If the Al and Mg touch...
Tony
You might want to insulate the al repair from the Mg - or else you will have the old problem over again! Even a good paint layer will do - at least for quite a few years!
Galvanic corrosion affects all dissimilar metals in contact - and magnesium is about the most reactive (and therefore protective - used as sacrificial anodes on metal boat hulls, props etc. ) Mg will dissolve in contact with Al ( or steel or copper) ; Al will dissolve in contact with steel etc. - there is a galvanic table.
Stop the electrical ("galvanic") contact and no problem even when moisture (or worse - salt) is present.
Though Mg has been around for more than 80 years in transportation, and we all know the Beetle engine block, it is still widely used - the instrument panel support in your car is almost certainly a Mg casting!
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