Effects of temperature and partial pressure of CO sub(2)/O sub(2) 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 Li sub(2)CO sub(3)-48 m/o Na sub(2)CO sub(3) salt in the temperature range of 520 degree C to 580 degree C, particularly in the presence of CO sub(2) and O sub(2). 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 degree C to 580 degree C suffer more corrosion attack than those tested in the temperature range of 600 degree C to 650 degree 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 degree C. Samples tested above 600 degree 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 degree C exhibit the typical active-passive behaviour, but the passive region disappears when the temperature is below 580 degree C. This is attributed to the formation of a porous LiFe sub(5)O sub(8) passive film. By contrast, the formation of a LiFeO sub(2) passive film, dense enough to provide protection, is observed with increasing temperature over 600 degree C. It is also found that the partial pressure of CO sub(2) affects markedly the corrosion rate, but the partial pressure of O sub(2) does not.