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Reason - the exterior cast aluminum WTC panels had been directly connected to the steel superstructure of the building, thus causing galvanic corrosion.
Originally posted by JIMC5499
I'll call this article "corn after it has been through a male cow's digestive tract". In my experience galvanic corrosion would eat through the aluminum instead of the steel. Ships actually have zinc electrodes that are designed to erode by corrosion to protect the steel in their hulls. I have seen many examples of this type of corrosion in aircraft structures. It is known in aviation as dissimilar metal corrosion. This usually occurrs around a steel bolt or bushing in an aluminum structure. Every example of it that I have seen has the aluminum pitted or fretted while the steel is in pristine condition.
Galvanic corrosion occurs when a galvanic cell is formed between two dissimilar metals. The resulting electrochemical potential then leads to formation of an electric current that leads to electrolytic dissolving of the less noble material. This effect can be prevented by electrical insulation of the materials, eg. by using rubber or plastic sleeves or washers, keeping the parts dry so there is no electrolyte to form the cell, or keeping the size of the less-noble material significantly larger than the more noble ones (eg. stainless-steel bolts in an aluminum block won't cause corrosion, but aluminum rivets on stainless steel sheet would rapidly corrode.