Effect of a Small Amount of Cr and Mo on Aqueous CO₂ Corrosion of Low-Alloyed Steel and Formation of Protective FeCO₃ in Near-Saturation Conditions Available to Purchase

The effect of a small amount (1 wt%) of Cr or Mo on aqueous CO₂ corrosion of low-alloyed steel and the formation of protective FeCO₃ corrosion product layers was investigated under controlled water chemistry conditions, where the bulk saturation value of FeCO₃ was maintained at near-saturated condition. Changes in CO₂ corrosion rate with exposure time were monitored by linear polarization resisitance measurements. The surface morphology and the composition of the corrosion product layers were analyzed by scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, and transmission electron microscopy. Results showed that the emergence of a continuous Fe₃C layer created favorable conditions at the surface of nonalloyed steel (containing no Cr and Mo) for semiprotective FeCO₃ to form even though the bulk saturation value of FeCO₃ was maintained at near-saturated condition. However, semiprotective FeCO₃ was not observed on the surface of 1% Cr steel and 1% Mo steel, but rather discontinuous and porous corrosion product layer was formed. Due to the hydrolysis reactions of Cr³⁺ and Mo³⁺, and the discontinuous structure of the corrosion product layers, the surface conditions for 1% Cr steel and 1% Mo steel were not favorable for the formation of FeCO₃ under the experimental conditions of this study.