Corrosion of Resulfurized Free-Machining Stainless Steels

The influence of manganese (0.3 to 2.4%), sulfur (0.01 to 0.4%), sulfide composition, and heat treatment on the corrosion resistance of 13% chromium, 17% chromium and 18% chromium-9% nickel steels has been investigated. Corrosion in acid solutions, high humidity, and salt spray is dependent upon sulfide composition which in turn is determined by the manganese to sulfur ratio in the steel. Low manganese resulfurized stainless steels which contain chromium rich sulfides are inherently more corrosion resistant than high manganese steels containing manganese rich sulfides. The influence of heat treatment on the corrosion resistance of resulfurized steels is similar to that for residual sulfur grades. Sulfide composition also controls the behavior of these steels during nitric acid passivation treatments. Highly oxidizing solutions must be used to avoid attack during these treatments particularly for steels with high manganese to sulfur ratios. These solutions do not dissolve the commonly used tool steels (particles of which might be imbedded in the surface of a machined part) but can improve subsequent corrosion resistance by removing deleterious manganese rich sulfides. These improvements are most marked in the high manganese 13% chromium alloys. Passivation treatments can increase the corrosion susceptibility of cross section areas in low manganese resulfurized stainless steels.