Comparison of corrosion scales formed on KO8OSS and N8O steels in CO2/H2S environment

Abstract

Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were applied to analyse the microstructure and composition of the corrosion scale formed on KO80SS and N80 tubes with carbon dioxide (CO₂) and hydrogen sulphide (SO₂). The corrosion scales of both KO80SS and N80 tubes were of the double layer structure, and not only uniform corrosion but also localised corrosion was observed. The crystal of the surface layer is laminar. The main phase in the outer layer is calcium carbonate (CaCO₃), and the inner scale consisted of iron carbonate (FeCO₃) for KO80SS steel and FeS_0·9 with a little amount of FeCO₃ for N80 steel respectively. Additionally, the electrochemical techniques were used to investigate the characteristics of the corrosion scales. The results indicated that the polarisation resistance R_p of KO80SS steel film was nobler than that of N80 steel film. Finally, the corrosion current I_corr of KO80SS steels was lower than that of N80 steels. Corrosion scale of KO80SS tube steels is more protective to the matrix than that of N80 tube steels.     

Corrosion and Stress Corrosion Cracking Behavior of X70 Pipeline Steel in a CO2-Containing Solution

The electrochemical corrosion and stress corrosion cracking (SCC) behaviors of X70 pipeline steel in CO₂-containing solution were studied by electrochemical measurements, slow strain rate tensile tests, and surface characterization. The results found that the electrochemical corrosion of X70 steel in aerated, alkaline solution is an activation-controlled process, and a stable passivity cannot develop on steel. Corrosion rate of the steel increases with the CO₂ partial pressure. The enhanced anodic dissolution due to the additional cathodic reaction in the presence of CO₂, rather than the film-formation reaction, dominates the corrosion process. The mass-transfer step through FeCO₃ deposit is the rate-controlling step in corrosion of the steel. The susceptibility of steel to SCC and the fracture brittleness increase with the CO₂ partial pressure. The enhanced fracture brittleness is attributed to the evolution and penetration of hydrogen atoms into the steel, contributing to crack propagation. The formed deposit layer is not effective in reducing hydrogen permeation due to the loose, porous structure.     

Corrosion of Three Commercial Steels Under ZnCl2 –KCl Deposits in a Reducing Atmosphere Containing HCl and H2S at 400–500°C

The corrosion of three commercial steels in a reducing atmosphere containing HCl plus H₂S in the presence of ZnCl₂–KCl deposits has been investigated at 400–500°C and compared with the corrosion of the same materials in a similar gas mixture free from H₂S. The presence of H₂S in the gas accelerated the corrosion of the three commercial steels beneath ZnCl₂–KCl deposits. All materials suffered from severe corrosion with partial detachment and spalling of the external scales. Degradation of the steels resulted from the penetration of chlorine-containing species through the scale formed initially down to the metal matrix, because chorine-rich species were detected close to the alloy/scale interface. Although the corrosion resistance generally increased with increasing Cr content, even the high-Cr stainless steel SS304 is still unable to provide good corrosion resistance against the ZnCl₂–KCl deposits in the presence of H₂S due to the bad adherence of the scales to the alloy. The mechanisms of attack are discussed on the basis of thermodynamic considerations and of the active-oxidation model.     

WC-Co-(Ni)-(Cr)硬质合金在中性溶液中的腐蚀行为

以WC粉、Co粉、Ni粉及Cr3C2粉为原料,采用粉末冶金方法制备了3组不同粘结相成分的WC-Co-(Ni)-(Cr)硬质合金,通过极化曲线测试和浸泡实验研究了3组合金在中性溶液中的腐蚀行为,并采用扫描电镜、能谱分析、X射线光电子能谱(XPS)和EBSD等手段对其腐蚀机理进行了探讨。结果表明,WC-Co和WC-Co-Cr硬质合金在中性溶液中主要发生粘结相Co的腐蚀,浸泡产生的腐蚀产物主要是Co(OH)2;添加Cr将提高WC-Co硬质合金在中性溶液中的耐腐蚀性能,这可能与Cr的添加明显降低了粘结相中密排六方Co的含量有关;同时添加Ni和Cr可进一步提高WC-Co合金在中性溶液中的耐腐蚀性能,在pH=7的Na2SO4溶液中浸泡480 h后,WC-Co-Ni-Cr合金发生很少量的腐蚀。     

Abrasion resistances of CO2 corrosion scales formed at different temperatures and their relationship to corrosion behaviour

The abrasion resistances of CO₂ corrosion scales formed on API P110 grade carbon steel at different temperatures and CO₂ corrosion behaviour of P110 carbon steel have been investigated utilising weight loss method, scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that as the film forming temperature increased, the abrasion rate of CO₂ corrosion scales increased firstly and then decreased, the variation trend was similar to that of the corrosion rate, and the maximal abrasion rate was present at 70°C rather than 100°C, at which the maximal corrosion rate was obtained and CO₂ corrosion scales were loose and porous and were composed of FeCO₃, Fe₃C and CaCO₃. The abrasion resistances of CO₂ corrosion scales formed at 100°C was reinforced by the sand particles embedded easily in the loose and porous layer, together with a very hard residual skeleton, Fe₃C. In addition, CaCO₃ was worn out easily and substituted by SiO₂ during abrasive wear.     

Fe3C influence on the corrosion rate of mild steel in aqueous CO2 systems under turbulent flow conditions

The corrosion and corrosion inhibition of mild steel in CO₂ saturated solutions were studied under turbulent flow conditions at different pH. Electrochemical measurements using a.c. and d.c. techniques in uninhibited solutions of pH 3.8 indicated the formation of protective surface films (FeCO₃) in short immersion times. However, as the exposure time was increased the corrosion rate always increased, an effect attributed to the increased surface area of Fe₃C residue from corrosion of the steel. At pH 5.5, the corrosion rate always increased with time, behaviour also associated with the presence of Fe₃C surface film. The huge cathodic area of Fe₃C seems to have a more important impact on the electrochemical behaviour than the poorly formed FeCO₃ products. The effect of Fe₃C on inhibition by a quaternary amine inhibitor at pH 3.8 is to increase the corrosion rate as the pre-corrosion time is increased. The Fe₃C causes either (a) a cathodic area increase reflected in the corrosion rate increase with time or (b) a potential gradient in the pores of the Fe₃C layer that prevents positively charge amine ions from reaching all anodic sites.     

The Corrosion Behavior of Four Commercial Steels in Reducing Atmospheres Containing HCl At 773–873 K

The corrosion of four Fe–Cr commercial steels with different chromium contents in a simulated waste-gasification atmosphere containing 0.5 vol. H₂, 0.5 vol. HCl, balance CO₂ has been investigated at 773 and 873 K. The same materials have also been tested in the same gas mixture free from HCl at both temperatures for comparison. The results show that the materials with low-chromium content (2.25CrMoV and NF616) undergo accelerated corrosion in the presence of HCl, while the stainless steel SS304 suffers very little corrosion. On the contrary a steel containing 12 wt. Cr (12CrMoV) corrodes rather rapidly at 773 K but quite slowly at 873 K. The beneficial effect of chromium on the corrosion resistance of the steels increases with the chromium content at both temperatures. The steels tested show corrosion rates generally decreasing with time, having kinetics which are approximately parabolic at 773 K but intermediate between parabolic and linear at 873 K. Only little or even no chlorine can be detected at the scale/metal interface at both temperatures for all materials corroded in HCl-containing atmospheres. The corrosion mechanism can be explained by the so-called active-oxidation model.     

Condensed phase corrosion of P91 and DSS 2205 steels at advanced oxygen-fired pressurized fluidized bed combustion plants

To achieve the targets of high energy efficiency and reduced CO₂ emission, advanced oxygen-fired pressurized fluidized bed combustion technology is being developed. The generated flue gas condensates are very corrosive, but very limited information is available to select appropriate alloys for the cost-effective construction and long-term safe operation of flue gas components. Thus, this study investigated the corrosion performance of P91 and DSS 2205 steels in the simulated condensates at 60°C–150°C. The dominant reactions on the two steels were considerable oxide formation and high chemical dissolution of the formed oxides instead of localized pitting. The increase in temperature leads to an exponential increase in the long-term corrosion rates of the steels. Benefited from its high Cr and Mo contents, DSS 2205 steel exhibited much better corrosion resistance, and the formed surface scales consisted of inner Fe-enriched and outer Cr-enriched oxides in which Cr₂O₃ was transformed into Cr(OH)₃ with the increase in temperature. The corrosion products on P91 steel consisted of inner Cr–Fr–Mo oxides and outer Fe-enriched oxides, which were porous and unable to protect the steel.     

Long term corrosion of X70 steel and iron in humid supercritical CO2 with SO2 and O2 impurities

Abstract

The corrosion of X70 steel and iron in supercritical CO₂/SO₂/O₂/H₂O environment were investigated after a 454 h exposure. Optical microscopy was applied to observe the morphology of etch pits and synthesise the three-dimensional morphology. X-ray diffraction and X-ray photoelectron spectroscopy were employed to detect the composition of product scales. Experimental results verified that the localised corrosion occurred on the X70 steel sample under corrosion product deposits. Ferrous sulphate, sulphur and iron sulphide were detected as the corrosion products.     

Corrosion rate of iron and iron–chromium alloys in CO2 medium

In the present paper corrosion of iron (99.95%) and iron–chromium alloys (1–5% Cr) were investigated by mass loss and ac impedance. These low-chromium content alloys have been proposed as an intermediate corrosion-performance material between C-steel and stainless steels to be used in oil plants. It was observed that small contents of chromium in the alloy decrease the corrosion attack only in very specific pH range 4 < pH < 5.In CO₂ medium, mass loss showed a linear behaviour with respect to time whereas in the presence of acetate the results were not monotonic. By ac impedance, it was found that and vary proportionally with respect to mass loss. A discussion concerning the online monitoring of the corrosion rate is presented.     

The high-temperature corrosion behavior of Nb-Al alloys in a H2/H2S/H2O gas mixture

The corrosion behavior of pure Nb and three Nb Al alloys containing 12.5, 25, and 75 at.% Al was studied over the temperature range of 800–1000°C in a H₂/H₂S/H₂O gas mixture. Except for the Nb-12.5Al alloy consisting of a two phase structure of -Nb and Nb₃Al, other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but fluctuated with increasing Al content. The Nb-75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 1.6–2.2 orders of magnitude lower than those of pure-Nb (depending on temperature). An exclusive NbO₂ layer was formed on pure Nb, while heterophasic scales were observed on Nb-Al alloys whose compositions and amounts strongly depended on Al content and temperature. The scales formed on Nb-12.5Al consisted of mostly NbO₂ and minor amounts of Nb₂O₅, NbS2, and -Al₂O₃, while the scales formed on Nb-25Al consisted of mostly Nb₂O₅ and some -Al₂O₃. The scales formed on Nb-75Al consisted of mostly -Al₂O₃ and Nb₃S₄ atT 900°C, and mostly -Al₂O₃ , Nb₃S₄ and some AlNbO₄ at 1000°C. The formation of -Al₂O₃ and Nb₃S₄ resulted in a significant reduction of the corrosion rates.     

Corrosion of X65 steel in CO2-saturated oilfield formation water in the absence and presence of acetic acid

Electrochemical corrosion behavior of X65 steel in CO₂-saturated formation water in the absence and presence of acetic acid was studied by electrochemical measurements, scanning vibrating micro-electrode (SVME), localized electrochemical impedance spectroscope (LEIS) and surface analysis techniques. It is found that, when steel is immersed in formation water, the dissolution of Fe dominates the anodic process and the steel is in active dissolution state. Adsorption of intermediate product on the electrode surface results in generation of an inductive loop in the low frequency range of EIS plot. As corrosion proceeds, the concentration of Fe²⁺ in the solution increases. When the product of [Fe²⁺] × [] exceeds solubility product of FeCO₃, FeCO₃ will deposit on the electrode surface, and protects the steel substrate from further corrosion. The steel is in a “passive” state. When the electrode surface is completely covered with FeCO₃ film, the inductive loop in the low frequency range disappears. In the presence of acetic acid in formation water, the cathodic reaction will be enhanced due to the direct reduction of undissociated acetic acid. Addition of acetic acid degrades the protectiveness of corrosion scale, and thus, enhances corrosion of steel by decreasing the FeCO₃ supersaturation in solution.     

Effects of sulfur pressure on the sulfidation behavior of 310 stainless steel

High-temperature sulfidation behavior of 310 stainless steel was studied over the temperature range of 700–900°C above a pure sulfur pool with the sulfurvapor range of 10^–4–10^–1 atm. The corrosion kinetics followed the parabolic rate law in all cases. The corrosion rates increased with increasing temperature and sulfur pressure. The scales formed on 310 stainless steel were complex and multilayered. The outer scale consisted of iron sulfide (with dissolved Cr), (Fe, Ni)₉S₈ and chromium sulfides (Cr₂S₃ and Cr₃S₄ with dissolved Fe), while the inner layer was a heterophasic mixture of Cr₂S₃, Cr₃S₄, NiCr₂S₄, and Fe₁–_xS. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations (Fe, Ni, and Cr), and the inner scale grew by the inward transport of sulfur. The formation of Cr₂S₃, Cr₃S₄, and NiCr₂S₄ partly blocked the transport of iron through the inner scale, resulting in a reduction of the corrosion rates as compared with the results in the literature.     

Effect of Na2SO4 and Water Vapor on the Corrosion of Ti3SiC2

The corrosion behavior of polycrystalline Ti₃SiC₂ was studied in the presence of Na₂SO₄ deposit and water vapor at 900°C and 1000°C. The mass gain per unit area of the samples superficially coated with Na₂SO₄ exposed to water vapor was slightly lower than that of the samples corroded without water vapor. The microstructure and composition of the scales were investigated by SEM/EDS and XRD. Pores were observed in the corroded sample surfaces. The main corrosion phases on the sample surface were identified by XRD as TiO₂, Na₂Si₂O₅ and Na₂TiO₃. After Ti₃SiC₂ corroded in the presence of the Na₂SO₄ deposit and water vapor, the scale had a three-layer microstructure, which was different from the duplex corrosion scale formed on Ti₃SiC₂ beneath the Na₂SO₄ film without water vapor. Because water vapor penetrated the corrosion layer and then reacted with SiO₂ to form volatile Si(OH)₄, an intermediate porous and TiO₂-enriched layer formed in the corrosion layer.     

High Temperature Corrosion of Chromium–Manganese Steels in Sulfur Dioxide

This work was aimed at explaining the corrosion mechanism of commercial Cr–Mn steels at 1073, 1173 and 1273 K in the atmospheres containing oxygen and sulfur. Three steels were selected for the investigations, two single-phase austenitic steels (Cr17Mn17 and Cr13Mn19SiCa) and a two-phase austenitic-ferritic steel Cr15Mn19. On all studied steels triplex scales were formed. The inner very thin, fine-grained part of the scale contained manganese, chromium and iron sulfides and oxides, the intermediate layer was built mainly of the MnCr₂O₄ spinel while MnO was the predominant constituent of the outer scale layer. According to the gravimetric measurements, after an initial incubation period, the oxidation of steel follows a parabolic rate law. Thermodynamic and kinetic aspects of the formation of oxide-sulfide and oxide layers were discussed. Oxidation was accompanied by depletion of the subscale region of the metallic core in manganese, which is the austenite former. Consequently austenite transformed into ferrite.     

The corrosion behavior of Fe-Nb-Al alloys in H2/H2O/H2S atmospheres

The corrosion behavior of eight Fe-Nb-Al ternary alloys was studied over the temperature range 700–980°C in H₂/H₂O/H₂S atmospheres. The corrosion kinetics followed the parabolic rate law for all alloys at all temperatures. The corrosion rates were reduced with increasing Nb content for Fe-x Nb -3Al alloys, the most pronounced reduction occurred as the Nb content increased from 30 to 40 wt.%. The corrosion rate of Fe-30Nb decreased by six orders of magnitude at 700°C and by five orders of magnitude at 800°C or above by the addition of 10 wt.% aluminum. The scales formed on low-Al alloys (3 wt.% Al) were duplex, consisting of an outer layer of iron sulfide (with Al dissolved near the outer-/inner-layer interface) and an inner complex layer of Fe_xNb₂S₄(FeNb₂S₄ or FeNb₃S₆), FeS, Nb₃S₄ (only detected for Nb contents of 30 wt.% or higher) and uncorroded Fe₂Nb. No oxides were detected on the low-Al alloys after corrosion at any temperature. Platinum markers were found to be located at the interface between the inner and outer scales for the low-Al alloys, suggesting that the outer scale grew by the outward transport of cations (Fe and Al) and the inner scale grew by the inward transport of sulfur. The scales formed on high-Al alloys (5 wt.% Al) were complex, consisting primarily of Nb₃S₄, Al₂O₃ and (Fe, Al)_xNb₂S₄, and minor amounts of (Fe, Al)S and uncorroded intermetallics (FeAl and Fe₂Nb). The formation of Nb₃S₄ and Al₂O₃ blocked the transport of iron through the inner scale, resulting in the significant reduction of the corrosion rates.     

On the fundamentals of electrochemical corrosion of X65 steel in CO2-containing formation water in the presence of acetic acid in petroleum production

Electrochemical corrosion behavior of X65 steel in CO₂-containing oilfield formation water in the presence of acetic acid (HAc) was investigated by various electrochemical measurements and analyses as well as thermodynamic calculations of ionic concentrations, reaction rate constants and equilibrium electrode potentials. A conceptual model was developed to illustrate corrosion processes of steel in oilfield formation water system. The anodic reactions of the steel contain a direct dissolution of Fe, Fe → Fe²⁺ + 2e, and the formation of corrosion scale, FeCO₃, by Fe + → FeCO₃ + H⁺ + 2e. The cathodic processes contain the reduction of H⁺, , H₂O and HAc, where reduction of HAc has the least negative equilibrium potential and thus dominates the cathodic process. With addition of HAc in the solution, both cathodic and anodic reaction rates increase remarkably. It is attributed to the fact that HAc inhibits or degrades the formation of protective scales due to the decrease of solution pH. Upon electrode rotation, the measured impedance decreases with the increase in HAc concentration. The FeCO₃ scale will not form on electrode surface. When HAc concentration is less than 1000 ppm, the adsorbed intermediate product is not significant, resulting in generation of a low-frequency inductive loop in EIS plots. When HAc concentration is more than 3000 ppm, the adsorption of intermediate product is significant, generating overlapped capacitive semicircles in EIS measurements.     

High temperature corrosion of Al<Subscript>3</Subscript>Ti−Cr intermetallics in an Ar−1 %SO<Subscript>2</Subscript> gas atmosphere

Ll₂-type Al₃Ti−Cr alloys of compositions 67Al−25Ti−8Cr, 66Al−24Ti−10Cr, and 59Al−26Ti−15Cr (in atomic percent) were corrosion tested between 800°C and 1100°C in an Ar−1% SO₂ gas atmosphere for up to 150h. Corrosion proceeded mainly via oxidation reactions. The scale consisted primarily of a thin Al₂O₃ barrier layer. The alloys tested had much better corrosion resistance than TiAl-base alloys or Hastelloy X. Virtually no sulfides were detected in the scale, because of the strong oxidizing tendency of the Al₃Ti−Cr alloys having high Al concentrations.     

The corrosion behavior of Fe-Al alloys in H2/H2S/H2O atmospheres at 700–900°C

The corrosion behavior of five Fe-Al binary alloys containing up to 40 at. % Al was studied over the temperature range of 700–900°C in a H₂/H₂S/H₂O mixture with varying sulfur partial pressures of 10^–7–10^–5 atm. and oxygen partial pressures of 10^–24–10^–2° atm. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with increasing Al content. The scales formed on Fe-5 and –10 at.% Al were duplex, consisting of an outer layer of iron sulfide (FeS or Fe_1–xS) and an inner complex scale of FeAl₂S₄ and FeS. Alloys having intermediate Al contents (Fe-18 and –28 at.% Al) formed scales that consisted of mostly iron sulfide and Al₂O₃ as well as minor a amount of FeAl₂S₄. The amount of Al₂O₃ increased with increasing Al content. The Fe 40 at.% Al formed only Al₂O₃ at 700°C, while most Al₂O₃ and some FeS were detected at T800°C. The formation of Al₂O₃ was responsible for the reduction of the corrosion rates.     

Influence of Pre-oxidation on the Corrosion and Mechanical Strength of Fe–25Cr and Fe–25Cr–20Ni Alloys at 973 K

The influence of pre-oxidation on the corrosion and mechanical strength of Fe–25Cr and Fe–25Cr–20Ni alloys was investigated in N₂–0.1SO₂ at 973 K with and without mechanical loadings. About 0.1μm-thick Cr₂O₃ scales formed on the Fe–25Cr alloy by pre-oxidation in Ar. However, spinel oxides of (Fe,Cr)₃O₄ remained on the Cr₂O₃ and voids formed at the oxide/metal interface on Fe–25Cr–20Ni after pre-oxidation in Ar. The preformed oxides are very effective in preventing corrosion of the alloy surfaces. The preformed oxides are also beneficial to increase the strength of the alloys in corrosive environments. The effects of pre-oxidation on Fe–25Cr are stronger than those of Fe–25Cr–20Ni due to the different characteristics of the preformed oxides.