Microstructural and selective dissolution analysis of 316L Austenitic Stainless Steel

Abstract Austenitic stainless steels (ASS) are among the most used structural materials because of their excellent mechanical and corrosive properties. In addition, 316L ASS has excellent weldability because of its low carbon content. This steel normally contains 5 to 10% residual delta ferrite to avoid hot cracking and micro-fissuring of the weld metal. Many studies show that selective dissolution of austenite or ferrite can occur in 316L steel. In a survey of the selective dissolution of phases in duplex stainless steel (DSS), Lo and coauthors studied the effect of electrolyte composition (H2SO4 + HCl solutions) on the active-passive transition. They showed that corrosion occurred only in the austenitic phase in solutions of 2M H2SO4 + 0.25-1.2M of HCl. Increasing the % of HCl, > 1.2%, austenite, and ferrite contributed to the corrosion process. The present authors did not find the effect of this electrolyte in the study of selective corrosion in 316L ASS welded in the literature Therefore, this study aims to determine how the phases behave on 316L ASS by applying 2M H2SO4 + 1.5M HCl solution. In this work, we welded the steel by the autogenous TIG process using three heat inputs. The purpose of welding was to obtain different amounts and morphology of ferrite and determine whether this affected the selective dissolution of the phases. After the corrosion tests, the samples showed selective austenite corrosion irrespective of the volume fraction and morphology of the ferrite. In summary, the corrosive etches severely consumed the austenite because of the significant difference in the electrochemical potential between the phases. Therefore, with more nickel, austenite functioned as an anodic region, and the δ-ferrite, with more chromium, worked as a cathodic region.