Corrosion properties of HVOF cermet coatings with bond coats in an aqueous chloride environment

Abstract

WC–17Co coatings with Ni–5Al bonding layers were deposited on Al–7075 by HVOF spraying. The top-coat consisted of layers comprising tungsten carbide particles embedded in a Co(W,C) matrix of varied composition.The coated specimens were subjected to potentiodynamic polarization in 3.5% aqueous NaCl at 25, 35 and 45 °C. The coatings exhibited pseudopassivity caused by the oxidation of tungsten, carbon and possibly cobalt. Chronoamperometric measurements indicated that the inhomogeneous binder composition induced active corrosion processes taking place simultaneously with pseudopassivity. Cyclic polarization suggested that the coatings were not susceptible to pit corrosion in the temperature range of 25–45 °C. The likely “critical pitting” temperature of the coatings was 60 °C. Higher testing temperatures led to lower corrosion potentials and faster corrosion kinetics.