In combating CO₂ corrosion, literature suggests specific micro-alloying (< 2 wt. %) of plain carbon steel to be the best choice in consideration of durability to cost ratio. In order to achieve optimized steel chemistry, the effects of number of alloying elements on the corrosion behaviour and protectiveness of the corrosion scale have been studied over the years. Amongst the various alloying elements, Cr has been found to be most effective. However, to date, no convincing mechanistic interpretation for the protective effect of Cr of the corrosion scale has been developed. A further question is whether a combination of alloying elements can yield superior behaviour, and if so what are the mechanistic determinants of behaviour.

In the present study, we investigated the effects of the micro-alloying of plain carbon steel with Cr and Mo on the corrosion behaviour and scale formation and protective nature in a CO₂ saturated (sweet) brine (0.5 M NaCl) environment, under controlled hydrodynamic conditions (0-1000 RPM), at 80^oC in a slightly acidic environment (pH 6.6). Materials were low alloy steels fabricated by incorporating small amounts of Cr (~ 1 wt. %) and/or Mo (~ 0.7 wt. %) in base plain carbon steel. Corrosion behaviour was investigated by chronoamperometric type current-transient characteristics, as well as by initial current measurement at various flow conditions under different anodic over-potentials with respect to the open circuit potential (E_oc + 20 to + 70 mV) and initial current measurement at various anodic over-potentials under low (100 RPM) and high (1000 RPM) flow conditions.

Potentiostatic current transients indicated that the presence of Cr induced an obvious decrease in the overall current flux throughout transient whilst the presence of Mo appeared to induce faster crystallization. Qualitative observation of the scale morphology using SEM indicated that the corrosion scales formed on Cr/Mo micro-alloyed steels are comparatively thinner and yet they render better protectivity and lead to faster passivation, especially under higher flow conditions (e.g. 1000 RPM). Although, in general the higher flow rates caused higher current-flux passing through the scales at the beginning of the transient, the final stage of the current transients for Cr/Mo micro-alloyed steels indicated quicker passivation at high flow rate (1000 RPM) compared to low flow rate (100 RPM). From the mechanistic perspective, we suggest that the addition of small amounts of Cr/Mo modulates the current due to dissolution of iron, as well as the current due to growth of a crystalline layer, by modifying the local environment at the steel surface, in terms of pH and degree of supersaturation. Modeling of this hypothesis is currently in progress.