Water diffusion is widely believed to be a driving factor in the breakdown of corrosion protection by polymer coatings. However, in the epoxy-phenolic system examined, water absorption into more cured, electrically resistive coatings is shown to increase, contradicting the common perception that hydrophobic coatings yield improvements in corrosion protection. Water uptake into epoxy-phenolic coatings was estimated as a function of time using the dielectric and resistive properties of the coating measured using electrochemical impedance spectroscopy (EIS). Bulk water uptake through the coating surface and the localised nature of corrosive failure was then confirmed using localised electrochemical impedance (LEIS). This technique also allowed comparison of resistance for coatings with different curing degrees. The greater degree of water absorption into highly cured coatings was attributed to diffusion into the polymer free volume, which increases with prolonged crosslinking. Evidence for this mechanism was found in the decreasing density of epoxy-phenolic coatings as a function of cure time. The effect of coating volume was therefore evaluated with respect to the dry coating dielectric properties. Cathodic delamination of the epoxy-phenolic coatings was achieved on tinplated steel substrates, whereas this was not possible from tin-free steel (ECCS). The electrochemical behaviour of these two substrates was examined under the same NaCl concentration, their surfaces were analysed by XPS prior to immersion, and XRD after potentiostatic polarisations. The electrochemical behaviour of the bare substrates was found to be unchanged by heating to the coating cure temperature (within the accuracy of polarisation tests carried out). Nonetheless, the delamination rate of epoxy-phenolic coatings deposited on them was dependent on the coating crosslinking (cure time). The reason coating adhesion selectively failed on tinplate substrates is believed to be the lower concentration of hydroxides on its relatively flat surface, allowing less bonding to take place at the interface with the coating. The impact of the substrate on coating resistivity was further investigated by volume resistivity measurements in the presence and absence of a metallic substrate. The resistivity of free standing films was tested in permeation cells, and showed a direct correlation between the polymer resistivity and the resistivity of the solution in which it was immersed. In comparison, attached films remained highly resistive at all examined concentrations. To conclude, the results presented here demonstrate that water uptake is not as critical to the breakdown in the protection offered by epoxy-phenolics as bonding at the metal-polymer interface. These results suggest that the complex interaction between these two dissimilar materials determines the overall coating electrochemical behaviour.
|Date of Award||1 Aug 2017|
- The University of Manchester
|Supervisor||Stuart Lyon (Supervisor) & Xiaorong Zhou (Supervisor)|
The impact of curing time on the electrochemical behaviour of intact epoxy-phenolic coatings on tinplate and tin-free steel
Kefallinou, Z. (Author). 1 Aug 2017
Student thesis: Phd