The role of alkaline-earth additives on the molten carbonate corrosion of 316L stainless steel

The corrosion behavior of 316L stainless steel in eutectic Li/Na molten carbonate containing various amounts of Mg, Ca and Ba ions has been evaluated by electrochemical techniques in combination with oxygen solubility determinations. Open circuit potential and corrosion rates have been correlated to the oxygen solubility properties of the carbonate melt for an understanding of the kinetic aspects of the corrosion process. It has been found that minor additions of Mg and Ca ions (1.5% molar fraction) distinctly promote a higher carbonate oxygen solubility, whereas Ba has only a marginal effect on it. In general, the electrochemical investigations showed that at 1.5% molar fraction addition there exist a strong correlation between steel corrosion rate and oxygen solubility indicating that (i) corrosion process takes place under a diffusion-limited cathodic reduction of dissolved O2 and (ii) corrosion rate is significantly increased with respect to the no-added Li/Na carbonate by the introduction of Mg and Ca ions that therefore serve as a sort of oxidizing agents. However, with larger Mg and Ca additions (up to 10% mol) the corrosion process was found to pass progressively under an anodic control despite decreasing oxygen solubility values. This effect is ascribed to the growth of an alkaline-earth doped lithium ferrite layer with enhanced barrier-like properties. In contrast, further additions of Ba cation in carbonate did not change the corrosion mechanism for its inability to react with the growing corrosion scale. Only minor reductions of corrosion rate are detected in agreement with the lower tendency of the melt containing high molar fractions of Ba to solubilize the oxygen gas.