A galvanized rectangular tube can pass a quick visual check and still fail in service. The outcome is usually decided before the coil is ever formed: pre-treatment chemistry, welding parameters and roll-set accuracy all leave fingerprints that a buyer can detect at goods-in without a laboratory.
Why Processing Decides Performance, Not Coating Mass Alone
Two tubes stamped with the same zinc coating mass of 275 g/m2 total (both surfaces) can behave very differently after two years outdoors. One holds its coating through bending, punching and site handling; the other flakes at the cut edge and at the weld. The variable is not the number on the mill certificate. It is the condition of the steel surface entering the zinc bath, the continuity of the iron-zinc alloy layer that forms in the first seconds of immersion, and the thermal history of the strip afterwards. Coating mass confirms how much zinc is present; processing decides whether that zinc stays in place.
For cold-formed hollow sections the relevant product standards are GB/T 3091-2015 for welded steel pipe for low-pressure fluid transport, and GB/T 6728 for cold-formed hollow sections; coating requirements follow GB/T 13912-2022 for hot-dip galvanized fabricated steel products. GB/T 3091-2015 holds wall thickness to plus or minus 10 percent of nominal and outside diameter to plus or minus 0.5 mm on small diameters and plus or minus 1 percent on larger sizes. Those are the tolerances a fabricator must plan around when matching tube to a bolted connection.
Judgment Point 1: Zinc Coating Adhesion
Adhesion is the single best proxy for bath and pre-treatment control. A thin, bright, well-alloyed coating resists a knife or key scratch with only a polished line; a compromised coating lifts in flakes. Flaking points to three common causes: residual oil, scale or oxide on the strip surface before fluxing; an aluminium content in the zinc melt outside the usual 0.15 to 0.25 percent window that governs formation of a continuous Fe-Al transition layer; or strip speed and bath temperature drifting outside the window written on the line's process card.
On site, the practical checks are a hammering test and a bend test. GB/T 13912-2022 and ISO 1461 both require the coating to withstand handling and forming without lifting, and ISO 1461 requires an average coating thickness of 85 micrometres on steel at least 6 mm thick with a minimum local thickness of 70 micrometres. Tubes produced for structural duty in continuous galvanizing lines are frequently specified to EN 10240, which sets defined coating classes for tubes galvanized in automatic plants.
Judgment Point 2: Weld Seam Integrity
The seam is where most premature corrosion starts, because the coating over a reflowed or oxidised weld is the first to break down. A sound seam on galvanized rectangular tube is smooth, free of pinholes and slag entrapment, and fully zinc-covered with no bare patch on either the outside or the inside of the section. A raised bead, a black oxide line along the weld, or a bright uncoated strip signals unstable welding current, worn squeeze rolls or weld flash that was not dressed before galvanizing.
Two field checks are worth the effort: a dye-penetrant wipe along the seam after light wire brushing, and a 180-degree flattening test on a short cut length. A seam that opens or that shows a continuous line of exposed steel under flattening will corrode from the inside out in wet service, and no external coating repair will stop it.
Judgment Point 3: Cross-Sectional Accuracy
Dimensional error is a processing signature, not a coincidence. If a tube marked 40 x 20 mm measures 39 x 19 mm, the forming rolls are under-set, the strip width is wrong, or the strip has excessive camber. Corner radii that vary along the length indicate unstable roll pressure. Wall thickness measured at four points around the section should sit inside plus or minus 10 percent of nominal under GB/T 3091-2015; the diagonal difference across the section should stay close enough that the tube squares up in a jig without forcing.
Buyers who record these three measurements over a few deliveries build a trend line. Drift in wall thickness usually appears before drift in coating appearance, which makes it the earliest warning of a forming line that needs attention.
Bench Checks for Incoming Inspection
| Check | Method | Acceptance basis |
|---|---|---|
| Coating mass | Weigh-strip-weigh on a 50 x 50 mm sample | ASTM A90/A90M; EN ISO 1460 |
| Local coating thickness | Magnetic gauge, 5 points per face | EN ISO 2178; GB/T 13912-2022 |
| Adhesion | Hammering and 180-degree bend | GB/T 13912-2022; ISO 1461 |
| Wall thickness | Micrometer, four points per section | GB/T 3091-2015 |
| Weld seam | Visual plus dye penetrant, flattening test | GB/T 3091-2015 |
| Length and squareness | Tape and try-square | Purchase drawing |
Frequently Asked Questions
Q: How can I tell whether flaking zinc comes from pre-treatment or from forming?
A: Flakes that appear as blisters before any forming point to pre-treatment or bath chemistry. Flakes that appear only on sheared edges or on the outside of a bend radius point to an over-thick or poorly alloyed coating, or to bending at too small a radius for the coating class.
Q: What coating mass should I specify for outdoor rectangular tube?
A: For general outdoor structural work, 275 g/m2 total both surfaces is the common commercial specification and corresponds to about 19 to 20 micrometres per side, using a zinc density of 7.14 g/cm3. Heavier classes are reserved for coastal or industrial atmospheres.
Q: Does a heavier coating mean better adhesion?
A: No. Adhesion depends on surface cleanliness, bath aluminium content and an intact Fe-Al transition layer. A very heavy coating applied over a contaminated surface can fail earlier than a lighter, well-bonded one.
Q: Why does the weld seam rust first?
A: The seam has a different thermal history, a different surface profile and often a thin coating. Porosity or entrapped oxide in the weld allows moisture to reach bare steel beneath the zinc, so the coating cannot protect the steel sacrificially.
Q: Is the wall thickness tolerance the same for all tube sizes?
A: No. GB/T 3091-2015 applies a percentage tolerance to wall thickness and a split tolerance to outside diameter, so the absolute allowance in millimetres changes with size. Always convert the percentage before accepting a shipment.
Q: What documentation should accompany a delivery?
A: A mill certificate giving substrate grade, coating mass or thickness per face, and mechanical properties, plus records of the adhesion and dimensional tests listed above. Certificates without measured coating values are not evidence of coating quality.