Abstract
Abstract The effect of dissolved oxygen and NO3− anions on the intergranular stress corrosion cracking (IGSCC) and transgranular stress corrosion cracking (TGSCC) of Type 304 stainless steel has been studied in water at 290 C by using the constant extension rate tests (CERT) method. A strain rate of 3.33×10−6 sec−1 was applied. TGSCC occurred in water containing less than 100 ppb dissolved oxygen, while IGSCC occurred at concentrations greater than 1 ppm dissolved oxygen. IGSCC was accelerated by the addition of small amounts of NO3− anions, but TGSCC was suppressed by the presence of this anion. The analyses of oxide films by the Ion Microprobe Mass Analyzer (IMMA) and the construction of potential-pH diagrams for Cr-H2O, Ni-H2O, and Fe-H2O systems at 290 C were performed to consider the effect of dissolved oxygen and NO3− anions on IGSCC and TGSCC. The IMMA studies revealed that the compositions of the oxide films of Type 304 stainless steel and Fe-12Cr-9.3Ni steel (the presumed composition of grain boundaries in sensitized Type 304 stainless steel) were Cr-enriched at 200 ppb O2 level. At 1 ppm O2 level, the oxide film of Type 304 stainless steel was composed of Cr-enriched phase, but the Cr content in the oxide film of Fe-12Cr-9.3Ni steel was low. At 10 ppm O2 level, the oxide film of Type 304 stainless steel was composed of Fe and Cr, but the oxide film of Fe-12Cr-9.3Ni steel was mainly composed of Fe. The potential-pH diagram for the Cr-H2O system at 290 C indicated that the corrosion potential of Type 304 stainless steel lay in the stable potential region of Cr2O3 at low oxygen levels. On the other hand, at high oxygen levels, the corrosion potential of Type 304 stainless steel lies near the unstable potential region of Cr2O3. This unstable region corresponds to the potential at which Cr2O3 dissolves as dichromate ion (Cr2O7−−) or chromate ion (CrO4−−). The relationship between the characteristics of oxide films and the IGSCC and TGSCC of sensitized Type 304 stainless steel is discussed.
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