Dependence of erosion–corrosion on carbide/metal matrix proportion for HVOF Cr₃C₂–NiCr coatings

A series of erosion–corrosion (EC) tests was carried out on HVOF Cr₃C₂–NiCr cermet coatings with different fractions of NiCr metal binder phase. Testswere conducted using a nozzle type elevated temperature erosion tester at test temperatures up to 450°C. Bed and fly ash retrieved from operating fluidised bed combustor boilers as well as SiO₂ angular quartz were used as the erodent materials for tests at both shallow and steep impact angles. The fly ash contained chemically active compounds and a corrosive environment was involved. The morphology of the specimens was examined by scanning electron microscopy. It was found that the cermet coatings were eroded by SiO₂ quartz and fly ash at a low velocity with peak wastage occurring at an intermediate value of carbide content at both the shallow and the steep impact angles. Erosion caused by the bed ash at a high velocity resulted in no peak wastage at either the shallow or the steep angles. Instead, the EC thickness loss increased with increasing metal binder content. The EC behaviour of the coatings is related to the morphology of the coatings and the characteristics of the erodent particles.