Corrosion of Ni–Al high velocity oxyfuel (HVOF) thermal spray coating by fly ash and synthetic biomass ash deposits

Abstract Corrosion of a thermal spray (HVOF) Ni–Al coating has been investigated at 600°C under both biomass generated fly ash and synthetic biomass ashes and has been monitored as a function of reaction time for up to 1000 h. Detailed microstructral and compositional analyses have been carried out to investigate the hot corrosion mechanism. Various microstructural defects and compositional inhomogeneities are found to play an important role in the initiation of hot corrosion. Deposition of molten reactants from the ashes creates the aggressive environment. Molten salts cause initial rapid hot corrosion via fluxing reactions between planar interlamellar porosity resulting in debonding of the surface lenticular splats followed by subsequent slow dissolution. Comparison between the performance of the coating in the fly ash and the synthetic ashes provides information for improved laboratory corrosion tests.