A mechanism of improvement in the corrosion resistance of ferritic stainless steels by Cu addition was studied by field exposure tests, XPS analysis of passive films and electrochemical evaluation.
Corrosion area ratio after a 1-month field exposure test was the smallest with 0.4%Cu addition among 21%Cr–0~2.0%Cu stainless steels, and chromium concentration of the passive films was higher with the Cu-bearing stainless steels than the Cu-free stainless steel. After an 18-month field exposure test, chromium concentration of the passive films of high Cr and Cu bearing stainless steels was higher than that of low Cr and Cu bearing stainless steels. This result suggested an existence of a synergistic effect between Cr and Cu.
Cu in ferritic stainless steels accelerates cathodic reaction and reduces anodic reaction when the steels dissolve in the active potential region. Therefore, Cu in the steels increases the open circuit potential at the passivation stage, and the increase of open circuit potential promotes the enrichment of chromium concentration of passive films on the Cu-bearing ferritic stainless steels. The cycle of dissolution and passivation strengthens the passive films on the Cu-bearing stainless steels.
On the other hand, the 1-month field exposure test revealed corrosion area ratio of stainless steels bearing more than 1% Cu was larger than that of a 0.4%Cu-bearing stainless steel. Stainless steels containing more than 1% Cu contain ε-Cu in matrix. It is considered that precipitation of ε-Cu deteriorate the corrosion resistance of these stainless steels.
