What Does Undercutting Mean?
Undercutting is a type of adhesion failure that involves the gradual penetration of corrosion underneath unprotected edges. Undercutting refers to corrosion byproducts from the substrate building up under the coating and breaking the coating at edges or pinholes. Corrosion progresses under the coating.
Undercutting, like blistering and peeling, corrodes the substrate as well as damaging structure.
Undercutting corrosion of a scribed surface during testing is a sign of poor paint performance. This particular kind of corrosion can be quite damaging since it combines all the elements of a crevice corrosion scenario.
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Corrosionpedia Explains Undercutting
Undercutting coating failure often happens when a coating has been applied over mill scale. Moisture and oxygen enter the coating and react with the scale, causing it to lose adhesion, and thus forms progressive corrosion 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
- Smooth surface (no surface profile)
- Coating incompatibility with the substrate surface
Most undercutting failures can be substantially eliminated by proper surface preparation prior to the application of the coating and the use of a coating with strong adhesion characteristics through the use of inhibitive primer on the substrate prior to applications of top coat.
One of the favored methods of preventing undercutting on steel surfaces is the application of an inorganic zinc coating as a permanent primer before the organic coating is applied. The inorganic binder reacts chemically 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.
Coating should be applied over a surface that has been abrasively cleaned or cleaned by water jet blasting. Use of a zinc-rich primer reduces undercutting.