Influence of Mg on the corrosion of Al

This article summarizes a series of experiments to determine the influence of Mg on the corrosion and electrochemical behavior of Al. Magnesium is com monly added to increase the strength of lightweight nonheat treatable Al al loys. However, these alloys are susceptible to grain boundary dissolution, stress corrosion cracking, or hydrogen induced embrittlement due to changes in the alloy structure and elemental distribution during processing, weldi ng, or in-service exposure to elevated temperatures. Auger electron spectro scopy and transmission electron microscopy measurements show that alloys ha ving a distribution of Al3Mg2 (beta phase) precipitates and segregated Mg o n grain boundaries are more susceptible to cracking. To understand the role s of Mg on the cracking process we compared the corrosion potential and fil m formation of pure Al, Al implanted with Mg, a 7 wt % Mg-Al alloy and pure Al3Mg2 phase; The surfaces of the specimens were cleaned and prepared in a surface analysis system and transferred in a vacuum transfer system to a c orrosion cell. After solution exposure and electrochemical measurement the specimens were returned to the spectrometer and analyzed by x-ray photoelec tron spectroscopy. The open circuit potentials for Al, Mg implanted Al, and the 7% alloy were nearly identical. However, the corrosion potential for t he beta phase differs significantly. The thickness of the film formed on ea ch of the samples is similar. Mg is observed to be depleted in the outer pa rt of the oxide films, but somewhat enhanced near the oxide-metal interface . The results suggest that segregated Mg plays little role in the cracking and that hydrogen production at the beta phase particles may be the most si gnificant factor. (C) 2000 American Vacuum Society. [S0734-2101 (00)01801-7 ].