It has been observed that during monsoon packed aluminium wire rod has tendency to attain black patches sometimes due to water seepage during transit or storage . What is this behavior of aluminium .
The most likely answer is 'tramp iron' staining of the oxide coating. This is caused by direct and indirect (through water seepage) of the aluminium with mild or medium carbon steels. It propagates in 2 ways;
Mechanically through iron oxides leaching into and staining the Alo2,
Electrolytically through moist contact with the steel.
One of the principal sources of the iron is the bandstrapping used to hold rolls of al. sheet or bundles of rods together. These should always be a soft stainless steel or heavyweight plastic for Aluminium