Zinc Coating in Corrosion under Insulation (CUI) Environments: Influence of Leached Anions in the Potential Reversal Phenomenon

Abstract Corrosion under insulation (CUI) is a significant challenge in industrial environments, particularly in the temperature range of 77-110°C according to API RP 581. Zinc-rich paint is commonly used in protective coatings for corrosion protection of carbon steel. This study investigates the behavior of zinc in simulated CUI environments at 80°C, focusing on the potential reversal phenomenon between zinc and carbon steel. Corrosion potentials of zinc and carbon steel were measured in solutions simulating anions leached from insulation materials. Additionally, electrochemical behavior of the galvanic couple were examined using a Zero-Resistance Ammeter. Corrosion potential measurements revealed that the potential of zinc is significantly influenced by the ion species and pH of the solution. Bicarbonate and nitrate ions, as well as higher pH levels, promoted zinc potential elevation. Effective galvanic protection required sufficient galvanic current, which was achieved with chloride and sulfate ions at high ion concentrations (10,000 ppm), but not with bicarbonate and nitrate ions. Consistently, carbon steel corrosion was observed with bicarbonate and nitrate ions that showed zinc potential elevation in the corrosion potential measurements. The results demonstrate the importance of sufficient galvanic current for effective protection, which is strongly influenced by both ion species and concentration in the environment.