Abstract
The purpose of this study is to investigate pitting and stress corrosion cracking (SCC) behavior of type 321 stainless steel (SS) in a simulated H2S-containing chloride solution (20wt%□ NaCl + 0.01M Na2S2O3, pH=2). In order to investigate the relationship between pitting and SCC as well as the feasibility of anodic protection for type 321SS in a simulated H2S-containing chloride solution (20wt%□ NaCl + 0.01M Na2S2O3, pH=4, 80℃), the slow strain rate tests (SSRT) at various applied potentials were carried out. For comparison, type 316SS, type 316LSS and 2205 duplex stainless steel (DSS) were also investigated in sulfide-containing chloride solutions. Results of this study can provide some useful information to prevent SCC when type 321SS is performed in H2S-containing chloride solutions.To prevent the corrosion damage of H2S to equipment, the suggestion of S2O3-2 as a substitute for H2S proposed by Japan Society of Corrosion Engineering (JSCE) was adopted. A series of simple immersion tests in deaerated 20wt%□ NaCl solution at 80℃ conducted by JSCE, indicating a good correlation between critical pitting concentration of S2O3-2 and critical H2S pressure for pitting. JSCE then proposed a standard simulated H2S-containing chloride environment as follows: deaerated 20wt%□ NaCl + 10-3□10-2 M S2O3-2 aqueous solution of pH=4 at 80℃. In order to simulate more acidified environment, pH=2 was used in this study.Results of the cyclic potentiodynamic anodic polarization tests for type 321SS are summarized as follows: (1) effect of Cl- concentration is to decrease Epit and Epp with increasing Cl- concentration; (2) effect of temperature is to decrease Epit and Epp but to increase the passive current density with increasing temperature; (3) effect of solution pH is to decrease Epit with decreasing solution pH, thereby extending the pitting region and reducing the imperfect region between Epit and Epp. Electron probe micro-analysis (EPMA) results of the pits formed on type 321SS after polarization test manifest the appearance of S at the bottom of pits. X-ray analysis result of the corrosion product formed on type 321SS after polarization test confirms that the composition of the dark corrosion layer is mainly of FeS. These results provide important information to understand the pitting mechanism of type 321SS in simulated H2S-containing chloride solutions: First, the chemical adsorption and agglomeration of Cl- on the metal surface become more active because of the high Cl- activity due to high temperature, and therefore Cl- penetrates the localized passive oxide film to form pits. Then, not only the localized acidification environments provided by the pits but also the decreasing solution pH will result in pit growth as well as aid S2O3-2 to decompose; besides, the increasing temperature will also aid S2O3-2 to proceed the following decomposition reactions:formation of S (pH＜5): S2O3-2 □ H□ □ S □ HSO3□S2O3-2 □ 6H□ □ 4e□ □ 2S □ 3H2Oformation of H2S: S □ 2H□ □ 2e□ □ H2S4S □ 4H2O □ 3H2S □ H2SO4Finally, H2S reacts with base metal to form FeS (Fe □ H2S □ FeS □ H2), causing the pits to become broader as well as deeper.Results of the SSRT tests for type 321SS indicate that the degree of environmental factors on SCC susceptibility decreases in the order, temperature effect >> solution pH effect > Cl- concentration effect, and the SCC mechanism induced by corrosion pits or TiC particles (5~10□m) is discussed. Results of the U-bend tests for type 321SS indicate that the susceptibility to SCC decreases with increasing temperature, which is related to more compact oxide film of Cr2O3 and corrosion product of Fe3O4.Results of the SSRT tests at various applied potentials for type 321SS indicate that the ultimate tensile strength (UTS) and uniform elongation (UEL) remain satisfactory in the passive potential range, but specimens were fractured near the yield strength at applied potentials above the critical potential Epit. This leads to the conclusion that SCC is induced by corrosion pits generated at applied potentials above Epit.Results of the cyclic potentiodynamic anodic polarization tests and the slow strain rate as well as U□bend tests for type 321SS, type 316SS, type 316LSS and 2205DSS are summarized as follows: (1) pitting resistance decreases in the order, 2205DSS > type 316SS□type 316LSS > type 321SS; (2) in the comparison of SCC resistance, type 321SS is inferior to both the high ductility type 316SS□type 316LSS and the high strength 2205DSS; (3) in the U-bend tests, decreasing the susceptibility to SCC with increasing temperature was discovered.