Influence of the HVOF spraying process on the microstructure and corrosion behaviour of Ni-20%Cr coatings

The microstructure and the aqueous corrosion resistance of coatings produced by High Velocity Oxy-Fuel (HVOF) spraying techniques has been investigated. Two types of spraying processes have been employed i.e., Topgun HVOF using propylene gas and Met-Jet II HVOF with kerosene liquid fuel together with two forms of Ni-20%Cr powders i.e., water and inert gas atomised. The oxide, porosity and the amount of melted material in the coatings were characterised using scanning electron microscopy (SEM) and X-ray diffraction (XRD), whilst the corrosion resistance of the coatings and the ability to protect the underlying mild steel substrate was evaluated by use of a salt spray chamber and potentiodynamic tests.MetJet II produced coatings from gas-atomised powder with a lower oxide content, a reduction in porosity and less melted material, as the residence time of particles in the combusted gas stream was shortened. Water atomised powder formed a higher volume fractions of unmelted material and porosity when compared with gas-atomised powder coatings. This was encouraged by the presence of a thin oxide layer, which formed during the production of the water-atomised powder. The orientation of oxides and pores in the coatings had a major effect on their aqueous corrosion behaviour. Better protection for the underlying steel substrate (>3000 h exposure in a salt spray test) was obtained with the coating produced from the gas-atomised powder with the MetJet II system, which had the lowest porosity/oxide content running perpendicular to the substrate surface. The major factor in preventing attack on the mild steel substrate is the amount of interconnecting porosity which allows the corrodant to percolate through the coating.