Effect of alloying (Cr, Mo, and V) on corrosion behaviors of API-grade steel in CO2-saturated aqueous solutions with different pH

Abstract

The corrosion behaviors of steels alloyed with Cr, Mo, and V were examined in two different solution chemistries: near-neutral (pH 6) and weakly acidic (pH 4.5) solutions saturated with CO2. In near-neutral solutions, Cr alloying hindered the formation of a stable corrosion scale on the surface, reducing the corrosion resistance compared to conventional low-carbon steel. On the other hand, the alloyed steel exhibited higher corrosion resistance in acidic solutions. In contrast to the conventional carbon steel, which had a single scale of crystalline FeCO3 scale on its surface, a more stable bi-layer of corrosion scale consisting of an inner layer of amorphous Cr(OH)3 and an outer layer of crystalline FeCO3 was developed on the alloyed steel. This was attributed primarily to the increased thermodynamic stability of Cr(OH)3 with an increase in Cr contents at this solution pH. Moreover, the increase in Cr with Mo addition resulted in a refinement of FeCO3 on the outer surface through increased hydrolysis reactions leading to a decrease in the local pH on the steel surface. Furthermore, adding V, even in minute quantities contributed to an increase in Cr-enrichment in the inner scale by suppressing M7C3 precipitation and leaving more Cr in the solid solution.