Chloride and Chromate Migration in Organic Coatings Measured Using Ion Selective Electrodes

  • Yishuang Yang

Student thesis: Phd

Abstract

This project investigates the migration/penetration of ionic species within organic coatings using ion-selective electrodes. Chloride and chromate are chosen to be studied for three reasons. Firstly, chloride is a very common aggressive specie and its movement from the external environment to the substrate is often the first step to the start of under-film corrosion. Secondly, chromate is a common and traditional species that acts as an effective corrosion inhibitor in coatings. Thirdly, both species can be determined, in principle, using the same method of a silver/silver chloride (or chromate) electrode. Solvent-free epoxy resin, waterborne epoxy resin and solvent-borne polyester resin were selected as three common types of coating. The overall objectives of this project are to prove that ions are detectable using this method, to measure the chemical influence (activity) of chloride and chromate in different organic coatings at the substrate after migration and to determine the influence of pigment amount in coatings on the migration of ions. Unfortunately, the last of these was not possible as a consequence of COVID. Ion-selective electrodes were manufactured using thin silver wire, thicker rod and flat plate with coating application by dipping (wire and rod) and spin/bar coating (plate). Any difference of reproducibility in the ion-selective electrodes of different shapes is not apparent. However, coatings applied on wire and rod electrodes are not uniform due to a retained droplet at the bottom of sample resulting in a difficulty in the measurement of actual coating thickness. Thus, flat plate samples were predominantly used with coating thickness measured by eddy-current electronic gauge. Before measurement of potential in ionic solutions of various concentrations, coating integrity was checked using electrochemical impedance. This is to avoid any errors in measurements caused by ion migration through pores and other small defects rather than through the bulk of the polymer coating. If the coating impedance was more than 10^6 ohm.cm^2 (water-borne coating) or 10^7 ohm.cm^2 (solvent borne coating) and maintained this value over the time of the experiment then it was assumed that the coating had no defects. The impedance of solvent-free epoxy resin coated electrodes was generally too high for the electrochemical potential of the electrode to be measured due to the insufficient input impedance (10^14 ohm) of the potentiostat/voltmeter. Therefore, work using the solvent-free system was abandoned. For waterborne epoxy resin on the chloride electrode, generally high impedance values (> 10^6 ohm.cm^2) were measured showed intact coatings. Moreover, the potentials measured for these coated electrodes are similar to those on uncoated electrodes and this indicates that the chloride was detected after migration through the coating and it was in thermodynamic equilibrium with the external chloride value. Changing the chloride value produced a relatively rapid change in potential on the coated electrode which was too fast for migration throughout the body of the coating. Therefore, it was interpreted as a short-circuit diffusion along the hydrophilic boundaries between the coalesced particles that comprised the coating. For the solvent polyester system, the coating impedance was often found to be too high for consistent measurement of potential and this may be due to a higher cross-link density or as the polymer coating is more homogeneous. However, there is evidence that if the coating is not too thin (so has holes) and is not too thick (so not a massive high resistance) that the electrode coating will respond to external chloride but over a longer time. For chromate detection the waterborne system was used as this seemed to have the greatest chance of success. The uncoated electrodes did not show expected straight line responses to logarithm of chromate concentration but with predicted slopes after linear fitting. However, a suitab
Date of Award1 Aug 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorStuart Lyon (Supervisor) & Michele Curioni (Supervisor)

Keywords

  • Corrosion
  • Organic Coating

Cite this

'