Influence of flame stoichiometry ratio on the corrosion resistance of chromium carbide-nickel chromium coatings processed by HVOF

High velocity oxy-fuel chromium carbide-nickel chromium coatings are popular for wear and corrosion resistant applications. In this work, the corrosion resistance of coatings processed by HVOF with gas atomized and agglomerated and sintered CrC-NiCr feedstocks is investigated for fuel rich, neutral and oxygen rich flames conditions, in simulated sea water media (NaCl 3.5%). Particle diagnostics showed that particle velocity diminishes and particle temperature increases with the fraction of oxygen in the flame, most likely due to a combination of a higher dwell time and a higher degree of decarburization (exothermic). Oxygen rich flames produce coatings with reduced hardness, higher susceptibility to peening, and more compressive residual stresses. Although flame conditions did not significantly affect the porosity of the coatings (higher for the atomized and sintered feedstock than for the gas atomized one), it had a systematic effect on the corrosion resistance. As the flame stoichiometry ratio changed from oxygen rich towards fuel rich, a progressive improvement in the corrosion resistance of the coatings was observed for both feedstocks. Results show that processing with fuel rich conditions can promote more positive open circuit potentials and corrosion potentials, smaller corrosion current densities and higher impedance values, improving the overall corrosion resistance.