PVD Aluminium Alloy Coatings: Environmentally Friendly Alternative to Protect Steel Parts against Corrosion

In the automotive and aeronautic industries, security is of prime importance and so corrosion prevention is essential. Steel parts were once protected with deposits such as cadmium or zinc - nickel produced using a chromatation treatment but recent environmental directives dictate that these methods are no longer acceptable. Aluminium sacrificial coatings have excellent corrosion behaviour but unfortunately they have poor mechanical characteristics and a fast dissolution rate. The present study investigated a possible solution: alloying the aluminium with a more noble element, chromium, in order to decrease the sacrificial galvanic effect of the deposit and therefore improve its lifetime. The corrosion protection afforded by such coatings in relation to their structure and mechanical properties was investigated. The coatings were produced on carbon steel by a vacuum PVD arc evaporation process at a pilot scale. Intrinsic electrochemical properties were determined on pure materials and on layers deposited on glass strip. Several elaboration configurations (pure as well as composite targets) were investigated. In most cases the coatings were stratified and composed of pure aluminium and numerous hardened Al _x Cr _y intermetallic phases. Chromium enrichment of aluminium based coatings induces not only a beneficial hardening effect on the surface characteristics (> 700 HV) but also significantly improves the corrosion behaviour of the coated pieces (increasing lifetime by up to three times compared to pure Aluminium).