Flux Inclusions & Flux Staining
Also called: flux burn · black flux spots
Appearance
Two modes, and they are not equally serious. INCLUSION: black/brown rough patches or crusty residue bonded into the coating, sometimes with bare steel underneath when picked off — flux stopped the zinc wetting the steel. STAINING: a broad translucent yellow-ochre film lying on top of otherwise sound coating, often with fine white crystalline speckles through it and the zinc's spangle still legible through the film, because nothing has been displaced. Look at the weld bead to tell flux from welding slag: with flux the bead is galvanized and the residue sits on top of sound zinc, whereas welding slag leaves a bare crusty ridge tracing the weld toe with no zinc on it at all.
Root cause
Flux carried on the work into the zinc (poor drying, flux too dense) or floating flux/skimmings contacting the work in wet-galvanizing practice; inadequate skimming. Flux liquor trapped in a crevice or joint can also weep out afterwards and run across finished coating, which produces the staining mode rather than an inclusion.
Acceptance verdict
ASTM draws the line by what the residue is, not how much of it there is. Flux DEPOSITS are listed among the things a shipped coating shall be free from — alongside uncoated areas and blisters, and with no 'gross' test, which flux is often wrongly given. Superficial bath skimmings riding on top of intact coating are the opposite case: expressly not cause for rejection, because the coating beneath is sound — they brush or weather off, or are ground off without taking the coating below its minimum. Where a removed deposit leaves bare steel, it is treated as an uncoated area and renovated within the standard's limits; beyond them the article is stripped and re-galvanized.
ISO 1461:2022 does not put flux residues in its shall-be-free-from list. It imposes a removal duty instead: flux residues and zinc ash shall be removed where they can affect the article's intended use or its corrosion resistance — with an access exception for surfaces that cannot be reached, such as inside hollow sections. Staining that affects neither is an appearance matter; where flux prevented coating, the bare area falls under the renovation limits, and past them the article is re-galvanized.
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. 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
Control flux bath density/iron/pH, ensure full drying before dipping, skim properly, review immersion speed.
Remedy at design
None.
Clause references
- ISO 1461:2022
- ASTM A123/A123M-24
- ASTM A780/A780M-20
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.