Electrochemical behaviour of cermet coatings with a bond coat on Al7075: Pseudopassivity, localized corrosion and galvanic effect considerations in a saline environment

Abstract

Abstract The behaviour of an HVOF WC–17Co/Ni–5Al coating on Al7075 in aqueous NaCl is investigated. The coating susceptibility to localized corrosion depended on the potential of polarization reversal. A two-stage pseudopassivity was observed for WC–17Co: At low overpotentials, oxidation occurred in the binder leading to surface films composed of anhydrous Co- and W-oxides. At high overpotentials, oxidation extended to the carbide phase leading to the formation of hydrated WO 3 films. Ni–5Al notably reduced the galvanic effect between WC–17Co and Al7075, whereas it hindered corrosion propagation into the substrate. The coating showed a high corrosion resistance during salt spraying for 49 days.