Abstract: Cathodic corrosion protection of a metallic structure in contact with the earth employs a backfill material which is a mixture of a carbonaceous solid and a clay. The backfill mixture is used to fill around at least one anode which is positioned in a borehole in the earth. A DC voltage is applied to the metallic structure and anode to provide the corrosion protection. The backfill mixture is highly electrically conductive and is relatively impermeable to water so as to assist in preventing intermingling, and possible contamination, of different aquifers through which the borehole penetrates.