Galvanic corrosion of earth electrodes — material choice copper versus steel

The guide on earth-electrode sizing and the guide on soil-enhancement material cover the geometry and soil resistivity of an earth electrode. This guide covers a different, often underestimated risk that is unrelated to the electrical resistance value measured at commissioning: galvanic corrosion, which can degrade or destroy the electrode years later.

The mechanism: a galvanic couple in the soil

When two different metals are electrically connected and both are in contact with an electrolyte — moist soil is an electrolyte — a galvanic couple forms. The metal with the lower (more negative) electrode potential in the galvanic series (the "less noble" metal) corrodes at an accelerated rate, while the more noble metal is protected (cathodically). Copper is considerably more noble than steel (including galvanised steel and reinforcing steel) in the galvanic series. As soon as a copper earth electrode or conductor is directly electrically connected to steel in the same moist soil, a galvanic couple forms in which the steel — not the copper — corrodes at an accelerated rate.

This explains why, for example, directly bonding a copper earth electrode to reinforcing steel in the soil (outside the context of a deliberately designed foundation earth electrode) can lead to accelerated corrosion of that reinforcing steel over time, with a risk of weakening the concrete structure itself — a risk that extends beyond the electrical function of the earthing alone.

Why this is particularly relevant with galvanised steel electrodes

A galvanised steel rod electrode is itself already designed to resist corrosion via its zinc coating, which acts as a sacrificial anode: the zinc layer corrodes preferentially instead of the underlying steel. However, if such a galvanised electrode is electrically connected to copper, not only does the steel beneath the zinc layer become less noble than copper, but the zinc layer itself is also sacrificed faster than would be the case for an isolated (not connected to copper) galvanised electrode. Once the zinc layer is locally depleted, the underlying steel is exposed and corrodes directly and rapidly through the galvanic couple with the copper.

Material choices that limit the risk

Consistent material selection: where possible, execute the earth electrode and its associated underground bonding conductors in the same metal (for example entirely copper, or entirely stainless steel of a suitable alloy) to avoid a galvanic couple between dissimilar metals in the soil.
Copper-bonded (copper-clad) steel rods: a rod electrode with a robust, metallurgically bonded copper layer around a steel core combines the mechanical strength of steel with a copper surface that functions in the soil; as long as the copper layer remains intact (no damage during driving), there is no direct galvanic contact between exposed steel and the surrounding soil.
Galvanic separation at the connection where a copper earthing conductor must nonetheless be connected to steel structural elements (for example for main bonding to a steel structure): make the connection itself above ground or outside the soil, so that the galvanic couple is not located in a moist, conductive soil environment where the corrosion current can actually flow — corrosion occurs primarily where both metals are jointly in contact with an electrolyte, not at a dry, above-ground clamped connection.
Exothermic weld connections instead of mechanical clamps at underground connection points, as covered in the guide on exothermic weld connections (Cadweld) for earthing connections: a properly made exothermic connection between like metals also reduces the risk of a local moisture-trapping crevice that can itself accelerate the corrosion rate of a galvanic couple.

Distinction from bimetallic corrosion at above-ground connections

This mechanism is related to, but not identical with, the bimetallic corrosion that occurs at aluminium-to-copper connections in terminals: there, the issue is a clamped connection between aluminium and copper conductors, usually above ground or enclosed, where moisture ingress into the terminal itself forms the electrolyte. With an earth electrode in the soil, the soil itself is permanently and extensively the electrolyte, which generally makes the corrosion process faster and harder to inspect, since the affected electrode is buried out of sight.

Typical mistakes

Directly connecting a copper earthing conductor to reinforcing steel outside the context of a deliberately designed foundation earth electrode, without accounting for the accelerated corrosion risk to that steel.
Combining galvanised rod electrodes with copper bonding conductors without a copper-bonded design or galvanic separation, causing the protective zinc layer to be sacrificed faster than expected.
Not recognising damage to the copper layer of a copper-bonded electrode during driving as a risk — a damaged copper layer locally exposes the underlying steel, where a galvanic couple with the rest of the copper surface can then still form.
Relying solely on the measured earth resistance at commissioning as proof of long-term integrity — a galvanic corrosion process can erode the electrode's cross-section unnoticed for years before the resistance value visibly deteriorates at a periodic re-inspection.