Galvanic corrosion is an electrochemical process that eats away at one metal when two different metals are in contact with each other in the presence of moisture. It’s one of the most common causes of unexpected metal failure in plumbing, construction, marine equipment, and everyday hardware. The metal that corrodes isn’t defective. It’s simply losing a chemical battle to its neighbor.
How Galvanic Corrosion Works
Every metal has a different willingness to give up electrons. When two metals with different electrical potentials touch each other and moisture is present, a tiny battery forms between them. One metal becomes the anode (the one that corrodes), and the other becomes the cathode (the one that’s protected). Electrons flow from the anode to the cathode through the metal contact, while charged particles move through the moisture. The anode gradually dissolves as a result.
Four things must be present simultaneously for galvanic corrosion to occur:
- An anode: the less “noble” metal, which will corrode
- A cathode: the more “noble” metal, which stays intact
- An electrolyte: any conductive liquid, from saltwater to rainwater to condensation
- A metallic path: direct contact or a conductive connection between the two metals
Remove any one of these four elements and galvanic corrosion stops. This is the foundation of every prevention strategy.
The Galvanic Series: Which Metal Loses?
Metals are ranked in what’s called the galvanic series, from most “active” (most likely to corrode) to most “noble” (least likely to corrode). Magnesium sits near the top as one of the most active metals. Zinc and aluminum are also relatively active. Stainless steel, titanium, and gold sit near the noble end.
The farther apart two metals are on this series, the faster the corrosion happens. A magnesium part bolted to stainless steel in wet conditions will corrode aggressively. Two metals close together on the series, like brass and copper, produce very little galvanic effect. The gap between them on the series is what determines severity.
Why Saltwater Makes It Worse
The electrolyte connecting the two metals plays a huge role in how fast corrosion progresses. Pure water is a poor conductor, so galvanic corrosion in dry or low-humidity environments tends to be slow. Saltwater, on the other hand, is highly conductive and dramatically accelerates the process. This is why galvanic corrosion is such a persistent problem on ships, offshore platforms, and coastal structures.
The thickness of the moisture layer matters too. Research on magnesium corrosion in salt solutions shows that when only a thin film of electrolyte covers the surface, galvanic corrosion stays concentrated in small areas right next to the contact point. But when the metal is submerged in a thick layer of electrolyte, the entire surface of the more active metal gets pulled into the reaction, causing much more widespread damage. Temperature and dissolved oxygen also speed things up, which is why warm, aerated seawater is one of the harshest environments for mixed-metal assemblies.
Common Places It Shows Up
One of the most frequent examples is in home plumbing, where copper pipes connect directly to galvanized steel pipes. The steel is more active than the copper, so it corrodes at the joint. Over time, this creates leaks, restricted water flow, and rusty discoloration. Older homes with mixed plumbing systems are especially vulnerable.
In construction, galvanized steel components like corner beads, framing, electrical conduit, and metal lath can all suffer when they contact less active metals. Brass fittings connected to galvanized pipe are a classic trouble spot. Even stainless steel fasteners driven into aluminum panels will cause the aluminum to corrode around the fastener hole, since aluminum is more active than stainless steel.
Marine environments see it constantly. Steel hulls, propellers, rudders, and engine cooling systems all involve different metals exposed to saltwater. Without protection, the more active metal in any pairing will degrade surprisingly quickly.
How Sacrificial Anodes Exploit the Process
One of the cleverest uses of galvanic corrosion is turning it into a deliberate protection strategy. By attaching a chunk of highly active metal to a structure you want to protect, you force the attached metal to corrode instead. These are called sacrificial anodes, and they’re used on ship hulls, pipelines, storage tanks, boat propellers, and offshore platforms.
The three most common sacrificial anode materials are magnesium, zinc, and aluminum. Magnesium has the most negative electrical potential of the three, making it the most “willing” to corrode. That property makes it best suited for onshore pipelines buried in soil, where the electrolyte (soil moisture) has higher electrical resistance and needs a stronger driving voltage to push the protective current. Zinc and aluminum anodes work better in saltwater, where the lower resistance means a smaller voltage difference is enough to provide protection.
Sacrificial anodes are designed to be replaced periodically. As long as the anode material remains, the structure it’s attached to stays protected. When the anode is consumed, the structure becomes vulnerable again.
Preventing Galvanic Corrosion
Since galvanic corrosion requires all four elements (two dissimilar metals, moisture, and a conductive path), prevention means eliminating at least one of them.
The most reliable approach in plumbing and piping is using dielectric fittings. A dielectric union places a non-conductive barrier between two different metals at a pipe joint, breaking the electrical path. These fittings use plastic or rubber inserts to keep the metals physically separated. However, they can leak under certain conditions, and if moisture bridges the gap, corrosion restarts. Dielectric flanges work on the same principle but require more careful installation. Every bolt hole needs a plastic sleeve, and the gaskets between the metal faces must stay fully intact. If a gasket shifts or splits during tightening, metal-to-metal contact is reestablished and the protection fails.
Coatings and paint offer another layer of defense by keeping moisture away from the metal surface. Powder coating, epoxy, and specialized marine paints are common choices. The key is coating the cathode (the more noble metal), not the anode. If you coat only the anode and the coating gets scratched, all the corrosion concentrates at that small exposed spot, making it worse than no coating at all.
Other practical strategies include:
- Using the same metal throughout a system: no dissimilar metals means no galvanic cell
- Choosing metals close together on the galvanic series: small potential differences produce minimal corrosion
- Keeping joints dry: eliminating the electrolyte stops the reaction entirely
- Using non-metallic washers, bushings, or gaskets: these break electrical contact at fastener points
Area Ratio: A Hidden Multiplier
One detail that catches people off guard is the effect of surface area ratio. If a small piece of active metal contacts a large piece of noble metal, corrosion at the small piece accelerates dramatically. The large cathode drives a strong electrical demand, and the small anode has to supply all of it from a tiny surface area. A single aluminum rivet in a large stainless steel panel, for instance, will corrode far faster than if the proportions were reversed.
The practical rule: if you can’t avoid mixing metals, make the more active metal the larger piece. A large aluminum panel with small stainless steel fasteners will corrode much more slowly at the contact points than the reverse arrangement, because the corrosion current gets spread across a bigger anode surface.