Study of the failure mechanism of an epoxy coating system under high hydrostatic pressure

•Failure mechanisms of two epoxy coatings were studied under 1 atm and 35 atm.•Mechanical properties of epoxy coating were more favorable under 35 atm.•Water diffusion was accelerated greatly by 35 atm.•Loss of wet adhesion has become a critical factor for coating failure.

The failure mechanisms of an epoxy varnish coating and an epoxy glass flake coating were investigated under ordinary pressure (1 atm) and high hydrostatic pressure (35 atm), from electrochemical behavior, water absorption, wet adhesion and mechanical characteristics. The results revealed that mechanical properties were more favorable under 35 atm, but pressure changed the electrochemical behavior and deteriorated coating protectiveness by accelerating water absorption, increasing diffusion coefficient and changing diffusion type. More importantly, loss of wet adhesion, induced by blisters and corrosion products on interface, became the main reason for coating failure and thus the key factor controlling coating lifetime.