Effects of different chloride ion concentrations on corrosion of mild steel in aerated environment

The corrosion of mild steel in aerated solutions containing different chloride concentrations (0.0585, 0.3, 3.5, 5.85, and 10% NaCl) has been investigated using different electrochemical techniques such as linear polarisation resistance (LPR), potentiodynamic polarisation (PDP), and electrochemical impedance spectroscopy (EIS). The three techniques gave similar results that explained the combined effects of both oxygen and chloride ions in determining the mechanism and kinetics of corrosion of mild steel. Both potentiodynamic polarisation and electrochemical impedance spectroscopy confirmed that increase in chloride concentration increases the electrical conductivity of the solution which has direct impact on the corrosion rate of the mild steel. The results obtained with linear polarisation resistance showed an increase in corrosion rate as the chloride ions concentration increase up to the threshold chloride concentration of 3.5% NaCl with corrosion rate of 0.24mmpy which is marginally higher compared to 0.189 and 0.181mmpy for potentiodynamic polarisation and electrochemical impedance spectroscopy respectively but there corrosion rate follow the same trend, and decrease with further increase in chloride concentration. Since the rate-determining step for the electrochemical reaction (corrosion) is oxygen reduction as the cathodic reaction, it was established that there was decrease in oxygen solubility at higher chloride concentrations that resulted in low corrosion rate. This is important in understanding how the chemistry of seawater environments and the condensates, which may be formed on mild steel, affect its corrosion resistance, so as to determine the amount of corrosion inhibitors to be injected to protect the steel in chloride environment.