Standard · Systems, environments and degradation
ISO 4628
Rating the degradation of coatings: blistering, rusting, cracking, flaking and chalking.
ISO 4628 gives a common language for how far a coating has broken down, so two inspectors rate the same panel or structure the same way.
The parts an inspector meets
- Part 2, blistering: density (0–5) and size (S1–S5), reported together, for example 2(S3).
- Part 3, rusting: Ri 0 to Ri 5 against reference pictures.
- Part 4, cracking: density, size and depth.
- Part 5, flaking: density, size and depth.
- Part 6, chalking: a tape method rated against pictures.
Rust in numbers
In the 2024 edition of Part 3, the Ri pictures stand for a rusted area of about 0.05 % (Ri 1), 0.5 % (Ri 2), 1 % (Ri 3), 8 % (Ri 4) and 35 % (Ri 5). Measured, the Ri 5 picture shows about 35 %, although loose rust flakes make it look like 40–50 % to the eye. When no picture fits, a second method rates the rusted area in per cent, from R% 0 to R% 5. White rust on zinc and duplex systems has its own scales, WRi and WR%.
Blisters in practice
Size S1 is visible only at ×10, so the pictures show sizes 2 to 5 at densities 2 to 5.
ASTM D610 (rusting) and ASTM D714 (blistering) are the US counterparts. Their scales run the other way round, so always name the standard with the rating.
15 practice items in the members area mention ISO 4628 (CIP 1: 2 · CIP 2: 6 · FROSIO written: 7).
Related
Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.
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Also published openly: Coatings Radar: ISO 4628-3, ASTM D610 and European rust scales.
This page describes the subject of ISO 4628 in our own words; it does not reproduce the standard. For the full text, buy the current edition from ISO. Checked on 2026-10-14; errors go in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.