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Zargarnezhad, Hossein

University of British Columbia

Barrier coatings protect metals from corrosion as long as they maintain integrity under service conditions. In the pipeline industry, the performance of coatings is largely determined by mass transport dynamics within the coating and the stability of the polymeric structure, with water infiltration posing a significant threat. Through extensive analytical methods, I examined the performance of powder coatings in humid conditions, specifically how water, with a permeation rate of 2×10ˉ¹³ mol/m-Pa-s at 65°C, affects the ingress of gaseous and salt components. Coating films and coated steel panels were analyzed using custom-built permeation cells and subjected to electrochemical and mechanical tests to assess performance in wet environments. This study is the first to demonstrate how oxygen can transport through coatings in wet environments. The interaction of epoxy with water vapor alters coating plasticization and gas permeation rates. I found that humidity levels above 60% block the gas transport within the coating at room temperature, thereby minimizing underlying corrosion. However, at elevated temperatures, gas diffusion increases significantly, with oxygen permeation rates reaching up to 2×10ˉ¹⁷ mol/m-s-Pa at 65°C, leading to early signs of under-coating corrosion on steel within 8 weeks of hydrothermal ageing. Unlike oxygen, CO₂ ingress in wet conditions involves not just physical absorption but also chemical interactions that compromise the coating’s barrier performance. At high CO₂ concentrations, approximately 90% at atmospheric pressures, permeation rates were around 1.5×10ˉ¹⁵ mol/m-s-Pa at 65°C. I also assessed the salt exposure resistance, revealing that the permeability of NaCl at 65°C is 9×10ˉ¹⁴ m²/s, compared to water at 1.1×10ˉ⁷ m²/s. This indicates a lower likelihood of salt reaching the substrate. Yet, accelerated exposure testing of coated panels showed that salt accumulation on the coating surface could lead to local defect formation due to salt osmotic effects. Overall, this work sheds light on the protective capabilities of epoxy-based coatings and lays a foundational framework for the remaining life assessment of these coatings. These findings suggest that with the integration of operational pipeline’s microclimate data and monitoring of in-line coupons, constructing a probabilistic failure analysis of coatings is feasible in the near future.

Text

2024-07-16

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/88644

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/