Deterioration mechanism of epoxy coating on EH36 ship steel during seawater freeze-thaw cycles

Abstract

• Epoxy coating on EH36 steel is degraded by freeze-thaw cycles of seawater. • Freeze-thaw cycles allow chloride ions to enter epoxy coating to corrode EH36 steel. • −4 °C/25 °C and −20 °C/25 °C cycles increase porosity by 37 % and 66 %, respectively. • Lower temperature cycle (−20 °C/25 °C) leads to more severe degradation of coating. Epoxy coatings are widely used on ships due to their excellent bonding and isolation properties. However, the performance of epoxy coatings in high-latitude marine environments, especially in the presence of freeze-thaw (F-T) cycles is still unclear. Herein, the deterioration behavior and mechanism of epoxy coatings on EH36 ship steel immersed in natural seawater at room temperature (RT) 25 °C, −4 °C/25 °C (−4RT) cycle, and −20 °C/25 °C (−20RT) cycle were investigated in this study. Results show that the F-T cycles led to severe deterioration of the epoxy coatings, and the degree of deterioration depended on freezing temperatures. | Z | 0.01 Hz for RT remained above 10 8 Ω cm 2 during the 30-d immersion, whereas | Z | 0.01 Hz corresponding to −4RT decreased to 2.5 × 10 6 Ω cm 2 and was further reduced to 5.3 × 10 5 Ω cm 2 by 20−20RT cycles. The maximum pit depth on EH36 steel with −20RT was 12.2 μm, which was 1.3 times that with −4RT (9.3 μm) and nearly 3 times that with RT (4.2 μm) after the 30-d immersion, respectively. In comparison with RT, the repeated crystallization of water in the coating increased the coating porosity by 37 % after −4RT cycles, which allowed chloride ions to enter the coating to corrode EH36 steel. −20RT further increased the coating porosity (66 % rise) and pore diameter, leading to more chloride ions reaching the substrate surface, thereby forming more corrosion products, which further accelerated the coating degradation. This study provides a theoretical basis for a new design of epoxy coating on ship steel in cold areas.

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