Corrosion behaviour of amorphous Al-Cr and Al-Cr-(N) coatings deposited by dc magnetron sputtering on mild steel substrate

Physical vapour deposition (PVD) Al alloys films are potential alternatives to conventional electrodeposited coatings for sacrificial protection of steel substrates. Even though the structure-mechanical relationships have already been studied for Al-Cr or Al-Cr-(N) coatings, there is no published data on the corrosion behaviour of these alloys deposited on mild steel substrates. Al-Cr or Al-Cr(N) coatings were deposited by d.c. magnetron sputtering of Al-Cr large size targets with different Ar-N-2 mixtures in an industrial PVD system. In this paper, the influence of both chromium and nitrogen contents on the substrate/coating galvanic coupling behaviour in 3% NaCl solution are studied. Only single-phased amorphous films have been synthesised and the microhardness can reach approximately 9 GPa. The results of potentiostatic and impedance spectroscopy electrochemical tests are discussed and linked to composition and morphology of coatings. It is shown that 20 at.% chromium in Al-Cr coating is the limit above which galvanic corrosion of steel substrate occurs. Nitrogen incorporation seems to influence the coating passive layer and favours its pitting resistance. The salt bath results (more than 800 h without corrosion for Al-Cr 18 at.% Cr) are in agreement with electrochemical behaviour. (C) 2004 Elsevier B.V. All rights reserved.