Cathodic Protection Current Demand

Cathodic protection works by supplying, from outside, the electrons the metal would otherwise have supplied by dissolving. The question this equation answers is how many: how much current the structure needs before its surface stops giving up iron. Everything downstream — the number of anodes, their mass, the rectifier rating, the cost — follows from this one number, which makes it the most consequential estimate in a cathodic protection design and the one made with the least certainty.

The area is the cathode's. It is every square metre of the structure being protected: the whole buried surface of the pipeline, the whole wetted surface of the hull, the whole submerged jacket. Taking the anode's area instead is the classic error in this calculation, and it is not a small one — on a pipeline the ratio between the two runs to several orders of magnitude, and a system sized that way will not polarize anything. The confusion is understandable, because the anode's area is what governs its output, but that is a different equation with a different purpose.

The current density is a property of the environment far more than of the steel. Buried steel in ordinary soil typically wants 10–30 mA/m² of bare surface; quiet seawater 50–90; flowing or aerated seawater considerably more; warm soil, tidal zones and bacterially active ground more again. What moves it is anything that speeds up the cathodic reaction — dissolved oxygen above all, then temperature, then movement that keeps replenishing oxygen at the surface. This is where published practice earns its keep, because these figures come from decades of field experience rather than from theory, and no calculation from first principles will produce them.

The coating breakdown factor is the term that makes coated structures economically protectable at all, and it is the reason coating and cathodic protection are designed together rather than as alternatives. A good coating exposes perhaps 1% of the surface when new, cutting current demand by a hundredfold; the anodes then only have to serve the holidays. But it degrades, and competent designs carry three factors — initial, mean and final — rather than one. The mean sizes the anode mass; the final sizes the anode count and the driving voltage, because the system must still work at end of life. A design using the initial factor throughout is not conservative in any direction that matters.

Two ways this number goes wrong in the field. Measured demand far higher than calculated usually means current is leaving somewhere it should not: a shorted casing at a road crossing, a metallic contact with a foreign structure, or a bond installed for one purpose and now draining the system. And demand can fall dramatically after commissioning, because cathodic protection raises the pH at the steel surface and precipitates a calcareous deposit from the hardness in the water — a chalky film that is itself protective and can cut the sustaining current to a fraction of what was needed to establish polarization. Marine systems are designed around this deliberately, with a high initial current to build the film and a much lower one to maintain it.