Barrier properties of spray-coated epoxy-graft-acrylic films on tin-plate in corrosive environments.

This thesis describes an investigation of corrosion processes occurring under thin polymeric layers (3 -5 mum) and suggests ways by which the films suppress the onset of corrosion of the bulk metal in corrosive aqueous environments. Synthetic solutions have been developed which replicate pH and conductivity conditions, likely to be found in beverage cans and which have been used to conduct corrosion studies of commercial lacquer systems. Studies carried out highlight the following points: i) The degree and nature of the film cross-linking has a role in the anti-corrosive action of the films. ii) The quantity of iron released into the beverage is much lower than expected when the corrosivity of the solution and the relative thinness of the film are considered. Work carried out indicates a possible means by which the iron might be complexed by the phenolic cross-linker component of the film with evidence having been obtained showing the ease with which phenolic cross-linker / iron complexes can be formed. iv) Optical Emission Glow discharge spectrometry has provided evidence that iron corrosion products are trapped within the polymer matrix. The work has also highlighted a means of accurately determining the film thickness. v) Water permeation across the cured films was followed using infra-red spectroscopy. The amount of water at found to migrate across films of different cross-linking densities was observed to vary with phenolic cross-linker level present vi) Permeation of ions across cured films has been followed with the use of ion selective electrodes. The films have also been shown to be perm-selective towards the diffusion of sodium ions in preference to chloride ions, in aqueous sodium chloride solutions, thus exhibiting a duality in the nature of the coatings towards the constituents of corrosive environments. vi) Alternating Current Impedance spectroscopy, has been used to observe the electrolyte penetration and corrosion initiation processes, in situ. A knowledge of the electrochemical processes occurring for these water-based epoxy-acrylate films during electrolyte penetration, saturation of the film and development of corrosion underneath the film has been obtained.