Z275, ISO 1461, ASTM A123, GB/T 13912: four names for one argument.

They are not interchangeable on a PO. A coil standard and a fabricated-part standard measure different objects.

The mistake that starts a claim

A buyer writes “Z275” and expects welded frames. Z275 is a coil designation: it describes the coating the mill applied to coil before it was formed. Once you weld it, the weld is not Z275. If the frame is welded and the requirement is Z275, the supplier either touches in the welds or ships bare welds — and both are technically “Z275 plus something else”.

How coating is measured

• Magnetic thickness gauge: quick, non-destructive, reads local thickness.
• Strip and weigh (gravimetric): destroys a sample, gives mass per area.
• Visual: coating uniformity, drain marks, bare spots, and whether cut ends were touched in.

An inspector usually does all three: gauge readings at defined points, one gravimetric sample, and a visual pass. The points that fail are almost always a weld, a cut end or a corner where the zinc could not drain.

Salt air and irrigation water

Zinc protects steel by sacrificial action, which means it consumes itself. In a coastal site with salt spray the coating lasts a fraction of what it lasts inland. That is not a defect — it is a design input. Where the environment is aggressive, specify a higher coating class rather than a thicker tube wall.