Metal dissolution ohmic drop

A technical application of potential measurements is the detection of corrosion damages of underground metal constructions like gas or water pipes. Two electrodes are put to the wet ground, one of them changes place to detect sites of intense dissolution currents and the related ohmic drops. These sites are the locations of corrosion damage. 

For Co electrodissolution in phosphoric acid, Sazou and Pagit-sasi32 i33 carried out a systematic study of the dynamic behavior in the voltage/extemal resistance parameter plane. The skeleton bifurcation diagram they found is typical for an NDR oscillator that is, bistability between stationary states occurs at high values of ohmic resistance, whereas oscillations are observed at relatively low values of the external resistance. However, from a chemical point of view, Co dissolution seems to be among the most complicated metal electrodissolution reactions because quite a number of different oxide species are involved. Explanations of the dynamics hardly go further than a general statement that the instabihties are due to the formation of a passive film in combination with an IR drop. 

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