Definition - What does Undercut mean?

Undercut is the progression of rust or corrosion on a material under the coating (i.e., where the paint, primer or metallic coating is scratched out or removed and the metal is exposed to potential corrosion). It is associated with corrosion occuring in a material due to undercutting.

Corrosionpedia explains Undercut

Undercut corrosion or undercutting corrosion is often due to a coating failure or an inadequate application of metal coating materials such as paint, primer, or adhesive on the metal surface. Moisture and oxygen enter the coating and react with the scale, causing it to lose adhesion and thus corrosion progressively forms beneath the coating. The use of coatings over a very smooth, dense surface where adhesion is critical also creates a situation inviting this type of failure.

Undercutting is the result of poor adhesion due to:

  • Improper surface preparation.
  • A smooth surface (no surface profile).
  • Coating incompatibility with the substrate surface.

Most undercutting failures can be substantially eliminated by proper surface preparation prior to coating and by using a coating material with strong adhesion characteristics through the use of an inhibitor primer on the substrate prior to applications of top coat.

One of the favorite methods to prevent undercutting on steel surfaces is to apply an inorganic zinc coating as a permanent primer before the organic coating is applied. The inorganic binder chemically reacts with the steel, providing very tight adhesion and preventing the undercutting, which is characteristic of most organic paints or coatings. This coating system has been successful on hundreds of millions of square feet of surface exposed to extreme marine or industrial conditions.

The coating should be applied on a surface that has been cleaned abrasively or cleaned by water jet blasting. A zinc-rich primer reduces undercutting.

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