Effects of temperature and partial pressure of CO2/O2 on corrosion behaviour of stainless-steel in molten Li/Na carbonate salt

The corrosion tests with AISI-type 316L and 310S stainless steels are carried out to understand the abnormal corrosion behaviour observed in a molten 52 m/o Li2CO3–48 m/o Na2CO3 salt in the temperature range of 520°C to 580°C, particularly in the presence of CO2 and O2. Two experimental methods, namely, an out-of-cell test and an electrochemical method, were employed to analyze the corrosion behaviour with varying gas composition as well as temperature. The samples tested in the temperature range of 520°C to 580°C suffer more corrosion attack than those tested in the temperature range of 600°C to 650°C. Optical microscope analysis of samples from out-of-cell tests for 100 h show that the surfaces of the samples, regardless of the type of stainless-steel, were corroded severely by pitting when the temperature is below 580°C. Samples tested above 600°C, however, do not suffer significant corrosion attack. This is also confirmed by potentiodynamic results. The polarization curves of 316L stainless-steel samples measured above 600°C exhibit the typical active–passive behaviour, but the passive region disappears when the temperature is below 580°C. This is attributed to the formation of a porous LiFe5O8 passive film. By contrast, the formation of a LiFeO2 passive film, dense enough to provide protection, is observed with increasing temperature over 600°C. It is also found that the partial pressure of CO2 affects markedly the corrosion rate, but the partial pressure of O2 does not.