Page 410 - Corrosion Engineering Principles and Practice

378 C h a p t e r 9 A t m o s p h e r i c C o r r o s i o n 379

FIGURE 9.49 Leaf-like exfoliation corrosion of corrugated A92024 aluminum
alloy. (Courtesy of Kingston Technical Software)

noble metal with an organic finish is sufficient to greatly reduce
galvanic couple corrosion. Due to the passive film on stainless steel,
it has been used in contact with aluminum with little expectation of
accelerated corrosion, despite the difference in potential.
As would be expected, constant exposure to moisture with a
limited supply of oxygen to the aluminum surface leads to the rapid
corrosion of any aluminum apparatus or equipment component. This
is due to the highly reactive nature of aluminum that leads to
formation of oxides or hydroxides. In the presence of oxygen, a
protective aluminum oxide film which is substantially unreactive,
develops on any aluminum surface. If the film is removed by
mechanical or chemical means and the aluminum exposed to water, a
rapid reaction sets in and the fresh aluminum surface is converted to
the hydroxide and subsequently to the oxide.

9.7.5 Zinc and Zinc Alloys
Zinc is exposed to the atmosphere in the form of galvanized sheet, as
in flashings on roofs; as die castings, and as coatings on steel, either
hot-dipped or electroplated. The general behavior of zinc metal and
zinc coatings is described in the ISO tables presented earlier. Note the
particularly low rates of attack on zinc as compared with steel in
marine exposures where chloride deposition is important. Such
excellent resistance is acquired by the hard, dense, protective products
of corrosion in a chloride atmosphere. Similar results cannot be
obtained in a sulfurous atmosphere where the corrosion products are
soft, voluminous, and non-protective.