Effect of resistivity on the corrosion mechanism of mild steel in sodium sulfate solutions

Electrochemical impedance spectroscopy (EIS) was used to study the corrosion behavior of mild steel samples immersed in 1, 0.1, 0.01 and 0.001% Na2SO4 aqueous solutions at room temperature in order to analyze the corrosion mechanism and obtain representative values of corrosion rates in environments with different resistivity. The EIS technique was used to measure corrosion current densities in 1 to 0.01% Na2SO4 solutions, and the measurement corresponding to 0.001% concentration gave a scattered Nyquist plot, whereas in the Bode representation a response associated with solution resistance was obtained. Other electrochemical techniques such as linear polarization resistance, LPR, Tafel extrapolation method and electrochemical noise (EN) were used in order to compare the results obtained by EIS. The ‘charge-transfer resistance’ (Rct) parameters obtained in the EIS technique agree well with the corresponding values of ‘polarization resistance’ (Rp) values obtained by the LPR technique, whereas the ‘electrochemical noise resistance’ (Rn) parameters obtained by the EN technique gave the highest values for all concentrations. These parameters generally tended to increase as the concentration of the solution decreased.