The polarisation resistance of a pair of electrodes is defined by the slope of the current - potential curve in the vicinity of the rest potential. If you want to use the polarisation resistance of the working electrode as a measure for the corrosion rate, the polarisation resistance of the counter electrode must be small with respect to that of the working electrode. A counter electrode made of gold or platinum which has a large surface compared to the working electrode would be nearly ideal for that purpose (for other possibilities see below).
As a standard procedure, you have to measure the rest potential first. The rest potential is then set potentiostatically, and then the working electrode is gradually polarised, both to the negative and then to the positive direction. The maximum polarisation should be small, some ten millivolts, in order to keep perturbations of the working electrode low.
In some cases, a very simple method is applicable. If the working electrode in the given electrolyte is not prone to local corrosion, it is allowed to use two (to the nearest) identical electrodes as working electrode and as counter electrode. In that case, the potential difference between both electrodes is equal to or near to zero. Connect the first electrode to the working electrode terminals and the other one to the counter electrode terminal, and also to the reference electrode terminal. Now polarise the working electrode, using either the built-in control source of the potentiostat or an external one. If you use the internal control source, you can e.g. set the polarisation to 10 mV, and then switch the source from zero to negative direction, and back, preceding by polarisation to the positive direction, and so on. The schematic is shown in fig. 3. If both electrodes have the same area, the total resistance is the sum of the two (more or less equal) polarisation resistances, that means that this total resistance must be divided by two. nhalt Textblock 1!
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