Originally Posted by
20grit
here i disagree. the architect in me kicks in and says aluminum and steel don't play well together. i've seen aluminum roofs that were damaged and the replacement roofing was galvanized steel. the steel was below the aluminum in the slope of the roof. the steel rusted like crazy. runoff from the aluminum causes serious oxidation of the steel, even galvanized stuff.
The shipwright in me says that anelectrolytic reaction took place between the zinc coating on the steel,the aluminium and the steel. Zinc being the least noble of the three metals will corrode first and expose the steel to oxygen which is what has caused the corrosion. Aluminium is in turn less noble than steel and will corrode next if the electrolytic reaction continues. Steel and aluminium of course will corrode even without an electrolytic reaction.
Ships and boats which live in the water have sacrificial zinc anodes for this reason.