Evaluation of impedance spectra of zinc and galvanised steel corroding under atmospheric environments

The evaluation of impedance spectra of zinc and galvanised steel corroded under wet–dry cyclic condition has been performed. The wet–dry cyclic test was performed by exposing samples to alternate conditions of 1 h immersion in 0·05M NaCl solution and 7 h drying at 298 K and 60% relative humidity (RH). During the corrosion test, the impedance responses of zinc and galvanised steel samples were recorded once per day for 10 days. The impedance spectra were evaluated on the basis of an equivalent circuit considering the charge and mass transfer consequences of the coupled corrosion processes and the porous oxide layer. After initial exposure, both samples showed the evolution of two capacitive loops. However, a sharp decrease in characteristic frequency, characterised by an increase in double layer capacitance in the range of several mF cm⁻² as corrosion progressed, was observed. This was explained by taking into account the coupling of characteristic frequencies of the two capacitive loops due to formation of porous rust on iron. The parameters for the diffusion impedance of the cathodic process were estimated from the low frequency capacitive loops attributed to a finite Warburg diffusion impedance. A sharp increase in impedance after several days of corrosion in the case of galvanised steel was attributed to the dissolution of Fe–Zn alloy layer that is formed during galvanising. The results are supported by cross-sectional analysis of corrosion products using FESEM-EDX.