Page 43 - MetalForming December 2019

metallics, are abbreviated as GA, HDGA or ZF. Electrogalvanized coatings are shown as EG, EGL or ZE. Other types of coatings used on steel for corrosion protection include an aluminum-sili- con (AS or AlSi) alloy commonly found in press hardening applications, and a zinc-aluminum-magnesium (ZAM or ZM) alloy now gaining in applications where cut-edge corrosion is a concern.
As for coating weights, G refers to pure zinc, as in the EG abbreviation for electrogalvanized or GI for hot-dip galvanized. The letter A refers to gal- vanneal, GA. The number either before or after this letter represents the coating weight. Syntax matters here. For exam- ple, the weight of a G60 coating differs from one designated as 60G.
A notation with the number first fol- lowed by G or A as a suffix describes the single-side coverage in g/m2. For exam- ple, a 60G coating signifies a minimum coverage of 60 grams of galvanized zinc on each square meter on each side of the steel. A notation such as 98G60G
indicates a minimum of 98 g/m2 of gal- vanized zinc on one surface and 60 g/m2 of galvanized zinc on the other surface.
Specifying G40, or any coating where the letter comes first, indicates a min- imum total zinc-coating coverage on both sides combined in oz./ft.2 divided by 100. For example, a G40 coating has a minimum total coating coverage of 0.40 oz./ft.2, or an average of 0.20 oz./ft.2 on each side.
Knowing the density of pure zinc, 446 lb./ft.3, allows for the conversion of the coating coverage to a thickness of 0.00034 in. per side, or 8.6 microns per side. Remember that only the total coverage must be 0.40 oz./ft.2, and it is reasonable to assume that each side will not measure exactly 0.20 oz./ft.2. To ensure sufficient corrosion protec- tion, the ASTM A653/653M specifica- tion also calls out a minimum single- side measurement as well. For G40, each individual side must measure at least 0.12 oz./ft.2, or slightly more than 5 microns. If this thickness isn’t suffi-
cient for the application, greater coat- ing coverage must be specified.
These differences in syntax mean that a 60G coating measures about the same as a G40 coating. The 60 g/m2 on one side equals 120 g/m2 combined top and bottom, which converts to 0.394 oz./ft.2, about equal to a G40 coating. As another example, a G60 coating weight signifies 0.30 oz./ft.2 of zinc on each surface, which approxi- mates a 90G coating weight.
In Europe, one or two letters (such as Z or ZF representing zinc or a zinc- iron-alloyed coating) followed by a num- ber represents the minimum total coat- ing mass combined on both surfaces expressed in g/m2. Using this notation, Z100 represents a hot-dipped galvanized zinc with a total of 100 g/m2 combined on both surfaces. In practice, the coating weight does not divide exactly evenly between the two surfaces.
Have ideas for topics in future columns? Email me at scienceofform- ing@eqsgroup.com. MF
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