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.