Effects of Alloying Elements on Corrosion Resistance and Microstructure of Fe-Cr Martensitic Stainless Steel

The combined effects of austenite and ferrite stabilizers were investigated for the enhanced corrosion resistance in martensitic stainless steels. Addition of austenite stabilizers such as Ni and N suppressed the formation of ferrite phases while decreasing the Ms temperature leading to the formation of retained austenite. From the results of Schaeffler diagram, the effective compositions for enhanced corrosion resistance were suggested to exist in the range of Cr equivalent less than 19 and Ni equivalent of 12 to 16. Outside those ranges, ferrite and austenite phases were observed to form, the former decreasing both hardness and corrosion resistance, whereas the latter, hardness value only. With the increase of N content, the degree of constitutional distribution between the two phases decreased, leading to homogeneous distribution. Corrosion tests revealed that pits were formed at the interface of ferrite and martensite phases, followed by growing into the ferrite phase. The ferrite phase is believed to be a preferential attack site and cause decrease in corrosion resistance in the present alloys. The addition of N and Mo showed a remarkable improvement in corrosion resistance in more severe environments