Glassy polymer coatings vulnerable to CO2 and humidity

Mechanism likely contributes to pipeline degradation

Metal pipelines – such as those used in the oil and gas industry – are typically coated and lined in epoxy-based materials to protect them from corrosion. However, in humid conditions, these protective coatings are susceptible to microstructural degradation, threatening the metal beneath. And while there has been a lot of research into how carbon dioxide is transported through different types of glassy polymers, the literature is sparse on any potential synergistic effects of CO₂ and water. Writing in Carbon [DOI: 10.1016/j.carbon.2023.118294], materials scientists from the University of British Columbia report on their examination of CO₂ permeation with and without the presence of H₂O, and discuss their combined effects on the degradation of epoxy barrier materials.

They focused on a fusion bonded epoxy (FBE), both in the form of free-standing films and as coatings applied to steel panels. The FBE composed of diglycidyl ether of bisphenol A (DGEBA) and dicyandiamide (DDA), with mineral agents such as wollastonite (CaSiO₃) added to it. To test the permeability of the FBE samples, they designed and built a continuous-flow, constant pressure variable volume set-up that could deliver a stream of CO₂ and or a ‘wet gas’ mixture (CO₂ + H₂O) to the test samples.

Dry-state permeation tests showed that the mobility (or diffusivity) of CO₂ in FBE was low; “…two orders of magnitude

less than that observed in high-density polyethylene”, and that this was independent of gas pressure. However, the permeability of CO₂ was found to decrease as the upstream pressure increased. Scanning electron microscope (SEM) / energy dispersive X-ray (EDX) analysis confirmed that the dry gas exposure had “…no adverse effect on the structural integrity of the FBE network or the filler component throughout the 120 h exposure period.”

Wet-state permeation tests showed that the presence of water in the feed volume did not result in a significant competition between water and CO₂ for transport. However, the team observed major variations in stead-state flux, which, they say, “…suggests a dynamic interplay between the two penetrants in terms of transport channels” SEM/EDX analysis showed some evidence of wollastonite leaching and transformation as a result of exposure to a CO₂ + H₂O environment. They credit this to a carbonation reaction involving both dissolution and precipitation mechanisms, which convert wollastonite into calcite and amorphous SiO₂. Their results suggest that, even at room temperature, this process opens new microchannels for gas/vapor transport inside the coating network, which could “…have implications for the long-term performance of FBE coatings in CO₂-rich environments.”

When exposed to a CO₂ + H₂O environment for an extended duration, the FBE coated steel samples also displayed signs of damage. Electrochemical analysis showed a decrease in coating resistance and a shift in phase angle to negative values, suggesting that the coating’s protective properties were compromised, rendering the underlying metal more susceptible to corrosion. The authors write, “Compared to earlier investigations on ageing in water alone, the changes in the coating impedance were significantly greater over this short exposure period, highlighting the adverse effects of the aggressive CO₂/H₂O atmosphere on the FBE’s barrier properties.” They caution that their exposure tests were accelerated and more severe than those experienced by coated metal pipes in service, concluding that degradation due to microchannel formation may take longer to manifest in real-world conditions.  

---  

Hossein Zargarnezhad, Kashif Mairaj Deen, Dennis Wong, C.N. Catherine Lam, Edouard Asselin. “CO₂ permeation through fusion-bonded epoxy coating in humid environments,” Carbon 214 (2023) 118294. DOI: 10.1016/j.carbon.2023.118294