Effect of Alloying Elements and Residuals on Corrosion Resistance of Type 444 Stainless Steel

The principal criteria for the corrosion resistance of intermediate-grade ferritic stainless steels (SS) were examined in a neutral chloride (Cl⁻) solution. The effect of increasing quantities of chromium and molybdenum was estimated for several heats in terms of the breakdown potential (E_b). The effect 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 alloying element trend, permitted derivation of an expression that integrated both phenomena. The expression represents the mutually opposing effects of the chromium/molybdenum passive film reinforcement as represented by the pitting resistance equivalent number (PREN), as well as incorporating the deleterious contribution of the inclusion density (ψ,/mm²). 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 resistance was via inclusion suppression. Corrosion studies of welded type 444 SS demonstrated that dual stabilization with low individual concentrations of titanium and niobium provided optimum corrosion resistance. This apparent synergism of niobium and titanium was independent of the surface of the welded materials, which were examined in the as-received, pickled, or polished states. The effect of the surface state in all cases was shown to exercise a critical effect on passive behavior.