Anode Current Output

This is Ohm's law, and it is doing the entire job of cathodic protection design. The voltage available divided by the resistance in the way gives the current that flows. What makes it interesting is that both terms are smaller and less obvious than they first appear.

Take the voltage. The open-circuit difference between a magnesium anode and bare steel might be a volt or more, but that is not what drives the mature system. Once the structure has polarized to its protection potential — conventionally −0.85 V against a copper/copper-sulphate reference for buried steel — the voltage still available is the anode's operating potential less that criterion. For zinc in soil that leaves about 0.2 V; for magnesium, about 0.7 V. That single difference is why magnesium is the standard anode for buried pipelines in ordinary and high-resistivity soil while zinc is used in low-resistivity soil and seawater, where 0.2 V is enough to push adequate current and zinc's much better current efficiency and lower cost win.

The resistance is almost entirely the anode-to-earth term in a normal installation, and Dwight's equation is what gives it. Cable resistance and the structure's own resistance to earth are usually small beside it — but "usually" is doing work in that sentence. A long, thin lead wire on a small anode, or a header cable serving many anodes in a deep groundbed, can quietly become the limiting element, and a corroded or poorly made connection is the single most common fault found on a system that has stopped protecting. When a system underperforms, measure the circuit resistance before redesigning anything: the number will usually name the culprit.

The self-regulating behaviour that follows from this equation is the quiet virtue of galvanic protection, and it is worth appreciating. If part of a coating breaks down, that area's potential drifts positive, the driving voltage across the couple rises, and the anode automatically delivers more current to exactly that spot. Nothing has to be adjusted and nobody has to notice. An impressed-current system, which supplies whatever the rectifier is set to, has no such feedback — it is far more powerful and can protect far larger structures, but it will happily over-protect a structure into coating disbondment and hydrogen embrittlement if the setting is wrong, and it needs monitoring that a galvanic system does not.

The corollary is that galvanic anodes have a hard output ceiling. If the required current exceeds what the available driving voltage can push through the circuit resistance, no quantity of anode metal fixes it — the anode simply lasts a very long time while the structure corrodes. That is the point at which the design moves to impressed current, and recognising it early is worth more than any refinement of the anode sizing.