# ISO 4628

Source: https://coatmentor.com/standards/iso-4628/ · Updated 2026-10-14

Rating the degradation of coatings: blistering, rusting, cracking, flaking and chalking.

**Publisher**: ISO

**Area**: Systems, environments and degradation

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

- **Concepts:** [Reports, nonconformances and traceability](https://coatmentor.com/learn/inspection-reports/)
- **Standards:** [ASTM D610](https://coatmentor.com/standards/astm-d610/)
- **Defects:** [Blistering](https://coatmentor.com/defects/blistering/) · [Rust-through (rusting)](https://coatmentor.com/defects/rust-through/) · [Cracking](https://coatmentor.com/defects/cracking/) · [Flaking and delamination](https://coatmentor.com/defects/flaking/) · [Chalking](https://coatmentor.com/defects/chalking/) · [Undercutting (underfilm corrosion)](https://coatmentor.com/defects/undercutting/) · [Alligatoring (crocodiling)](https://coatmentor.com/defects/alligatoring/) · [Checking](https://coatmentor.com/defects/checking/) · [Fading and discoloration](https://coatmentor.com/defects/fading/)

## About this page

**Also published openly:** [Coatings Radar: ISO 4628-3, ASTM D610 and European rust scales](https://www.coatingsradar.com/general.html).

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](https://coatmentor.com/method/). CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.