The influence of CO₂ at HPHT conditions on properties and failures of an amine-cured epoxy novolac coating

Using a three-phase batch reactor with coated steel panels, this investigation studies the influence of carbon dioxide (CO₂), present in the gas phase at conditions of high pressure and high temperature (HPHT), on the degradation of an amine-cured epoxy novolac coating (EN). The combined effect of a gas, a hydrocarbon, and a seawater phase compromises the coating and leads to underfilm corrosion. Consequently, an understanding of the role of each of the phases is essential for the effective design of superior epoxy-based coatings for HPHT applications in the petroleum and other industries. On exposure to the three phases individually, at a low pressure of N₂, the EN network remained unaffected and impervious. However, in the hydrocarbon-exposed zone, a combination of para-xylene, representing the hydrocarbon phase, and CO₂ at HPHT, initiated a glass transition temperature depression with subsequent softening of the EN network. This allowed dissolved CO₂ gas to diffuse into the EN network, thereby generating pinholes at the coating surface. The seawater-exposed zone, in the presence of CO₂ at HPHT, suffered from an increased seawater ion diffusion, leading to blister formation.
Moreover, the most detrimental subzone for the EN network was when CO₂, para-xylene, and seawater were synergistically interacting at its hydrocarbon-seawater interface. This combination resulted in an increased chain motion of the EN network, subsequently allowing CO2 and seawater ions to diffuse into the EN network to the steel substrate, imposing underfilm corrosion. In the absence of CO₂, blisters were formed at the interface subzone, but no corrosion was detected. The results are of high relevance to the petroleum industry, but also for the protection of transport pipelines and process equipment in the next-generation Carbon Capture and Storage (CCS) technologies.