Galvanic Corrosion: Why Dissimilar Metals Fail (and How to Stop It)
How galvanic corrosion destroys assemblies of dissimilar metals - the galvanic series, the cathode-to-anode area trap, and the design rules to prevent it: matching, isolation, area ratio and coatings.
An assembly can be built from two perfectly good metals and still rot at the joint within months. The culprit is galvanic corrosion — when two dissimilar metals touch in the presence of moisture, one of them corrodes far faster than it would alone. It is one of the most common and most preventable causes of premature failure in mechanical assemblies, and it traces back to a choice made on the drawing: which metals were allowed to touch. This guide explains how galvanic corrosion works, the galvanic series that predicts it, and the design rules that stop it.
How Galvanic Corrosion Works
When two different metals are in electrical contact and bridged by an electrolyte (water, humidity, salt spray), they form a small battery. The less noble metal becomes the anode and corrodes, while the more noble metal becomes the cathode and is protected. The bigger the difference in nobility between the two metals, the stronger the driving voltage and the faster the anode is eaten away. So it is not that one metal is “bad” — it is the pairing, plus moisture, that causes the damage.
The Galvanic Series
Metals can be ranked from least noble (anodic, corrodes) to most noble (cathodic, protected). A simplified order:
| End | Metals (typical order) |
|---|---|
| Anodic / least noble (sacrificed) | Magnesium → Zinc → Aluminium → Carbon steel |
| Middle | Cast iron → Lead → Tin |
| Cathodic / most noble (protected) | Brass/Copper → Stainless steel (passive) → Titanium → Gold |
The further apart two metals sit on this list, the worse the galvanic couple. Aluminium bolted to stainless steel, or steel fasteners in a copper part, are classic high-risk pairings — the aluminium or steel becomes the sacrificial anode and corrodes.
The Area Ratio Trap
One rule matters more than any other: the ratio of cathode area to anode area. A small anode connected to a large cathode corrodes very fast, because all the galvanic current concentrates on a tiny area. The practical lesson: a small fastener of the less-noble metal in a large noble part is the worst case — an aluminium rivet in a stainless panel will fail quickly, while a stainless fastener in an aluminium panel is far safer (large anode, small cathode). When in doubt, make the fastener the more noble metal.
How to Prevent It
- Choose metals close together on the galvanic series, or the same metal throughout. This is the cleanest fix — see our materials guide.
- Isolate the metals. Non-conductive washers, sleeves, gaskets or coatings break the electrical path so no galvanic cell forms.
- Keep the joint dry. No electrolyte, no galvanic corrosion — seal joints and design so water drains rather than pools.
- Get the area ratio right. Make fasteners and small parts the more noble metal; never put a small anodic part against a large cathodic one.
- Use protective finishes. Zinc plating, anodising or paint adds a barrier; sacrificial zinc coatings protect steel by corroding first — compare options in our finishing guide.
- Passivate stainless so its protective oxide layer is fully formed and it stays firmly on the noble end — see passivation.
Common Real-World Pairings to Watch
Aluminium with stainless or copper (common in marine and outdoor hardware), steel fasteners in aluminium or copper parts, and any carbon-steel part bolted to stainless in a damp environment are the joints that fail first. In each case the fix is the same: separate them, match them, coat them or dry them. For aluminium assemblies specifically, alloy choice and finish matter — see 6061 vs 7075.
The Bottom Line
Galvanic corrosion is a design decision disguised as a material problem. Two dissimilar metals plus moisture form a battery, and the less-noble one pays the price — fastest of all when it is a small part against a large noble one. Keep paired metals close on the galvanic series, isolate them when you can’t, control the area ratio so fasteners are the noble metal, keep joints dry and add protective finishes. Design the couple out and the assembly lasts; ignore it and the joint quietly corrodes from the inside.
MechPart Pro helps you select compatible metals, fasteners, isolation and finishes so galvanic corrosion never reaches your assembly, with plating, anodising and passivation in-house. Share your assembly and our engineers will flag risky metal pairings as part of our free design-for-manufacturability review. Explore corrosion-resistant options in our stainless grades and materials guides.
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