Galvanized pipe coating weight: how to verify 350–500 g/m²
Short answer: coating weight is the mass of zinc on a measured area of pipe, written in
g/m². The reliable way to check it is the gravimetric test — cut a short sample, weigh it, strip the zinc off, weigh it again,
and divide the mass lost by the coated area (ISO 1460 / A90). A magnetic gauge gives a
fast reading in microns instead, so convert it: µm × 7.14 = g/m². Our hot-dip standard is
350–500 g/m² inside and outside, and the measured figure for each lot is printed on the certificate.
What the number actually means
Three units describe the same coating, and mixing them up is where most coating disputes start. g/m² is a mass per area (the language of EN orders), µm is a thickness (what a gauge shows you), and oz/ft² is the US way of writing the same mass per area. The bridge between mass and thickness is the density of zinc, about 7.14 g/cm³.
| Coating weight quoted | Thickness if it applies to one surface | Thickness if it is the inside + outside total | Same figure in oz/ft² |
|---|---|---|---|
| 350 g/m² | ≈ 49 µm | ≈ 25 µm per surface | ≈ 1.15 oz/ft² |
| 400 g/m² | ≈ 56 µm | ≈ 28 µm per surface | ≈ 1.31 oz/ft² |
| 450 g/m² | ≈ 63 µm | ≈ 32 µm per surface | ≈ 1.48 oz/ft² |
| 500 g/m² | ≈ 70 µm | ≈ 35 µm per surface | ≈ 1.64 oz/ft² |
Conversion rules worth memorising: g/m² ÷ 7.14 = µm · µm × 7.14 = g/m² · 1 oz/ft² = 305 g/m². Our hot-dip galvanized pipe is supplied at 350–500 g/m² inside and outside; pre-galvanized (coil-coated) tube is a different product at roughly Z100–Z275 on the strip — see hot-dip vs pre-galvanized.
Three ways the coating weight gets verified
Strip and weigh. Cut a short sample, degrease it, weigh it, strip the zinc with an inhibited acid solution, rinse and dry it, weigh it again. Mass lost ÷ coated area = g/m². This is the method in ISO 1460 and ASTM A90, and it is the number that stands up if there is a claim.
Magnetic thickness gauge. Fast, works on the loading dock, reads in microns. Calibrate it on the same curvature as the pipe, take 5–10 readings and average them, then multiply by 7.14 to get g/m². A gauge checks uniformity; it does not replace the gravimetric test.
Zinc consumption. The line records how much zinc a batch absorbed, divided by the surface area galvanized. It is a good process control on the day, but it is not a lot certificate — ask for the measured figure per lot.
How to run the check so the result stands
| Step | What to do | Why it matters |
|---|---|---|
| Sampling | One sample per lot, from the pipe body — not from the threads, the cut face or the last few centimetres at the end, where zinc builds up. | The sample has to be representative of what the whole lot carries. |
| Inside surface | For the inside coating, cut the sample lengthwise so the bore can be reached and stripped. | Inside coating is the part that cannot be seen, and the part that fails first in small sizes. |
| Area measurement | Measure the coated area after the sample is cut (length × outside circumference, plus the bore if the figure covers both surfaces). | A 5 % error in area is a 5 % error in the result — usually larger than the difference being argued about. |
| First weighing | Degrease with solvent and dry fully before weighing. | Oil, dust and moisture are counted as zinc and push the result up. |
| Stripping | Use the stripping solution the method specifies, and stop when the zinc is gone — not when the steel is being attacked. | Over-stripping removes steel and inflates the coating weight; under-stripping leaves zinc behind and understates it. |
| Balance | Use the same balance and the same procedure for both weighings. | Two balances, two errors, one argument. |
| Retest | Agree in advance what happens if a sample fails — retest from two other pipes, or reject the lot. | Agreeing the rule before the result exists keeps the shipment moving. |
When two measurements disagree
Per surface or both surfaces. The same coating is 400 g/m² or 200 g/m² depending on the convention. A factor of two, and the most common disagreement in the file.
A gauge on a hot-dip surface. Hot-dip zinc is rough; the gauge reads the peaks and usually reports slightly high. On a curved or small-diameter pipe, an uncalibrated gauge reads low.
The sample came from a thread or a cut end. Threading and cutting remove zinc, so a sample taken there always measures under the specification.
The area was taken from the drawing. Nominal outside diameter and nominal length are not the measured sample. Cut first, then measure.
The sample was not degreased. Oil from the threader, dust from the yard and a drop of rainwater all add mass that is not zinc.
Two products are being compared. Pre-galvanized coil at Z120 is not hot-dip at 400 g/m², whatever the quotation calls it — see EN 10255 vs ASTM A53 for how the ordering systems differ.
What to write in the purchase order
| Line to add | Example wording |
|---|---|
| Coating weight and convention | Zinc coating 350–500 g/m², inside and outside surfaces, both surfaces added together. |
| Test method | Coating weight determined by the gravimetric method to ISO 1460; magnetic gauge readings permitted for routine control only. |
| Sampling plan | One sample per lot, taken from the pipe body; the inside surface to be tested on a lengthwise-cut sample. |
| Acceptance and retest | If one sample fails, two further samples from the same lot to be tested; both must pass. |
| Inspection | Third-party witness of the coating weight test before shipment (SGS / BV / buyer's inspector). |
| Documents | Mill certificate stating the measured zinc weight per lot, plus the coating mass test report. |
A single sentence — “galvanized” — leaves the coating open to interpretation. Naming the figure, the convention and the method takes ten minutes at the enquiry stage and prevents the whole discussion at the port.
What we supply and what comes with it
| Product | Standard | Size range | Coating |
|---|---|---|---|
| Hot-dip galvanized round pipe | EN 10255 | DN8 – DN150 | 350 – 500 g/m², inside and outside |
| Hot-dip galvanized round pipe | ASTM A53 | ½″ – 8″ | Schedule 40 / 80, hot-dip galvanized |
| Threaded & socketed lengths | ISO 7/1 or NPT | DN15 – DN100 | Threaded before dipping, coupling fitted, caps on |
| Scaffolding pipe | EN 39 / EN 10219 | 48.3 × 3.2 / 4.0 mm | Hot-dip galvanized, 6 m or cut to order |
| Pre-galvanized square & rectangular tube | EN 10346 / ASTM A653 | 20×20 – 100×100, 40×80 … | Z100 – Z275 coil coating (dry indoor service) |
Every hot-dip lot ships with the measured zinc weight on the certificate. If your client requires a witnessed test or a third-party report, tell us at the enquiry stage and it is arranged at the galvanizing plant before loading — see current price tiers for how packing and documents are handled.
Questions we answer every week
How is galvanized coating weight measured?
Gravimetrically: degrease the sample, weigh it, strip the zinc with an inhibited acid solution, rinse, dry, weigh again, and divide the mass lost by the coated area. That is the method in ISO 1460 / ASTM A90. A magnetic gauge reads thickness in microns for quick checks — multiply by 7.14 to get g/m².
Is 350 g/m² per surface or both surfaces?
It depends on the specification and on how the enquiry was written — and the difference is a factor of two. Ask which convention the number follows and put it in the order. Our hot-dip supply is quoted as 350–500 g/m² inside and outside.
What thickness is 400 g/m²?
About 56 µm on one surface (400 ÷ 7.14). If the 400 g/m² is the inside-plus-outside total, each surface carries about 200 g/m², or about 28 µm.
Can I check the coating without cutting the pipe?
Yes — a calibrated magnetic gauge gives a fast reading along the length and spots thin areas. For the certificate, though, the gravimetric test is the one that is normally accepted, because it measures mass rather than thickness and can be witnessed by a third party.
Send the coating requirement with the size list
Tell us the standard, sizes, ends and the coating weight you need — and whether your figure covers one surface or both. We reply within 24 hours with the coating system, the certificate wording and a price.