High temperature chlorine-induced corrosion of Ni50Cr coating: HVOLF, HVOGF, cold spray and laser cladding

To combat the high temperature chlorine-induced corrosion of materials in the biomass and waste fuel-fired thermal power plants NiCr coatings can be used. In this study, a commercially available Ni50Cr gas atomised powder was sprayed onto a power plant alloy using a liquid fuelled and a gas fuelled high velocity oxygen fuel thermal spray (HVOF), cold gas dynamic spray (CS) and laser cladding (LC). High temperature corrosion test was performed at 700 degrees C in a controlled environment corrosion test with 500 ppm HCl, 5 vol% O-2 and bal. N-2 for 250 h. KCl was deposited onto certain samples to understand the mechanism of high temperature chlorine induced corrosion. The corrosion products were characterised in a SEM with EDX and XRD. A wide range of oxide morphology and distribution was found on the coating top surface which was primarily due to their processing techniques. Cross-section examination of the coatings showed various degrees of internal attack and scale growth. In the absence of any deposit on the surface, cold-sprayed coating showed the poorest performance since the corrosive ion penetrated similar to 600 mu m after 250 h exposure, which is due to the inter-connected porosity. In presence of KCl deposit, laser cladded coating showed the best corrosion performance because the grooving pattern (traces of corrosive ion penetration) only penetrated 80 mu m, which is predominantly due to its pore free microstructure.