Study on the effect of mischmetal (La,Ce) on the micro-galvanic corrosion of AZ91 alloy using multiscale methods

The relative potential difference can be treated as an index of local corrosion of alloys. Since local corrosion is driven by the micro-galvanic coupling, which is consist of the micro-constituent phases and the matrix. They present distinct potential difference normally. In the present work, the relationship between the Volta potential and the corrosion effect of the dominated intermetallic phase in the AZ91 Mg alloy with (La, Ce) MM addition was investigated. A multiscale approach coupling global and local measurements was used to study the corrosion behavior of the alloy and its intermetallic phase. Results showed that the microstructure of AZ91 Mg alloy with (La, Ce) MM addition was complex, with the presence of Al₄(La, Ce) phases in acicular or rod shape, a small number of Al₈Mn₄Ce, β-Mg₁₇Al₁₂ and α-Mg solid solution. In this case, the improved corrosion resistance of the alloy was achieved. Scanning Kelvin probe force microscopy (SKPFM) analysis revealed that all the intermetallics were noble comparing with the α-Mg solid solution. In-situ electrochemical atomic force microscopy (EC-AFM) observation for the initial stages of corrosion showed the α-Mg matrix surrounded by β-Mg₁₇Al₁₂ or Al₈Mn₄Ce was susceptible to pitting corrosion. No noticeable corrosion occurred at the α-Mg adjacent to Al₄(La,Ce) phase, in spite of the relatively higher difference of Volta potential between Al₄(La,Ce) phase and the matrix. Volta potential is insufficient to determine whether the phase is an effective cathode or anode.