Coating defect

Undercutting (underfilm corrosion)

Corrosion that spreads under the coating from a scratch, edge or holiday, lifting the film as the rust grows.

Undercutting (underfilm corrosion): the surface as you see it, and a schematic cross-section.

Causes

Damage, holidays or thin film at edges that let water reach the steel.
Rust, mill scale or salts left under the coating.
Poor adhesion of the primer.

Prevention

Prepare to the specified grade and test for salts.
Stripe coat edges; repair damage promptly.

How it is assessed

Measure how far the corrosion and loss of adhesion extend from the damage. Probe with a knife to find the edge of sound coating (outside CIP 1).

Telling it apart

Filiform corrosion: Filiform corrosion is narrow threads under a thin film, each with a growing head. Undercutting is a broad front of rust.
Cathodic disbondment: Under cathodic protection, coating lifts round a holiday because the current makes the steel surface alkaline. Undercutting is rust creeping under the film where no such current flows.

Repair

Remove all lifted coating back to firmly adherent coating, prepare the steel, feather the edges and rebuild the system. The specification and the coating manufacturer have the final word on the repair procedure.

6 practice items in the members area mention undercutting (CIP 1: 2 · CIP 2: 2 · FROSIO written: 2).

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.