Mechanical Aspects of Anti - Corrosive Coatings Performance Tests

Protective coating corrosion failure is a complicated process that involves various phenomena which are hard to test for separately. An adequately protected pipe can be in service for a relatively long time without exhibiting any signs of corrosion failure, and then all of a sudden it corrodes over a short period of time. Failure will definitely happen if the interface between the pipe surface and its protective coating system is somehow compromised or exposed to the environment. While corrosion reactions have quite a bit of variability in terms of time, the situation is further complicated when corrosion is coupled with the protective coating mechanical failure. Similar to materials fracture, corrosion failures are governed by the laws of probability, where multiple variables control the ultimate outcome. The coating system needs to be adequately tested before placing it in service, so there is a strong need for a meaningful corrosion test. A typical test would incorporate exposing scratched coating surface to certain corrosive environments for a fixed amount of time, trying to simulate the real conditions as close as possible. Sometimes, a so-called “freeze” step is introduced to “stress” the coating prior to exposing it to the test environment, ultimately causing its failure. Due to the differences in thickness, elastic, thermal and adhesion properties, various coatings would exhibit different results when exposed to a fixed freeze temperature. One needs to understand the mechanics of the freeze in order to properly utilize it. This paper considers the coating delamination and fracture mechanics, including the freeze step mechanics in terms of the strain energy release rate, and coating sub-critical debonding. This research is sponsored by the NACE University Research Seed Grant.