CO2 permeation through fusion-bonded epoxy coating in humid environments

Abstract

The effects of water and CO2 exposure on the performance of epoxy-based coatings under conditions commonly found in pipeline applications are investigated. The permeability of fusion bonded epoxy (FBE) decreases as CO2 pressure increases and the presence of water facilitates gas transport through the coating. The latter is in conflict with theories of competitive transport in gas/vapor systems, in which water is the predominant permeant for its lower kinetic diameter and higher condensability. Our results show that this anomaly was due to the dynamic transformation of permeable channels in the coating structure. Microstructural characterization of FBE after exposure to CO2/H2O mixtures showed that carbonation of wollastonite fillers results in a change in shape and chemical composition of these filler particles. To verify these findings for the coating in the presence of adhesion forces, electrochemical impedance spectroscopy (EIS) was also used. Initially, the carbonation of fillers led to an increase in the pore resistance of the coating, which was attributed to the plugging of micropore channels on the coating surface. However, the subsequent decreasing trend of this parameter suggested that water infiltration into the coating had increased due to this degradation. This transformation facilitates the easier penetration of water and dissolved gas to the underlying substrate. Consequently, the accumulation of water inside the coating increases the dielectric constant, resulting in a higher capacitance of the coating. It appears that high concentrations of CO2 in wet conditions, even at low pressures, can have negative impacts on the barrier performance of the coating.