Effect of Alloying Elements Addition to the Corrosion Property of 316L Stainless Steel based Alloys

The effects of added molybdenum (5-20 wt. %) and nickel (5-10 wt. %) on the corrosion behavior of 316L-based alloys in an aggressive acid-brine environment (pH 4) were measured. Electrochemical methods that included potentiodynamic, potentiostatic, and electrical impedance spectroscopy procedures were used. Two types of Mo-rich intermetallic phases precipitated as the solid solubility of Mo in austenite was exceeded. Sigma phases (21-29 wt. % Mo) formed first with 5 wt. % Mo addition and Laves phases (33-40 wt% Mo) precipitated with additions exceeding 5 wt% Mo. The Ni additions did not yield new phases and only austenite remained. Alloys with 15 wt. % Mo or more showed poor corrosion resistance that is attributed to the abundance of Laves phase, whereas, 10 wt. % Mo addition showed best corrosion resistance containing more sigma phase than Laves phase. Analyses of the corroded samples using SEM/EDS showed preferential corrosion of the sigma and Laves phases due to galvanic corrosion. Nickel additions were not beneficial for the corrosion properties instead extensive pitting corrosion occurred. Long-term tests showed that molybdenum added alloys achieve passivation by forming chromium and molybdenum rich oxide layer. An electric circuit model was developed for predicting long-term corrosion rates.