An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part Ⅱ. EIS interpretation of corrosion behaviour

摘要

In Part Ⅰ, of this work the equivalent circuits for electrochemical impedance spectroscopy (EIS) modelling of PVD coated steels in 0.5 N NaCl solution were established. In this paper, Part Ⅱ, the EIS spectra of such coated systems are modelled using the equivalent circuits. The circuit parameters obtained are correlated with the dielectric characteristics, and microstruc-ture of steels and PVD hard coatings. Coating porosity and localised corrosion with exposure time have also been determined using the corrosion potential difference (ΔE_(corr)) between mild steel and PVD coatings and polarisation resistance R_p, which was obtained through EIS modelling using equivalent circuits. In addition, diffusion rates of the reactants (e.g. oxygen) through 'permeable' defects (e.g. pores) are studied by introducing the diffusion impedances W and O in EIS modelling. It has been found that the usage of impedances W and O is closely related to the crystallite features of PVD coatings. Warburg impedance (W) is most suitable for columnar crystallites, while the co-tangent-hyperbolic diffusion impedance (O) is best for the equiaxed crystallite structure. Finally, visual inspection, SEM examination, and the scanning reference electrode technique were employed to observe the corrosion progress of PVD coated steels with immersion time, in order to validate the EIS interpretation.