# Edge corrosion (thin film at edges)

Source: https://coatmentor.com/defects/edge-corrosion/ · Updated 2026-10-14

Rust spots or streaks along cut edges, flanges and corners while the flat faces are still sound.

*Figure: Edge corrosion (thin film at edges): the surface as you see it, and a schematic cross-section.*

## Causes

- Wet paint pulls away from a sharp edge as it dries and shrinks, so the film on the edge is much thinner than on the face.
- No stripe coat on edges.

## Prevention

- Round sharp edges, for example to a 2 mm radius where ISO 8501-3 grade P3 applies.
- Apply a stripe coat to edges and let it reach touch dry before the full coat.

## How it is assessed

Visual; measure the film on and near edges.

## Telling it apart

- **[Steel surface imperfections](https://coatmentor.com/defects/steel-surface-imperfections/):** The sharp edge or the weld is the imperfection before coating. Edge corrosion is what it causes in service.
- **[Undercutting (underfilm corrosion)](https://coatmentor.com/defects/undercutting/):** Undercutting creeps under the film from a defect anywhere. Edge corrosion starts on the thin film of the edge itself.

## Repair

Prepare the edge, stripe coat it and recoat. The specification and the coating manufacturer have the final word on the repair procedure.

**8** practice items in the members area mention edge corrosion (CIP 1: 4 · CIP 2: 1 · FROSIO written: 3).

## Related

- **Concepts:** [Stripe coats](https://coatmentor.com/learn/stripe-coat/) · [Surface cleanliness grades](https://coatmentor.com/learn/surface-cleanliness/)
- **Standards:** [ISO 8501](https://coatmentor.com/standards/iso-8501/) · [NACE SP0178](https://coatmentor.com/standards/nace-sp0178/)

## About this page

Written from coating practice and the published rating standards, in our own words; the drawing is schematic. 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.