Corrosion of API-X100 steel heat affected zones (HAZs) in CO₂-saturated solutions - EIS investigations

This paper evaluates and models the corrosion products growth on the HAZs of API-X100 steel in CO₂ solutions. It first outlines the aqueous dissociation of CO₂ to generate H₂CO₃, H⁺, the adsorption and dissolution processes on the ferrite grains, the cathodic reactions, the cathodic role(s) of Fe₃C, and the precipitation and growth of the corrosion products. With the use of EIS, over after 5,000, 10,000 and 15,000 s, the charge transfer resistance, double-layer capacitance, and the resistance of the porosity of the growing corrosion products, were measured to be fitted with equivalent circuits and to model the corrosion interactions with proper interfaces. The HAZs cooled down from 950°C at 10°C/s showed a continuous decay in the corrosion resistance which was lower than that of the base steel. The 30 and 60°C/s showed intrinsically high charge-transfer resistance and evidence for thin corrosion products.