Effect of the thermal oxide on the stress corrosion cracking susceptibility of stainless steels welds in chloride-containing media

The stress corrosion cracking (SCC) susceptibility of a non-polished 316 L austenitic stainless steel (ASS) welded joint, i.e., covered by a thermal oxide film, was studied by carrying out uniaxial tensile tests in a 1 M NaCl solution (pH 4, 60 °C). Experiments were also performed on a polished weld, i.e., covered by a native oxide film, for comparison. The thermal oxide film presented a bi-layer structure similar to that of the native oxide film, but the ratio Fe/Cr in the outer layer of the thermal oxide film was higher. Moreover, the thermal oxide film was thicker than the native oxide film, but it was more porous and showed a gradient in chemical composition and morphology as a function of the distance from the fusion line of the weld. The defective structure of the thermal oxide film was associated with a degradation in the corrosion behaviour of the non-polished weld, as well a change in the crack propagation mode during SCC tests, from intergranular to transgranular even though the cracks formed preferentially in the heat affected zone (HAZ) for both polished and non-polished samples. Furthermore, crack propagation was promoted, with cracks extending across multiple grains, in the HAZ of the non-polished sample near the fusion line, where the thermal oxide was more porous and richer in Fe compared to the HAZ far from the fusion line. The results therefore highlighted the major influence of the thermal oxide properties on the corrosion behaviour and SCC susceptibility of the 316 L weld.