Influence of fuel and shroud gas on the corrosion resistance of HVOF nickel coatings

The corrosion and passivation behavior of high-velocity oxy-fuel (HVOF) thermally sprayed nickel coatings is studied by means of potentiodynamic polarization curves and electrochemical impedance spectroscopy. During the nickel thermal spray process, various fuel gases (propane, methane, and ethylene) as well as two different shroud gases (air and nitrogen) are used at two spraying distances (210 and 300 mm). The aim of this work is to explore the effect of these spraying parameters on the corrosion resistance and passive region of pure HVOF nickel coatings. Nitrogen shrouding does not lead to improved corrosion properties. Especially, propane fueling results in poor corrosion and passivation characteristics due to the sub-melting-point deposition temperature. Improved corrosion behavior as well as a wide passive region is determined for methane-fueled thermally sprayed nickel coatings, for which air is used as the shroud gas.