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Anode Materials for Impressed Current Cathodic Protection

3.4.1 Requirements for Anode Materials

Impressed current cathodic protection is an effective method to prevent corrosion of underground metal structures such as pipelines and storage tanks. The auxiliary anode is a crucial component in the impressed current system, serving to transmit the protective current through the medium to the surface of the structure being protected.

Anodes for impressed current cathodic protection of underground structures are typically not directly buried in the soil; instead, they are surrounded by carbonaceous backfill to form an anode ground bed. Carbonaceous backfill usually includes metallurgical coke, petroleum calcined coke, and graphite particles. The backfill helps reduce the grounding resistance of the anode bed and extends the anode's service life.

Considering the working environment of the anode and the requirements of practical engineering, an ideal buried auxiliary anode should possess the following properties:

① Good electrical conductivity, high operating current density, and low polarization;

② Excellent chemical and electrochemical stability in harsh environments, low consumption rate, and long service life;

③ Good mechanical properties, resistance to damage, and ease of processing, manufacturing, transportation, and installation;

④ Low overall protection cost.

3.4.2 Scrap Steel Anode

Steel anodes refer to anodes made from angle steel, flat steel, channel steel, steel pipes, or other discarded steel structures used as anodes, with a consumption rate of 8–10 kg/(A·a). Scrap steel was a commonly used anode material in early impressed current cathodic protection due to its wide availability and low cost. As a soluble anode, it rarely releases gases on the surface, so there is no gas blockage issue in the ground bed. Its drawback is the high consumption rate and short service life, making it suitable mainly for temporary protection or in high-resistivity soils.

3.4.3 Graphite Anode

Graphite is a crystalline material formed by heating carbon at high temperatures, usually impregnated with paraffin, linseed oil, or resin to reduce electrolyte penetration and increase mechanical strength. After impregnation, the consumption rate of graphite anodes is significantly reduced. The allowable current density for graphite anodes in ground beds is 5–10 A/m2, making them suitable for drier soil environments.

The performance of the anode should comply with the specifications in Table 3-1. The anode lead cable should be connected to the anode (tubular) at its midpoint, with a contact resistance of less than 0.01 Ω and a pull-off force greater than 1.5 times the anode's own weight. The connection must be reliably sealed. The anode cable length should not be less than 1.5 m, and the anode surface should have no obvious defects.

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