Matte Grey / Dull Coating
Also called: grey coating · mottled coating · reactive steel finish
Appearance
Uniform or patchy dark grey, matte surface — no bright spangle; sometimes a cellular or net-like pattern; often thicker than normal.
Root cause
Reactive steel chemistry (Sandelin-range or high silicon) driving zinc-iron alloy layers to grow through the free zinc layer; slow cooling of heavy sections has the same effect.
Acceptance verdict
Fully acceptable — appearance is not a rejection criterion where coating is continuous and meets thickness; grey coating usually EXCEEDS minimum thickness.
Fully acceptable — ISO 1461 explicitly does not require uniform appearance/brightness.
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
Faster cooling where practical; manage customer expectations at quotation when the MTC shows reactive chemistry.
Remedy at design
Specify steel chemistry at purchase: Si ≤ 0.03 % with P < 0.02 % (Category A), or the Sebisty range Si 0.14–0.25 % (Category B) where reactive steel cannot be avoided — per ISO 14713-2:2020 Table 1.
Clause references
- ASTM A123/A123M-24
- ISO 1461:2022
- ISO 14713-2:2020
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.