Are unexpected chloride ions in molten salt really harmful to stainless steel?

Commercial mixed salt generally contains the impurity chloride ion (Cl−) due to the influence of the preparation process of the single component salt that constitutes the mixed salt. The impurity Cl− can increase the corrosion of molten salt to metal materials. In order to investigate the effect of molten salt with different content of Cl− on the corrosion of metal materials, a low melting point quaternary nitrate and solar salt were selected as the base salt to analyze the corrosion rule of Cl−. Using the weight loss method, the corrosion behaviors of the two molten nitrate salts with and without Cl− to 316L stainless steel were studied. The results of the corrosion test for 1000 h show that the quaternary mixed molten salt with low Cl− content of 0.03 wt% has the lowest corrosion rate on 316L stainless steel than that of the other two salts with Cl− content of 0.01 and 0.05 wt% for the existence of calcium nitrate. Calcium ions (Ca2+) are introduced to solar salt with the same Cl− content of 0.03 wt% to confirm its corrosion effect on stainless steel. The results show that the addition of a small amount of Cl− and Ca2+ can reduce the corrosion rate of solar salt on 316L stainless steel. The corrosion products morphology analysis by SEM, EDS, and XRD test show that the calcium rich oxide layers containing CaFeO4 was found in the corrosion layer of stainless steel, which inhibits the further corrosion of stainless steel in molten salt. This finding has the far-reaching implications for inhibiting corrosion of molten salt with Cl− on stainless steel.