Influence of Ni, Cr and Ni-Cr20 coatings on the electrochemical parameters and corrosion behaviour of Ni–Fe–Cu anodes at 800 °C under aluminium electrolysis conditions

Kurzfassung

The electrochemical corrosion behaviour of uncoated, Ni, Ni-Cr20 and Cr-coated Ni42Fe38Cu20 wt.% anodes was investigated with respect to corrosion current density, wear rate and scale morphology in a NaF-KF-AlF3-Al2O3 melt at 800 °C, for up to 1 h of electrolysis. Ni-Cr20 and Cr-coated anodes were found to be the most resistant to oxidation in air due to Cr2O3 formation and reduced interdiffusion with substrate elements, whereas Ni-coated anodes demonstrated significant interdiffusion and oxide penetration. Electrochemical measurements of exchange current densities on the clean and oxidised anode surface were used to calculate the wear rate (WR), where the Ni-Cr20 coating was deemed to be detrimental to anode performance due to the catastrophic tendency towards electrode passivation via NiF2 formation. For the Ni and Cr-coated anodes, whilst a slight increase in the electrochemical parameters and wear rates was observed compared to their uncoated counterparts, SEM-EDX analysis of used-anode cross-sections revealed a significant reduction in electrolyte penetration for these two coatings as well as the formation of a more uniform oxide scale with less extensive blistering. Overall, the increase in corrosion parameters for the Ni and Cr-coated anodes appears to derive from the sacrificial dissolution of the coating material, whilst a noticeable degree of protection to the bulk is provided. The longer-term corrosion resistance implications with respect to changes in corrosion scale and desirable NiFe2O4 formation are discussed, and a robust research methodology for future studies on suitable coatings for inert anodes is displayed.