Effect of Alloying Elements on the Corrosion Behavior of Carbon Steel in CO2 Environments

The objective of the present study was to evaluate the effect of alloying elements (Cr, Mo, and Cu) on the corrosion behavior of low carbon steel in CO2 environments. Six samples were prepared with varying Cr content from 0 wt% to 2 wt% and with added 0.5 wt% of Mo and Cu; the specimens had ferritic/pearlitic microstructures. Steel samples were exposed to a CO2-saturated 1 wt% NaCl solution with different combinations of pH and temperature (pH 4.0 at 25 degrees C, pH 6.6 at 80 degrees C, and pH 5.9 at 70 degrees C). Changes in corrosion rate with time were determined by linear polarization resistance measurements. The surface morphology and the composition of the corrosion product layers were analyzed by surface analysis techniques (scanning electron microscopy and energy dispersive x-ray spectroscopy). Results showed that the presence of Cr and Cu showed a slight positive effect on the corrosion resistance at pH 4.0 and 25 degrees C. At pH 6.6 and 80 degrees C, regardless of the alloying elements, the trend of corrosion rate with time was similar, i.e., the corrosion rate of all specimens decreased with time resulting from the formation of protective FeCO3. A beneficial effect of Cr presence was clearly seen at "gray zone" conditions: pH 5.9 and 70 degrees C, where steel sample without Cr showed no decrease in corrosion rate with time. The presence of Cr in the steel promoted the formation of protective FeCO3 with Cr enrichment and it decreased the corrosion rate.