Volta potential of second phase particles in extruded AZ80 magnesium alloy

Abstract

Magnesium alloys show strong susceptibility to localized corrosion when immersed in aggressive solutions (e.g. chlorides). The existence of second phase particles in the microstructure might represent initiation sites for localized corrosion. This is due to the formation of galvanic couples between the particles and the matrix. Extruded AZ80 magnesium alloy has been investigated by means of scanning Kelvin probe force microscopy (SKPFM) in order to measure the Volta potential of different phases relative to the matrix. The phases present in the alloy have been identified by optical microscopy and scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDXS). Three different phases were observed: Al 8Mn 5, Mg 2Si and Mg 17Al 12 (β phase). All phases exhibited positive Volta potentials relative to the matrix indicating a cathodic behaviour. The Volta potential depends on the composition of second phase particles. The Al 8Mn 5 intermetallics showed the strongest cathodic behaviour. Based on the SKPFM results, it is expected that the cathodic phases are effective sites for the initiation of localized corrosion in extruded AZ80 magnesium alloy.