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

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 WO3 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.