Improvement of the SCC resistance of FCC alloys: influence of pre-fatigue on the SCC resistance of the austenitic stainless steel-316L in a MgCl2 boiling solution at 117 deg C

The aim of this study is to analyse the effect of pre-fatigue of FCC materials on their mechanical and electrochemical response to better understand and delay the SCC damage. The material/environment couple tested is the 316L polycrystalline austenitic stainless steel in boiling MgCl2 at 30% mass. Samples are pre-strained in low cycle fatigue under plastic strain control, with a p/2 value of 0.4%, for various number of cycles (25%, 75% and at the number of cycles to reach saturation during pre-fatigue). It was found that only pre-fatigue at saturation improves the SCC resistance of the material, both on SSRT and constant load tests. A delayed crack initiation up to 10% of strain. which increases strain to failure by half. mostly accounts for this beneficial effect, during SSRT tests. Furthermore, other pre-straining only resulted in loss of strain to fracture and no delay in crack initiation. We related the crack initiation delay to the surface strain state due to pre-fatigue. It provides fine parallel slip bands. homogeneously located at the surface of the samples. This surface state induces an increasing anodic surface-cathodic surface ratio which lowers the kinetics of localised corrosion. thus that of crack initiation. We also show some experiments implying that pre-fatigue at saturation decreases the SCC crack growth velocity which can be understood through the CEP (Corrosion Enhanced Plasticity) Model. We also show that this beneficial effect is probably available on other fcc material/environment couples, such as OFHC Cu/ 1 M NaNO2 at pH 9. (author)