Effect of minor Ge addition on microstructure and localized corrosion behavior of Al-Zn-Mg alloy sheet

The effect of minor Ge addition on microstructure and localized corrosion behavior of Al-Zn-Mg alloy sheet was investigated. Ge leads to the formation of Mg2Ge phase, which can stimulate recrystallization nucleus, and a higher density of smaller Al3Zr dispersoids, which can inhibit recrystallization effectively. Consequently, a lower recrystallization fraction and smaller recrystallized grains are obtained in the Ge-added sheet. Ge reduces the density of η′ strengthening precipitates in the interior of grains but increase the length and spacing of η precipitates at grain boundaries. The corrosion potential is similar for Mg2Ge phase and the matrix, and therefore Mg2Ge phase and the matrix around it do not dissolve during immersion in NaCl solution. Corrosion immersion test and electrochemical test results show that Ge can improve exfoliation corrosion resistance. The reason has been discussed based on the modification of microstructure caused by Ge addition.