Low Coating Thickness
Also called: thin coating · under-thickness · below grade
Appearance
None. That is the whole point of this entry — coating thickness has no appearance. A thin coating on non-reactive steel looks bright, smooth and entirely normal, and no amount of looking will separate it from a conforming one. It is found with a gauge, a magnetic thickness meter or a cross-section, or it is not found at all. This is the one condition in the gallery you cannot photograph: the picture here is an instrument reading, because that is the only honest way to show it.
Root cause
The zinc-iron reaction never ran long enough to build the coating. On thin plate the usual culprit is insufficient immersion time — thin work heats quickly and gets run fast, and it is withdrawn before the coating has grown. Withdrawal too rapid, a bath below temperature, or steel at the low-silicon end of the range (which grows less coating than reactive steel) do the same thing. Where insufficient immersion is the cause, expect poor adhesion alongside it: the same shortfall that starves the thickness also starves the bond.
Acceptance verdict
There is no argument to have — the coating either meets the grade required for the article's steel thickness and category, or it does not. Grades come from ASTM A123/A123M-24 Table 1. But it is judged over a specimen, never from one spot: a specimen's thickness is the average of five or more widely dispersed readings, a specimen may sit one grade below the table if the article's average still makes the grade, and the standard says outright that no individual reading or cluster of readings is cause for rejection on its own. A single low number is the flag to measure properly, not the verdict.
Same discipline, different machinery. ISO 1461:2022 Table 3 sets a local minimum and a mean minimum by steel thickness — but 'local' means the AVERAGE over a reference area of at least 10 cm2, taken from at least five readings, and the standard states expressly that one gauge reading below the local value is not relevant on its own. A reference area averaging below the local minimum, or a control sample averaging below the mean, is non-conforming; a failed control sample is retested at twice the sample size before the lot is condemned.
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
Give the work the immersion time the reaction needs rather than the time the schedule wants — thin plate is the classic victim, because it looks ready long before it is. Hold bath temperature, control withdrawal speed, and gauge the work before it leaves. A thin coating found at the customer is a re-galvanize; found at the plant it is a re-dip.
Remedy at design
Little to do at design, beyond knowing the requirement is set by the steel's own thickness — a thinner section is entitled to less coating, and asking for more than the table requires needs saying so explicitly at order.
Clause references
- ASTM A123/A123M-24
- ISO 1461:2022
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.