Zinc Flaking / Poor Adhesion
Also called: peeling · delamination · coating shedding
Appearance
Flat patches where the bright free zinc has come away and left the darker, duller zinc-iron alloy layer showing beneath. The trap is that it barely looks like flaking: the free zinc layer is only tens of microns thick, so what it leaves is a change of TONE with an angular boundary, not a lifted flake casting a shadow. Look at texture rather than relief — the shed area is smooth and uniform where the surviving zinc is chalky and granular, and that difference is two materials, not one. In worse cases the coating sheds in visible flakes or lifts under a blade.
Root cause
The coating never formed a sound metallurgical bond to the steel. On thin plate the usual cause is insufficient immersion time: the work is withdrawn before the zinc-iron alloy layers have grown, so the free zinc rests on an underdeveloped bond and sheds. Expect low coating thickness on the same article — one cause, two symptoms, because the reaction that builds the thickness is the same reaction that builds the bond. (The opposite extreme also flakes: an over-thick, brittle alloy layer on reactive steel loses adhesion for the reverse reason.)
Acceptance verdict
Both standards land in the same place — a coating that peels or flakes under normal handling fails, and no thickness reading rescues it. ASTM states it as a requirement: the coating shall withstand handling consistent with its nature and thickness and the article's normal use, without peeling or flaking; its referee test is prying with a stout knife, and flaking off in layers ahead of the point is the defined failure — while paring small particles at an edge or corner is expressly NOT one. ISO puts it that adequate bonding is characteristic of galvanizing itself and the article should withstand normal handling without peeling or flaking, with adhesion testing only by prior agreement — and bending or forming after galvanizing does not count as normal handling. Not an A780 case: this is a failure of the coating's bond over an area, so the article is stripped and re-galvanized.
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
Immersion time — give the reaction long enough to build the alloy layers, and resist running thin plate fast because it looks ready early. Hold bath temperature. If an article is shedding, gauge it too: the same shortfall usually shows up as under-thickness, and finding both confirms the cause rather than leaving it to argument.
Remedy at design
Little at design. Where reactive steel is unavoidable and the alloy layer runs thick and brittle instead, specify chemistry at purchase — see the Steel Reactivity Checker.
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.