Effects of Cl− Ions on the Corrosion Behaviour of Low Alloy Steel in Deaerated Bicarbonate Solutions

The effect of Cl− ions on corrosion evolution of NiCu low alloy steel during immersion tests (up to 70 days) in deaerated 0.05 mol/L bicarbonate solutions was investigated by in-situ electrochemical measurements combined with X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) characterisations. The results showed that due to the acceleration of substrate dissolution in the presence of Cl−, corrosion of NiCu low alloy steel underwent only two stages, i.e., a quick oxidation process followed by a final metastable passive state, without the initial slow anodic dissolution as observed in the Cl−-free bicarbonate solution. The main components of the formed rust layer in the Cl−-free bicarbonate solution were α-FeOOH and Fe3O4, while apart from α-FeOOH, Fe6(OH)12CO3 was found evident instead of Fe3O4 in the Cl−-containing solution. Metastable pits were only found in the Cl−-containing solution where Cl− accumulated after the immersion test, confirming the attack of Cl− on the substrate after penetrating the formed corrosion product layer.