Application of Scanning Kelvin Probe for Studying Corrosion under the Polymeric Coatings

摘要

In this research, corrosion protection of different polymeric nanocomposites coated on mild steel was investigated by the application of scanning Kelvin probe (SKP). The samples were mild steel coated with a neat epoxy, polyanilne embedded epoxy and clay embedded epoxy respectively. The potential difference between the tip and sample which was defined as Volta potential was measured by SKP. Potential decrease is an apparent phenomenon caused by corrosion induced Fermi level misalignment at the interface between the tip and the Iron substrate. Contact potential difference between the conducting tip and sample, is called 61ta potential difference. Promoting corrosion in presence of electrolyte leads to metal surface potential drop that can be measured by means of Kelvin probe. Although the corrosion potential calculation is not easy, the relation between the Volta potential difference measured by the Kelvin probe and the electrode potential at the buried interface is straightforward, Volta potential differences between the probe and the sample were measured by means of scanning Kelvin probe. SKP plots were obtained at different exposure times with 121 points for each measurement. The average Volta potential difference of each sample was also calculated and plotted versus exposure time. At initial times, Volta potential difference for all the samples were about -200 mVvs. SCE. It means that before water diffusion and corrosion initiation, samples surfaces were in similar conditions. But 61ta potentials were different in samples due to their composition and corrosion role after exposure for long hours. Different capacitors between the probe and conductive substrate may form in the case of the coated steel exposed to aqueous corrosive media. Coating capacitance vary as a consequence of water diffusion. Initial drop in surface 61ta potential differences of all the coated steels were due to diffusion of electrolyte and corrosive ions through the coating defects. CV was introduced as an index in order to compare the deviations of Volta potential differences from their mean values for the coated steels with or without PANs. Results are plotted in.CV defines deviation of Volta potential differences from the mean value in each sample for different exposure times in humidity chamber. If CV equals 1, there is no deviation from average, which means a completely uniform corrosion. On the other hand, if CV=0, a complete localized corrosion is anticipated. These two ideal situations would never take place but this could be a good method for comparing corrosion uniformity or localization in different conditions beneath the coatings by the application of SKP.