CO2 permeation through fusion-bonded epoxy coating in humid environments

This study investigates the effect of water and CO2 exposure on the performance of epoxy-based coatings under conditions commonly found in pipeline applications. 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 membrane selectivity and 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 filler particles results in a change in shape, volume, and chemical composition of these filler particles. The chemical reaction between wollastonite and CO2 in the presence of water led to a degradation that caused changes in porosity, permeability, and filler volume. To verify these findings for the coating in the presence of adhesion forces, electrochemical impedance spectroscopy (EIS) was also used. The tests conducted showed that exposure to a CO2/H2O mixture causes filler particles within the coating to undergo carbonation, leading to the formation of new microchannels within the epoxy network. 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. The carbonation of wollastonite within the FBE coating leads to a volume change in the fillers, resulting in the creation of small voids or gaps in the epoxy matrix. This, in turn, facilitates the easier penetration of water and dissolved gas to the underlying substrate. Consequently, the accumulation of water inside 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.