CP of steel in soil: temporospatial pH and oxygen variation as a function of soil porosity

This paper documents an investigation of pH variations in water saturated sandy soil with steel samples under impressed current cathodic protection (CP). The aim of this work was to evaluate the temporal and spatial variation in pH and oxygen under different protection current density and different soil porosity. Carbon steel samples were buried in quartz sand saturated with a simulated soil solution. The pH variation was determined by means of planar optical sensors and iridium oxides (IrOx) sensors. The oxygen variation was determined by means of fiber optical oxygen sensors. Different soil porosities were studied by dosing quartz sand with various granulometry. The increase of pH at the sample surface occurred within the first 10 hours from CP application independently from the soil porosity. The alkalinity produced from CP, and thus the near-field pH, was dependent on the cathodic current density. The pH gradient as a function of the distance from the steel surface was dependent on the soil porosity. The oxygen consumption rate at the steel surface was dependent on the cathodic current density. The oxygen concentration at the steel surface was reduced by at least 50% within 20 hours of CP application independently from the soil porosity. Thus, this work confirms that CP increases the pH and reduces the oxygen concentration at the working electrode, thereby strengthening the established view that corrosion protection can be achieved through generating conditions that facilitate passivation