Key parameters and mechanism of blistering of coil-coatings in humid-hot laboratory environments

Abstract

Blistering of organic coatings on hot dip galvanized (HDG) steel is of high practical relevance, especially in warm-humid environments. The aim of this work was to study the key parameters of blister formation and growth by accelerated laboratory testing at 60 and 70 °C. Following a statistical design of Taguchi, the components of a polyurethane (PU) primer formulation were varied and applied to hot-deep galvanized (HDG) steel. The presence of a topcoat was found to be a precondition for blistering. Degradation started at microcracks within the primer and was significantly influenced by the adhesion between primer and topcoat. A high content of anticorrosive pigments resulted in enhanced blister formation, while blister growth is mainly controlled by mass transport processes and shows a strong temperature dependence. This study focuses on one specific blistering mechanism, osmotic blistering, which is indicated by blister kinetics and leachate analysis, estimated pressure values inside the blister are in reasonable agreement by using two different calculation approaches.