Effect of Si Addition in NiCrAl Coating on Corrosion in Molten Nitrate Salt

Abstract The materials used in concentrating solar power (CSP) systems are becoming of interest because of high energy efficiency of energy storage. It can apply molten salts as heat storage media and heat transfer fluid in a CSP system. In molten salts, steel alloys such as vessels, pipe-lines, and heat exchangers are very vulnerable to hot corrosion. To protect steel alloys, coating is an excellent strategy to lengthen the service lifetime and reduce the maintenance cost. NiCrAl coatings are suitable candidates for a high temperature environment. Thus, this research aims to investigate the corrosion behaviour in molten salt of NiCrAl coatings on AISI304 stainless steel with Si addition in the coating. NiCrAl coatings with and without Si addition were deposited using the high-velocity oxygen fuel (HVOF) technique. The corrosion test was performed using the immersion test in the modified molten salt (solar salt + 0.5%NaCl) at 400, 500 and 600°C. The gravimetric method was used to evaluate weight change for the immersion test. At 400°C, more evident weight gain due to molten salt infiltration was observed rather than oxidation reaction. At 600°C, oxidation became more dominant and oxidation products NiO and sodium-aluminium-silicate were detected. Si addition resulted in supporting the formation of the protective oxide, sodium-aluminium-silicate which was acting as a blockade against molten salt infiltration. Furthermore pore spaces were replaced by the protective oxide in the coating surface. The reactions related to oxide formation and hot corrosion were further discussed.