Failure evolution and life assessment of organic protective coating under temperature alternation conditions

This study investigates the degradation of a zinc-rich epoxy/acrylic polyurethane system iwhen exposed to a simulated marine-industrial atmosphere (0.05% NaCl + 0.35% (NH 4 ) 2 SO 4 ) under three conditions: constant 25 °C, and cyclic temperatures of 35/25 °C and 45/25 °C (with a 12 h interval for each stage). The results show that degradation rate follows the order of 45 °C > 35 °C > 25 °C. Elevated temperatures were found to accelerate electrolyte penetration and saturated water uptake, while concurrently reducing chemical stability through the cleavage of urethane and ester bonds within the topcoat, as well as the hydrolysis of ether bonds in the primer. Moreover, thermal cycling further compromises coating integrity by inducing expansion–contraction stresses, which lead to increased porosity. On the basis of low-frequency impedance extrapolated using the GM(1,1) model, the predicted service life under 35 °C cycling is estimated to be 20–30% of that at 25 °C, and merely ∼10% under 45 °C cycling. These findings underscore the big influence of temperature on the long-term durability of organic protective coatings.