Monitoring cathodic protection of buried pipeline by means of a potential probe with an embedded zinc reference electrode

Abstract

According to international standard, cathodic protection efficiency is controlled by monitoring IR-free potential. Nowadays commercial probes are available to perform correct potential reading, eliminating the ohmic drop contribution. In this paper, a new probe with a zinc reference electrode embedded in a proper backfill is proposed. Zinc is a promising material due to the low overvoltage in active condition and the high exchange current density. Laboratory tests were performed to select a stable backfill, in order to avoid any risk of shrinkage and mass loss. Sixteen mixtures based on gypsum, clay and other additives were considered. Gypsum, specifically sulfate ions, were used to maintain zinc in active condition; bentonite, absorbing humidity from the surrounding soil, guarantee a constant proper level of water content. Four additives, pure cellulose, an air-entraining agent, expanded glass granules and silica-based sand, were added to study the effect on water absorption, mass loss and electrical conductivity. Two of them were selected to prepare two probe prototypes with incorporated zinc electrode in order to verify the stability of both backfill and zinc during continuous cathodic protection monitoring.