Selection of Zinc Coatings

After the decision is made to use a zinc coating for corrosion protection, some factors must be considered to ensure that the proper coating is selected for the intended application and service environment. Obviously, zinc coating processes that are limited to small parts such as fasteners or other small hardware, or operations limited to continuous lines in steel mills, such as continuous galvanizing and electrogalvanizing, cannot be considered for the protective coating of structural steel members.

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.

It is also important to remember that for all continuous galvanized sheet materials, including electrogalvanized, the coating weight is given in weight per unit area of sheet. To obtain the amount of zinc per unit area of surface, the weight given must be divided by two, assuming equal distribution on both sides. For example, an ASTM A 653 Class G90 sheet contains 0.90 oz. zinc/ft2 of sheet or about 0.45 oz./ft2 on a surface. A G210 (2.10oz/ft2) sheet would have to be specified to obtain about 1 oz/ft2 on each side of the sheet.

While the coating densities for some of the different types of zinc coatings are nearly identical, others differ considerably. The coating densities, in terms of thickness required to equal 1 oz. of zinc per square foot of surface, are:

Hot-dip galvanizing (batch or continuous), electrogalvanizing, zinc plating	1.7 mils (43 µm)
Zinc Spraying (Metallizing)	1.9 mils (48 µm)
Mechanical plating	2.2 mils (55 µm)
Zinc-rich paint	3-6 mils (75 – 150 µm)

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 3-6 mils (depending on the paint formulation) of zinc-rich paint, assuming bond strength and edge protection are not factors.

The figure below lists the different types of zinc coatings and representative applications for each. While a coating is not limited to those uses listed, the applications represent the most common types of products coated by the process.

1 Total for both sides of sheet
2 Range based on ASTM minimum thicknesses for all grades, classes, etc., encompassed by the specifications.
3 Range based on ASTM and CSA minimum thicknesses for all grades, classes, etc., encompassed by the specifications.

Selection from the wide range of coatings available for steel will normally depend on the suitability of the coating for the intended use and the economics of the protective system. Factors that affect the economics for a particular application include:

Initial cost of the coating
Coating life to first maintenance
Cost of maintenance
Hidden costs, such as accessibility of the site, production loss due to maintenance re-coating, and rising wages for labor-intensive coatings, such as metal spraying and painting

The choice of the most economical system should include both an initial and life-cycle cost analysis. The American Galvanizers Association has developed an online calculator, which taking data from a paint industry survey conducted by KTA Tator, Inc. and a galvanized industry survey conducted by AGA, will provide an initial and life-cycle cost comparison of hot-dip galvanizing to a number of paint systems.