Electrochemical evaluation of the corrosion protectiveness and porosity of vacuum annealed CrAlN and TiAlN cathodic arc physical vapor deposited coatings

The corrosion behavior of cathodic arc physical vapor deposited CrAlN and TiAlN coatings were examined in 1 M HCl solution before and after vacuum annealing at 700, 800, 900, and 1000 °C. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods were used to study the corrosion behavior and porosity of the coatings in comparison with the bare steel substrate (304SS). Structural and mechanical characterization of the coatings were also conducted. It is found that with increasing annealing temperature, the mechanical properties of TiAlN increased due to age hardening caused by spinodal decomposition while the hardness of CrAlN decreased as result of relaxation. Similarly, EIS and PDP results revealed that the as-deposited and annealed coatings offer higher corrosion resistance as compared to the bare 304SS substrate. The coatings susceptibility to corrosion is reduced after annealing as indicated by the increasing nobility of E_corr. Both PDP and EIS tests revealed that CrAlN coating annealed at 1000°C exhibited superior corrosion resistance properties. It is found that the reduced current density for CrAlN coating annealed at 1000°C was due to the reduction in the porosity. Annealed TiAlN coating follows similar behavior until an optimum annealing temperature of 800°C. Beyond this temperature, porosity enlargement and an increase in the number of pores subsequent to structural changes deteriorated the corrosion resistance of TiAlN coating.