Coating defect

Filiform corrosion

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

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): Undercutting is a broad creep of rust under the film from a defect. Filiform corrosion is narrow threads, each with a growing head.
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).

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.

Take the free readiness check See the tracks

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. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.