Corrosion characteristics of 316L stainless steel in oxide-rich molten solar salt at 600 °C

An attempt has been made in this work, to observe the influence on alloy aging by the sodium oxide (Na 2 O) in solar salt (60 wt% NaNO 3 + 40 wt% KNO 3 ). The accelerated aging was established by adding Na 2 O (0.005, 0.07, 0.135 and 0.2 wt%) to the solar salt and their effect on corrosion of 316L stainless steel (SS) at 600 °C in that oxide-rich solar salts for 168 h in synthetic air was investigated. Corrosion is significantly more in oxide-rich solar salt compared to pure solar salt. Strikingly, the oxide scale-base metal interface is wavy in solar salt containing 0.005 % Na 2 O clearly shows the oxide addition to salt melt influences Cr-rich inner oxide layer formation and its selective dissolution at early stage that leads to non-uniform corrosion. Interestingly, with increase of Na 2 O to 0.07 %, steel corrosion proceeded uniformly by accelerated disintegration of Cr-rich inner layer and subsequent dissolution. Severe scale spallation and weight loss in nitrate melt containing 0.2 % Na 2 O fostering more rapid corrosion, alarming that substantial tolerance of oxide content in solar salt is ≥ 0.135 % for an acceptable corrosion of 316L SS. Despite preferential dissolution of Cr and scale degradation/spallation with increased oxide content in solar salt, the corrosion layer in all cases comprised of sodium ferrite, and Cr-rich Cr-Fe mixed oxides with the Ni enrichment at the scale-metal interface. Competing processes between oxide scale growth, degradation and dissolution or even spallation has been discussed with an emphasis of Na 2 O addition to solar salt. • Corrosion of 316L stainless steel at 600 °C in Na 2 O-added solar salt was explored. • Corrosion upon oxide-rich salt exposure was analyzed by weight change, GIXRD, FIB and SEM-EDX. • 0.005 wt% Na 2 O in solar salt show non-uniform corrosion at scale-metal interface. • Oxide content tolerance was found to be ≥ 0.135 wt% for an acceptable corrosion. • Strong basicity of 0.2 % Na 2 O-added solar salt deteriorates corrosion resistance.