Solid particle erosion-corrosion behaviour of a novel HVOF nickel aluminium bronze coating for marine applications-correlation between mass loss and electrochemical measurements

This paper investigates the solid particle erosion-corrosion performance of an experimental high velocity oxy-fuel (HVOF) sprayed nickel-aluminium bronze (NAB) coating using conventional gravimetric techniques as well as in situ electrochemical analysis. The coating consists of HVOF powders from three alloys: stainless steel alloy, nickel-based alloy and aluminium bronze alloy. It is a candidate coating for marine applications as a cost effective replacement of existing castings and to improve component life. The coating was subjected to pure erosion, flow corrosion and erosion-corrosion tests. A jet impingement slurry erosion rig was used to carry out the experiments; the effects of jet velocity were investigated. By gravimetric analysis the degree of synergy was evaluated and a constant was revealed which described to what extent the presence of corrosion products/films reduces the erosivity and promotes negative synergy. Likewise, standard deviations of electrochemical current measurements are shown to reveal the presence of protective film formation under flow corrosion and film breakdown under erosion-corrosion conditions. Separation of the erosion-enhanced corrosion component revealed that at high erodent kinetic energies, erosion-enhanced corrosion dominates and generates a positive synergy. At lower energies, this coating system forms a protective film which reduces the contact conditions on impingement and a negative synergy results. Overall, correlations between the mass loss and electrochemical measurements have been established and were used to identify and quantify synergy.