Testing procedure to obtain reliable potentiodynamic polarization curves on type 310S stainless steel in alkali carbonate melts



Potentiodynamic polarization measurements have been employed to evaluate the anodic behavior of a type 310S stainless steel in the eutectic Li/K molten carbonate. In general, the electrochemical tests yield useful information to predict the stability of the oxide films formed on the surface at the initial period of corrosion, although some precaution is required in the testing procedure as the reproducibility of results is seen to be adversely affected by the passage of large currents. Especially when the steel is in a passive state, erratic results are easily observed if the corrosion layer is being damaged by uncontrolled large currents. This is because the acid-base properties of the melt are susceptible to deep changes by applied currents in the milli-ampere range resulting in hysteresis phenomena in the polarization plot. Hysteresis is caused, on one hand, by acidic dissolution of the passive layer at high anodic currents and, on the other hand, by increased melt basicity due to oxide ion build-up at high cathodic currents. An optimized testing procedure is therefore suggested that minimizes these effects by imposing a 2 mA/cm2 threshold current during polarization measurements. Moreover, the conditions for the applicability of the linear polarization technique to estimate kinetic parameters have been discussed in relationship with the corrosion mechanisms analysed by impedance spectra. It is concluded that the presence of diffusional impedance terms and formation of surface resistive films in molten carbonates may result in not reliable polarization resistance values obtained with the linear polarization.