Design and testing of a solid backfill of potential probes for cathodic protection monitoring in soil

The detailed assessment of cathodic protection efficiency is based on IR-free potential measurement, i.e., without the voltage drop contribution in soil. For buried structures, as pipes for the transport of methane, the ohmic drop contribution is considerable, due to the medium-high resistivity of soil and the presence of different current sources (protection current, interference current, etc.). Among the different techniques adopted to measure the IR-free potential, the installation of potential probes is continuously increasing, thanks to their accuracy in the potential measurements and the possibility to assess cathodic protection efficiency even in the presence of stray currents. A potential probe consists of a fixed (local) reference electrode (mainly saturated Cu/CuSO4) and a coupon (or multiple coupons) close to it. The coupon is permanently connected to the structure in cathodic protection and simulates a coating defect. One drawback of the use of the potential probe is related to the leakage of the copper sulphate-based electrolyte (liquid or gels) contained in the plastic case of the probe through the porous septum. At the same time, contaminants, as chloride ions, could diffuse through the septum polluting the internal electrolyte. In this research, funded by an Italian company operating in the field of cathodic protection and anti-corrosion systems, a new internal solid electrolyte (backfill) for potential probes has been proposed. In the first part, some mixtures consisting of copper sulphate and a solid binder, i.e., gypsum or concrete, in different proportion with water were studied. Then, tailored tests (conductivity and mass loss measurement) were carried out in order to investigate the stability of the backfills to the exposure in dry or wet soil. The most promising backfills were selected for the preparation of Cu/CuSO4 reference electrodes, which potential accuracy was measured by laboratory long-term trials. Results show the higher stability of the potential of the reference electrodes prepared with the proposed solid backfill. Moreover, experimental data from field tests on the potential probe are discussed, too.