Depth-profiling electrochemical measurements of low temperature plasma carburised 316L stainless steel in 1 M H2SO4 solution

The electrochemical corrosion behaviour of the hardened layer on 316L austenitic stainless steel produced by low temperature plasma carburising has been studied in 1 M H2SO4 solution. The present work focuses on the variation of the corrosion behaviour of the carburised case with depth from the surface and with carbon concentration. Electrochemical tests were performed on the surfaces created at various depths across the carburised case by removing successive layers of different thicknesses. The results show that although the as-carburised surface exhibits deteriorated corrosion resistance due to the existence of a superficial film, the bulk of the carburised case exhibits anodic dissolution and transpassive behaviour similar to the untreated surface. Carbon concentration does not affect the general passivity and transpassivity of the investigated 316L steel, but increased carbon concentration is beneficial in increasing corrosion potential, accelerating passive film growth, reducing corrosion current density and decreasing charge carrier densities in the semiconducting passive film. The results are discussed in terms of lattice expansion of the expanded austenite and the effects of carbon dissolved in austenite on the semiconducting properties of the passive film.