Hydrothermal degradation study of phenolic polymer coatings by advanced analytical methods

Abstract

Resolve-type phenolic polymer coatings were deposited on carbon steel panels, and then were exposed for 15 days to a simulated geothermal brine with pH 1.6 at 150°, 175°, and 200°C. The phenolic polymers were hydrothermally oxidized. To comprehensively understand the mechanisms of this oxidative degradation of the coatings, the modern analytical techniques of XPS, contact angle, TGA, SEM-EDX, and EIS were used in combination. The oxidative degradation of polymer took place in the three-step oxidation routes: first, the bridging methylene linkages in the network polymer structure were replaced by the benzlhydrol-type linkages; second, the benzlhydrol-type linkages were transformed into the benzophenone-type linkages; and finally, the C-C-C linkage in the benzophenone derivative ruptured to form salicylic acid derivatives as the ultimate degradation products. Hydrothermal temperatures of >175°C promoted the degree of such oxidative degradation, causing the coating surface to become susceptible to moisture, to absorb more brine, and also to allow corrosive electrolytes to permeate easily. Consequently, iron oxides as the corrosion products from the underlying steel were yielded at a critical interfacial zone between the coating and steel. The excessive growth of iron oxides led to the generation of internal stress-induced cracks in the coating film, thereby resulting in the failure of these corrosion-preventing barriers.