid and Anhydrous Hydrogen Fluoride; NACE International: Houston, TX, 2007.

–200

–400

10–4

10–3 10–2 10–1 1 Current density (mA cm–2)

10

102

Figure 5 Potentiodynamic polarization curves for steels in 90% (w/w) hydrofluoric acid at 90  C. Adapted from Chirinos, G.; Turgoose, S.; Newman, R. C. Effects of Residual Elements on the Corrosion of Steels in HF, Paper 97513, Corrosion ’97; NACE International: Houston, TX, 1997.

Corrosion in Hydrogen Halides and Hydrohalic Acids

practices that have been developed for the control of the similar problems that are experienced in wet H2S, including the control of base metal chemistry and the hardness of welds and HAZs within threshold levels and, if appropriate, postweld heat treatment (PWHT).16 The risks of HIC and SOHIC in the base materials are mitigated by controlling steel chemistry and microstructure (including limits on sulfur content and inclusion shape control) and manufacture (in particular rolling conditions), and confirming resistance by appropriate testing.17–19 Gray and ductile cast irons have corrosion resistances broadly similar to steels in hydrohalic acids, but find no application in HF acid service, not least in the case of gray irons because of safety concerns relating to their poor ductility. High-silicon cast irons, containing typically 14–16% Si, are resistant to all concentrations of hydrochloric acid up to 40% (w/w) at ambient temperatures and at higher temperatures in more dilute acids, as shown in the isocorrosion curves for some metals and alloys with exceptional corrosion resistance to aqueous hydrochloric acid, described later in Figure 14. Their exceptional corrosion resistance is due to the formation of robust, siliceous films that have considerable resistance to erosion and abrasion. However, they find relatively limited application due to their poor ductility that renders them difficult to fabricate and requires them to be protected from thermal and/or mechanical shock. Table 1

1211

2.22.3 Stainless Steels The compositions of the relevant commercial grades of stainless steels are summarized in Table 1. In anhydrous hydrogen chloride gas, stainless steels perform in a similar manner to steels in that they form chloride films that obey parabolic kinetics up to temperatures of 500–600  C, above which the vaporization of the chlorides results in a switch to linear kinetics. However, the films contain CrCl3 and NiCl2 in addition to, or instead of FeCl2, depending on the alloy and the temperature.10 They form more slowly and are much more protective than the corresponding films on steels at temperatures below 500  C. As a result, the corrosion rates for stainless steels are lower than for steels, and the design temperature limits are correspondingly higher, exemplified by those reported in Figure 2.10 At temperatures above 500  C, stainless steels offer no significant advantage over carbon or low alloy steels in anhydrous hydrogen chloride gas. The performances of austenitic stainless steels in AHF liquid and vapor are good up to a temperature of 100  C. All of the basic grades 304 (S30400), 304L (S30403), 316 (S31600) and 316L (S31603), and their cast equivalents show good resistance and find significant application, although due caution to prevailing velocities is necessary as in the case of steels. However, austenitic stainless steels that can develop a-martensite as a result of cold working are vulnerable

Compositions of some wrought stainless steels that are relevant to hydrohalic acid applications

UNS no.

Common name

Fe

Ni

Cr

Mo

S31803 S32205 S32750 S32760 S30400 S30403 S34700 S31600 S31603 S31700 S31703 S31254 S30900 S31000 N08904 N08926 S31277 N08020 N08031 R20033

2205

Balance

4.5–6.5

21–23

2.5–3.5

2507

Balance

6–8

24–26

3–4.5 3–4

304 304L 347 316 316L 317 317L 254SMO 309 310 904L 25–6MO 27–7MO 20 31 33

Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance

8–10.5 8–12 9–13 10–14 10–14 11–15 11–15 17.5–18.5 12–15 19–22 23–28 24–26 26–28 32–35 30–32 30–33

18–20 18–20 17–19 16–18 16–18 18–20 18–20 19.5–20.5 22–24 24–26 19–23 19–21 20.5–23.0 19–21 26–28 31–35

Cu

Si

Other N

<0.5 0.5–1.0

N W, N <0.08C Nb/Ta < 0.08C <0.08C

2–3 2–3 3–4 3–4 6–6.5

0.5–1.0

4–5 6–7 6.5–8.0 2–3 6–7 0.5–2.0

1–2 0.5–1.5 0.5–1.5 3–4 1.4 0.3–1.2

<0.08C N Mn Mn N Mn, N N N

Liquid Corrosion Environments

12 10 8 6 4 2

Carbon steel

0 100

200 300 Temperature (⬚C)

The compositions of the relevant commercial grades of nickel and its alloys are summarized in Table 2. The reaction of nickel with anhydrous hydrogen chloride gas as a function of temperature is shown in Figure 10.21 Evidently, reaction kinetics is parabolic up to a temperature of 450  C, associated with the formation of protective NiCl2. In the temperature range of 450–550  C, there is a transition to linear kinetics associated with the relatively high vapor pressure and progressive evaporation of NiCl2. Alloying, in general, reduces the temperature at which there is a transition to unacceptable evaporation rates. The effects are relatively small for the commercial nickel–chromium 120 Boiling point

100 80 25–6MO

60 2507

40 316L

20 0

254SMO 904L

2 4 6 8 Hydrochloric acid concentration % (w/w)

10

Figure 7 Isocorrosion charts (0.1 mm year
1) for austenitic and duplex stainless steels in hydrochloric acid. Adapted from Corrosion Handbook, Stainless Steels; AB Sandvik Steel, 1994.

14 Corrosion rate (mm year–1)

304 Stainless steel

2.22.4 Nickel and Nickel Alloys

Temperature (⬚C)

to HSC, and some users favor the higher nickel 316/316L grades for this reason.8 Above 100  C, the rates of corrosion of austenitic stainless steels in AHF increase to unacceptable levels and they are outperformed by carbon steel, as shown in Figure 6. The performances of stainless steels in aqueous hydrohalic acids are generally poor because they are unable to form and sustain the passive, chromium oxide films upon which they depend for corrosion resistance and, as a result, corrode actively. Ferritic and martensitic stainless steels have poor resistance under all conditions. Austenitic and duplex stainless steels offer some limited resistance, as shown in the isocorrosion diagrams for various grades in HCl and HF acids in Figures 7 and 8 respectively. The basic 316L (S31600) grade is clearly limited to very dilute acids at relatively low temperatures. The grades with higher levels of nickel, molybdenum, and copper offer progressive extension of the boundaries for stable passivity, but more than 30% nickel and significant contents of molybdenum and copper are required to provide resistance to hydrochloric acid at all concentrations up to 40% (w/w) at close to ambient temperatures, exemplified by the isocorrosion diagram for alloy 31 (N08031) which is shown in Figure 9.20 Overall, taking into account the additional risks of localized corrosion and stress corrosion cracking (SCC) presented by the halide anions, stainless steels are weak options for aqueous, hydrohalic acid service.

100

80 Temperature (8C)

1212

60 25–6MO 254SMO

40

904L

400

Figure 6 Corrosion rates of carbon steel and type 304 stainless steel as a function of temperature in static, anhydrous hydrofluoric acid. Adapted from Jennings, H. S. Corrosion by Hydrogen Fluoride and Hydrofluoric Acid. In ASM Handbook Corrosion: Environments and Industries; ASM International, 2006; pp 690–703, Vol. 13C.

316L

20 0

2 4 6 8 Hydrochloric acid concentration % (w/w)

10

Figure 8 Isocorrosion charts (0.1 mm year
1) for austenitic stainless steels in hydrofluoric acid. Adapted from Corrosion Handbook, Stainless Steels; AB Sandvik Steel, 1994.

Corrosion in Hydrogen Halides and Hydrohalic Acids

and nickel–chromium–molybdenum alloys, but more significant for nickel–molybdenum and particularly nickel–copper alloys because of the relatively low melting point and high volatility of copper chloride. This is reflected in the relatively high, proposed, design temperature limits proposed in Figure 2 for nickel 200 (N02200) and alloy 600 (N06600) compared with alloy 400 (N04400) for which the proposed limits are little better than those for carbon steel.10 The resistance of nickel and its alloys to AHF gas at elevated temperatures is dependent on the formation

Temperature (8C)

120 Boiling point 80

40 31 0 0

10 20 30 Hydrochloric acid concentration % (w/w)

40

Figure 9 Isocorrosion chart (0.13 mm year
1) for austenitic alloy 31 in hydrochloric acid. Adapted from Agarwal, D. C., Alves, H. Applications of Alloys 59 (UNS N06059) and 31 (UNS N08031) in Mitigating Corrosion Risks in the CPI and Petrochemical Industries, Paper 07186, Corrosion 2007; NACE International: Houston, TX, 2007.

Table 2

1213

of protective metal fluoride films. Nickel and molybdenum fluorides are stable and protective relative to chromium and iron fluorides that have lower melting points and higher volatilities, and this is reflected in the corrosion rates for various metals and alloys in AHF at 600  C that are charted in Figure 1122 which links to the data reported in Figure 6. Alloys can suffer internal as well as external attack in high temperature hydrogen fluoride related to fluoride diffusivity and solubility in the metal and, in addition to chromium and iron, the elements niobium, tantalum, and titanium also exacerbate the vulnerability of an alloy to internal attack due to the lower melting points and higher volatilities of their fluorides. Figure 11 confirms that the chromium-rich types 309 (S30900) and 310 (S31000) and the niobium/ tantalum-containing type 347 (S34700) stainless steel suffer extremely high rates of corrosion. Type 304 (S30400) stainless steel is confirmed as having unacceptable rates relative to steel, and the most resistant materials are nickel 200 (N02200) and nickel alloys 400 (N04400) and 600 (N06600). In AHF liquid, the leaner nickel alloys such as alloys 20 (N08020), 825 (N08825), and the G family are highly resistant to corrosion at temperatures up to 125  C, even under flowing conditions, and find significant application, subject to appropriate mitigation of risks relating to preferential weld corrosion and HSC of heavily cold-worked material.6 The more

Compositions of some wrought nickel alloys that are relevant to hydrohalic acid applications

UNS no.

Common name

N02200 N02201 N04400 N05500 N08825 N06007 N06985 N06030 N06600 N06690 N10276 N06455 N06022 N06200 N06059 N06686 N06625 N10001 N10665 N10675

200 201 400 500 825 G G-3 G30 600 690 C-276 C-4 C-22 C-2000 59 686 625 B B-2 B-3 Hybrid BC-1

Fe

<2.5 <2.0 >22 18–21 18–21 13–17 6–10 7–11 4–7 3 2–3

<5 <5 5.5 <2 1.5 2

Ni >99 >99 Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance

Cr

19.5–23.5 21–23.5 21–23.5 28.0–31.5 14–17 27–31 14.5–16.5 14–18 20.5–22 23 23 19–23 20–23

1.5 15

Mo

2.5–3.5 5.5–7.5 6–8 4–6

15–17 14–17 13–14 16 16 15–17 8–10 28 28 28.5 22

Cu

28–34 27–33 1.5–3.0 1.5–2.5 1.5–2.5 1.0–2.4

Si

Other <0.15C <0.02C Mn Al, Mn Ti Co, Nb Co W W

W, Co Co W, Co 2 W Nb Co W, Co

Liquid Corrosion Environments

450 ⬚C 400 ⬚C

5 Weight change (g m–2)

Corrosion rate (mm year–1)

1214

0

500 ⬚C

–5 –10 550 ⬚C

–15 600 ⬚C

–20

650 ⬚C

–25 0

12

24

36 48 Time (h)

60

72

Corrosion rate (mm year–1)

Figure 10 Weight change of nickel exposed to anhydrous hydrogen chloride gas as a function of temperature. Adapted from Ihara, Y.; Ohgame, S.; Sakiyama, K.; Hashimoto, K. Corros. Sci. 1982, 22, 901.

0.6 0.5 0.4 0.3 0.2 0.1 0

Anhydrous HF (15–40 8C) Aluminum Austenitic SS 80–20 Cu–Ni Nickel 200

Magnesium Steel Alloy 400 70–30 Cu–Ni

Copper Low alloy steel Alloy 600

Figure 12 Corrosion rates for various metals and alloys tested for 6–40 days in AHF at 15–40  C. Adapted from Jennings, H. S. Corrosion by Hydrogen Fluoride and Hydrofluoric Acid. In ASM Handbook. Corrosion: Environments and Industries; ASM International, 2006; pp 690–703, Vol. 13C.

350 300 250 200 150 100 50 0 Type 310 Alloy 400

Anhydrous HF (600 8C) Type 347 Alloy 600

Type 309 Copper

Type 304 Nickel 200

Steel

Figure 11 Corrosion rates for various metals and alloys tested for 4–15 h in anhydrous hydrogen fluoride (AHF) at 600  C. Adapted from Myers, W. R.; Delong, W. B. Chem. Eng. Prog. 1948, 44, 359.

nickel-rich alloys 600 (N06600), C-22 (N06022), and C-276 (N10276) are even more resistant, as illustrated in Figure 12 that shows corrosion rates for various metals and alloys tested for 6–40 days in AHF at 15–40  C.6 The nickel–chromium and nickel–chromium– molybdenum alloys are used at temperatures up to at least 150  C, but appropriate precautions are also necessary with the nickel-rich alloys to avoid environmental cracking. Thus, the nickel–copper alloys 400 (N04400) and 500 (N05500) have excellent general corrosion resistance but are vulnerable to environmental cracking in the presence of air. Alloy 600 (N06600) has shown a vulnerability to cracking in the presence of chlorine and copper fluoride after cold forming.

Such risks can be mitigated by appropriate heat treatment procedures.6 The basic electrochemistry of the classes of alloys of commercial significance for aqueous hydrohalic acid duties is illustrated by the polarization curves in Figure 13 for nickel and three nickel alloys in aqueous hydrochloric acid. Evidently, the corrosion potentials and polarization characteristics of nickel 200 (N02200) and the nickel–copper alloy 400 (N04400) in deaerated 5N (18%, w/w) hydrochloric acid at room temperature are very similar.23 Both display very active corrosion potentials and anodic polarization behavior characteristic of active dissolution of the base elements with no tendency to passive film formation: Ni ¼ Ni2þ þ 2e
Cu ¼ Cu2þ þ 2e
The polarization curves in Figure 13 and Figure 3 confirm the much greater resistance to active, anodic dissolution that nickel displays relative to iron under similar conditions. That is the basis of the generally better performances of nickel-based compared with iron-based alloys in hydrochloric acid. Regarding the specific effects of alloying with copper, the curves in Figure 13 suggest slightly more polarization of both anodic and cathodic processes in the case of alloy 400 (N04400) than for nickel 200 (N02200), but this depends very much on acid concentration and temperature. Alloy 400 (N04400) is more sensitive to increases in concentration and less sensitive to increases in temperature than nickel 200. In practice, both alloys

Corrosion in Hydrogen Halides and Hydrohalic Acids

1215

1200 1000

Potential (mV) (SCE)

800 Alloy C-2000 600 400 Alloy B-3 200 0

0

–200

–200

–400

–400

Alloy 400

–600

(a )

10–4

1 10 10–3 10–2 10–1 Current density (mA cm–2)

(b)

Ni 200

10–2

10–1

1

10

Figure 13 Polarization behavior of (a) alloys B-3 and C-2000 in deaerated 20% (w/w) hydrochloric acid at 25  C. Adapted from Nacera, S. M.; Crook, P.; Klarstrom, D. L.; Rebak, R. B. Effect of Ferric Ions on the Corrosion Performance of Nickel Alloys in Hydrochloric Acid Solutions, Paper 04430, Corrosion 2004: NACE.

have acceptable resistance to corrosion in relatively dilute (<10%, w/w) acids at ambient temperatures but at higher concentrations and temperatures, and particularly if oxygen or other oxidizing agents are present, their application is limited. The more important alloying elements for resistance to aqueous hydrochloric acid are molybdenum and chromium, exemplified by the polarization curves in Figure 13 for the nickel–molybdenum alloy B-3 (N10675) and the nickel–chromium–molybdenum alloy C-2000 (N06200) in deaerated 20% (w/w) hydrochloric acid at 25  C.24 Both alloys exhibit relatively active corrosion potentials, albeit more noble than for nickel 200 (N02200) or the nickel–copper alloy 400 (N04400) under similar conditions. Active dissolution is clearly inhibited in the nickel– molybdenum alloy B-3 (N10675) relative to nickel 200 (N02200), arising from the significant contribution of molybdenum to overall dissolution: Mo ¼ Mo3þ þ 3e
The mechanism via which molybdenum inhibits dissolution has been variously attributed to the relatively high Mo–Mo bond energy that blocks active sites25,26 and the promotion of salt film formation due to the relative insolubility of molybdenum chlorides.27 It has been suggested that the pseudo-passive behavior,

exemplified by the anodic polarization curve for alloy B-3 (N10675) in Figure 13, may be due to the formation of a molybdenum dioxide (MoO2) and/or trioxide (MoO3) film24 that breaks down under further anodic polarization. Whatever the mechanism, the inhibiting effect of molybdenum on the anodic dissolution of nickel in hydrochloric acid is of significant practical benefit, exemplified by the isocorrosion diagrams in Figure 14 that show the performance of alloy B-2 (N10665) relative to other metals and alloys with exceptional resistance to aqueous hydrochloric acid.28 However, the absence of the capacity of nickel– molybdenum alloys to passivate, which is apparent in the anodic polarization curve for alloy B-3 (N10675) in Figure 13 signals a sensitivity to the presence of oxidizing agents which limits their practical application, as described later. In contrast to alloy B-3 (N10675), the anodic polarization curve for the chromium-containing alloy C-2000 (N06200) in Figure 13 shows a pronounced capacity to passivate, which offers the prospect of useful corrosion performance in environments that have sufficient oxidizing capacity to exceed the critical current density for passivation and raise the potential into the passivation range. The active portion of the curve describes active dissolution of the alloy to produce, in addition to Ni2þ and Mo3þ, soluble

1216

Liquid Corrosion Environments

Ta

220

Temperature (8C)

Zr 180 Nb 140

100

Boiling point Si iron

60

20

B-2

0

10 20 30 Hydrochloric acid concentration % (w/w)

40

Figure 14 Isocorrosion charts (0.13 mm year
1) for various metals and alloys in hydrochloric acid. Adapted from Hunkeler, F. J. Tantalum and Niobium. In Process Industries Corrosion; Moniz, B. J., Pollock, W. J., Eds.; NACE International: Houston, TX, 1986.

chromous ions as follows: Cr ¼ Cr3þ þ 3e
The active dissolution curve is less polarized than alloy B-3 (N10675) due to its lower molybdenum content. At a potential in the 50–100 mV range, the current density drops by more than two orders of magnitude, signaling the formation of a protective passive film. The passive films that form on nickel– chromium and nickel–chromium–molybdenum alloys have been shown to be thin (<10 nm), and to consist mostly of chromium oxide, Cr2O3. An Auger electron spectroscopic (AES) study showed the presence of outer nickel and iron oxides over an inner chromium oxide layer on alloy 600 (N06600), but nickel oxide only in the case of alloy C-4 (N06455).29 An X-ray photoelectron spectroscopic (XPS) study differentiated inner chromium oxide and outer hydroxide layers.30 A study using both XPS and time of flight secondary ion mass spectrometry (TOF SIMS) suggested a threshold concentration of chromium in an alloy of 20% to promote strong passivity, above which thicker oxides developed with layered structures consisting of inner Cr–Ni and outer Cr–Mo oxides. Molybdenum and tungsten exert little discernible effect on passivation at potentials below 200 mV, when Cr3þ is the only possible chromium dissolution

product. However, they appear to suppress passive film dissolution at potentials > 500 mV, when transpassive oxidation to Cr6þ is possible.31 The relative performances of nickel–chromium and nickel–chromium–molybdenum alloys in aqueous hydrochloric acid will depend on their compositions, as illustrated in the isocorrosion diagrams in Figure 15 for a range of alloys relative to the stainless steel 25–6MO (N08926) (note that these are isocorrosion diagrams for a corrosion rate of 0.5 mm year
1 which is not an acceptable corrosion rate for most practical applications).32 Clearly, all of the alloys have significant limitations and none perform as well as the nickel– molybdenum, B family of alloys at concentrations above 25% (w/w) pure acid. Alloy 825 (N08825) is the leanest in relation to both nickel and molybdenum contents, and is, therefore, the poorest performer. Alloy 625s (N06625) poor performance relates mainly to its relatively low molybdenum content. The remainder has similar resistances, the marginal differences amongst them reflecting mainly their relatively high, but differing molybdenum and tungsten contents. There is relatively little published data on the behavior of nickel alloys in aqueous hydrobromic acid, but such data suggest that their relative performances are broadly similar to those in hydrochloric acid. However, hydrobromic acid appears to be less aggressive than hydrochloric acid, despite being a stronger acid, exemplified by the isocorrosion diagrams (0.1 mm year
1 for alloy C-22 (N06022) in hydrochloric and hydrobromic acids at concentrations up to 5M that are shown in Figure 16.33 In practice, the performances of nickel alloys in aqueous hydrohalic acids are very often determined by the presence of oxygen or other oxidizing agents that can accelerate markedly the corrosion rates of nickel–molybdenum alloys, or reduce the corrosion rates of chromium-containing alloys by promoting passivity. For example, corrosion rates for alloy B (N10001) tested in aerated and deaerated aqueous hydrochloric acid at 65–70  C are shown in Figure 17, and the pronounced accelerating effects of aeration, particularly at concentrations below 25% (w/w), are evident.34 The most pronounced effects are produced by stronger oxidizing agents such as ferric ions, the effects of which on the corrosion of alloys B-3 (N10675) and C-2000 (N06200) in deaerated 20% (w/w) hydrochloric acid at 25  C are summarized in Figure 18.

Corrosion in Hydrogen Halides and Hydrohalic Acids

1217

120

Boiling point

Temperature (8C)

100

80

686

C276

C22, 59

60 825

625 40 25–6MO 20

0

10

20

30

40

Hydrochloric acid concentration % (w/w) Figure 15 Isocorrosion charts (0.5 mm year
1) for nickel alloys in hydrochloric acid. Adapted from High Performance Alloys for Resistance to Aqueous Corrosion; Publication SMC-026, Special Metals Corporation, 2000.

120 Corrosion rate (mm year–1)

1.25

Temperature ( 8C)

100 80 HBr

60 40

HCl

20

Alloy B 1.00

0.50

0

1

2 3 Acid concentration (M)

4

5

~40% (w/w) HBr ~18% (w/w) HCl

Figure 16 Isocorrosion charts (0.1 mm year
1) for alloy C-22 in hydrochloric and hydrobromic acids at concentrations up to 5 M. Adapted from Meck, N. S.; Pike, L.; Crook, P. Corrosion Performance of a New Age-Hardenable Ni–Cr–Mo Alloy, Paper 08181, Corrosion 2008; NACE International: Houston, TX, 2008.

Evidently, the addition of up to 200 ppm Fe3þ increases the corrosion rates of both alloys by increasing their corrosion potentials in the active potential range. Further additions produce progressive increases in the corrosion potential and corrosion rate of alloy B-3 (N10675) which remains active throughout the full potential range, as revealed in the polarization curves for the alloy with and without

Unaerated, 65 8C

0.25 0

0

Aerated, 70 8C

0.75

0

10 20 30 Hydrochloric acid concentration % (w/w)

40

Figure 17 Corrosion rates for alloy B tested in aqueous hydrochloric acid at 65–70  C. Adapted from Friend, W. Z. Corrosion of Nickel and Nickel-Base Alloys; John Wiley & Sons: London, 1980.

1000 ppm Fe3þ in Figure 18. In the case of alloy C-2000 (N06200), further additions also produce progressive and significant increases in the corrosion potential, but reductions in the corrosion rate by promoting passivation. This is apparent in the polarization curve in Figure 18 for alloy C-2000 (N06200) in the presence of 1000 ppm Fe3þ which shows no evidence of an active/passive transition because the material is spontaneously passivated at the outset of the test, as confirmed by its relatively high corrosion potential and the correspondingly low applied current densities required to raise the potential above the corrosion potential.24

Liquid Corrosion Environments

102

Corrosion rate (mm year–1) Potential (mv) (SCE)

Alloy B-3 1000 ppm Fe3+

0 –200 –400 10–5

Alloy C-2000

1000

10–1 C-2000

10–2 10–3 600

C-2000

400 200

10–1

10

600 1000 ppm Fe3+

200

B-3 0

–200 0 200 400 600 800 1000 Ferric ion concentration (ppm)

10–3

1400

1

Corrosion potential (mv) (SCE)

400 200

B-3 10

Potential (mv) (SCE)

1218

–200

103 Current density (mA cm–2)

–600 10–5

10–3

10–1

10

103

Figure 18 Corrosion rates, corrosion potentials, and polarization behaviors of alloys B-3 and C-2000 in deaerated 20% (w/w) hydrochloric acid with various additions of ferric ions at 25  C. Adapted from Nacera, S. M.; Crook, P.; Klarstrom, D. L.; Rebak, R. B. Effect of Ferric Ions on the Corrosion Performance of Nickel Alloys in Hydrochloric Acid Solutions, Paper 04430, Corrosion 2004; NACE International: Houston, TX, 2004.

Nickel and its alloys are among the more corrosion resistant materials in aqueous HF acid, and their relative performances up to the relevant atmospheric boiling point are summarized relative to the stainless steel type 316L (S31603) in the isocorrosion diagrams in Figure 19 (note that these are isocorrosion diagrams for a corrosion rate of 0.5 mm year
1 which is not an acceptable corrosion rate for most practical applications).35 Corrosion rates increase generally with concentration and temperature, and the nickel–copper and nickel–molybdenum alloys outperform significantly the nickel–chromium–molybdenum alloys in the pure acid. Behavior is influenced strongly by the formation and persistence or otherwise of pseudopassive, fluoride films. However, as with the other acids, the presence of oxygen and other oxidizing agents play a key role and can dominate materials performance, particularly in the cases of nickel–copper and nickel–molybdenum alloys. As a result, the performances of alloys can vary significantly between

liquid and vapor phases as shown in the corrosion rates for various nickel alloys tested in and above 10% (w/w) HF acid at 24 and 76  C that are charted in Figure 20.36 Thus, all the alloys perform relatively well in the liquid phase but nickel 200 (N02200) and alloys 400 (N04400) and 600 (N06600) perform very poorly in the vapor phase at both temperatures. The corrosion rates of alloy C-22 (N06022) also increase in the vapor phase, but not to the same extent as the other alloys that were tested. These effects are all due to the increased availability of oxygen in the thin, liquid films that condense in the vapor phase. In the case of the chromium-free alloys 200 (N02200) and 400 (N04400) or chromiumdeficient alloy 600 (N06600), corrosion product accumulation in the liquid films promotes progressively increased rates of corrosion. Thus, in the case of alloy 400 (N04400), cuprous ions (Cuþ) that are the initial products of dissolution are oxidized by oxygen to the cupric state (Cu2þ) in which they induce further

Corrosion in Hydrogen Halides and Hydrohalic Acids

1219

120 Alloy 400 < 0.5 (mm year–1) at all concentrations below boiling point

Boiling point 100 Temperature (8C)

200, C-276, C-22, 59, 686 80

B-2

60 600 40

825 G-30

20

690 625

316L 0

20

40 60 80 Hydrofluoric acid concentration % (w/w)

100

Figure 19 Isocorrosion charts (0.5 mm year
1) for nickel alloys in aqueous hydrofluoric acid. Adapted from Crum, J. R.; Smith, G. D.; McNallan, M. J.; Hirnyj, S. Characterisation of Corrosion Resistant Materials in Low and High Temperature HF Environments, Paper 382, Corrosion ’99; NACE International: Houston, TX, 1999.

9 Corrosion rate (mm year–1)

8 7 6 5 4 3 2 1 0 Ni 200 24 8C, L

Alloy 400

Alloy 600

24 8C, V

76 8C, L

Alloy C-22 76 8C, V

Figure 20 Corrosion rates for various nickel alloys tested in (liquid, L) and above (vapor, V ) 10% (w/w) hydrofluoric acid at 24  C and 76  C. Adapted from Pawel, S. J. Corrosion 1994, 50, 963–971.

increases in corrosion potential and corrosion rate. In contrast, the chromium-containing alloy C-22 (N06022) performs better in the vapor phase because passive film formation, albeit relatively nonprotective, is possible. Clearly, this is a significant consideration in selecting nickel alloys for aqueous HF acid duties. Other significant factors include: 1. Niobium has an adverse effect on the corrosion resistance of nickel alloys, exemplified by the relatively poor performance of alloy 625 (N06625) that

contains a nominal 3.7% niobium, as shown in Figure 19. 2. Segregation of specific elements, not least niobium, can result in preferential corrosion of welds in nickel alloys.6 3. SCC is a significant threat to nickel alloys in aqueous HF acid. Nickel 200 (N02200) and alloys 400 (N04400), 500 (N05500), 600 (N06600), B-2 (N10665), and C-276 (N10276) have all been reported as vulnerable under specific environmental conditions, in particular in vapors over 48% (w/ w) acid. The risks of SCC are exacerbated by cold work, and the presence of oxygen and Cu2þ and can be mitigated by appropriate heat treatment.6 The very different behaviors of the nickel– molybdenum B and nickel–chromium–molybdenum C families of alloys have stimulated interest in alloy compositions that might provide both the resistance to anodic dissolution of the B family and the capacity to passivate of the C family. For example, a proprietary alloy Hybrid BC-1 has recently been developed37 containing 22% molybdenum and 15% chromium that outperforms significantly the other members of the C family of alloys in aqueous hydrochloric and hydrobromic acid solutions, and maintains its performance in the presence of oxygen. For example, corrosion rates for the new alloy and various members of the C family of alloys in tests in hydrochloric acid at concentrations in the range 5–20% (w/w) at 80  C are shown in Figure 21. Clearly, the

1220

Liquid Corrosion Environments

2.0

2.5

C-22

Copper

1.5 C-4 C-276 1.0

Corrosion rate (mm year–1)

Corrosion rate (mm year–1)

C-2000

70–30 Cu–Ni

0.25

Alloy 400 0.025

0.5 Hybrid BC-1 0 0 5

10 15 20 Hydrochloric acid concentration % (w/w)

Figure 21 Corrosion rates for various nickel–chromium– molybdenum alloys tested in aqueous hydrochloric acid at 79  C. Adapted from Crook, P.; Meck, N. S.; Koon, N. E. The Corrosion Characteristics of a Uniquely Versatile Nickel Alloy, Paper 08190, Corrosion 2008; NACE International: Houston, TX, 2008.

new alloy performs significantly better than any of the other alloys tested, but the prevailing rates of 0.5 mm year
1 that apply across much of the concentration range are of limited practical benefit.

2.22.5 Other Metals and Alloys 2.22.5.1

Copper and Copper Alloys

Copper has limited resistance to anhydrous hydrogen chloride gas, with a suggested upper temperature limit for continuous service below 100  C. Copper and some of its alloys are, however, much more resistant to AHF gas and liquid, as shown in Figures 11 and 12. Copper has been used at ambient temperatures to handle AHF liquid and vapor and the cupro–nickels (C71000, C71500), in particular, have useful resistance, and have been used for heat exchanger tubing because of their relative resistance to flowing AHF.6 Copper might be expected to resist aqueous hydrochloric acid because it is relatively noble and does not normally displace hydrogen from acid solutions. However, its resistance is reduced significantly in the presence of air or other oxidizing agents, not least its own corrosion product, Cu2þ, which can stimulate corrosion autocatalytically. Silicon bronzes (for example, C65500) have found some use at

10 20 30 40 50 60 70 Hydrofluoric acid concentration % (w/w)

Figure 22 Corrosion rates for copper alloys in static, aqueous hydrofluoric acid under air at 60  C. Adapted from Materials Selector for Hazardous Chemicals. In Hydrogen Fluoride and Hydrofluoric Acid; MTI Publication MS-4, Materials Technology Institute of the Process Industries Inc., 2000; Vol. 4.

concentrations < 20% (w/w) at moderate temperatures5 but, in general, applications for copper and its alloys in aqueous hydrochloric acid are limited. Some copper alloys have useful resistance to aqueous HF acid but only at relatively low velocities in the absence of oxygen or other oxidizing agents. Corrosion rates for some copper alloys in static, aqueous HF acid under air at 60  C are shown in Figure 22.3 Other tests have shown increased rates of corrosion in vapors over aqueous acids due to the presence of oxygen. Copper, cupronickels (C71000, C71500), and aluminum/aluminum–silicon bronzes have all found some application in aqueous acids, but there have been problems with dealloying and SCC.6 2.22.5.2

Titanium and Titanium Alloys

Titanium and its alloys are dependent on the formation of protective oxide films consisting mostly of titanium dioxide, TiO2, for their resistances to corrosion. Such films require the presence of oxygen or water to form, and as a general rule anhydrous conditions are best avoided. Titanium and its alloys thus have useful resistance to hydrogen chloride gas containing water but anhydrous gas or condensates from water-containing gas can present significant corrosion risks, depending on the temperature.2 One of the conditions for protective film formation in aqueous hydrohalic acids, namely the availability of a

Corrosion in Hydrogen Halides and Hydrohalic Acids

cathodic process to elevate the corrosion potential above the potential for passive oxide formation, is satisfied under most conditions because this potential is below the hydrogen evolution potential. However, the solubility of the passive film increases with concentration and temperature. It has also been shown that the protection afforded by the film depends on the conditions under which it forms, in particular the formation rate.38 Titanium dioxide is attacked strongly by HF acid to a degree that precludes the application of titanium and its alloys in acidic environments containing HF acid or fluorides. The performance of titanium and its alloys in other hydrohalic acids is dominated by the stability of the titanium dioxide film, and applications are limited to relatively weak acids. Figure 23 shows isocorrosion curves (0.13 mm year
1) for commercially pure titanium (ASTM grade 2, R50400), a titanium–nickel–molybdenum alloy (ASTM grade 12, R53400), and a titanium– palladium alloy (ASTM grade 7, R52400) in naturally aerated hydrochloric acid at concentrations up to 35% (w/w).39 Evidently, pure titanium is restricted

Boiling point

100 85

Grade 7 70

Temperature (8C)

55

Grade 12 Grade 2

40 25

to concentrations below 5% (w/w) even at ambient temperatures. Alloying with palladium significantly improves the corrosion resistance arising from the well established cathodic modification effects of palladium on promoting and sustaining the passivity of titanium, which are described elsewhere in this book. There is little published data relating to the performance of titanium and its alloys in hydrobromic acid. Apart from the order of hydrohalic acid strengths, it is known that the breakdown potentials of titanium in aqueous halide-containing solutions are in the order: chlorides > bromides > iodides,40 so hydrobromic acid might be expected to be more aggressive than hydrochloric acid. Notwithstanding that, titanium and its alloys do find commercial application in processes that involve exposure to hydrobromic acid, such as in the manufacture of terephthalic acid from p-xylene using metal bromide catalysts. Oxidizing agents in the acid such as dissolved oxygen, chlorine, nitric and chromic acids, and metal cations in higher oxidation states such as ferric, Fe3þ or cupric, Cu2þ ions also promote significant increases in corrosion resistance by promoting and sustaining passivity. For example, the effects of progressive additions of up to 125 ppm ferric ions on the performance of commercially pure titanium, (ASTM grade 2, R50400), in aqueous hydrochloric acid is also shown in Figure 23. Evidently, the addition of 125 ppm ferric ions extends the acceptable performance boundary to match the performance of the titanium–palladium alloy, (ASTM grade 7, R52400), at lower temperatures.39 However, even the temporary absence of oxidizing agents can result in a loss of passivity and the addition of palladium to titanium provides a much more robust mechanism for the mitigation of corrosion risk than additions of oxidizing agents to the acid.

Boiling point

100

2.22.5.3

85 Grade 2 Ti 70

125 ppm Fe3+ 60

55 30 40

1221

0

25

0 10 20 30 Hydrochloric acid concentration % (w/w)

Figure 23 Isocorrosion charts (0.13 mm year
1) for titanium and various titanium alloys in hydrochloric acid and in hydrochloric acid containing various levels of ferric ions. Adapted from Corrosion Resistance of Titanium; Timet Metals Corporation, 1997.39

Zirconium and Zirconium Alloys

Zirconium and its alloys are similar to titanium in their dependence on oxide films, composed mainly of ZrO2, to resist corrosion. They are less resistant to anhydrous hydrogen halide gases than titanium and they find no practical application. Zirconium dioxide is also attacked strongly by HF acid to a degree that also precludes the application of zirconium and its alloys in acidic environments containing HF acid or fluorides. The performance of zirconium and its alloys in other aqueous hydrohalic acids is superior to titanium because the zirconia film is much less soluble than the corresponding titania film on titanium.

1222

Liquid Corrosion Environments

Indeed, zirconium is second only to tantalum in its resistance to aqueous hydrochloric acid, exemplified by the isocorrosion diagram for commercial purity zirconium (Zr702, R60702) in Figure 14. The performances of the commercial alloys containing tin (Zr704, R60704) and niobium (Zr705, R60705) are broadly similar, but they may display inferior resistance toward the performance boundary for commercial purity zirconium (Zr702, R60702) in Figure 14. The main threats to the successful performance of zirconium and its alloys in hydrochloric acid are: 1. Strongly oxidizing conditions can result in the potential exceeding the breakdown potential, resulting in the initiation of localized corrosion. Aeration does not present a significant threat in this respect but stronger oxidizing agents such as cupric and ferric ions in sufficient quantities can stimulate significant pitting attack. Galvanic coupling with nobler materials such as graphite can present a similar threat. 2. Galvanic coupling to more active materials can result in the uptake of cathodically produced hydrogen, resulting in the embrittlement and hydriding of zirconium and its alloys, with a consequential loss of ductility. These risks and their mitigation are described in more detail in the chapter on zirconium and its alloys in this book. As with titanium, there is little published data relating to the performance of zirconium and its alloys in hydrobromic acid. However, in contrast to titanium, the breakdown potentials of zirconium in aqueous halide-containing solutions are in the order: iodides > bromides > chlorides41 and it might thus be expected to be less vulnerable to oxidizing agents than in hydrochloric acid. 2.22.5.4

Tantalum and Tantalum Alloys

Tantalum and its alloys are similar to zirconium and titanium in their dependence on the formation of a protective oxide, specifically tantalum pentoxide (Ta2O5), to resist corrosion. Tantalum is resistant to hydrogen chloride and bromide gases up to at least 150  C, and even at higher temperatures if the gases contain some water. However, in keeping with the other reactive metals, tantalum’s protective oxide is attacked strongly by anhydrous and aqueous HF acid to a degree that precludes its application in acidic environments containing HF acid or fluorides.

Tantalum pentoxide is very stable in aqueous hydrochloric acid, and tantalum (R05200) has exceptional corrosion resistance to the acid, as illustrated in Figure 14. Niobium is similar to tantalum, being protected from corrosion by a film of niobium pentoxide, Nb2O5. The film is less protective than tantalum pentoxide, as a result of which niobium is considerably less resistant to corrosion in hydrochloric acid than tantalum or zirconium, as shown in Figure 14. Binary alloys of tantalum and niobium deliver performances intermediate between those of the parent metals. The performance of the commercial 2.5% tungsten alloy (R05252) is broadly similar to tantalum metal. The main threat to the successful performance of tantalum and its alloys in hydrochloric acid is hydrogen embrittlement due to uptake of cathodic hydrogen from the corrosion of tantalum itself or a more active metal to which tantalum is coupled electrically. Tantalum has a high solubility for hydrogen and forms hydrides. It can absorb hydrogen at ambient temperatures but embrittlement arising from the self corrosion of tantalum and its alloys is usually associated with higher temperatures at which significant corrosion rates are experienced. The rate of embrittlement depends on the corrosion rate but the effects are cumulative, and it has been reported5 in 25% (w/w) hydrochloric acid at 190  C in which the corrosion rate is no more than 0.025 mm year
1. Hydrogen embrittlement of tantalum can be prevented by electrical isolation from more active equipment items using insulated flanges or by coupling with a more noble metal with a low hydrogen overvoltage such as platinum, either mechanically, or by resistance or spot welding, or brush plating.2 An area ratio of 1000:1 Ta:Pt has been shown to be effective, not only in eliminating embrittlement, but also in reducing the corrosion rates of both tantalum and the noble metal, as shown in Figure 24.42 The presence of oxidizing agents also prevents hydrogen embrittlement by raising the potential of the tantalum. Thus, embrittlement does not occur in chlorine-saturated hydrochloric acid.2 Tantalum is also highly resistant to hydrobromic acid but there is much less data in the public arena than for hydrochloric acid. Like titanium, the breakdown potentials of tantalum in aqueous halidecontaining solutions are in the order: chlorides > bromides > iodides,40 so tantalum might be expected to be more vulnerable to the presence of oxidizing agents in hydrobromic than in hydrochloric acid.

Corrosion rate (mm year–1)

Corrosion in Hydrogen Halides and Hydrohalic Acids

2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

2.22.6.1 Thermoplastic and Reinforced Thermosetting Materials

Concentrated HCl (190 8C) Ta uncoupled Pt uncoupled

Ta coupled Pt coupled

Figure 24 Corrosion rates for uncoupled and coupled tantalum and platinum in concentrated hydrochloric acid at 15–40  C. Adapted from Bishop, C. R.; Stern, M. Hydrogen Embrittlement of Tantalum in Aqueous Media; 17th Annual NACE Conference; National Association of Corrosion Engineers, 1966.

2.22.5.5

1223

Noble Metals

Silver, gold, and platinum are highly resistant to the anhydrous hydrogen halides. In anhydrous hydrogen chloride, upper temperature limits of 230, 870, and 1200  C have been suggested for silver, gold, and platinum, respectively.5 Silver has found significant application in equipment for AHF in the absence of oxidizing agents and sulfides.6 Silver has good but not unlimited resistance to the aqueous hydrohalic acids below the atmospheric boiling point, depending on the temperature, velocity, and the presence or otherwise of oxidizing agents. Gold and platinum are highly resistant below the atmospheric boiling point, although gold is vulnerable to the presence of very strong oxidants such as ferric ions or nitric acid, and platinum is slightly attacked in 36% (w/w) acid at the boiling point5,6 and more significantly so at higher temperatures, exemplified by the data in Figure 24.

2.22.6 Nonmetallic Materials Given the limitations of most classes of alloys that might be considered for handling hydrohalic acid, and the high cost of the more resistant materials such as nickel, zirconium, and tantalum alloys, it is inevitable that nonmetallic materials play major roles in hydrohalic acid applications. The application of these materials is covered in more detail in the relevant chapters in this book but the more significant materials are as follows.

In practice, chemical compatibility is but one factor that determines the suitability or otherwise of a thermoplastic material for a specific application. Other factors such as mechanical and fabrication properties, thermal expansion characteristics, permeation properties, etc. are significant design considerations. In practice, other than for small scale equipment, thermoplastics are used more commonly as linings on stronger substrates rather than in solid form, because of reliability and integrity concerns. Permeation is a particular issue in the case of HF acid at higher concentrations and temperatures, and needs to be managed by, for example, the use of manifolded vent holes through the metal7 in the case of lined metal systems. Most thermoplastics of commercial significance find applications in the handling of hydrohalic acids. Readers interested in specific applications should consult materials suppliers and the more detailed sources are referenced,2–8 but the more significant materials are as follows: 1. Polyethylene (PE), depending upon its molecular weight, and polypropopylene (PP) have useful resistance. PE tends to be restricted to temperatures close to ambient, but PP has been used at much higher temperatures as a lining material in the handling, for example, of hydrochloric acid. 2. Polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC) also find uses at lower temperatures, for example, as linings in glass reinforced plastic tanks for the storage of hydrochloric acid. Unplasticized grades are normally specified because plasticized grades have lower chemical resistance. 3. Fluoroplastics are highly resistant to hydrohalic acids, depending upon their degree of fluorination. The highly fluorinated materials polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), and perfluoroalkoxy (PFA) are resistant to all concentrations of the acids up to and well-beyond their atmospheric boiling points. The less highly fluorinated materials ethylene chlorotrifluoroethylene (ECTFE), ethylene tetrafluoroethylene (ETFE), and polyvinylidene fluoride (PVDF) are also highly resistant but tend to have lower temperature limits for service than the fully fluorinated grades. Reinforced polyester, epoxy, phenolic, and furane resins all find application in the handling of hydrohalic

1224

Liquid Corrosion Environments

acids. Glass reinforced materials (GRP) based on polyester or vinyl ester resins, possibly lined with PVC (so called dual laminate construction), are widely used for the storage and handling of hydrochloric acid at temperatures up to 90  C, depending on the acid concentration and the specific resin. Glass is attacked by HF acid but constructions using epoxy vinyl esters and corrosion resistant barriers such as carbon veils can be used for handling weaker acids at lower temperatures.

HF acid duties. Carbon bricks with appropriate mortars and fluoroplastic membranes have found applications as linings for steel in high temperature duties. Silicon carbide also has excellent hydrohalic acid resistance and has been used in heat exchange duties.

References 1.

2.22.6.2

Elastomers

Natural rubber is used widely as a lining for carbon steel in the storage, transport, and handling of hydrochloric acid. Softer rubbers tend to be restricted to lower temperatures, but hard rubber can tolerate temperatures as high as 90  C. Natural rubbers are limited to ambient temperatures in HF acid. Chloro- or bromobutyl rubbers are preferred as linings on carbon steel for the storage and transport of up to 70% (w/w) HF acid, up to maximum service temperatures that depend on concentration. Other elastomers such as butyl, ethylene propylene diene terpolymer (EPDM), and chlorosulphonated PE (such as Hypalon) find niche applications in hydrohalic acids where, for example, the resistance of natural rubber is limited by factors such as the presence of organic contaminants, higher temperatures, or flexibility requirements. Additives such as silica or magnesia compounds are unsuitable for HF acid service, and have to be substituted by materials such as carbon in special grades. Fluoro- and perfluoroelastomers provide the best resistance to hot and/or concentrated hydrohalic acids, but their high cost tends to limit their use to smaller components such as gaskets and other sealing components. 2.22.6.3

2.

3.

4. 5.

6.

7.

8.

9. 10. 11. 12. 13.

Inorganic Materials 14.

Borosilicate glass is highly resistant to hydrochloric and hydrobromic acids up to temperatures well beyond their atmospheric boiling points, and solid or glass lined equipment finds significant use for handling the acids. However, glass is heavily attacked by HF acid for which it is unsuitable. Impervious graphite, filled with impregnants such as phenolic resins, PTFE, or carbon, is highly resistant to hydrohalic acids and finds significant use in heat exchange duties for temperatures well-beyond atmospheric boiling. Carbon–carbon composites have excellent resistance to hydrohalic acids and found application for internal components, particularly in

15.

16.

17.

18.

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