One of the most confusing aspects of coated steel sheet products is coating weight designations and what they mean, particularly with respect to product performance. This article is intended to clarify this issue.
Coating Weight [Mass] Designation Systems
The appropriate ASTM standard specifies the coating weight identification method for each coated steel sheet product. The most widely used ASTM metallic-coated sheet standard is A653/A653M, which covers hot-dip galvanised items. One of the coating weight identification methods in this standard uses G60, G90, and other descriptors. The letters “G” stand for galvanised (zinc) coating, while the numbers reflect the weight of zinc on the surface of the steel sheet in inch-pound (English/Imperial) units. According to the triple spot test, the coating weight on one square foot of sheet (total-both-sides of the sheet) must be a minimum of 0.90 ounces (TST). If uniformly coated on both sides of the sheet, there would be a minimum of 0.45 ounces per square foot of surface.
The other measurement system in widespread use today is the SI [Metric] system. The conversion from the inch-pound weight1 in ounces per square foot (oz/ft2) to the SI mass2, 3 in grams per square metre (g/m2) is:
1.00 oz/ft2 = 305 g/m2 To convert from oz/ft2 to g/m2 , multiply by 305 Example: G90 (0.90 oz/ft2 ) = Z275 (275 g/m2 )
Why don’t ASTM standards employ thickness measurements if we’re interested in coating thickness? The reason for this is that measuring thickness correctly and precisely is challenging. A G90 coating, for example, adds around 1.6 mils (0.0016 inches, or 42 microns) to the total thickness of the coated sheet. This translates to around 0.0008 inches (21 microns) of zinc on each surface for a coating evenly applied to both sides of the sheet.
The coating thickness must be measured, the coating peeled off, and the steel substrate thickness measured with a gauge capable of reading to the nearest tenth of an inch to accurately compute the coating thickness. This is extremely difficult to achieve with precision. Using the “weigh-strip-weigh” procedure, the most realistic manual method of measuring the quantity of coating present is to measure its weight [mass] on a bigger and specified surface area. The most representative manual method of determining the amount of coating present is to measure its weight [mass] on a larger and given surface area using the “weigh-strip-weigh” technique. On-line equipment that can do this non-destructively is available (see sidebar), but the most representative manual method of determining the amount of coating present is to measure its weight [mass] on a larger and given surface area using the “weigh-strip-weigh” technique.
Weighing-strip-weighing is a technique of determining weight [mass] loss that involves weighing a standard size sample of the product on a very precise scale, stripping the coating in an inhibited acid without removing any of the substrate, then reweighing the coupon. This is the original method for calculating coating weight [mass], and it’s currently used to examine and calibrate non-destructive on-line and laboratory coating thickness gauges as a referee method.
Any zinc-based coatings now in commercial production may be weighed using weigh-strip-weigh processes. ASTM defines these procedures for the most common commodities.
Galvanized and galvannealed sheet, 55 percent aluminium-zinc alloy-coated sheet, zinc-5 percent aluminium alloy-coated sheet, and zinc-aluminium-magnesium alloy-coated sheet are all covered by standard A90/A90M. Other forms of alloy coatings, such as aluminized and zinc-nickel alloy electroplated sheet, need unique methods. Other ASTM standards cover these topics.
Galvanized and Galvannealed Sheet Product Designation System energise – G30, G40, G60, G90, G115 are standard inch-pound coating weight designations for galvanised sheet (ordered as A653 or A1063). These designations describe the minimum average TST, total-both-sides, tested under ASTM A924/A924M, e.g., G90 demands a minimum average TST of 0.90 oz/ft2 total-both-sides. TST samples must be obtained at predetermined places at the edge-centre-edge of the as-coated sheet, according to the specification.
G165 and G210 are designations for thicker coatings that are utilised for particularly specific applications and are often not available on thinner gauge sheet.
In g/m2, the similar coating mass designations for galvanised sheet in SI units (arranged as A653M or A1063M) are Z90, Z120, Z180, Z275, Z350, Z450, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M, Z450M The minimum average TST, total-both-sides, tested per A924/A924M is specified by these designations, for example, Z275 needs a minimum average TST of 275 g/m2 total-both-sides. ASTM A653/A653M now includes the option of purchasing single side, single spot test (SST) coating designations.
The minimum and maximum allowed coating mass per side for every SST are specified by these SI designations (sorted to A653M). They take the traditional form of automotive coating designations (numerical characters first, indicating a need per side). Because single side coatings are generally purchased in SI quantities alone, no inch-pound designations are utilised.
Zinc-Iron (Galvanneal)
– For galvannealed sheet (zinc-iron alloy-coated), the standard inch-pound coating weight designations (listed as A653) are: A25, A40, and A60 A40, for example, demands a minimum average TST coating weight of 0.40 oz/ft2, total-both-sides, much like galvanised product designators. While the coating includes around 8% to 10% iron, the density is somewhat greater than zinc and the coating thickness is slightly less than G40 galvanise, the difference is too tiny to be of concern. In the section on 55 percent Al-Zn coatings, the impact of density is explored.
– The conventional inch-pound coating weight designations (specified as A653) for galvannealed sheet (zinc-iron alloy-coated) are as follows, in oz/ft2: The numerals A25, A40, and A60 can all be used to signify different things. In the same way, as galvanised product designators need a minimum TST coating weight of 0.40 oz/ft2, total-both-sides, A40 requires a minimum TST coating weight of 0.40 oz/ft2, total-both-sides. While the coating contains around 8% to 10% iron, the density is somewhat higher than a zinc coating, and the coating thickness is slightly lower than a G40 galvanise coating, the differences are minor. The influence of density is discussed in the section on 55 percent Al-Zn coatings.
Keeping ASTM Designation Systems Straight
With the increased usage of hot-dip galvanised and galvannealed coatings in the automobile sector, it became common practise to produce these goods to meet single side, SST g/m2 standards, as required by vehicle manufacturers. Total-both-sides, TST, inch-pound designations are still used to order products for construction and other general end purposes. As we’ve seen, ASTM utilises “G” (preceding the numbers) for inch-pound coatings and “Z” for SI coatings for hot-dip galvanise – total-both-sides in each instance. Electro galvanize, on the other hand, is denoted by the letter’s “G” (following the numerals) and “Z” (following the numerals) – single spot, single side in each case.
The employment of both dimensional units, as well as the reversal of “G” and “Z” between TST hot-dip and single side, SST EG in ASTM standards, would undoubtedly generate confusion in the marketplace.
TST Coatings on Both Sides vs. SST Coatings on One Side Because the ASTM total-both-sides SST requirements allow for an uneven coating split (one side SST will typically be at least 40% of the specified minimum SST total-both-sides coating weight), it’s impossible to convert them precisely to SST single side designators, which specify mandatory minimums per surface.
However, an approximate approximation based on the total coating thickness on both surfaces is occasionally beneficial. For example, a G60 coating has a minimum total both-sides thickness of around 1.0 mil, which is about twice as thick as a 90G90G coating’s minimum per side thickness (2 x 12.5 microns). Remember that a G60 coating is an average of three readings (TST) and can have an uneven coating thickness split, but a 90G90G coating must have a minimum of 12.5 microns on either side for any single area.
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