Structural optimisation and electrochemical behaviour of AlCrN coatings

•AlCrN coatings deposited on 316L stainless steel by multi-arc ion plating•Structural optimisation by vacuum annealing to improve corrosion resistance•Excellently protective efficiency to stainless steel in both acid and seawater environments

AlCrN coatings were deposited on 316L stainless steel by multi-arc ion plating. The phase structures were controlled and optimised by vacuum annealing at 700 °C, 800 °C, and 900 °C, each for 2 h. The microstructures and morphologies of these coatings were examined by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscope, in association with adhesion strength measurement, residual stress measurement and potentiodynamic polarisation with 3.5% NaCl and 10% H2SO4 solutions. Results show that the solid solution hcp-(Cr, Al)N phase of as-deposited AlCrN coatings decomposes into hcp-AlN and CrN phases, then further decompose into N and Cr with increasing annealing temperature. Metal particles and droplets on the coating surface were gradually removed simultaneously when compared with as-deposited coatings. The highest adhesion strength was obtained after annealing at 700 °C. It decreased with increasing annealing temperature due to the σ-FeCr phase and Cr2N phases, and the residual stress decreasing. The potentiodynamic polarisation measurements showed that the corrosion resistance of such coatings was significantly improved after annealing: the coatings annealed at 800 °C showed the best protective efficiency because some corrosion resistance phases (h-AlN, Al2O3, CrN, and Cr2N) were generated.