# Filiform corrosion

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

Thin, thread-like tracks under a thin film, spreading from a scratch or cut edge. On aluminium the tracks are white.

*Figure: Filiform corrosion: the surface as you see it, and a schematic cross-section.*

## Causes

- A break in the film, salts, and high humidity, roughly 70 to 95 % relative humidity.
- Thin organic films on aluminium, steel or magnesium.

## Prevention

- Good surface pretreatment and sealed cut edges.
- Avoid damage to the film.

## How it is assessed

Visual. Filiform tracks do not cross each other. Filiform corrosion needs a break in the film to start, so look first at cut edges, scratches and fasteners.

## Telling it apart

- **[Undercutting (underfilm corrosion)](https://coatmentor.com/defects/undercutting/):** Undercutting is a broad creep of rust under the film from a defect. Filiform corrosion is narrow threads, each with a growing head.
- **[Blistering](https://coatmentor.com/defects/blistering/):** Blisters are domes in the film; filiform corrosion is tracks along it.

## Repair

Remove the coating over the tracks, clean the metal and recoat. The specification and the coating manufacturer have the final word on the repair procedure.

**2** practice items in the members area mention filiform corrosion (CIP 1: 1 · CIP 2: 1).

## Related

- **Concepts:** [Galvanic corrosion](https://coatmentor.com/learn/galvanic-corrosion/)
- **Standards:**

## 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.