Effects of Minor Alloying Elements (Cr and Mo) on the Corrosion Behaviors of API Grade Steel in CO₂-Saturated Brine Environments

Abstract

This study examined the effects of minor alloying elements (Cr and Mo) on the corrosion behaviors of API grade steel in CO₂-saturated near-neutral aqueous solutions. Various experimental and analytical methods were applied to gain insights into the underlying CO₂ corrosion mechanism of the steels. The findings revealed that steel with a minute quantity of Mo (0.1 ~ 0.15 wt%) exhibited the lowest corrosion current density and the highest polarization resistance. This outcome can be primarily attributed to the formation of a thin layer of Mo-based oxides/hydroxide, covered with a fine FeCO₃ scale, consequently enhancing corrosion resistance in CO₂ environments. On the other hand, the addition of Cr (0.4 ~ 0.5 wt%) in combination with Mo resulted in degraded corrosion resistance due to the competitive precipitation of amorphous Cr(OH)3 and crystalline FeCO₃. This led to an uneven interface and the growth of FeCO₃ particles, consequently reducing polarization resistance and increasing the corrosion rate. Based on the obtained results, it is recommended that cost-effective steels for CO₂ transportation can be developed by adding a small amount of Mo along with a higher quantity of Cr. This optimized combination of alloying elements is expected to significantly improve the anti-corrosion performance of the steel in near-neutral brine environments with CO₂.