Understanding on accelerated corrosion of 316H stainless steel in molten solar salt containing micro-sized SiO2 impurity

Sand ingress is a common issue affecting the safe service of structural alloys used in the system of concentrating solar power (CSP) plants. Abnormal corrosion caused by the addition of micro-sized silica (SiO 2 ) in high temperature molten solar salt environment has attracted much attention. However, the specific corrosion effect and related mechanism have not been thoroughly elucidated. This work systematically investigates corrosion behaviours of 316H stainless steel (SS) in 565 °C molten solar salt with different concentrations of micro-sized SiO 2 (10 μm). The results of mass gain show that SiO 2 addition markedly accelerates the high-temperature oxidation and corrosion of 316H SS samples exposed to the solar salt. Analyses on corrosion morphologies reveal that SiO 2 addition significantly thickens the oxide scale, induces a loose and porous outer layer and promotes interfacial cracking. Meanwhile, the oxide film exhibits a complex three-layer structure, consisting of an outermost layer of NaSiO 3 and NaFeO 2 , an intermediate Fe-O layer and an innermost Cr-rich layer. Addition of SiO 2 (0.5 wt.%) promotes the reaction of NO 2 - with SiO 2 in molten solar salt, leading to an increase in activity of O 2- ions in the solar salt and enhancing its attack on protective chromium oxide film. With a higher concentration of SiO 2 (1.5 wt.%), the produced SiO 3 2- diffuses into the oxide layer to react with chromium oxide and simultaneously interacts with Na + to form the non-protective Na 2 SiO 3 phase, ultimately weakening the protective ability of the oxide layer and accelerating the degradation of 316H SS alloys exposed to molten solar salts.