About Corrosion Macrocells Formed on Underground Pipelines at the Switched off Cathodic Protection

Abstract

Experimental modeling in grounds showed that during the pauses in cathodic protection, dangerous corrosion macrocells, suppressed by the cathodic polarization, may regenerate and some new macrocells may originate due to the cathodically induced passivation of steel at some parts of a pipeline. The transformation of an active cathode into a passive one in a cell formed because of the difference in the types of ground slightly affects the degree of its danger estimated from the dissolution rate of the anode and results in completely ceasing the local corrosion of the cathode. The cells which form (during pauses in the cathodic protection) due to the difference in the protective potentials along the pipeline, i.e., from –1.1 to –0.85 V within the protection zone, are not dangerous, because the more negative the polarization potential, the higher the degree of the cathodically induced passivity of the metal. The less passivated electrode soon becomes an anode in such a cell; in a week it may become active and the cell may transform into a dangerous one if the cathode is still passive.