Blisters & Pimples
Also called: hydrogen blisters
Appearance
Small raised bumps or blisters in the coating, intact when probed or occasionally hollow.
Root cause
Hydrogen absorbed during pickling erupting at galvanizing temperature (worst on hard/high-strength steel), or steel surface defects and subsurface inclusions opening at 450 C.
Acceptance verdict
Both standards prohibit blisters outright — this entry is stricter than most people expect. ASTM lists blisters among the things a shipped coating shall be free from, alongside uncoated areas and flux deposits, with no test of size or grossness. ISO defines them precisely — raised areas without solid metal beneath — and puts them in its shall-be-free-from list the same way. There is no acceptable-if-continuous reading: a blister is a void under the coating, and an article presented with them does not conform. Small localised blistering may be ground out and renovated within the renovation limits like any other bare area; widespread blistering is a strip-and-re-galvanize, and on hard or high-strength steel it is also the cue to investigate hydrogen before anything is re-dipped.
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 pickling time and acid strength (use inhibitors), avoid over-pickling hard steels.
Remedy at design
For high-strength (>~1100 MPa) components follow ASTM A143 embrittlement safeguards; consider abrasive cleaning instead of long pickling.
Clause references
- ASTM A143/A143M-22
- 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.