The Synergistic Effect of Iodide and Sodium Nitrite on the Corrosion Inhibition of Mild Steel in Bicarbonate–Chloride Solution

The effect of potassium iodide (KI) and sodium nitrite (NaNO₂ inhibitor on the corrosion inhibition of mild steel in chloride bicarbonate solution has been studied using electrochemical techniques. Potentiodynamic polarisation data suggest that, when used in combination, KI and NaNO₂ function together to inhibit reactions at both the anode and the cathode, but predominantly anodic. KI/NO₂- concentration ratios varied from 2:1 to 2:5; inhibition efficiency was optimized for a ratio of 1:1. The surface morphology and corrosion products were analysed using scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The latter shows that the addition of I- to NO₂ facilitates the formation of a passivating oxide (γ-Fe₂O₃) as compared to NO₂- alone, decreasing the rate of metal dissolution observed in electrochemical testing. The synergistic effect of KI/NO₂- inhibition was enhanced under the dynamic conditions associated with testing in a rotating disc electrode.