Coating defect

Edge corrosion (thin film at edges)

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

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: The sharp edge or the weld is the imperfection before coating. Edge corrosion is what it causes in service.
Undercutting (underfilm corrosion): 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).

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.