Blistering behaviour of waterborne DTM coatings in water and NaCl solution: a comparative study

This study investigated the blistering failure mechanisms of waterborne direct-to-metal (DTM) coatings in deionized water (DI water) and 1 M NaCl solution. Distinct blistering kinetics were observed during 96 h immersion tests: rapid initiation and stabilization of blistering in DI water versus delayed yet pronounced blistering in NaCl solution. The results indicate that blistering was triggered by localized substrate corrosion. In DI water, γ-FeOOH formed anodically at blister centers, while Fe₃O₄ precipitated in peripheral regions. These corrosion products generated substantial interfacial stresses, ultimately causing coating delamination. In NaCl solution, the persistence of soluble species, together with the absence of a blocking corrosion-product layer and the accumulation of cathodic OH⁻, degraded the coating adhesion. Consequently, osmotic pressure buildup within the blister cavity, driving continuous water ingress and accelerating blister growth.