A study on effects of elastic stress on protective properties of marine coatings on mild steel in artificial seawater

• Elastic stress can have a significant influence on marine coating properties.
• The effects of stress magnitude and direction on coating properties were evaluated.
• Disparity in the evolution of water uptake and diffusion with stress was discussed.
• The apparent water diffusion coefficient is a thermo-activated function of stress.

The effects of macro-elastic stress on protective properties of two marine coating systems applied on mild steel plates in artificial seawater at room temperature were investigated through the measurement of electrochemical impedance spectroscopy (EIS). One coating system contained zinc-rich epoxy primer, micaceous iron oxide anchor coat and aliphatic polyurethane top coat and the other was composed of zinc-rich epoxy primer, micaceous iron oxide anchor coat and acrylic polyurethane top coat. Two different elastic stress levels (tensile and compressed modes) were applied on coated substrates by using bent samples and non-bent coated samples were used as references. The obtained results indicated that elastic stress could have a significant influence on coating resistance, water absorption and diffusion coefficient, and meanwhile the extent of this influence depends on both the magnitude and direction of elastic stress. Furthermore, the disparity in the evolution of water uptake and diffusion coefficient with elastic stress was discussed, and a thermodynamic approach was employed to evaluate the enthalpic and entropic contributions of the water diffusion coefficient under various elastic stress levels.