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Corrosion failures of AISI type 304 stainless steel in a fertiliser plant
Engineering Failure Analysis 10 (2003) 329–339 www.elsevier.com/locate/engfailanal
Corrosion failures of AISI type 304 stainless steel in a fertiliser plant H. Shaikh*, R.V. Subba Rao, R.P. George, T. Anita, H.S. Khatak Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102, India Received 16 July 2002; accepted 14 October 2002
Abstract A urea plant, operating on ammonia and carbon dioxide (CO2) gases, had to be shutdown due to corrosion in the intercooler and aftercooler of its CO2 gas cleaning circuit. Extensive general corrosion of AISI type 304 stainless steel parts, such as sealing strips, fins, demisters and the shell, of these two components which were in contact with the duplex stainless steel tubes, caused the shutdown of the fertiliser plant within 6 months. Investigations of the corrosion products by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques showed the presence of carbon and ammonia based compounds, thus suggesting the role of ammonia and CO2 gases, or the product of their reactions, in the corrosion of type 304 stainless steel. Electrochemical polarisation studies showed that duplex stainless steel possessed a more positive open circuit potential and a nobler critical pitting potential than type 304 stainless steel thus confirming that the corrosion of type 304 stainless steel was caused by the galvanic action with the duplex stainless steel heat transfer tubes. Hence, it was recommended that (i) the same material (type 304 stainless steel) be used for all parts of the intercooler and aftercooler to avoid galvanic corrosion, (ii) condense water carried over by CO2 gas by cooling it to low temperatures immediately after it comes out from the scrubber, (iii) slight modification of the process to add up to 0.8% oxygen in the CO2 gas before entry into the intercooler, which will help in retaining/formation of an effective passive film on type 304 stainless steel. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Chemical-plant failures; Galvanic corrosion; Stainless steel; Corrosion products; Corrosion protection
1. Introduction Fertiliser industry is playing a crucial role in ensuring sustained growth in agricultural production and growing fertilizer consumption over the years has been one of the major factors for India having become self-sufficient in food grains. Fertiliser plants in India can be classified into two broad categories: (1) nitrogenous fertilizers, and (2) phosphatic and mixed fertilizers. The fertilizer industry uses a wide variety of feed stocks eg. naphtha, coal, natural gas, fuel oil, coke oven gas, rock phosphate, sulphuric acid, phosphoric acid, nitric acid, hydrochloric acid etc. for the production of a wide variety of nitrogenous, * Corresponding author. Fax: +91-4114-80301. E-mail address: [email protected] (H. Shaikh). 1350-6307/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00076-6
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phosphatic and mixed fertilizers. During the manufacturing process, a wide variety of corrosive environments, like fluorine, hydrofluorosilicic acid, SO2, SO3 and NOx gases, acid mist etc, are formed. The different forms of corrosion which have been experienced in fertilizer plants are uniform corrosion, galvanic corrosion, intergranular corrosion, stress corrosion cracking, cavitation, fretting corrosion, high temperature corrosion, hydrogen embrittlement, metal dusting etc. Urea amounts to about 90% of the total nitrogenous fertilizers. Liquid ammonia and gaseous carbon dioxide form the two basic raw materials for the manufacture of urea. Since carbonic acid does not form a stable ammonium salt, unlike acids such as sulphuric and nitric acid, the reaction between ammonia and CO2 gas has to be carried out under high temperature and pressure to first form ammonium carbamate, a portion of which then dehydrates to form urea and water, as per the following reactions [1]: 2 NH3 þ CO2 $ NH2 COONH4 ðammonium carbamateÞ
ð1Þ
NH2 COONH4 $ COðNH2 Þ2 þH2 O
ð2Þ
Whereas the first reaction is highly exothermic and rapidly goes to completion, the second reaction is endothermic and always incomplete. The conversion of ammonium carbamate to urea increases with increase in reaction temperature and pressure, increasing molal ratio of NH3/CO2 and the residence time, and decreases with increase in ratio of H2O/CO2 of the feed reactants. This paper deals with the analysis of the failure due to extensive corrosion of AISI type 304 stainless steel parts in the intercooler and aftercooler of the carbon dioxide gas cleaning circuit of a urea production plant. 1.1. Operational history The normal operating condition of the intercooler and aftercooler is as follows: The carbon dioxide coming from the ammonia plant is impure and has N2, H2, CO, Ar and CH4 as impurities. The impure CO2 is then cleaned in a purification circuit. The purification starts by absorption of CO2 gas in Lean solution, which is specified to have 29.16% w/w K2CO3, 5.98% w/w KHCO3, 0.31% w/w pentavalent vanadium, 0.70% w/w diethanol amine and 0.69% w/w glycine, 57 ppm Fe and 6 ppm chloride present in it. This absorption is carried out at high pressure and low temperature. The impurity gases are not absorbed here and are released to atmosphere. The CO2 gas absorbed in the Lean solution is then regenerated in another chamber at high temperature and low pressure. The CO2-saturated Lean solution enters this chamber at 406 K and CO2 gas with entrainments of Lean solution leaves this chamber at 373 K. The regenerated CO2 gas is then scrubbed with water from the cooling tower containing ammoniacal nitrogen, chlorides and sulphates. The CO2 gas enters this scrubber at 355 K and leaves it at 305 K. The scrubbed CO2 gas, also containing ammoniacal nitrogen, water vapour and chlorides, then gets compressed and enters the intercooler at 461 K at a pressure of 3.5 kg/cm2 and leaves it at 313 K at the same pressure. Then, it is again compressed and enters the aftercooler at a pressure of 15 kg/cm2 and temperature of 461 K, before leaving at the same pressure but reduced temperature of 313 K. Fig. 1 gives the schematic of the intercooler and aftercooler, in which the hot CO2 gas exchanges heat with water. In both these components, the carbon dioxide is in the shell side while the water flows in tubes. The tubes additionally have suspended fins to aid the heat transfer. Demisters function to trap the water particles in the CO2 gas. Sealing strips help prevent escape of gases from the main functional area to other parts of the intercooler and aftercooler. 1.2. Background of failure The intercooler and aftercooler of the carbon dioxide cleaning circuit were found to have undergone extensive corrosion. The shell, sealing strips and fins were made of AISI type 304 stainless steel while the tubes were made of duplex stainless steel. Earlier, the tubes were also made of AISI type 304 stainless steel.
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Fig. 1. A schematic of the intercooler and aftercooler.
However, the problem of chloride-induced stress corrosion cracking (CSCC) after 10 years of service, led to change in the tube material to duplex stainless steel. In the present case, failure occurred within 6 months of replacing the fins, demisters and sealing strips. However, before the change over of the tube material to duplex stainless steel, extensive general corrosion of shell, sealing strips and fins was not observed.
2. Experimental procedures Visual examination of the failed intercooler and aftercooler was carried out. The fins and sealing strips were examined for their microstructure using optical microscopy. X-ray photelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses of the corrosion products were carried out. The chloride content in the Lean solution was determined by titrating against AgNO3 solution, using sodium chromate as indicator. The technique was standardized by titrating NaCl solution of known concentration. The corrosion products were analysed using XPS and XRD techniques. In the XPS technique, the elements were scanned over a large range of binding energies from 0 to 1000 eV. In the XRD technique, the scanning was done over a wide range of angles from 5 to 44.7 in steps of 0.05 , with a signal collection time of 10 s per step.
3. Results 3.1. Visual examination Visual examination of the failed parts showed general corrosion of fins, demisters, sealing strips and shell. Extensive thinning of the 0.5–1.0 mm thick fins, sealing strips and demisters was observed. In some locations, total loss of thickness was observed (Fig. 2). This suggested extensive general corrosion on fins,
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Fig. 2. Stereo micrographs showing general corrosion. Arrows show regions of loss of thickness in the sealing strip. Such features were also observed on the fins.
demisters and sealing strips. Also, a few cracks, which appeared to be brittle in nature, were detected on the sealing strips. These cracks were covered with corrosion products, making it difficult to indicate whether they were mechanically or corrosion-induced cracks. However, the duplex stainless steel tubes were free of both general corrosion and cracking. About 5 gunny bags (nearly 100 kg of corrosion products) were removed from both the intercooler and aftercooler. 3.2. Chemical analysis Table 1 gives the chemical composition of AISI type 304 stainless steel and 3RE60 duplex stainless steel. The chemical composition of AISI type 304 stainless steel conformed to its documented specification. 3.3. Optical microscopic examination Optical microscopic examination of the fins and sealing strips suggested that the materials were well annealed and there was no evidence of sensitization. Fig. 3 shows the microstructure of the sealing strip. Fig. 4 shows the presence of ferrite in the austenite matrix of the duplex stainless steel. 3.4. Determination of chloride content by titration technique The chloride content in the Lean solution, as determined by the titration technique, was found to be 11,100 ppm as against the specified amount of 6 ppm..
Table 1 Chemical composition of AISI type 304 stainless steel of the sealing strip and 3RE60 duplex stainless steel used in the polarization test Element
C
Cr
Ni
Mo
N
Si
Mn
Cu
P
S
Type 304 SS 3RE60 SS
0.05 0.03
18.05 17.5
8.42 5.2
0.15 3.0
– 0.08
– 1.4
– 1.6
– 0.2
– 0.04
– 0.004
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333
Fig. 3. Optical micrograph showing a well annealed microstructure of sealing strip material.
Fig. 4. Optical micrograph of duplex stainless steel showing the presence of ferrite in austenite matrix.
3.5. Analysis of corrosion products Fig. 5(a) and (b) shows the broad spectra of corrosion products from intercooler and aftercooler respectively. Compounds of carbon, oxygen, chromium, iron and nickel were present in all the corrosion products. Indium peaks were due to the indium foils used to contain the corrosion products during tests. High resolution narrow band scans were carried out for each of the peaks of interest to measure the exact deconvoluted binding energies (shifts) of each minor peak within. These were compared with the standards to identify the compound. Table 2 shows the whole range of spectra of binding energies that were determined and compounds that were identified against them.
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Fig. 5. Broad spectrum of the XPS pattern of corrosion products from (a) intercooler and (b) aftercooler.
Fig. 6(a) and (b) shows the XRD patterns of corrosion products from the aftercooler and the fins to contain Fe2O3 and (Fe,Cr)2O4. By the analysis of corrosion products using XPS and XRD techniques, the presence of large quantities of Fe2O3, (Fe,Cr)2O4, and minor amounts of Fe(OH)O, Fe(CO)5, Cr(CO)6, Ni(CO)4, Cr(OH)3, Fe(OH)3, NiO, FeCOOH, Fe(CO)2(NO)2, (NH2)2CO, FeCH2COOH and FeCH2NH2 were identified. It was clear from the analysis that CO and NH2 based compounds dominated the corrosion product apart from oxides of iron and chromium.
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H. Shaikh et al. / Engineering Failure Analysis 10 (2003) 329–339 Table 2 Compounds identified in the corrosion product Binding energy peak centre (eV)
Identified in corrosion product of
Element identified
Compound identified
714.1 713.4 534.2 531.3 529.6 289.4 288.2 287.7 287.1 285.4 284.6 284.4
AC IC IC IC IC AC AC IC AC AC IC AC
Fe Fe O O O C C C C C C C
Fe2O3, Fe(OH)O Fe2O3, Fe(OH)O Fe(CO)5, Cr(CO)6, Ni(CO)4 Cr(OH)3, Fe(OH)3 NiO, Fe2O3 FeCOOH Fe(CO)2(NO)2, (NH2)2CO Fe2CO FeCH2COOH FeCH2NH2 Elemental C Elemental C
IC intercooler; AC aftercooler.
4. Discussion The accumulation of large amounts of corrosion products in the intercooler and aftercooler, which were made of stainless steels, in just about 6 months suggested that general corrosion caused the failure. Substantial thinning down of the fins and sealing strip was observed. Such a rampant corrosion implied that something was drastically wrong with the process or the materials used in the components. Chemical analyses of sealing strips and fins confirmed that they conform to the AISI type 304 stainless steel specification for composition. Metallographic examination indicated a well-annealed microstructure. In fact, in many urea plants, types 304 and 316 stainless steels and their variants are the most favoured materials of construction. Hence, choice of material for the application could not be faulted. For the above reasons, this unusual amount of corrosion of the fins, sealing strips and the shell of the intercooler and aftercooler, all of which were made of type 304 stainless steel, was baffling and called for a thorough investigation. The first point that needed to be investigated was the environment that could have caused such largescale general corrosion of stainless steels. The intercooler and aftercooler could possibly have seen any of the following environments that could have mixed with carbon dioxide gas: (i) Lean solution, (ii) water from the cooling tower, used to scrub the regenerated CO2 gas, containing ammoniacal nitrogen, chlorides and sulphates, and (iii) wet CO2 gas, and products of reactions that could occur between CO2 gas and ammonia in the scrubbing water under the temperature and pressure of operation in the intercooler and aftercooler. Analysis of chloride content indicated that the Lean solution had about 11,000 ppm of chlorides as against the specified amount of 6 ppm, suggesting that the role of Lean solution in the extensive corrosion of type 304 stainless steel could not be ruled out. Analysis of corrosion products suggested absence of chlorides in the corrosion products. The corrosion product analysis showed that Fe2O3 was the most dominant corrosion product. Potassium, carbonates, bicarbonates and vanadium ions were not detected in the corrosion product, thus indicating that there was very little carry over of Lean solution due to efficient scrubbing of the regenerated CO2 gas by the water from the cooling tower. For this reason, chlorides were not expected to carry over since all the Lean solution was scrubbed off from the CO2 gas. A number of CO and NH2 based compounds were found to be present in the corrosion products. This suggested that ammonium ions present in the water of the cooling tower and CO2 gas were responsible for the corrosion. Ammonia is known to be harmful only under conditions of high temperature and high pressure. Ammonia could dissociate into atomic nitrogen and atomic hydrogen. Atomic nitrogen could
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Fig. 6. XRD patterns from corrosion products of (a) aftercooler and (b) fins.
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lead to nitriding while atomic hydrogen could cause decarburisation [2]. Unpressurised ammonia is known to cause nitriding at temperatures above 723 K and pressurized ammonia causes nitriding and subsequent embrittlement above 623 K in Cr–Ni steel [2]. Under the present conditions of operating temperatures and pressures, nitriding of the stainless steel was ruled out. Electrochemical polarisation tests on type 304 stainless steels in the ammonia containing scrubbing water showed extremely low passive current density and a very noble open circuit and pitting potentials. This suggested that condensation of water containing ammonia would not cause general or pitting corrosion of type 304 stainless steel parts. Table 3 shows that corrosion rates of type 304 stainless steel in anhydrous ammonia is < 20 mpy up to 250 C while the corresponding value in ammonia gas is < 2 mpy up to 90 C [3]. In the present case no ammonia gas was present. All the ammonia was dissolved in the water. Type 304 stainless steel undergoes a corrosion of about 20 mpy up to 100 C in both dry and wet CO2 gas [3] (Table 3). Corrosion rate of 20 mpy corresponds to gnawing down of the material to the extent of about 0.5 mm/year, which is the thickness of the fins and demisters. The presence of urea and urea-based compounds alongwith CO and NH2-based compounds suggest the formation of ammonium carbamate. Although urea as such is not corrosive to stainless steel, the ammonium carbamate formed is highly corrosive to it, more so with increase in temperature [4]. In case of type 304 stainless steel, high corrosion rates have been reported in ammonium carbamate in the absence of passivation or lower concentration of oxygen [5]. During the period, when type 304 stainless steel tubes were being used, corrosion, though not extensive, occurred at sections in the intercooler and aftercooler where the water vapour condensed such as at sealing strip etc. However, corrosion of fins was not observed then. The extensive corrosion of fins occurred only after changeover to duplex stainless steel tubes. This suggested that the galvanic coupling between tube and fins increased the rate of corrosion. Electrochemical polarisation tests to confirm the galvanic action between type 304 stainless steel and duplex stainless steel under actual service conditions could not be carried out due to experimentation difficulties. However, attempts were made to confirm the galvanic action between the type 304 and duplex stainless steel by carrying out polarisation tests separately on these steels in a laboratory environment i.e. acidified 1 M NaCl solution (pH=1.61) at room temperature. The results of the electrochemical polarization studies indicated that the open circuit potential of duplex stainless steel was almost 200 mV nobler than type 304 stainless steel in acidified NaCl solution. This suggested that type 304 stainless steel was more prone to corrosion than duplex stainless steel. The passive current density was nearly ten times lower for the duplex stainless steel vis-a`-vis type 304 stainless steel (Figs. 7 and 8), thus suggesting better passivity in corrosive environments for the former. The above results indicate that the carbon dioxide/water vapour/ammonia environment, and/or the ammonium carbamate produce acidic conditions that would depassivate type 304 stainless steel more easily than duplex stainless steel. The higher Cr+Mo content of the duplex stainless steel prevents such a depassivation occurring on the surface. Thus, the fins corroded due to the galvanic action with the duplex stainless steel tubes while sealing strip, demisters and shell experienced general corrosion due to loss of passivity in the environment encountered in the intercooler and aftercooler. Table 3 Corrosion rates of AISI type 304 stainless steel in various environments [3] Environment
Up to a temperature of ( C)
Corrosion rate (mpy)
Wet carbon dioxide Dry carbon dioxide Satuarated ammonium carbonate Ammonium bicarbonate Ammonia gas Anhydrous ammonia
93 99 93 93 99 249
<20 <20 <20 <20 <2 <20
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Fig. 7. Anodic polarisation curve of type 304 stainless steel in acidified 1 M NaCl solution.
Fig. 8. Anodic polarisation curve of duplex stainless steel in acidified 1 M NaCl solution.
5. Conclusions The extensive corrosion of type 304 stainless steel parts of the intercooler and aftercooler was not caused by either the Lean solution or the scrubbing water from the cooling tower. The CO2 gas and/or the reaction product of CO2 gas+ammonia+water vapour, probably ammonium carbamate, aided by the galvanic effects between type 304 and duplex stainless steels, caused the extensive corrosion of the type 304 stainless steel components, particularly of the fins, demisters and sealing strips.
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6. Recommendations Recommendations to minimize/delay future failures were made considering the economics of the material and minimization of downtime of the plant vis-a`-vis corrosion of fins and their replacement every 6 months, and chloride stress corrosion cracking of coolant tubes and their replacement every 10 years. The following were the recommendations: 1. Use the same materials for all the parts of the intercooler and aftercooler to avoid galvanic corrosion. Seamless tubes may be used. The duplex stainless steel tubes may be replaced with wellannealed AISI type 304 stainless steel tubes. In case welding is involved at any stage during manufacturing of tubes, AISI type 304L stainless steel tubes may be used to avoid the risk of sensitization. 2. Type 304 stainless steel is expected to undergo CSCC. The water chemistry of the cooling water in the tubes could be improved to reduce the chloride content. 3. Before the carbon dioxide enters the intercooler, ie immediately after scrubbing of the carbon dioxide with water, inject about 0.6–0.8% oxygen into the carbon dioxide gas. This need be implemented only as an additional precautionary measure. Addition of oxygen is reported to be effective in formation of a passivating film. 4. Condense the water carried over by CO2 gas by cooling it to low temperatures immediately after it comes out from the scrubber. This would prevent ammonia from getting into the IC and AC, thus reducing the possibility of formation of ammonium carbamate.
Acknowledgements The authors wish to acknowledge the help rendered by Mrs. N. Sivaibharasi of our laboratory, Mr. Subramaniam of Water and Steam Chemistry Laboratory, BARC Facilities, Kalpakkam and Mr. G.L.N. Reddy of Materials Science Division, IGCAR, Kalpakkam, in carrying out the XPS and XRD experiments.
References [1] Bansal RK, Karmarkar AK. Technological and engineering aspects of urea plants. Chemical Engineering World 1976;11(1):51– 68. [2] Behrens D, editor. DECHMA corrosion handbook, vol 2. FRG: VCH Publishers, 1988. [3] Schweitzer PA. Corrosion resistance tables, part A, IVth ed. USA: Marcel Dekker Inc; 1995. [4] Mall ID. Corrosion problems and selection of material of construction in fertilizer industry. Chemical Engineering World 1997; 32(9):153–65. [5] Somani SC. Corrosion problems in urea plant at FCI, Gorakhpur. Chemical Age of India 1979;30(7A):667.
Corrosion failures of AISI type 304 stainless steel in a fertiliser plant H. Shaikh*, R.V. Subba Rao, R.P. George, T. Anita, H.S. Khatak Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102, India Received 16 July 2002; accepted 14 October 2002
Abstract A urea plant, operating on ammonia and carbon dioxide (CO2) gases, had to be shutdown due to corrosion in the intercooler and aftercooler of its CO2 gas cleaning circuit. Extensive general corrosion of AISI type 304 stainless steel parts, such as sealing strips, fins, demisters and the shell, of these two components which were in contact with the duplex stainless steel tubes, caused the shutdown of the fertiliser plant within 6 months. Investigations of the corrosion products by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques showed the presence of carbon and ammonia based compounds, thus suggesting the role of ammonia and CO2 gases, or the product of their reactions, in the corrosion of type 304 stainless steel. Electrochemical polarisation studies showed that duplex stainless steel possessed a more positive open circuit potential and a nobler critical pitting potential than type 304 stainless steel thus confirming that the corrosion of type 304 stainless steel was caused by the galvanic action with the duplex stainless steel heat transfer tubes. Hence, it was recommended that (i) the same material (type 304 stainless steel) be used for all parts of the intercooler and aftercooler to avoid galvanic corrosion, (ii) condense water carried over by CO2 gas by cooling it to low temperatures immediately after it comes out from the scrubber, (iii) slight modification of the process to add up to 0.8% oxygen in the CO2 gas before entry into the intercooler, which will help in retaining/formation of an effective passive film on type 304 stainless steel. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Chemical-plant failures; Galvanic corrosion; Stainless steel; Corrosion products; Corrosion protection
1. Introduction Fertiliser industry is playing a crucial role in ensuring sustained growth in agricultural production and growing fertilizer consumption over the years has been one of the major factors for India having become self-sufficient in food grains. Fertiliser plants in India can be classified into two broad categories: (1) nitrogenous fertilizers, and (2) phosphatic and mixed fertilizers. The fertilizer industry uses a wide variety of feed stocks eg. naphtha, coal, natural gas, fuel oil, coke oven gas, rock phosphate, sulphuric acid, phosphoric acid, nitric acid, hydrochloric acid etc. for the production of a wide variety of nitrogenous, * Corresponding author. Fax: +91-4114-80301. E-mail address: [email protected] (H. Shaikh). 1350-6307/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00076-6
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phosphatic and mixed fertilizers. During the manufacturing process, a wide variety of corrosive environments, like fluorine, hydrofluorosilicic acid, SO2, SO3 and NOx gases, acid mist etc, are formed. The different forms of corrosion which have been experienced in fertilizer plants are uniform corrosion, galvanic corrosion, intergranular corrosion, stress corrosion cracking, cavitation, fretting corrosion, high temperature corrosion, hydrogen embrittlement, metal dusting etc. Urea amounts to about 90% of the total nitrogenous fertilizers. Liquid ammonia and gaseous carbon dioxide form the two basic raw materials for the manufacture of urea. Since carbonic acid does not form a stable ammonium salt, unlike acids such as sulphuric and nitric acid, the reaction between ammonia and CO2 gas has to be carried out under high temperature and pressure to first form ammonium carbamate, a portion of which then dehydrates to form urea and water, as per the following reactions [1]: 2 NH3 þ CO2 $ NH2 COONH4 ðammonium carbamateÞ
ð1Þ
NH2 COONH4 $ COðNH2 Þ2 þH2 O
ð2Þ
Whereas the first reaction is highly exothermic and rapidly goes to completion, the second reaction is endothermic and always incomplete. The conversion of ammonium carbamate to urea increases with increase in reaction temperature and pressure, increasing molal ratio of NH3/CO2 and the residence time, and decreases with increase in ratio of H2O/CO2 of the feed reactants. This paper deals with the analysis of the failure due to extensive corrosion of AISI type 304 stainless steel parts in the intercooler and aftercooler of the carbon dioxide gas cleaning circuit of a urea production plant. 1.1. Operational history The normal operating condition of the intercooler and aftercooler is as follows: The carbon dioxide coming from the ammonia plant is impure and has N2, H2, CO, Ar and CH4 as impurities. The impure CO2 is then cleaned in a purification circuit. The purification starts by absorption of CO2 gas in Lean solution, which is specified to have 29.16% w/w K2CO3, 5.98% w/w KHCO3, 0.31% w/w pentavalent vanadium, 0.70% w/w diethanol amine and 0.69% w/w glycine, 57 ppm Fe and 6 ppm chloride present in it. This absorption is carried out at high pressure and low temperature. The impurity gases are not absorbed here and are released to atmosphere. The CO2 gas absorbed in the Lean solution is then regenerated in another chamber at high temperature and low pressure. The CO2-saturated Lean solution enters this chamber at 406 K and CO2 gas with entrainments of Lean solution leaves this chamber at 373 K. The regenerated CO2 gas is then scrubbed with water from the cooling tower containing ammoniacal nitrogen, chlorides and sulphates. The CO2 gas enters this scrubber at 355 K and leaves it at 305 K. The scrubbed CO2 gas, also containing ammoniacal nitrogen, water vapour and chlorides, then gets compressed and enters the intercooler at 461 K at a pressure of 3.5 kg/cm2 and leaves it at 313 K at the same pressure. Then, it is again compressed and enters the aftercooler at a pressure of 15 kg/cm2 and temperature of 461 K, before leaving at the same pressure but reduced temperature of 313 K. Fig. 1 gives the schematic of the intercooler and aftercooler, in which the hot CO2 gas exchanges heat with water. In both these components, the carbon dioxide is in the shell side while the water flows in tubes. The tubes additionally have suspended fins to aid the heat transfer. Demisters function to trap the water particles in the CO2 gas. Sealing strips help prevent escape of gases from the main functional area to other parts of the intercooler and aftercooler. 1.2. Background of failure The intercooler and aftercooler of the carbon dioxide cleaning circuit were found to have undergone extensive corrosion. The shell, sealing strips and fins were made of AISI type 304 stainless steel while the tubes were made of duplex stainless steel. Earlier, the tubes were also made of AISI type 304 stainless steel.
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331
Fig. 1. A schematic of the intercooler and aftercooler.
However, the problem of chloride-induced stress corrosion cracking (CSCC) after 10 years of service, led to change in the tube material to duplex stainless steel. In the present case, failure occurred within 6 months of replacing the fins, demisters and sealing strips. However, before the change over of the tube material to duplex stainless steel, extensive general corrosion of shell, sealing strips and fins was not observed.
2. Experimental procedures Visual examination of the failed intercooler and aftercooler was carried out. The fins and sealing strips were examined for their microstructure using optical microscopy. X-ray photelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses of the corrosion products were carried out. The chloride content in the Lean solution was determined by titrating against AgNO3 solution, using sodium chromate as indicator. The technique was standardized by titrating NaCl solution of known concentration. The corrosion products were analysed using XPS and XRD techniques. In the XPS technique, the elements were scanned over a large range of binding energies from 0 to 1000 eV. In the XRD technique, the scanning was done over a wide range of angles from 5 to 44.7 in steps of 0.05 , with a signal collection time of 10 s per step.
3. Results 3.1. Visual examination Visual examination of the failed parts showed general corrosion of fins, demisters, sealing strips and shell. Extensive thinning of the 0.5–1.0 mm thick fins, sealing strips and demisters was observed. In some locations, total loss of thickness was observed (Fig. 2). This suggested extensive general corrosion on fins,
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Fig. 2. Stereo micrographs showing general corrosion. Arrows show regions of loss of thickness in the sealing strip. Such features were also observed on the fins.
demisters and sealing strips. Also, a few cracks, which appeared to be brittle in nature, were detected on the sealing strips. These cracks were covered with corrosion products, making it difficult to indicate whether they were mechanically or corrosion-induced cracks. However, the duplex stainless steel tubes were free of both general corrosion and cracking. About 5 gunny bags (nearly 100 kg of corrosion products) were removed from both the intercooler and aftercooler. 3.2. Chemical analysis Table 1 gives the chemical composition of AISI type 304 stainless steel and 3RE60 duplex stainless steel. The chemical composition of AISI type 304 stainless steel conformed to its documented specification. 3.3. Optical microscopic examination Optical microscopic examination of the fins and sealing strips suggested that the materials were well annealed and there was no evidence of sensitization. Fig. 3 shows the microstructure of the sealing strip. Fig. 4 shows the presence of ferrite in the austenite matrix of the duplex stainless steel. 3.4. Determination of chloride content by titration technique The chloride content in the Lean solution, as determined by the titration technique, was found to be 11,100 ppm as against the specified amount of 6 ppm..
Table 1 Chemical composition of AISI type 304 stainless steel of the sealing strip and 3RE60 duplex stainless steel used in the polarization test Element
C
Cr
Ni
Mo
N
Si
Mn
Cu
P
S
Type 304 SS 3RE60 SS
0.05 0.03
18.05 17.5
8.42 5.2
0.15 3.0
– 0.08
– 1.4
– 1.6
– 0.2
– 0.04
– 0.004
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333
Fig. 3. Optical micrograph showing a well annealed microstructure of sealing strip material.
Fig. 4. Optical micrograph of duplex stainless steel showing the presence of ferrite in austenite matrix.
3.5. Analysis of corrosion products Fig. 5(a) and (b) shows the broad spectra of corrosion products from intercooler and aftercooler respectively. Compounds of carbon, oxygen, chromium, iron and nickel were present in all the corrosion products. Indium peaks were due to the indium foils used to contain the corrosion products during tests. High resolution narrow band scans were carried out for each of the peaks of interest to measure the exact deconvoluted binding energies (shifts) of each minor peak within. These were compared with the standards to identify the compound. Table 2 shows the whole range of spectra of binding energies that were determined and compounds that were identified against them.
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Fig. 5. Broad spectrum of the XPS pattern of corrosion products from (a) intercooler and (b) aftercooler.
Fig. 6(a) and (b) shows the XRD patterns of corrosion products from the aftercooler and the fins to contain Fe2O3 and (Fe,Cr)2O4. By the analysis of corrosion products using XPS and XRD techniques, the presence of large quantities of Fe2O3, (Fe,Cr)2O4, and minor amounts of Fe(OH)O, Fe(CO)5, Cr(CO)6, Ni(CO)4, Cr(OH)3, Fe(OH)3, NiO, FeCOOH, Fe(CO)2(NO)2, (NH2)2CO, FeCH2COOH and FeCH2NH2 were identified. It was clear from the analysis that CO and NH2 based compounds dominated the corrosion product apart from oxides of iron and chromium.
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H. Shaikh et al. / Engineering Failure Analysis 10 (2003) 329–339 Table 2 Compounds identified in the corrosion product Binding energy peak centre (eV)
Identified in corrosion product of
Element identified
Compound identified
714.1 713.4 534.2 531.3 529.6 289.4 288.2 287.7 287.1 285.4 284.6 284.4
AC IC IC IC IC AC AC IC AC AC IC AC
Fe Fe O O O C C C C C C C
Fe2O3, Fe(OH)O Fe2O3, Fe(OH)O Fe(CO)5, Cr(CO)6, Ni(CO)4 Cr(OH)3, Fe(OH)3 NiO, Fe2O3 FeCOOH Fe(CO)2(NO)2, (NH2)2CO Fe2CO FeCH2COOH FeCH2NH2 Elemental C Elemental C
IC intercooler; AC aftercooler.
4. Discussion The accumulation of large amounts of corrosion products in the intercooler and aftercooler, which were made of stainless steels, in just about 6 months suggested that general corrosion caused the failure. Substantial thinning down of the fins and sealing strip was observed. Such a rampant corrosion implied that something was drastically wrong with the process or the materials used in the components. Chemical analyses of sealing strips and fins confirmed that they conform to the AISI type 304 stainless steel specification for composition. Metallographic examination indicated a well-annealed microstructure. In fact, in many urea plants, types 304 and 316 stainless steels and their variants are the most favoured materials of construction. Hence, choice of material for the application could not be faulted. For the above reasons, this unusual amount of corrosion of the fins, sealing strips and the shell of the intercooler and aftercooler, all of which were made of type 304 stainless steel, was baffling and called for a thorough investigation. The first point that needed to be investigated was the environment that could have caused such largescale general corrosion of stainless steels. The intercooler and aftercooler could possibly have seen any of the following environments that could have mixed with carbon dioxide gas: (i) Lean solution, (ii) water from the cooling tower, used to scrub the regenerated CO2 gas, containing ammoniacal nitrogen, chlorides and sulphates, and (iii) wet CO2 gas, and products of reactions that could occur between CO2 gas and ammonia in the scrubbing water under the temperature and pressure of operation in the intercooler and aftercooler. Analysis of chloride content indicated that the Lean solution had about 11,000 ppm of chlorides as against the specified amount of 6 ppm, suggesting that the role of Lean solution in the extensive corrosion of type 304 stainless steel could not be ruled out. Analysis of corrosion products suggested absence of chlorides in the corrosion products. The corrosion product analysis showed that Fe2O3 was the most dominant corrosion product. Potassium, carbonates, bicarbonates and vanadium ions were not detected in the corrosion product, thus indicating that there was very little carry over of Lean solution due to efficient scrubbing of the regenerated CO2 gas by the water from the cooling tower. For this reason, chlorides were not expected to carry over since all the Lean solution was scrubbed off from the CO2 gas. A number of CO and NH2 based compounds were found to be present in the corrosion products. This suggested that ammonium ions present in the water of the cooling tower and CO2 gas were responsible for the corrosion. Ammonia is known to be harmful only under conditions of high temperature and high pressure. Ammonia could dissociate into atomic nitrogen and atomic hydrogen. Atomic nitrogen could
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Fig. 6. XRD patterns from corrosion products of (a) aftercooler and (b) fins.
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lead to nitriding while atomic hydrogen could cause decarburisation [2]. Unpressurised ammonia is known to cause nitriding at temperatures above 723 K and pressurized ammonia causes nitriding and subsequent embrittlement above 623 K in Cr–Ni steel [2]. Under the present conditions of operating temperatures and pressures, nitriding of the stainless steel was ruled out. Electrochemical polarisation tests on type 304 stainless steels in the ammonia containing scrubbing water showed extremely low passive current density and a very noble open circuit and pitting potentials. This suggested that condensation of water containing ammonia would not cause general or pitting corrosion of type 304 stainless steel parts. Table 3 shows that corrosion rates of type 304 stainless steel in anhydrous ammonia is < 20 mpy up to 250 C while the corresponding value in ammonia gas is < 2 mpy up to 90 C [3]. In the present case no ammonia gas was present. All the ammonia was dissolved in the water. Type 304 stainless steel undergoes a corrosion of about 20 mpy up to 100 C in both dry and wet CO2 gas [3] (Table 3). Corrosion rate of 20 mpy corresponds to gnawing down of the material to the extent of about 0.5 mm/year, which is the thickness of the fins and demisters. The presence of urea and urea-based compounds alongwith CO and NH2-based compounds suggest the formation of ammonium carbamate. Although urea as such is not corrosive to stainless steel, the ammonium carbamate formed is highly corrosive to it, more so with increase in temperature [4]. In case of type 304 stainless steel, high corrosion rates have been reported in ammonium carbamate in the absence of passivation or lower concentration of oxygen [5]. During the period, when type 304 stainless steel tubes were being used, corrosion, though not extensive, occurred at sections in the intercooler and aftercooler where the water vapour condensed such as at sealing strip etc. However, corrosion of fins was not observed then. The extensive corrosion of fins occurred only after changeover to duplex stainless steel tubes. This suggested that the galvanic coupling between tube and fins increased the rate of corrosion. Electrochemical polarisation tests to confirm the galvanic action between type 304 stainless steel and duplex stainless steel under actual service conditions could not be carried out due to experimentation difficulties. However, attempts were made to confirm the galvanic action between the type 304 and duplex stainless steel by carrying out polarisation tests separately on these steels in a laboratory environment i.e. acidified 1 M NaCl solution (pH=1.61) at room temperature. The results of the electrochemical polarization studies indicated that the open circuit potential of duplex stainless steel was almost 200 mV nobler than type 304 stainless steel in acidified NaCl solution. This suggested that type 304 stainless steel was more prone to corrosion than duplex stainless steel. The passive current density was nearly ten times lower for the duplex stainless steel vis-a`-vis type 304 stainless steel (Figs. 7 and 8), thus suggesting better passivity in corrosive environments for the former. The above results indicate that the carbon dioxide/water vapour/ammonia environment, and/or the ammonium carbamate produce acidic conditions that would depassivate type 304 stainless steel more easily than duplex stainless steel. The higher Cr+Mo content of the duplex stainless steel prevents such a depassivation occurring on the surface. Thus, the fins corroded due to the galvanic action with the duplex stainless steel tubes while sealing strip, demisters and shell experienced general corrosion due to loss of passivity in the environment encountered in the intercooler and aftercooler. Table 3 Corrosion rates of AISI type 304 stainless steel in various environments [3] Environment
Up to a temperature of ( C)
Corrosion rate (mpy)
Wet carbon dioxide Dry carbon dioxide Satuarated ammonium carbonate Ammonium bicarbonate Ammonia gas Anhydrous ammonia
93 99 93 93 99 249
<20 <20 <20 <20 <2 <20
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Fig. 7. Anodic polarisation curve of type 304 stainless steel in acidified 1 M NaCl solution.
Fig. 8. Anodic polarisation curve of duplex stainless steel in acidified 1 M NaCl solution.
5. Conclusions The extensive corrosion of type 304 stainless steel parts of the intercooler and aftercooler was not caused by either the Lean solution or the scrubbing water from the cooling tower. The CO2 gas and/or the reaction product of CO2 gas+ammonia+water vapour, probably ammonium carbamate, aided by the galvanic effects between type 304 and duplex stainless steels, caused the extensive corrosion of the type 304 stainless steel components, particularly of the fins, demisters and sealing strips.
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6. Recommendations Recommendations to minimize/delay future failures were made considering the economics of the material and minimization of downtime of the plant vis-a`-vis corrosion of fins and their replacement every 6 months, and chloride stress corrosion cracking of coolant tubes and their replacement every 10 years. The following were the recommendations: 1. Use the same materials for all the parts of the intercooler and aftercooler to avoid galvanic corrosion. Seamless tubes may be used. The duplex stainless steel tubes may be replaced with wellannealed AISI type 304 stainless steel tubes. In case welding is involved at any stage during manufacturing of tubes, AISI type 304L stainless steel tubes may be used to avoid the risk of sensitization. 2. Type 304 stainless steel is expected to undergo CSCC. The water chemistry of the cooling water in the tubes could be improved to reduce the chloride content. 3. Before the carbon dioxide enters the intercooler, ie immediately after scrubbing of the carbon dioxide with water, inject about 0.6–0.8% oxygen into the carbon dioxide gas. This need be implemented only as an additional precautionary measure. Addition of oxygen is reported to be effective in formation of a passivating film. 4. Condense the water carried over by CO2 gas by cooling it to low temperatures immediately after it comes out from the scrubber. This would prevent ammonia from getting into the IC and AC, thus reducing the possibility of formation of ammonium carbamate.
Acknowledgements The authors wish to acknowledge the help rendered by Mrs. N. Sivaibharasi of our laboratory, Mr. Subramaniam of Water and Steam Chemistry Laboratory, BARC Facilities, Kalpakkam and Mr. G.L.N. Reddy of Materials Science Division, IGCAR, Kalpakkam, in carrying out the XPS and XRD experiments.
References [1] Bansal RK, Karmarkar AK. Technological and engineering aspects of urea plants. Chemical Engineering World 1976;11(1):51– 68. [2] Behrens D, editor. DECHMA corrosion handbook, vol 2. FRG: VCH Publishers, 1988. [3] Schweitzer PA. Corrosion resistance tables, part A, IVth ed. USA: Marcel Dekker Inc; 1995. [4] Mall ID. Corrosion problems and selection of material of construction in fertilizer industry. Chemical Engineering World 1997; 32(9):153–65. [5] Somani SC. Corrosion problems in urea plant at FCI, Gorakhpur. Chemical Age of India 1979;30(7A):667.