Paint undercutting accelerated test (PUCAT)—test development and mechanistic information

Undercutting is paint film blistering resulting from accelerated corrosion of the metallic coating on painted ZINCALUME ® (55% Al-Zn alloy coating on a steel substrate) or galvanized steel. It is caused by galvanic coupling of the metallic coating and steel substrate at an exposed edge, causing a crevice under the paint film. Once the crevice has formed, undercutting can continue by a crevice corrosion mechanism. This paper describes the development of a laboratory Paint UnderCutting Accelerated Test (PUCAT), in two versions—full immersion and cyclic (wet/dry) PUCAT. The relative importance of galvanic cell and crevice corrosion mechanisms is discussed for these tests, the ASTM standard salt spray test and for an atmospheric severe marine exposure site along with types of paint film blister pattern and corrosion of the metallic coating. It is shown that the salt spray cabinet is the most aggressive, with indications that the crevice corrosion mechanism becomes dominant for ZINCALUME. Full immersion PUCAT and cyclic PUCAT are not as aggressive as the salt spray test, but can be designed to be of similar aggressiveness to each other. Cyclic PUCAT results in a finer paint film blister pattern than either full immersion PUCAT or the salt spray test, and more closely resembles that which occurs at the atmospheric severe marine exposure site. Indications are that the crevice corrosion mechanism operates to a greater extent in full immersion PUCAT than in cyclic PUCAT. Cyclic PUCAT is the most appropriate test for reproducing undercutting in the laboratory.