Limits of the ON-OFF technique for the assessment of cathodic protection of buried pipeline

The general criterion used to verify the cathodic protection (CP) condition of a structure is based on the IR-free potential measurement, in compliance with the protection criteria provided by international standards. The measurement techniques to eliminate ohmic drop are based on the reduction of the distance between reference electrode and structure (local or fixed reference electrode, Luggin capillary and potential probes) or on the interruption of the circulating current (ON-OFF technique in impressed current CP systems). Laboratory tests were carried out on cathodically protected carbon steel specimens in order to evaluate the effectiveness of ON-OFF potential technique for the assessment of CP condition. The limitations of the OFF-potential measurement are discussed: the technique is not reliable in overprotection condition, with the risk to underestimate the problems involved in overprotected buried pipes (cathodic disbonding, hydrogen embrittlement of susceptible steels, AC-induced corrosion). The effect of the presence of a low-pass filter in the electrical circuit of the data acquisition system on OFF-potential measurement is discussed.