Hot Corrosion Behavior of Plasma-Sprayed WC-CoCr Coatings on AISI 316L Steel Substrate in Na2SO4-25% NaCl Salt Environment

Abstract

This study investigates the hot corrosion resistance characteristics of plasma-sprayed WC-CoCr coatings deposited on AISI 316L stainless steel. Coated and uncoated samples were exposed to a fused Na2SO4-25%NaCl salt deposit at 700 °C in cyclic conditions for fifty thermal cycles. The corrosion kinetics were determined using a thermogravimetric method. To determine the hot corrosion rate, weight-change measurements were taken after every cycle. X-ray diffraction, energy dispersive spectroscopy, and scanning electron microscopy were performed on samples to examine the hot corrosion characteristics. With a fused salt deposit at 700 °C, the uncoated AISI 316L steel substrate samples experienced faster high-temperature oxidation as compared to coated samples. It was noticed that the uncoated steel substrate gained a little weight during the first few cycles, afterwards as the number of thermal cycles increased, the weight gain followed a linear kinetics. The WC-CoCr coating reduced the weight gain of steel substrate by about 75% in a corrosive salt environment. The plasma-sprayed WC-CoCr coatings had adhesion strength and microhardness of 10.8 ± 2.4 MPa and 410 ± 25 HV0.5, respectively. The hot corrosion-affected coatings displayed slow-scaling kinetics. The oxides of the coatings, formed at the surface were rich in chromium and tungsten. These oxides helped in providing hot corrosion resistance in the corrosive environment as they worked as obstacles to the penetration/diffusion of corrosive elements through coatings. The higher content of tungsten inside the coating promoted WO3 formation, which improved resistance to corrosion at elevated temperatures. Chromium in the coating caused the formation of spinel CoCr2O4 and Cr2O3 the spinel CoCr2O4 that emerged as an intermediate phase also contributes to the maintenance of corrosion resistance. Some oxides such as Fe2O3, FeCr2O4, and CoSO4 were found. Apart from that some traces of Na were also found.