The effect of laser surface melting of stainless steel grade AISI 316L welded joint on its corrosion performance in molten Solar Salt

Abstract

Whereas there are numerous studies dedicated to the corrosion performance of different grades of austenitic stainless steel (ASS) exposed to the conditions of thermal storage of solar energy, there is no data concerning the resistance of the welding joints, which are unavoidable when fabricating large structures. Because the microstructure of a weld bead can significantly differ from that of the base metal and the heat-affected zone (HAZ), increased susceptibility to localized corrosion can be expected. This work explores corrosion resistance of a welding joint of ASS grade 316 L when exposed to molten Solar Salt (60% NaNO3 – 40% KNO3) at 565 °C. Further, the application of laser surface melting (LSM) is proposed as a method to modify the microstructure of the weld bead and HAZ to enhance the corrosion resistance of the joint. Isothermal exposure to molten Solar Salt was implemented in static immersion, in the presence of atmospheric air, for the duration of up to 1008 h (6 weeks). The gravimetric analysis revealed a corrosion rate of 54 μm/y at the weld joint, which is at least twice as high as that expected for the same non-welded alloy in similar conditions. Although LSM reduces the rate of uniform corrosion in the first 2 weeks, there is no long term benefit in this regard. However, the onset of localized corrosion of the intergranular type is effectively prevented, presenting a potential approach for diminishing the risk of catastrophic failure of HAZ by fracture.