Effect of alloying elements and residuals on corrosion resistance of type 444 stainless steel

The principal criteria for the corrosion resistance of intermediate-grade f erritic stainless steels (SS) were examined in a neutral chloride (Cl-) sol ution. The effect of increasing quantities of chromium and molybdenum was e stimated for several heats in terms of the breakdown potential (E-b). The e ffect of inclusions (particularly the oxide-sulfide type) in type 444 SS ([ UNS S44400] 19% Cr-2% Mo-Nb or 19% Cr-2% Mo alloy), combined with the alloy ing element trend, permitted derivation of an expression that integrated bo th phenomena. me expression represents the mutually opposing effects of the chromium/molybdenum passive film reinforcement as represented by the pitti ng resistance equivalent number (PREN), as well as incorporating the delete rious contribution of the inclusion density (Psi, /mm(2)). Aluminum reduced the total inclusion content, which was associated with an increase in E-b. Since no aluminum was detected in the passive film of high aluminum steels , it appeared likely that the prime effect of this element on, corrosion re sistance was via inclusion suppression. Corrosion studies of welded type 44 4 SS demonstrated that dual stabilization with low individual concentration s of titanium and niobium provided optimum corrosion resistance, This appar ent synergism of niobium and titanium was independent of the surface of the welded materials, which were examined in the as-received, pickled, or poli shed states. The effect of the surface state in all cases was shown to exer cise a critical effect on passive behavior.