Galvanized Steel Wire: How to Specify Zinc Coating Weight in g/m2

Why the Word Galvanized Tells You Nothing About Coating

Two suppliers quote galvanized steel wire at the same price per ton and the buyer picks the cheaper one. One length lasts five years outdoors and the other shows red rust in one season, because galvanized is a process name, not a coating specification. The zinc coating on wire can range from about 10 g/m2 for a thin electroplated finish to over 300 g/m2 for a heavy hot dip coating, and that range is the entire service life difference.

If your purchase order says galvanized wire and nothing else, you are buying whichever coating the mill happens to be running. The fix is a single line on the specification: the coating class, or the coating weight in g/m2, plus a stated test method.

Reading a Zinc Coating Spec: g/m2, Micron and Class

Coating can be written in three ways and they convert predictably.

Unit	Meaning	Conversion basis
g/m2	Mass of zinc per unit of wire surface	Direct specification
Micron (um)	Coating thickness	1 um is about 7.14 g/m2 (zinc density 7.14 g/cm3)
Class / Type	Named coating grade in a standard	Defined by a g/m2 range

So a 60 um hot dip coating is roughly 428 g/m2, and a 20 g/m2 electroplated coating is roughly 2.8 um. Note that wire coating is usually quoted as total zinc on all surfaces, unlike sheet where per-side values are common, so always confirm the basis before comparing two quotes.

Standards define named classes: ASTM A641 uses Class 1, 2 and 3; EN 10244-2 uses classes such as A, B and AB; GB/T 3082 defines its own coating grades. When you buy to a class, ask the supplier to state the g/m2 range that class means, because class definitions vary between standards.

Coating Weight by Diameter: Where the Money Is

The required coating depends on the wire diameter and the environment. Thin wire needs a proportionally lighter coating simply because there is less surface to protect and the wire cannot carry a heavy zinc load without affecting mechanical properties.

Wire type	Typical zinc coating (g/m2)	Service environment
Electro galvanised, thin	10-25	Indoor, dry, decorative
Electro galvanised, heavy	25-60	Sheltered outdoor
Hot dip, light	60-120	Rural and urban outdoor
Hot dip, medium	120-200	Industrial and coastal
Hot dip, heavy	200-300+	Marine, high chloride

Electro galvanised wire has a uniform, bright finish and a thin coating; hot dip wire has a thicker, duller coating with better corrosion life. If the application is outdoors near the coast, heating the price down to electro galvanised is a false saving, and the customer will discover it in the first wet season.

Test Methods You Can Actually Ask For

Do not accept the word galvanized on a certificate. Specify a test.

Test	Method	What it proves
Coating weight	Gravimetric stripping, ISO 1460	Zinc mass per unit area, g/m2
Adhesion	Wrap test around a mandrel	Coating does not flake when bent
Salt spray	ISO 9227 neutral salt spray	Hours to first red rust
Coating thickness	Magnetic gauge (wire over 2 mm)	Thickness in micron
Uniformity	Preece or copper sulphate dip test	No bare or thin spots

The gravimetric stripping test is the definitive one: a known length of wire is stripped of zinc in acid, and the weight loss is converted to g/m2. Ask for it on the pre-shipment inspection, not only at the mill. Salt spray hours are useful for comparison, but clarify whether the result is to first white rust, first red rust, or five percent red rust, because the three give very different hour counts.

Surface Quality Acceptance: What to Reject

Coating weight is necessary but not sufficient. The surface has its own failure modes.

• Bare spots and uncoated patches. Reject; these are the start of red rust.
• Lumps and zinc drips. Excessive zinc build-up that flakes off and blocks the customer’s machine.
• Cracks in the coating. Usually from poor drawing after coating; affects fatigue life.
• Poor adhesion. Visible flaking after a simple bend, a hard fail.
• Diameter after coating. Confirm the finished diameter and its tolerance, and that it matches the ordered size.

Ask for a pre-shipment sample of every size, bend it over a mandrel by hand, and look at the outside of the bend. If the zinc flakes, the lot will not survive the voyage or the customer’s process.

Putting It on the Purchase Order

• Construction. Electro galvanised or hot dip galvanised, stated explicitly.
• Coating. Class and the g/m2 range, or the g/m2 range directly.
• Diameter and tolerance. Finished diameter and the tolerance standard.
• Tensile strength. Range in MPa, since galvanising can alter properties.
• Test and certificate. Stripping test to ISO 1460 and EN 10204 3.1.
• Packing. Coil weight, spool or coil form, and protection against white rust in transit.

Tell us the wire diameter, the coating class or g/m2 you need, the tensile range and the tonnage, and we will confirm what the mill can hold and quote with the stripping test and salt spray results included.