Cause

Atmospheric Corrosion

The chemical reaction of a metal with the environment (that causes deterioration) is corrosion. Aluminum is a very reactive metal. Powdered Aluminum is used as rocket propellant!

Aluminum is naturally resistant to the environment due to the Aluminum oxide film that forms on it’s surface. The film is extremely thin but is sufficient to protect it from oxidizing elements such as water. When aluminum is coated however the oxide is absorbed into the coating reducing it’s barrier effect. If the coating fails it exposes the Aluminum to moisture.

 

Fix

Exposure to moisture is a common form of corrosion for Aluminum. All areas of the metal corrode at a similar rate. Over a period of time, the exposed metal undergoes oxidation by aggressive ions (such as chloride ions), until the metal fails, due to thinning.

If the metal is not too thin it should be cleaned with a weak acid solution. There are commercial products available such as Eastwood Aluminum Prep. More about that hereA Zinc rich primer should be applied followed by a quality enamel top coat, solvent or latex.

More information about coating Aluminum can be found here