Metals corrode according to the galvanic series. Those high on the scale are most reactive when coupled with materials lower on the list. Magnesium is very high and gold is lowest. All other engineering materials are in between. Aluminum is below magnesium so if these two are in contact with an electrolyte (water, especially salt water), the mag corrodes first. If you look at alum alloys like 6061 or 2024, the 6000 series has mag as alloying constituent. Since mag and alum are close on the galvanic series, there isn't too much activity. With 2000 series, the alum is coupled with copper as alloying element, which is lower on the scale, so the reactivity is higher. Therefore, 2024 has higher galvanic activity and corrodes easier. So you have to protect 2000 series more than 6000 series. By cladding the reactive alum alloy with pure alum ( cladding) you reduce the galvanic activity at the surface and hopefully reduce corrosion.