|MadSci Network: Chemistry|
The answer to your question "Why is AlMg3 better at preventing corrosion in marine environments than Aluminum alone" is: Magnesium is above aluminum in the electromotive series of metals, that is it tends to loose electrons (corrode) more readily than metals below it.
In the Mg2Al3 complex, the magnesium acts as a sacrificial anode, that is it corrodes and protects the aluminum. This same process is used to reduce the rate of corrosion in hot water tanks. A magnesium rod is placed in the tank, it corrodes and protects the metal in the tank. The process is also used in protecting underground pipelines. The extensive random precipitation of the Mg2Al3 throughout the metal grain boundary, and along slip planes caused by plastic deformation (working the metal i.e. bending and shaping) decreases the rate of localized attack. As a result, this alloy is generally as resistant as commercially pure aluminum and even more resistant to salt water and some alkaline solutions, such as carbonates and amines.
I hope I have answered your question.
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