Effects of alloying elements (Cu, Ni) on the passivity of stainless steels

Abstract

In this thesis, the effects of alloying elements (Cu, Ni) on the passivity of stainless steels were explored. The major findings of this work are summarized below;

Effects of Cu on the Passivity of Fe-20Cr-xCu (x=0, 2, 4 wt.%) Alloys in dilute $H_2SO_4$ solution

The addition of Cu to Fe-20Cr alloy is beneficial in improving the corrosion resistance of the alloys in active region, as confirmed by the significant decrease in critical anodic current density ($I_c$) as well as corrosion rate due primarily to the enrichment of Cu on the surface in the alloys. In passive region, the effects of Cu on the passivation behavior of the alloys was dependent on the applied potential; at potentials active to -100 $mV_{SCE}$, the passivity of the alloys is improved with Cu content, which is confirmed by the increase in passivation rate with a decrease in oxidation tendency. In contrast, above -100 $mV_{SCE}$ where the oxidation reaction of $Cu^+^ to $Cu^{2+}$ occurs, the passivity of the alloys was significantly degraded with Cu content, as confirmed by the increase in passive current density as well as the significant decrease in passivation rate. However, the deleterious effects of Cu on the passivation behavior of Fe-20Cr-xCu (x= 0, 2, 4 wt.%) decreased at potentials noble to 500 $mV_{SCE}$ due primarily to the significant decrease in Cu content in passive film and hence to the decrease in the oxidation reaction of $Cu^+$ to $Cu^{2+}$. The stability of passive film decreases with an increase in Cu content, suggesting that the film formed on the Cu-containing alloys was less stable than that of Fe-20Cr alloy. The rapid potential decay or the $1^{st}$ stage-potential decay to approximately -300 $mV_{SCE}$ in potential decay curves of Cu-containing alloys is due primarily to the Cu-dissolution from the film, which ultimately increased the defect density of the passive film and hence decreased its stability. After the 1st stage-potential decay, Fe-20Cr alloy exhibited the 2...