Effect of Ni and Cu addition on corrosion behaviors of pre-oxidized ultra-strong steel for automotive applications

The incomplete elimination of oxide scale formed during the normalizing process can result in severe degradation of the surface and mechanical properties of ultra-strong steels. This study revealed that the formation of oxide scale was highly dependent upon the alloying elements Ni and Cu. Ni-bearing steel showed a much higher oxidation rate, resulting in much thicker scale than Cu-bearing steel. Uneven scale/steel interfaces and oxide penetration along the grain boundary of the steel were clearly observed in the Ni-bearing steel. The difference in the solid solubility limit of Ni and Cu to austenite could lead to the enrichment of Ni and precipitation of Cu at scale/steel interfaces in the two types of steels, respectively. This resulted in the different structure and properties of the oxide scales, which influenced the subsequent corrosion behavior in chloride containing conditions. Linear polarization resistance measurements showed that the addition of Ni and Cu to the steel had a beneficial influence on the corrosion resistance, by suppressing the anodic dissolution and cathodic reduction reactions of Ni and Cu-bearing steels, respectively. This study provided a proposed mechanism for the difference in corrosion behaviors between the two types of steel.