The effect of nitrogen and carbon on the stress corrosion cracking performance of sensitized AISI 304 stainless steel in chloride and sulfate solutions at 250°C

Abstract

Slow strain rate tests (SSRT) were conducted on sensitized AISI 304 stainless steels (SS) with varying nitrogen and carbon contents in order to study their susceptibility to stress corrosion cracking (SCC). The tests were performed in de-aerated 0.01 M NaCl at 250°C, at a strain rate of 2 × 10 −6s −1 and at various applied potentials in the range −0.4 to 0.1 V(NHE), after which scanning electron microscopy (SEM) was used to observe the fracture surfaces of the SSRT specimens. SSRT results show that SCC occurs above a certain critical potential ( E SCC) which depends on the carbon and nitrogen contents; E SCC is in the range −0.3 to 0 V(NHE), with nitrogen additions up to 0.16 wt% increasing E SCC and carbon additions decreasing E SCC. This implies that the degree of sensitization (DOS) is the major factor which determines E SCC. The DOS also determines the fracture mode obtained above E SCC; at low DOS transgranular stress corrosion cracking (TGSCC) occurs, while at high DOS intergranular stress corrosion cracking (IGSCC) is the predominant mode. The potential ranges in which (1) IGSCC and (2) simultaneous IGSCC, TGSCC and shallow pitting occurred corresponded to (1) the passive range and (2) potentials above the breakdown of passivity on the polarization curves. The results of this investigation are compared with those obtained from similar tests in sulphate solutions [ T. A. Mozhi et al., Corrosion 42, 197 (1986)], and possible mechanisms discussed.