A NEW THERMODYNAMIC CRITERION AND A NEW FIELD METHODOLOGY TO VERIFY THE PROBABILITY OF AC CORROSION IN BURIED PIPELINES

One of the current and great challenges faced by cathodic protection professionals is to access the corrosion probability of cathodically protected buried pipes subjected to alternating current interferences (AC corrosion). In practice, it is very common for cathodically protected pipes to be buried along high power voltage lines and electric energy distribution systems. These electric systems produce stray AC current in the soil which can cause severe corrosion on buried pipes which are supposed to be catholically protected. There are several criteria cited by the literature to evaluate the probability of AC corrosion but these criteria are inefficient, since failures due to AC corrosion have been reported in pipelines which presented electric and electrochemical parameters within the acceptable limits of those criteria. This work has as an objective to propose a new thermodynamic criterion and presents all the necessary equipment for obtaining, in the field, the necessary electric and electrochemical parameters to apply in a safe way the proposed criterion. Those parameters are obtained from the waveform of pipe-soil interface AC+DC off potential. Two devices were developed: a probe composed of a permanent modified Cu/CuSO₄ reference electrode coupled with corrosion coupons and an electronic alternating switching device. Some measurements conducted in Petrobras pipelines are also presented.

Keywords: cathodic protection, AC current, AC corrosion.