Effects of Alloying Additions on Corrosion and Passivation Behaviors of Type 304 Stainless Steel

The effects of copper (Cu), silicon (Si), molybdenum (Mo), and nitrogen (N) as alloying elements on the microstructure and corrosion behavior of type 304 (UNS S30400) austenitic stainless steel (SS) in deaerated dilute acidic chloride (Cl–) solutions at 30°C and 60°C was investigated using potentiodynamic, scanning electron microscopy (SEM), and energy dispersive x-ray analysis (EDXA) techniques. The addition of 2% Cu decreased the corrosion and critical current densities sharply. Surface analysis showed the presence of insoluble cuprous chloride (CuCl) dispersed on the steel surface in severe conditions. The additive Cu had no measurable effect on the other passivation parameters. The presence of 3% Si promoted the formation of some δ-ferrite phase, but the Si-rich film on the surface was sufficient to improve the general and pitting corrosion resistance. A combined beneficial effect was brought about by alloying 0.8% Mo with high-nickel (Ni) type 304 SS containing 2% Cu + 3% Si. SEM revealed the segregation of N-rich phases in the presence of 0.24% N in these steels. However, N addition shifted the pitting potential in the positive direction, extending the passive range of the steel.