Potential probes are sensors consisting of a reference electrode and a metal coupon close to the electrode used for measuring the protection potential of underground metal structures in cathodic protection. They allow the reading of the potential purified by the ohmic drop contribution, the so-called IR-free potential, a useful parameter for assessing the effectiveness of a cathodic protection system. Aim of this work is to compare the accuracy of the potential measurements of commercial probes, which differ in geometry, extension of the porous plug, number and position of the metal coupons. Measurements of reference electrode potential stability in the presence of cathodic protection and ability of the probes to compensate the ohmic drop were performed. The results showed that not all potential probes are able to measure the IR-free potential, as the measurement contains an ohmic drop contribution, greater for more negative cathodic polarization values.
Cathodic protection (CP) is an electrochemical corrosion prevention technique, which is implemented by circulating a direct current between an electrode (anode) placed in the environment and the structure to be protected (the cathode): this current causes a lowering of the potential of the metal and reduces its corrosion rate, until it stops. 1
The general criteria used to verify CP efficiency is based on potential measurement, performed by using a reference electrode placed in contact with the environment surrounding the structure (soil, water, electrolyte or concrete) and a high impedance voltmeter whose positive pole is connected to the structure and its negative one to the reference electrode.
Potential measured through a reference electrode depends on electrode position with respect to the structure. In the presence of a cathodic protection current, I, the measured value, EON, is the sum of three contributions: the equilibrium potential of the metal (Eeq), the overvoltage (η) related to the cathodic current density, and the ohmic drop (IR, where R is the electrical resistance), the latter depending on the reference electrode position, environment resistivity and the circulating current: