What the G60 and G90 Designations Actually Mean
In ASTM A653/A653M, hot-dip zinc-coated steel sheet is ordered by coating designation rather than by coating thickness. The letter G identifies a pure zinc coating produced on a continuous hot-dip line; the number that follows is the minimum coating weight expressed in ounces per square foot of sheet, total for both surfaces. G60 therefore means 0.60 oz/ft2 minimum and G90 means 0.90 oz/ft2 minimum. Because 1 oz/ft2 equals approximately 305 g/m2, the two designations convert to about 183 g/m2 and 275 g/m2. That is the reason G60 is quoted against the metric equivalent Z180 and G90 against Z275 under EN 10346 and ISO 3575. The coating weight is a minimum, verified on a triple-spot basis under ASTM A924/A924M and measured by the stripping method of ASTM A90/A90M, not an average of the coil.
Coating Weight, Thickness and Typical Applications
| Designation | ASTM A653 coating weight | Metric equivalent | Nominal zinc per side | Typical exposure |
|---|---|---|---|---|
| G40 | 0.40 oz/ft2 | Z120 | approx. 8 um | Interior panels, dry indoor use |
| G60 | 0.60 oz/ft2 | Z180 | approx. 13 um | Indoor appliances, ducting, non-washed parts |
| G90 | 0.90 oz/ft2 | Z275 | approx. 19 um | Roofing, outdoor structures, coastal-adjacent builds |
| G115 | 1.15 oz/ft2 | Z350 | approx. 25 um | Severe industrial and humid service |
The thickness figures follow from the density of zinc, 7.14 g/cm3: a coating mass of 137.5 g/m2 per side equals about 19 um of zinc. Buyers who specify coating thickness in micrometres rather than mass should confirm which convention the mill is using, because a Z275 coil measured on one side only will read about half the total mass.
Corrosion Performance and Service Life
Zinc protects steel in two ways. It acts as a barrier that keeps moisture and chlorides away from the substrate, and it provides sacrificial (cathodic) protection at cut edges and scratches, where the zinc corrodes preferentially and the exposed steel remains protected until a substantial amount of adjacent zinc has been consumed. Doubling the coating mass does not double service life in every environment, but time to first red rust increases roughly in proportion to zinc mass in rural and urban atmospheres. In marine or heavily industrial atmospheres the consumption rate is higher and G90 or a heavier coating becomes the practical minimum. Where the part is formed after coating, coating adhesion and the risk of micro-cracking matter as much as coating weight.
How to Specify G60 or G90 Correctly
An order that leaves the coating grade open will normally be filled with the lightest acceptable coating. A complete specification should state: the coating designation (G60 or G90), the applicable product standard (ASTM A653/A653M or the EN 10346 / ISO 3575 equivalent), the base-steel grade such as DX51D, SGCC or SPCC, the surface treatment (chemically passivated, oiled, anti-finger print, or untreated), the spangle condition (regular, minimized or skin-passed), and the mechanical properties required for the forming operation. Table 1 the coating-type range in ASTM A653 also covers zinc-iron alloy (galvannealed, "A" prefix) coatings, which are preferred for painted parts because the matte, phosphateable surface accepts paint and welds better than pure zinc.
Frequently Asked Questions
Q: Is G90 better than G60 for every application?
A: No. G90 lasts longer outdoors, but it costs more and the heavier coating can flake during severe bending, so interior and lightly formed parts are usually specified as G60.
Q: Does G90 equal Z275 exactly?
A: They are commercial equivalents, not identical clauses. G90 is a minimum of 0.90 oz/ft2 under ASTM A653, which converts to about 275 g/m2, and Z275 is the EN 10346 designation for the same nominal coating mass.
Q: How is coating weight verified on a delivered coil?
A: By the stripping method of ASTM A90/A90M, with acceptance under ASTM A924/A924M. Instruments that measure coating thickness require calibration to the same mass basis.
Q: Can a G60 coil be supplied with a Z275 marking?
A: The marking must follow the standard actually applied. Buyers should ask for the mill test certificate to state the coating designation and the measured triple-spot coating weight.
Q: Which coating suits prepainted (PPGI) production?
A: A G90-equivalent or heavier zinc coating, or a galvannealed surface, gives the paint system a stable base; the zinc is normally chemically passivated before the primer is applied.
Q: Does the base-steel grade change the corrosion life at equal coating weight?
A: Only indirectly. At identical coating mass the zinc determines time to red rust; the substrate grade mainly controls strength and formability, and any difference in surface roughness changes how quickly the coating is consumed.