Cathodic protection – carbonaceous backfill and anodes

Cathodic protection is active corrosion protection: the protected structure is shifted to a negative potential and becomes the cathode, while the anodic reaction is transferred to an auxiliary anode. Carbon plays a key role here – the carbonaceous backfill of the anode groundbed is what actually discharges the current into the ground.

We supply coke breeze in fine and coarse grades and calcined petroleum coke as backfill for anode groundbeds, and graphite for anodes, conductive coating and cementitious anode systems and conductive grouts.

Carbonaceous backfill for anode groundbeds

Impressed current anodes are installed in a bed of granular carbonaceous material. The backfill performs four functions at once:

• It reduces the anode-to-earth resistance. The groundbed discharges more current at the same source voltage.
• It enlarges the effective anode surface. The conductive column of backfill becomes part of the anode – the current leaves into the soil from its surface, not from the surface of the electrode.
• It extends anode life. The oxidation reaction moves to the backfill/soil interface. The backfill is consumed, not the expensive electrode.
• It prevents gas blockage and drying out around the anode.

The condition is careful tamping of the backfill around the electrode – without anode-to-backfill contact none of these effects occur.

Material selection by groundbed type:

Anode groundbed type	Recommended material	Indicative resistivity
Horizontal and vertical, shallow	coke breeze, coarse grade	tens of Ω·cm
Deep well (drilled)	calcined petroleum coke, pumpable	units to tens of Ω·cm
Prepackaged (canister) anodes	fine calcined coke, graphite	to specification

Grain size is a compromise, not a detail. Fine, dense grains improve the electrical contact between electrode and backfill. Coarser grains, on the other hand, keep the column permeable so that anodic gases can escape even after tamping. For deep well groundbeds the material must also be pumpable.

Graphite anodes

Graphite anodes are among the classic materials for impressed current cathodic protection, particularly in soil. They are produced by firing coke at high temperatures and are impregnated with linseed oil or resin to reduce porosity and increase oxidation resistance. Compared with scrap steel used as an anode, their consumption rate is an order of magnitude lower.

Cathodic protection of steel in concrete

EN ISO 12696 recognises conductive coating anodes and conductive cementitious anodes among the anode systems. In both cases the function is provided by a carbon filler – graphite powder, fine activated carbon or carbon fibres – dispersed in a binder applied to the surface of the structure. The same principle is used in electrochemical chloride extraction and realkalisation.

This also includes conductive grouts for deep well anode groundbeds: a mixture of calcined petroleum coke, Portland cement and graphite flakes gives a pumpable, conductive and impermeable grout that also seals the borehole against leakage between aquifers.

Forms of supply

• Coke breeze, fine and coarse grades for shallow horizontal and vertical anode groundbeds
• Calcined petroleum coke for deep well groundbeds and prepackaged anodes
• Graphite – powder, flakes, granules; for anodes, conductive coating and cementitious anode systems and grouts
• Pre-blended conductive grouts – carbon filler with cement in the required ratio
• Packaging: 20 and 25 kg bags, 1 t big bags, bulk

Please contact us and we will propose a solution.