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 commonly added to increase the strength of lightweight nonheat treatable Al alloys. 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, welding, or in-service exposure to elevated temperatures. Auger electron spectroscopy and transmission electron microscopy measurements show that alloys having a distribution of Al 3 Mg 2 (β phase) precipitates and segregated Mg on grain boundaries are more susceptible to cracking. To understand the roles of Mg on the cracking process we compared the corrosion potential and film formation of pure Al, Al implanted with Mg, a 7 wt % Mg–Al alloy and pure Al 3 Mg 2 phase. The surfaces of the specimens were cleaned and prepared in a surface analysis system and transferred in a vacuum transfer system to a corrosion cell. After solution exposure and electrochemical measurement the specimens were returned to the spectrometer and analyzed by x-ray photoelectron spectroscopy. The open circuit potentials for Al, Mg implanted Al, and the 7% alloy were nearly identical. However, the corrosion potential for the β phase differs significantly. The thickness of the film formed on each of the samples is similar. Mg is observed to be depleted in the outer part 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 β phase particles may be the most significant factor.