Three Characteristics Define Every Galvanized Product
Metal-coated steel from a hot-dip line is described by three characteristics: the base steel material, the coating type and the coating weight. Every national standard arranges those three in a slightly different order and uses a different code, which is why the same physical coil can carry three different-looking designations. Approximately half of the zinc produced worldwide is consumed by the surface protection of steel and stainless steel products, including the various zinc series coatings and a large volume of dipped steel components, so the designation systems matter to a very wide range of buyers.
The base steel is where the first decision sits. Commercial quality material is adequate for bending and profiling, drawing quality is required for deeper forming, and structural quality is required where the coated sheet carries load. In the Japanese system these appear as SGCC, SGCD and SGCE in JIS G3302; in the European system as DX51D through DX57D plus the S250GD to S550GD structural series in EN 10346; in the United States as commercial steel, drawing steel and structural steel in ASTM A653; and in China as the DX and S series in GB/T 2518.
Coating Type and Coating Weight
The coating type identifies the alloy, and the coating weight identifies how much of it is present. Pure zinc remains the most common, but 5% aluminium-zinc, 55% aluminium-zinc and zinc-aluminium-magnesium alloys are all supplied from the same lines. Coating weight is quoted in grams per square metre over both surfaces in the metric standards and in ounces per square foot over both surfaces in the ASTM system.
| Coating weight, both sides | JIS G3302 designation | ASTM A653 designation | EN 10346 and GB/T 2518 |
|---|---|---|---|
| 80 g/m² | Z08 | - | - |
| 120 g/m² | Z12 | G40, 0.40 oz/ft² | - |
| 180 g/m² | Z18 | G60, 0.60 oz/ft² | Z180 |
| 275 g/m² | Z27 | G90, 0.90 oz/ft² | Z275 |
| 350 g/m² | Z35 | G115, 1.15 oz/ft² | Z350 |
| 500 g/m² | Z50 | G185, 1.85 oz/ft² | Z450 to Z600 range |
The conversion factor is the trap. One ounce per square foot equals approximately 305.15 g/m², so G90 at 0.90 oz/ft² is about 275 g/m² over both surfaces. A buyer who reads G90 as 90 g/m² and then orders a metric equivalent of 90 g/m² receives roughly one third of the intended coating weight and a correspondingly shorter service life. The reverse error also occurs when a metric designation such as Z275 is read as an imperial figure, leading to an unnecessarily heavy coating and a cost penalty. Every purchase order should state the unit system next to the designation, not just the letter and number.
Marking and Additional Letters
Complete designations also carry letters for surface condition, passivation and oiling. Surface condition distinguishes normal spangle from minimized spangle and from a skin-passed surface, passivation distinguishes chromate from chromium-free treatment, and the oiling code records whether a protective oil film is present. A designation that omits these characters is incomplete and can result in a coil that meets the coating requirement but fails the paint adhesion requirement at the customer's line. Surface quality classes add a further distinction between general, improved and best appearance, which matters for visible parts and is frequently overlooked on orders for profiled sheeting.
Ordering, Testing and Pitfalls
Verify the base steel by chemical analysis and tensile test against the named standard, with JIS G3302 SGCC requiring a tensile strength of at least 270 MPa and EN 10346 DX51D specified at 270 to 500 MPa tensile with a minimum elongation A80 of 22%. Verify the coating by a triple-spot gravimetric test on specimens taken from the driving and non-driving ends of the coil, which is the method that detects an uneven coating before it reaches a forming line. Check adhesion with a bend or impact test, since a coating that passes the mass test can still flake at the radius if the intermetallic layer is too thick. Confirm spangle and surface condition visually against an approved sample, and confirm passivation by a suitable chemical spot test or supplier declaration. Alongside the designation, state the coil inner diameter, maximum weight, edge condition and packing method, and require a mill test certificate to EN 10204 3.1 with the heat number linked to each coil label.
Recurring Ordering Errors and How to Avoid Them
Three recurring errors account for most disputes. Reading an imperial coating designation as a metric value, as described above, is the first. Omitting the spangle and passivation codes is the second, since paint adhesion and appearance depend on them. Confusing structural and commercial base qualities is the third, because a coating that meets the specification on a commercial substrate still fails a structural requirement, irrespective of how heavy the zinc layer is.
Frequently Asked Questions
Q: What does SGCC Z27 mean?
A: SGCC is commercial quality hot-dip galvanized steel to JIS G3302, and Z27 indicates a coating mass of 275 g/m² over both surfaces, which is the same coating weight as G90 in ASTM A653 and Z275 in EN 10346.
Q: Why do the imperial and metric designations differ by about three times?
A: Because the units differ. G90 means 0.90 oz/ft², which converts to about 275 g/m². Reading 90 as a gram figure under-specifies the coating by roughly a factor of three.
Q: How is coating weight verified?
A: By stripping the coating from a known area of a specimen and weighing the mass loss, using samples from both ends of the coil. This triple-spot method catches uneven coating that a single sample would miss.
Q: Does a heavier coating always mean a longer life?
A: In the same environment, broadly yes, because zinc is consumed at a roughly constant rate. Coating type matters too, which is why an aluminium-zinc or zinc-aluminium-magnesium coating can outlast a heavier pure zinc coating in some atmospheres.
Q: What else must be stated on the order besides the coating designation?
A: The base steel grade and standard, spangle condition, surface quality class, passivation type, oiling, coil inner diameter, weight limit, edge condition and packing, together with the unit system used for the coating weight.
