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

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.