Galvanic Driving Voltage

Put two different metals in the same electrolyte and connect them, and one of them will corrode faster than it would have alone while the other corrodes more slowly. The difference in their potentials is what drives it, and the galvanic series is the list that tells you which way round. It is one of the oldest pieces of practical corrosion knowledge and one of the most consistently misused.

The misuse comes from treating the series as a table of constants. It is not. It is a ranking measured in one specific electrolyte, and the ranking itself changes when the electrolyte does. The series everyone reproduces is for flowing seawater at ambient temperature; series for soil, for soft potable water, for acids and for hot water are different lists with different orders. Steel and zinc are the notorious case: in cold water zinc is anodic to steel, which is the whole basis of galvanising, but above roughly 60 °C the polarity reverses in many waters and the zinc coating on a hot-water tank becomes the cathode while the steel beneath it corrodes. Alloys with passive films get two entries, "passive" and "active", separated by half a volt or more, and which one applies depends on whether the film is intact — which depends on the chloride, the oxygen and the flow.

Even within one series the numbers are ranges, not values. Surface condition, temperature, aeration, flow velocity and the age of the film all shift a metal's potential by tens of millivolts. This is why a competent designer uses the series to answer "which one corrodes?" and "roughly how hard?" and then measures if the answer matters. A single potential quoted to three decimal places from a table on the internet, with no electrolyte named, is worth almost nothing.

Then there is the thing the driving voltage does not tell you at all: the rate. That is set by the circuit resistance and, dominantly, by the area ratio. The total galvanic current flowing is shared over whatever anode area exists, so a large cathode wired to a small anode concentrates the entire cell's current onto very little metal and destroys it quickly. Practically: steel fasteners in a copper or stainless plate fail fast, while copper or stainless fasteners in a steel plate are almost harmless, and the driving voltage is identical in both cases. Coating the anode in a galvanic couple is actively dangerous for the same reason — every holiday in the coating becomes a tiny anode serving a huge cathode. If only one member can be coated, coat the cathode.

The last thing worth saying is that all of this is exploitable rather than merely avoidable. Cathodic protection is a galvanic couple built on purpose: choose a metal active enough to be reliably anodic, give it plenty of area, accept that it will be consumed, and the structure becomes the cathode. The same physics that destroys a steel bolt in a bronze fitting is what keeps every buried pipeline in the country intact.