Development of NI3AL corrosion resistant coatings for SS347 heat storage components in presence of molten nitrate salt.

Abstract

Climate change is an inevitable global issue with long term consequences for the sustainable development. It is a crucial time to review this climate issue with ensured determination. There is a need and demand for alternative sources to generate power rather than the conventional burning of fuels due to impact on environment. Renewable energy sources are those natural reserves that are refilled continually, including wind, solar, biomass and geothermal. A number of technologies have been developed to use solar energy for power generation. Among them, an important feature of concentrated solar power plants is the potential to incorporate thermal storage. Thermal energy storage allows generation beyond sunset and in times of cloud cover. Several possibilities for heat transfer fluid and thermal energy storage have been identified. From a wide range of materials, molten nitrate salt is selected because of adequate heat storage and transfer capability. Different approaches to prolong life by suppressing corrosion have been suggested in the literature, coating is a promising option because coatings are believed to provide shield to suppress corrosion. Among different coatings, nickel aluminide has been claimed to possess high-temperature mechanical strength and it has a remarkable oxidation resistance performance as substrate component. Moreover, nickel aluminide has low solubility in the molten nitrate salt. Ni₃Al coatings are much preferred to be used as corrosion resistant coatings as they possess strength at high temperature, oxidation protection and creep properties. Among different deposition techniques, plasma spray has been identified as most applicable because it is versatile, adaptable, cost effective. It also has high deposition rate, deposition efficiency and less environmental impact, more importantly it is easy to scale up. Corrosion behaviour of stainless steel 347 (SS347) and Ni₃Al coated SS347 was investigated in molten nitrate salt (60wt% NaNO₃ + 40wt% KNO₃) immersion at 565oC for 500 hours intervals up to 3000 hours. A growth of stratified oxide layers was observed on SS347 sample surface comprising of NaFeO₂ , Fe₂ O₃ and Fe₃O₄ . The Ni₃Al coated SS347 samples were observed to undergo rapid oxidation within first 500 hours. Apparent Mass change for bare SS347 was 4 mg/cm²/yr, equivalent to oxide growth rate of ~ 5 µm/yr. Mass change for Ni₃Al coated SS347 was 29.8 mg/cm²/yr, equivalent to oxide growth rate of ~ 44.6 μm/yr for first 500 hours and 0.5 mg/cm²/yr, equivalent to oxide growth rate of ~ 0.7 μm/yr for 500 to 3000 hours. The results presented in this study suggest that Ni3Al coating supresses the formation of oxide layers on the surface of stainless- steel substrates and can be used to suppress corrosion in presence of molten nitrate salts. The fact, that Ni₃Al coated SS347 gives mass change of one order of magnitude lower than the bare SS347, it means that these coatings can be used to prolong the lifetime of bare SS347 in molten nitrate salt at 565oC, which is of relevance to thermal energy storage applications. The Engineering Doctorate portfolio is structured as an innovation report and five submissions. A personal profile and a report on international industrial placement are also included in the portfolio.