Corrosion characteristics of 316L stainless steel in oxide-rich molten solar salt at 600 °C
Srinivasan Swaminathan, Sumit Kumar, Axel Kranzmann, René Hesse, Hennig Goldbeck, Andrea Fantin
Abstract
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.