Peeling of Galvanized Coating
Peeling of the galvanized coating occurs when the outer free zinc layer separates from the intermetallic layers. When a coating thickness measurement is taken in an area that has peeled, there is some coating left, usually in the range of 2 to 6 mils. One cause of peeling is when a newly galvanized piece cools extremely slowly or when the steel is subjected to high temperatures (excess of 400F) for prolonged periods. When galvanized steel cools very slowly and the part remains in excess of 600F, the galvanizing reaction can continue. The eta layer serves as the source of zinc for further reactions and can be consumed by the galvanizing reaction. This consumption of the eta zinc layer can create voids between it and intermetallics, which then causes peeling of the outer zinc layer, as seen in Figure 1. When peeling is caused in this manner it is called the Kirkendall Effect.
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