Effect of Al additions in WC-(Cr_1-xAl_x)N coatings on the corrosion resistance of coated AISI D2 steel in a deaerated 3.5 wt.% NaCl solution

Multilayered WC-(Cr_1-xAl_x)N coatings were deposited on AISI D2 steel using cathodic arc ion plating (CAIP) process. Five kinds of WC-(Cr_1-xAl_x)N coatings were prepared: WC-Cr_0.6Al_0.4N, WC-Cr_0.57Al_0.43N, WC-Cr_0.53 Al_0.47N, WC-Cr_0.48Al_0.52N and WC-Cr_0.45Al_0.55N. The Al concentration could be controlled by using evaporation source for Al targets and fixing the evaporation rate of the other metals (WC alloy and Cr). In this study, the corrosion behavior in deaerated 3.5 wt.% NaCl solution was investigated by electrochemical corrosion tests (potentiodynamic polarization test, galvanic corrosion test, electrochemical impedance spectroscopy (EIS)) and surface analyses (glow discharge optical emission spectroscopy, X-ray diffractometry, scratch adhesion test, scanning electron microscopy, electron probe micro-analyzer). The results of potentiodynamic polarization test showed that the WC-Cr_0.48Al_0.52N coating with lower porosity exhibited the lower corrosion current density. The galvanic corrosion current between the coating and the substrate showed low values. In EIS measurements, the charge transfer resistance (R_ct) value of WC-Cr_0.48Al_0.52N coating only increased with the immersion time, when compared to the other coatings. It can be due to the corrosion products plugging the pores and increasing the pathway resistance.