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 degrees C to 580 degrees C, particularly in the presence of CO2 and O-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 degrees C to 580 degrees C suffer more corrosion attack than those tested in the temperature range of 600 degrees C to 650 degrees 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 degrees C. Samples tested above 600 degrees 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 degrees C exhibit the typical active-passive behaviour, but the passive region disappears when the temperature is below 580 degrees 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 degrees C. It is also found that the partial pressure of CO2 affects markedly the corrosion rate, but the partial pressure of O-2 does not. (C) 2000 Elsevier Science S.A. All rights reserved.