The water permeation kinetics of two common epoxy-based powder coating systems for pipelines across a range of industrially-relevant temperatures (from room temperature to 80 °C) has been studied. The nonlinear dependency of water transport on the vapour concentration at 65 °C was also analysed. The vapour transport analysis of epoxy coatings demonstrated a turning point around this temperature, perhaps due to clustering of water molecules. At higher temperatures, break-up of water clusters and plasticisation of the polymer expedited the transport.
The researchers also examined microstructural changes of the epoxy network due to water transport and found evidence for irreversible damage to epoxy coatings under hydrothermal exposures. It appears that the combination of thermal exposure and internal stresses in the glassy epoxy leads to a phase separation of filler particles from the epoxy matrix, as well as to a distinctive cavity formation in the coating membrane. The results indicate that hydrothermal exposure is likely to increase aggregate porosity of the coating. Analysis of wet-state permeation is not only crucial for protection of transport pipelines, but it also is of high relevance to process equipment and underground storage tanks.
The study was published in Progress in Organic Coatings, Volume 168, July 2022.