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The usual criterion for determining the expected service life of zinc coatings is thickness: the thicker the coating, the longer the service life. This is an acceptable criterion when comparing zinc coatings produced by the same process.

When comparing zinc coatings produced by different processes, the thickness criterion cannot be used without considering the amount of available zinc per unit volume. It is also important to keep in mind various ASTM or other specifications as they relate to coating weight or thickness, and reduce the coating requirements to a common denominator prior to making a comparison of different zinc coatings.

While the coating densities for some of the different types of zinc coatings are nearly identical, others differ considerably. The coating thicknesses required to equal 1 oz of zinc/ft² of surface, are displayed in the Coating Densities table below.

Each of these thicknesses, representing the same weight per unit area of zinc, would be expected to provide equivalent service life; i.e. 1.7 mils of hot-dip galvanized would give about the same service life as 2.2 mils of mechanical plating or 3 to 6 mils (depending on the paint formulation) of zinc-rich paint. It is also important to remember that for all continuous galvanized sheet materials, including electrogalvanized, the coating weight is given for the total zinc weight for both sides of the sheet. To obtain the amount of zinc per unit area of surface, the weight given must be divided in two, assuming equal distribution on both sides. For example, an ASTM A 653 Class G90 sheet contains 0.90 oz zinc/ft² of zinc or about 0.45 oz/ft² per side, as seen in the table below.

See Also: Zinc Coatings Publication