Corrosion Evolution of Low Alloy Steel in Deaerated Bicarbonate Solutions

Corrosion evolution during immersion tests (up to 43 days) of NiCu steel in deaerated 0.1 mol/L bicarbonate solutions was investigated by electrochemical measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results show that NiCu steel transformed from the anodic dissolution in the early stage of immersion to a metastable passive state in the final stage as the open-circuit potential value shifted positively, which was aroused by the precipitation of corrosion products. This process was mainly promoted by the trace amount of oxygen. Simultaneously, dominant cathodic reaction transformed from the hydrogen evolution in early stage to reduction processes of corrosion products in later stages. Possible corrosion processes were discussed with the assistance of a corresponding Pourbaix diagram.