Bare Spots (Uncoated Areas)
Also called: misses · black spots
Appearance
Exposed steel with no zinc — from pinhead spots to patches; edges of the surrounding coating are usually sound.
Root cause
Two different failures produce the same bare steel, and they belong to different people. CONTAMINATION THE PICKLE CANNOT REMOVE — paint, oil, weld slag, anti-spatter, marker ink, rolled-in mill defects — arrives on the steel already there, and is the fabricator's to keep off it. INCOMPLETE PICKLING is the galvanizer's own: the iron oxide and mill scale were perfectly removable, but the pickle did not remove them — acid too weak or too cold, immersion too short, iron loaded up in the bath, or the work shielded where it was bundled — so the zinc meets oxide instead of steel and never wets it. Also flux failure spots, and work touching work in the bath. Which family you are looking at decides who pays, so establish it before the argument starts.
Acceptance verdict
Repairable within A123 limits, and they are tighter than most people remember. Each uncoated area may exceed 25 mm (1 in.) in only ONE of its two dimensions — a long narrow miss can pass where a square patch of the same area fails. Total uncoated area: at most 0.5% of the accessible surface AND at most 256 cm2 per metric ton (36 in.2 per short ton) of piece weight. Repair per A780: metallizing or zinc solder to the full Table 1 grade; zinc-rich paint at 75-100 um. Beyond the limits, or where the bare area cannot be reached for repair, the article is rejected for stripping and re-galvanizing. The size limits bind only at the galvanizing facility — once the work has shipped, damage found later has no size cap on renovation.
Repairable when the total bare area is at most 0.5% of the article's surface and each single area at most 10 cm2; beyond that the article is re-galvanized unless purchaser and galvanizer agree otherwise. Repair by zinc-rich paint, zinc paste or alloy stick, or thermal spray, to a minimum average of 100 um unless otherwise agreed — for example matched to the surrounding coating where the article will be painted over.
This is the question the standards actually answer — describe the defect, then decide: accept, repair, or reject.
Repairable?
Yes — within the standard's renovation limits, using ASTM A780 methods: zinc-alloy solder, zinc-rich paint, thermal spray. Repair only within the size and total-area limits set by the governing standard; beyond them the article is re-galvanized.
Remedy at the plant
Split the two causes and fix the right one. For contamination arriving on the steel: inspect incoming work for paint, marker and slag and reject it to the fabricator BEFORE pickling, and verify degreasing performance. For incomplete pickling — the one that is yours: hold acid concentration and temperature, give the work the immersion the scale actually needs rather than the schedule's, watch the iron loading, and hang work so the acid reaches every face instead of bundling it. Maintain the flux bath (density, iron, pH).
Remedy at design
Use water-soluble welding fluxes or grind slag; no paint or grease markers on steel bound for the kettle.
Clause references
- ASTM A123/A123M-24
- ASTM A780/A780M-20
- 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.