Hot-dip galvanized and electrogalvanized coatings contain uniform chemical composition through the entire coating thickness, unlike galvannealed coatings. Galvannealed surface phases, either delta or zeta, are created by different iron-zinc alloys and feature different characteristics. Compared to zeta-phase surfaces, galvannealed steels with a delta-phase surface exhibit lower friction and contain higher iron levels in the coating. Achieving delta as the surface phase requires the mill to use higher galvannealing temperatures and/or a slower line speed. These conditions promote an increase in powdering, or fracturing within the GA coating at the gamma-delta interface due to compressive or bending stresses. Lower iron content minimizes powdering. All of these conditions and constraints translate into a narrow time/temperature process window in order to achieve a lower friction delta surface with acceptable powdering performance.
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Coating Syntax Matters
ASTM A653/A653M provides for coating-weight minimum requirements. The specific per-side minimum requirements are a function of the total coating weight, as listed in ASTM Specification A653.
Different metallic coatings can be applied to steel surfaces, with each offering specific characteristics and designated by different letter codes and numbers. With different specifications, the descriptors can differ as well. Hot-dipped galvanized coatings of pure zinc may be abbreviated as GI, HDG or Z. Hot-dipped galvannealed coatings, which contain zinc-iron intermetallics, 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-silicon (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 galvanneal, GA. The number either before or after this letter represents the coating weight. Syntax matters here. For example, the weight of a G60 coating differs from one designated as 60G.
A notation with the number first followed by G or A as a suffix describes the single-side coverage in g/m2. For example, 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 galvanized 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 minimum 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. 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 protection, the ASTM A653/653M specification 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 sufficient for the application, greater coating 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 approximates 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 number represents the minimum total coating 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.
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Technologies: Materials
Specifying Hot Dip Galvanized Steel
For the vast majority of applications, hot dip galvanizing can be simply specified using the following reference: Hot dip galvanized to EN ISO : by a member of Galvanizer Association (Galco, Sperrin). This standard contains coating thickness requirements as shown in table below which will typically be sufficient to achieve acceptably long coating life.
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Minimum coating thicknesses achieved by hot dip galvanizing to EN ISO (not centrifuged)
Microstructure of a typical hot dip galvanized coating
Thicker Coatings by Surface Roughening
For instances where an increased coating thickness is required to achieve a longer coating life, or where work will be exposed in a severe service environment, steelwork may be grit blasted prior to galvanizing. This produces a coarse profile so opening up more steel surface to react with molten zinc resulting in a thicker galvanized coating. Typically the following reference may be used: Grit blast to Sa2½ with G24 chilled angular iron grit before hot dip galvanizing to BS EN ISO : to achieve a nominal coating thickness by a member of Galvanizing Association (Galco, Sperrin). It should be noted that the term `nominal` implies a target to be aimed for but not guaranteed, the value of which can vary with the steel section thickness, with upper limits being indicated in table below. For steel section thickness well in excess of 6 mm, a coating thickness in excess of 140 μm might be achieved.
Nominal coating thicknesses on samples that are grit blasted prior to hot dip galvanizing
Microstructure of a thick coating obtained by grit blasting steel prior to galvanising
Use of a Reactive Steel
The silicon (and to a lesser extent the phosphorous) content of a steel can affect its reactivity so causing the galvanized coating to grow more rapidly during immersion in the galvanizing bath. Typically steel with a silicon content in excess of 0.25% may be reactive and in the very rare cases where prior grit blasting will not achieve a sufficiently thick coating the specification of a steel`s chemistry can enable a still thicker galvanized coating to be specified using the following reference: Use a high silicon steel hot dip galvanized to BS EN ISO : to achieve a nominal coating thickness of 200 μm, by a member of Galvanizers Association (Galco, Sperrin). The 200 μm requirement would only apply for heavier steel section thicknesses. For light gauge steelwork a more realistic figure might be circa 120 μm. Where such a specification is used the coating may be comprised fully of zinc-iron alloy such that it will be dull grey in appearance and might be more susceptible to mechanical or handling damage. As a result, increased care should be taken when handling such product. You should contact your steel stockholder to discuss material availability.
Microstructure of thick coating obtained on a reactive steel
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You can find more information about Hot Dip Galvanizing on Galvanizing Association website
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