Failure behaviors of vinyl ester resin-based glass flake coatings on mild steel by repeated tensile loading

The inner bottom plates of oil storage tanks are protected from corrosion by organic coatings because highly corrosive electrolyte solutions containing drain water and aggressive ions tend to accumulate on the plates. Although organic coatings are extensively used, their protective performance progressively degrades, eventually leading to the progression of under-film corrosion. In addition, there is a risk of unpredictable damage to coating systems when large earthquakes occur, particularly in seismically active regions. In this study, the failure behavior of vinyl ester resin-based glass flake coatings on steel was investigated under repeated tensile loading in 3.5 wt% NaCl solution. Five coating systems, differing in resin formulation, curing agents, and glass flake content, were examined with varying thicknesses. Four-point bending tests combined with in situ open-circuit potential monitoring enabled the quantitative identification of the corrosion-initiation cycle number, defined as the cycle at which cracks or interfacial defects allowed electrolyte access to the steel substrate. The results revealed a power-law relationship between strain amplitude and corrosion-initiation cycle number, analogous to the Coffin–Manson equation. Coating thickness significantly influenced fatigue resistance. In addition, four distinct damage modes were identified, among which interfacial debonding occurred at extremely low cycle numbers. Electrochemical impedance spectroscopy demonstrated that coating failure led to a significant reduction in impedance, corresponding to the loss of barrier performance. These results provide new insights into the coupled mechanical–electrochemical degradation of protective coatings and offer a basis for integrity assessment of oil storage tanks subjected to seismic loading.