A Study on the Hot Corrosion of Ni/Na2SO4 System by Electrochemical Methods

Hot corrosion behavior of pure Ni wire specimen dipped in 1200 .deg. K pure Na2SO4 melt under Ar atmosphere has been studied by means of electrochemical methods. Ag(s)/Ag2SO4(1) half cell electrode contained in a mullite membrane was used as a reference electrode. Potentiodynamic polarization curves for the specimens demonstrated a definite active-passive transition but soon after the transition they showed rather a transpassive region than a passive region with increasing potential indicating the passive film is non-protective in the melt. Ni wire specimen under freely corroding condition spontaneously passivated, which suggests Na2SO4 itself functions as an oxidizing agent. EPMA revealed that nickel sulfide was formed beneath surface oxide scale and in grain boundaries in freely corroding conditions as well as during potentiostatic anodic polarization at potentials in the passive and transpassive region. Discussion on the reduction mechanisms of sulfate ion is given in the text. It was interesting that Ni was cathodically protected at potentials slightly negative to the open circuit potential in the Na2SO4 melt