A localized electrochemical impedance technique has beendeveloped based on a measure of the local a.c. potentialgradient in solution by a high-resolution probe. The probeconsists of a pair of microelectrodes or a single vibratingmicroelectrode with a tip size of about 10 mm. Compared withthe experimental set-up using the bielectrode probe, the set-upusing the single vibrating-electrode probe has a highersensitivity and can be used in very low frequency range, butthe measurements by this set-up are more complex.
Some aspects of this technique are also discussed in thethesis. The measured local a.c. potential gradient in solutionmay be related not only to the local a.c. surface currentdensity at the same location by simply using Ohms law asdescribed in the literature but also to the convolutionintegral of the a.c. surface current density distribution. Inorder correctly to interpret the surface current distribution,a deconvolution of the measured potential differencedistribution is necessary. The deconvolution using thebase-line stripping procedure and the Fourier Transform methodhas been investigated. The results showed that a more accuratesurface current density distribution could be obtained by thedeconvolution of the measured potential gradient distribution.In order to achieve a higher spatial resolution, the probe isnormally located as close as possible to the surface of thespecimen. However, when the probe is placed very close to thesurface, e.g. within the diffusion layer, the measurement maybe influenced by the heterogeneous distribution of ionicspecies in this range of the solution. This influence has alsobeen demonstrated and simulated. It is found that the influencecan be d.c. current-dependent when the influence ispredominately due to the variation of solution conductivity orless d.c. current-dependent when the influence is predominatelydue to the redox potential difference at different locations inthe solution. It is also noted that the influence is morepronounced in the low frequency range than in the highfrequency range.
The technique has been used to study the pitting corrosionof Fe-Cr alloys and the degradation of organic coatings. Theresults indicate that this technique can be used not only toinvestigate the heterogeneities over the surface of a specimenwhich may be related to the localized attack, but also toobtain information about the mechanism of the localizedcorrosion by positioning the probe at the interested locationand recording the localized impedance as a function offrequency.
Key words: corrosion, micro-electrode, electrochemicalimpedance, localized impedance technique, localized corrosion,pitting corrosion, coatings, deconvolution, diffusion.
Stockholm: Materialvetenskap , 1998. , 47 p.