The Influence of Silicon on the Pitting Corrosion Resistance of an 18Cr-8Ni Stainless Steel

This paper describes the effect of silicon additions to an 18Cr-8Ni stainless steel (SS) on the resistance to pit initiation in halide media. Electrochemical polarization measurements conducted in aerated 3.5 wt% sodium chloride at 25 C indicate a progressive increase in the critical breakdown potential (Ec) for 18Cr-8Ni alloys over the range 1.01 to 4.45 wt% silicon (0 to 17% δ-ferrite), thereby predicting increasing resistance to pit initiation. Pitting corrosion tests conducted in acidified 10% ferric chloride solution confirmed the trends predicted from electrochemical studies. Corrosion rates decreasing from 1016 to 50 mdd were observed over the silicon range of 1.01 to 4.45 wt%. The latter figure was approximately 20 times better than that observed on AISI 304 SS. Analysis of cyclic potentiodynamic anodic polarization curves revealed that although a significant increase in pit initiation resistance was observed on the duplex alloys, increasing silicon additions (and δ-ferrite content) may increase their susceptibility to crevice corrosion.