Influence of Silicon, Carbon and Phosphorus on Intergranular Corrosion of High Purity Austenitic Stainless Steels Under Transpassive Conditions

Abstract
Precipitate-free Fe-Cr-Ni f.c.c. alloys exhibit strong intergranular corrosion in acid solutions at electrochemical potentials from the transpassivity range. Segregation of impurity atoms to grain boundaries is generally considered to be responsible for this specific kind of localized damage. A study of the influence of silicon, phosphorus and carbon on the intergranular transpassive corrosion of the solution treated Fe-17Cr-13Ni alloy in the 2N sulphuric acid at a fixed electrochemical potential is presented. No localized attack occurs in a high purity base alloy nor in one containing phosphorus. The amount of intergranular corrosion is determined by the silicon content : according to previous results a maximum is observed for about 1 wt.% (2 at.%) Si. Carbon additions between 100 and 200 ppm cause some intergrannlar corrosion in silicon-free alloys and amplify the corrosion intensity in those containing silicon. No localized corrosion occurs for silicon contents higher than 2 wt%. An analysis of the segregation kinetics of silicon is presented and the relation between intergranular corrosion and bulk silicon concentration is discussed.