Coating Weight Designations for Zinc-Coated
Sheet Steels

Many manufacturing industries use a variety of zinc and zinc alloy-coated sheet steels in their products, including electroplated and hot-dip-coated sheet. With the increasing use of metric units and the advent of new coating standards, some confusion has arisen over the definitions of and conversions between the terms and units used to specify coating weights, masses and thicknesses. This Technical Bulletin will define those terms and provide a quick reference for the conversions between the units found in the applicable ASTM standards.

Coating Mass and Coating Weight:
Although mass and weight are technically different, they can be considered equivalent for all practical purposes such as metallic coatings. The main differences are in the units used to specify the amount of coating. Electroplated coatings such as zinc and zinc alloys are usually designated in units of mass per unit area on each side of the sheet, or more specifically, grams per square meter (g/m²) per side of sheet. For example, a 60G60G product would have 60 g/m² of zinc on each surface. On the other hand, hot-dipped coatings such as galvanized are given in units of weight per unit area (oz/ft²) on both sides of the sheet. For example, a G90 coating means that there would be a total of 0.90 oz/ft² of zinc on both sheet surfaces added together. To convert from oz/ft² of coating on both sides of the sheet to g/m² of coating on both sides of the sheet, multiply by 305.

Electrogalvanized coatings are specified in the United States based on ASTM Specifications A591 and A879. A591 is an older standard for very thin electrogalvanized coatings and, therefore, will not be considered here; all conversion and definitions given apply equally to A591. Hot-dipped galvanized coatings are described generally in Specification A525. Typical designations are given below:

Electroplated Coatings:
Electroplating can be controlled to produce different coating masses on each side of the strip and this is reflected in the standards. For example, a zinc coating specified to 40G20G under A879 means that one side will have a minimum of 40 g/m² of zinc and the other a minimum of 20 g/m². Electrogalvanized coating masses are generally specified to the nearest whole number, i.e., 60G60G, 40G20G, etc. The number used in the designation is the minimum expected amount of zinc on each sheet surface. Electrogalvanized sheet has not generally been used with coating masses in excess of 100 g/m² per side of sheet.

Hot-dipped Coatings:
A hot-dipped coating as specified in A525 denotes the total amount of zinc to be found on both sides of the sheet. ASTM A525 further refines a hot-dipped galvanized coating weight distribution with the stipulation "…the minimum triple-spot average coating mass on any side shall not be less than 40% of the single-spot test requirement". For example, the single-spot check limit for G90 is 0.80 oz/ft², so 40% of that value is 0.32 oz/ft². This means that a G90 hot-dipped galvanized coating will have no less than 0.32 oz/ft² on each side of the sheet. In most cases, the coating is relatively equally divided between both sides of the sheet.

There is also a metric version of A525, numbered A525M, which defines hot-dipped galvanized coatings in metric units following guidelines similar to those in A525. Coatings specified in this manner have a prefix letter of Z. For example, a Z275 coating would have 275 g/m² of zinc total on both sides of the strip.

A Word of Caution:
Although we have categorized the two coating systems as electroplated and hot-dipped, obviously the designations are interchangeable. We are starting to see hot-dipped products ordered with coating amounts in units of g/m² per side of sheet. Care must be taken not to confuse, for example, 60G60G with G60…the amount of zinc actually specified for each is very different.

Coating Thickness:
Although there are very slight differences in the densities of the coatings, electrogalvanized and hot-dipped galvanized can be considered to have equal densities for the purpose of coating thickness conversions. An electrogalvanized coating is essentially pure zinc which has a density of 7.14 g/cm³. By using this value and other conversions, coating weights and masses can be converted to coating thicknesses as shown in the quick reference conversion table below. Some useful conversion factors and definitions are listed below.

The conversion table can also be used to convert from coating mass or weight to thickness for alloy coatings. These alloys include zinc-iron and zinc-nickel, among others. Although the densities are different for the alloy coatings, the variations are minimal for the alloys currently finding widespread use; e.g., 10-18% Fe-Zn and 11-13% Ni-Zn. For most calculations or specification purposes, the values contained in the table should be sufficiently accurate.

¹ Coating mass, weight and thickness values reflect the amount of zinc present on one side of the sheet.

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