ABSTRACT

A common test for the evaluation of corrosion resistance of coatings over steel substrates is underfilm corrosion creep, typically measured by determining how far corrosion emanates on a substrate from a holiday under a coating film when subjected to corrosion conditions. This is a very commonly specified property in coatings specifications.

Another test which is somewhat related to underfilm corrosion creep is cathodic delamination. Cathodic delamination is the de-adhesion of a coating from a substrate occurring at a defect in the coating film in a corrosive environment. The cause of this delamination has been associated with the generation of an alkaline substance produced at the cathodic half cell reaction of the corrosion process. This property, however, is rarely, if ever, specified in coatings specifications. This fact is puzzling and somewhat troubling because cathodic delamination has the potential to becoming a serious corrosion issue, more so than underfilm corrosion creep.

This paper looks at the possible mechanisms of underfilm corrosion creep and cathodic delamination over steel surfaces. Also examined is the relationship between these two properties. The paper further looks into how both of these properties are affected by coatings that are designed to protect steel from corrosion and possible mechanisms of that protection. And finally, the key question is asked, why is cathodic delamination not specified in coatings specifications?

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