NOT A COATING DEFECT

Distortion & Warpage

Also called: warping · twist after galvanizing

Thin galvanized steel plates hanging on wires after the kettle, buckled into deep S-curves and waves along their length while the zinc coating on them stays bright and sound

Stacked galvanized brackets, an open slot running along the plate-to-section joint on the nearest two, each traced with a red marker line — The same standard, a different failure: here the assembly stayed straight and the joint let go instead. A thin plate intermittently welded to a heavy section — the welds held, the unwelded lengths between them did not, and the plate lifted clear. Marked in red at inspection. The zinc is sound throughout.

Appearance

Members bowed, twisted, or panels oil-canned after galvanizing — plus a third mode that catches people out: the assembly stays straight but a joint lets go. Where a thin plate is intermittently welded to a heavy section, the unwelded lengths between the welds lift away from it and leave an open slot along the interface. In every case the coating itself is sound — what moved is the steel.

Root cause

Relief of residual fabrication/rolling stresses at 450 C, amplified by asymmetric sections, mixed thin/thick material in one assembly, and thin flat panels. In a mixed-thickness assembly the thin part reaches bath temperature and expands well before the heavy section does, and the joint between them has to carry that difference. Intermittent welds concentrate it: the welds hold, the unwelded lengths between them do not, and the thin plate lifts clear of the section.

Acceptance verdict

ASTM A123 & ISO 1461

Not a coating defect — a dimensional non-conformance judged against fabrication tolerances, not against ASTM A123 or ISO 1461. Prevention is a shared design/galvanizer responsibility under ASTM A384. Read the badge as whose defect it is, not how serious it is: a joint that has opened is a structural matter and goes back to the fabricator and the designer for assessment. It is not something to wave through because the zinc happens to be sound.

This is the question the standards actually answer — describe the defect, then decide: accept, repair, or reject.

Repairable?

Not an A780 repair case. Address it at the process, not with a patch — see the remedies below.

Remedy at the plant

Support long members properly on jigs; agree double-dip sequence for oversize work; control immersion speed.

Remedy at design

Follow ASTM A384: symmetrical sections, avoid large thickness mixes, temporary bracing on thin panels, stress-relieve critical weldments. Where a thin plate must meet a heavy section, continuously weld the joint instead of relying on intermittent welds — intermittent welds leave the plate free to lift between them. The gap that opens is also an unsealed overlap — A385's own warning: a gap too tight for zinc to enter holds pickling liquor instead and causes rust staining once the work weathers — so the same detailing decision costs you twice.

Clause references

ASTM A384/A384M
ASTM A385/A385M
ISO 14713-2:2020

Standard designations are the property of ASTM International and ISO. This page states the acceptance outcome for this condition — it reproduces no standard text or tables. Verify against the current edition at astm.org · iso.org.

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