Localized corrosion mechanisms of the multilayered coatings related to growth defects

Multilayered WC–Ti1−xAlxN coatings were deposited on AISI D2 steel using cathodic arc deposition method. These coatings contain structural defects such as pores or pinholes. Thus, the substrate is not completely isolated from the corrosive environment. These growth defects in the coatings are detrimental to corrosion resistance of the coatings used in severe corrosion environments. The localized corrosion of the coatings was studied in deaerated 3.5 wt.% NaCl solution using classical electrochemical technique (potentiodynamic polarization test). Coating characteristics were examined by means of glow discharge optical emission spectroscopy, scanning electron microscopy, auger electron spectroscopy and transmission electron spectroscopy. The porosity was calculated from a result of potentiodynamic polarization test of the uncoated and coated specimens. The calculated porosity is higher in the WC–Ti0.6Al0.4N than others, which is closely related to the packing factor. The positive effects of greater packing factor act on inhibiting the passage of the corrosive electrolyte to the substrate and reducing the localized corrosion kinetics. From the electrochemical tests and surface analyses, the major corrosion reaction of coatings is caused by defects (pores, pinholes and crevices), coating delamination and galvanic effect between the droplet and the coating.