October 21, 2026 · Indumor
Galvanic corrosion: corrosion that occurs when two different metals are in electrical contact and wetted by an electrolyte — water, moisture, salt. They form a battery: the less noble metal acts as the anode and corrodes faster than it otherwise would, while the more noble one, the cathode, is protected. The definition is correct, and it sounds like a lab. In practice it is far more everyday: the bolt that ate its hole, the aluminum sheet pitted around every screw, the brass fitting that stripped the zinc off a galvanized connection.
In a given medium, every metal has its own electrical potential. Ranked, they form the galvanic series. In seawater, from most active to most noble, the list starts with magnesium and zinc, continues with aluminum, carbon steel and cast iron, then lead, tin, bronze and copper, and ends with passive stainless steel, titanium and graphite. The farther apart two metals sit on the list, the stronger the battery. As a reference, zinc sits around −1.03 V and carbon steel around −0.61 V, measured in seawater against a calomel electrode: nearly half a volt apart, more than enough for the zinc to sacrifice itself.
That sacrifice is exactly what galvanizing takes advantage of: zinc is less noble than steel, so if the coating gets scratched, the zinc around it corrodes and protects the bare steel. The same physics that is a problem in a badly designed joint is the solution in a galvanized coating.
Two more things matter as much as the series. The first is the electrolyte: no water, no battery. Two different metals in a dry interior live together for years without trouble; the same two metals outdoors, near the sea or in a plant with vapors, attack each other quickly. The second is the area ratio. A small anode attached to a large cathode corrodes very fast, because all the current concentrates on it. The other way around, a large anode with a small cathode barely notices. That is why a carbon steel bolt in a stainless plate is a bad idea, and a stainless bolt in a carbon steel plate generally is not. An easy way to remember it: the fastener should always be the more noble metal, never the less noble one, because it is small and must not be the anode.
There is a nuance with stainless. It sits so high in the series because its passive film makes it noble; if it loses that film — in a crevice without oxygen, under a gasket, under a deposit of wet mud — it starts behaving as an active metal and drops down the list. That is why stainless clamped against carbon steel in a wet corner can end up corroding too, whatever the table says. The galvanic series is a guide, not a guarantee.
And the environment changes everything. ISO 9223 classifies atmospheric corrosivity from C1, a dry interior, to C5 and CX, aggressive industrial or marine environments. In C1, almost no galvanic couple is a concern; in C4 or C5, even a moderate couple shows up within a few years.
In the shop we see it often in repair jobs, and it is nearly always one of these four cases.
Careful with assuming that “indoors” means dry. In process equipment, water is not only rain: there is condensation on cold pipes, pressure washing and leaks. A galvanic couple under a roof can spend more time wet than one outdoors. And when you can neither isolate nor change the metal, there is a classic fallback: a sacrificial anode of zinc, magnesium or aluminum, which wears away instead of the metal that matters and gets replaced when it is used up. Ship hulls carry them, and so do some tank bottoms. The anode does the dying so the structure does not have to.
The golden rule of painting also comes from the theory, and it is counterintuitive: if only one of the two metals is going to be painted, paint the cathode, the more noble one, not the anode. Painting the anode leaves tiny pores and scratches that become minuscule anodes facing a huge cathode, and there corrosion drills through fast. In practice, the best option is to paint both or isolate them.
The solutions, in order of preference:
And one detail that gets forgotten in the field: an insulating washer is useless if the bolt touches the hole. The isolation has to be complete, with a sleeve on the shank, or the battery forms anyway through the side. It is easy to check: put a multimeter between the two metals after assembly. If it shows continuity, there is contact somewhere, and it is better to find it before corrosion does. Isolation kits also age: gaskets crush and washers crack over the years, so on a joint that matters, the check gets repeated at maintenance.
If you have a joint between dissimilar metals that is being eaten away, send us photos, close up and from a distance, and we will tell you how to isolate it. And in what we build, joints between dissimilar metals are solved at the design stage, with their isolation included.
Send us photos and we will tell you how to isolate it.
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