How Is Zinc Coating Weight Tested on Galvanized Wire?

Zinc coating weight on galvanized wire is measured most reliably by the gravimetric strip method, which dissolves the zinc from a known length of wire and converts the mass loss to grams per square meter; this is the reference procedure in ASTM A90/A90M and EN 10244-2, and Aogang also offers non-destructive magnetic and XRF checks on its galvanized wire range. The number you get back, usually expressed as g/m², is the single strongest predictor of how many years the wire will resist rust before the steel core is exposed.

Summary. Coating weight is the mass of zinc per unit surface area, not a paint thickness, and it decides corrosion life. The audit-grade method is the gravimetric strip test standardized as ASTM A90/A90M and EN 10244-2; express methods such as the PCT (copper sulfate, or Preece) test and non-destructive magnetic or XRF gauges are used for incoming inspection. Aogang runs a dedicated metal products testing laboratory backed by a long-term SGS third-party testing contract and issues a complete factory test report for every batch.

Why coating weight, not thickness alone, decides service life?

Buyers often ask for coating thickness in microns, but weight in g/m² is what standards actually specify because it captures the total protective mass regardless of how evenly it sits on the wire. A heavier coating simply lasts longer in the field: in a mild rural atmosphere a 60 g/m² layer may give several years of protection, while a 200–300 g/m² heavy coating is what you want for marine, salty, or industrial environments where corrosion is aggressive. The relationship is not linear and depends on humidity, salt, and abrasion, but the rule holds that more zinc mass equals more sacrificial protection before the base steel starts to corrode.

Field experience from Aogang's projects shows the cost of under-specifying is rarely the wire itself but the downtime when a fence or structure rusts through years early. A buyer who orders the lightest class to save a small percentage on price often pays several times more in replacement labor. The practical approach is to match the class to the worst condition the wire will see, not the average, because corrosion is driven by the harshest exposure point such as a splash zone or coastal wind. Aogang's team can suggest a class from the application and local environment rather than leaving the choice to chance.

Which test method is the recognized reference?

The gravimetric strip method is the reference everywhere it is traded. In the United States the method is ASTM A90/A90M, in Europe it is EN 10244-2, and general hot-dip coatings reference ISO 1461. These standards all weigh the wire, strip the zinc chemically, weigh the bare steel, and calculate mass per area. The PCT or copper sulfate dip test is a fast field check that tells you only whether a minimum coating is present; it turns the wire black where zinc is stripped and leaves copper color where the coating is too thin. It is useful for a pass-or-fail screen on site but it does not produce a weight number, so it cannot replace the gravimetric result on a certificate.

How the gravimetric strip test is performed step by step?

The test is straightforward but must be done on a representative sample. First, cut a known length of wire and record its dimensions accurately. Second, degrease the sample and weigh it as received, call this W1. Third, dissolve the zinc in an inhibited acid such as antimony trichloride in hydrochloric acid, which attacks zinc but leaves steel largely untouched. Fourth, rinse, dry, and weigh the bare steel core, call this W2. The coating mass is W1 minus W2. The surface area comes from the wire length multiplied by its circumference, and the final result is reported as grams per square meter. Aogang performs this in its in-house laboratory and can also commission an SGS report when a buyer needs an independent certificate.

Can coating weight be checked without destroying the wire

Yes, and most incoming inspection is done non-destructively. Magnetic thickness gauges and X-ray fluorescence (XRF) meters read the coating on the finished wire in seconds and convert thickness to an approximate weight using zinc density of about 7.14 g/cm³. Eddy current probes are another option for production-line checks. These tools are fast and good for sorting a shipment, but they measure local thickness, not total mass, and they can be thrown off by a rough or uneven surface. For an audit, a dispute, or a first-article approval, the gravimetric strip remains the method everyone agrees on. Aogang uses both: non-destructive gauges for daily control and the strip method for the reported certificate.

How to read the result and what counts as a pass?

Standards usually set a minimum average coating across the sample and, in some cases, a minimum on any single point so that thin spots cannot hide inside a good average. Coatings are grouped into light, medium, and heavy classes. The table below shows typical minimum average values and where each class is normally used; Aogang supplies the full range including a high-zinc grade up to 300 g/m².

When you review a mill certificate, check that the reported average meets or exceeds the class you ordered and that the single-point minimum is not breached. A certificate that reports only one measurement is weaker evidence than one that reports average plus range across the coil.

What to put on the purchase order?

Being explicit on the order prevents most disputes. The table lists the parameters worth writing down, why each matters, and a workable example drawn from Aogang's standard offering.

What Aogang recommends for coating verification?

Aogang operates a dedicated metal products testing laboratory and holds a long-term quality inspection contract with SGS, so factory-finished product testing reports are complete and third-party reports can be arranged on request. Because all raw material comes from the Aosen Special Steel Group base, the substrate chemistry is stable, which keeps coating results consistent coil to coil. For corrosive duty, Aogang recommends specifying a heavy class in writing and requesting the gravimetric certificate rather than relying on a visual or dip test alone. Details of the test setup are on the testing capabilities page, and the FAQ covers how reports are delivered.

Frequently asked questions

Q: How often should coating weight be tested? 

A: At minimum on first-article approval and then by lot. Aogang issues a factory test report per batch, and you can add SGS third-party testing for critical projects.

Q: Is the copper sulfate (Preece) dip test enough on its own? 

A: No. It only shows whether a minimum coating exists at the tested spot. It cannot give you a g/m² value, so keep it for quick screening and rely on the gravimetric method for certificates.

Q: Does coating weight include both sides of the wire? 

A: The wire is round, so the measured g/m² is the total zinc mass over the full surface area of the sample, which is what protects the steel in all directions.

Q: Can I order a coating heavier than the standard class? 

A: Yes. Aogang supplies high-zinc galvanized wire up to 300 g/m² and can customize the class to match a specified environment or standard.

Q: How does coating weight relate to diameter tolerance? 

A: The zinc layer adds to the finished outside diameter, so tolerance should be specified on the coated wire. If you measure the bare steel you will read a smaller value than the ordered finished size.

Talk to Aogang about your galvanized wire specification

If you need galvanized wire with a verified coating weight for a corrosive or long-life application, Aogang can supply the class, the standard, and the test report to match. Send your requirement to aosenmetal123@gmail.com or use the contact us page, and the Aogang team will return a specification and a quote, typically within one working day