Effect of segregation on initiation of corrosion of aluminium sacrificial anode containing mischmetal

Abstract

The effects of rare earth (RE) elemental additions in the form of mischmetal, added into an Al–Zn–In sacrificial anode alloy, were studied in 3% NaCl solution. Scanning electron microscopy and energy dispersive X-ray analysis were employed to analyse the microstructure of all Al alloys. Potentiodynamic polarisation of the anode material and a model alloy that simulated the segregated grain boundary composition were also investigated. The results showed that the microstructure of Al–Zn–In–RE alloy exhibited a typical dentritic form with small precipitates in an Al matrix and eutectic in interdendritic regions. Preferential attack from anodic segregation produced the early pitting and minor element enrichment on the surface, thus activating the anodes. Thus, using a RE mischmetal, which is much cheaper than pure RE elements, Al–Zn–In–RE alloys obtained desirable electrochemical properties for Al sacrificial anodes.