Any medium that conducts electricity by moving ions rather than electrons. In corrosion work it is the soil, water, moist concrete, condensation film or process fluid that completes the circuit between anodic and cathodic areas. Its resistivity, pH, oxygen content and chloride or sulfate levels govern how fast corrosion runs and how far CP current spreads, which is why techs speak of structure-to-electrolyte potential.
That wet clay is a nasty electrolyte, low resistivity, which is why every bare spot on this section is eating up.
Related terms
Soil resistivity
How strongly soil resists electric current, in ohm-cm, the main guide to soil corrosivity and anode design.
IR drop
Voltage error in a potential reading caused by current flowing through the soil between pipe and electrode.
Anode
Electrode where oxidation happens and metal dissolves; in CP, the hardware installed to discharge current.
Cathode
Electrode where reduction happens and current is collected, so the metal there does not corrode.