Evaluation of epoxy-based coating degradation under thermal insulation at elevated temperatures on different steel substrates

Corrosion under insulation (CUI) remains one of the most critical issues in the petrochemical industry. In a typical CUI system, protective coatings (organic or metallic) are used primarily as the last barrier to prevent the rapid corrosion of the metallic substrate. However, organic coatings exposed to high operating temperatures and thermal cycling conditions are susceptible to failure and thus require coating performance evaluations under representative thermal insulation conditions. This study presents experimental designs to investigate the degradation of an organic polyamine-cured epoxy coating under accelerated laboratory test conditions. Additionally, a systematic CUI evaluation protocol for high-temperature organic coating performance is presented. The method involved specially designed apparatus to simulate CUI systems and characterisation techniques, such as visual inspection, adhesion test, peel-off test, scanning electron microscopy, electrochemical impedance spectroscopy, and chemical analysis using Fourier-transform infrared spectroscopy, and differential scanning calorimetry. The results showed that polyamine-based epoxy coating experienced thermal degradation starting at temperatures above 130 °C under mineral wool insulation in accelerated and cyclic CUI tests. Chemical degradation was the primary mode of degradation, with the formation of carbonyl functional groups and increased glass transition temperature observed at higher exposure temperatures. The proposed combination of electrochemical, spectroscopic, mechanical, and microscopy techniques allows quantifying coating performance under insulation and subsequently provides insights into predicting coatings' service lifetime.