Surface morphological studies on hot corrosion behaviour of pre-oxidized plasma sprayed WC-CoCr coating on AISI316L steel in Na₂SO₄/NaCl molten salt environment

Abstract

The hot corrosion behaviour of plasma sprayed WC-CoCr coatings on AISI 316L steel substrate is studied in two corrosive salt environments namely 100% Na2SO4 and 75% Na2SO4+25%NaCl at 1000 °C. Also, on WC-CoCr coated AISI 316L steel substrate, a persistent Cr2O3 barrier scale is created employing pre-oxidation at 1200 °C for ten hours with the expectation that it would withstand hot corrosion in a Na2SO4 salt environment at 900 °C. X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), electron micro probe analysis (EPMA) and scanning electron microscopy (SEM) are performed on samples to examine the pre oxidation and hot corrosion characteristics of plasma sprayed WC-CoCr coatings at elevated temperatures. The findings indicate that the presence of both salt environment affects the degradation process of WC-CoCr coatings. The coatings in 100% Na2SO4 and the mixture of 75% Na2SO4+25%NaCl has a weight change of 0.14 mg/cm2 and 0.33 mg/cm2, respectively, after hot corrosion at 1000 °C for 50 cycles. Each cycle includes 1 hour heating in furnace and 20 minutes cooling in ambient air. Corrosion kinetics using thermogravimetric method showed that non-pre-oxidized samples gained 52.5 % more weight and more severely affected by hot corrosion than pre-oxidized ones. During hot corrosion after 2 hours, there was relatively negligible corrosion, but after 4 and 8 hours, deposits of Na2WO4, WO3, Cr2O3 and CoSO4 were produced. Oxides were primarily composed of Cr2O3 and WO3. Cr2O3 and WO3 acted as barriers to the penetration and diffusion of corrosive elements through coatings, which contributed to the hot corrosion resistance in the corrosive area. The hot corrosion deterioration of WC-CoCr coatings may be effectively reduced by introducing pre oxidation. Slower reaction rate of pre oxidized Cr2O3 scale may operate as a barrier which separates hot corrosion by Na2SO4 salt.