A positron annihilation lifetime spectroscopic study of the corrosion protective properties of epoxy coatings

Positron annihilation lifetime spectroscopy (PALS) was used to measure the free volume cavity sizes and free volume fractions of bisphenol A and polyglycol diepoxide crosslinked epoxy coatings on steel before and after saturation with liquid water at 23 degrees C. The effect of water saturation on the coatings' free volume was studied under the assumptions: (i) the free volume is the major pathway by which water passes through an intact coating, (ii) water uptake is related to the corrosion protective properties of polymer coatings. A linear relationship was found between the equilibrium volume fraction of water absorbed and the dry relative free volume fraction of bisphenol A epoxy coatings. The free volume cavity sizes and the number of free volume cavities per unit volume of these epoxies decreased by 3-4% and 3-19%, respectively, when fully water saturated. These decreases are ascribed to the occupation of 13-17% of the free volume cavities by 2-4 water molecules per cavity. The small percentage of free volume occupied by water and the small number of water molecules capable of filling voids after water saturation correlate to the high 0.01 Hz impedance values and the 'better' corrosion protection of these coatings than the polyglycol diepoxide coatings, suggesting that water passes through the bisphenol A epoxy coatings by slower diffusion in the connected free volume cavities. The free volume cavity sizes of polyglycol diepoxide coatings were found to increase by 4-14% after water saturation. This increase is ascribed to the expansion of the free volume cavities by water, which is substantiated by the macroscopic swelling observed in the coatings. An inverse, linear relationship was observed between the equilibrium water uptake and the relative free volume fraction of the coatings. This result indicates that other factors (solvent interactions and swelling), not the magnitude of the free volume fraction, affect the amount of water absorbed by epoxy coatings. Increases in the free volume cavity sizes of the polyglycol diepoxide coatings after water saturation correlate to the low 0.01 Hz impedance values and the 'poorer' corrosion protection of these coatings than the bisphenol A epoxy coatings. Increases in the free volume cavity sizes suggest that water creates additional free volume (defects) in polyglycol diepoxide coatings allowing easy migration of water to the coating-metal interface.