Anodic polarization of stainless steel alloys in molten NaNO3-KNO3 eutectic mixture

This article presents the galvanostatic anodic oxidation of two types of stainless steel alloys, ferritic (15.03% Cr) and austenitic (20.45% Cr, 8.37% Ni), in molten NaNO3-KNO3 eutectic mixture at different temperatures ranging from 673-873K. At a temperature of 6731< the shape of polarization curves for the alloys is complex, while at higher temperatures it is simple. The passivity potential range was calculated as the difference between the passivation potential, E-p, and the breakdown potential, E-b The value of E-b - E-p decreases with the increase of temperature. The amount of iron, chromium and nickel dissolved in the melt was determined after each experiment using atomic absorption spectroscopy. The composition and structure of the corrosion products formed on the surface of electrodes were examined by X-ray diffraction analysis. Corrosion parameters derived from the polarization curves are calculated; these are: polarization resistance at low current densities, R-p, exchange current density, i(o), corrosion current density, i(corr), passivation current density, i(p). It was found that the increase of temperature increases i(o), i(corr), and i(p) while R-p, decreases. From these results it was found that, under the given conditions, the austenitic stainless steel alloy is more corrosion resistant than the ferritic one. The activation energy of corrosion was estimated for the two alloys.