An a.c. impedance study on PVD CrN-coated mild steel with different surface roughnesses

CrN coatings produced by physical vapour deposition (PVD) technology on mild steel of different surface roughness have been characterized using a.c. electrochemical impedance spectroscopy (EIS). It was found that as the roughness of the substrate decreased, the number of flaws, evidenced by the frequency and size of initial corrosion sites, also decreased. Thus it was concluded that coating porosity controlled the initiation of corrosion on the underlying substrate. In addition, the coating adhesion increased substantially as the substrate roughness decreased. As corrosion was seen to progress via an undermining mechanism with volume expansion and subsequent cracking of the coating, it was concluded that coating adhesion greatly influenced the propagation of corrosion on PVD-coated metals.