Galvanic Corrosion: How to Mix Metals Safely
What galvanic corrosion is, the galvanic series, why stainless fixings on aluminium or galvanized steel can cause problems, area ratio effects and how to isolate dissimilar metals.
Saudi Steel Work Editorial Team · · 3 min read
Short answer: Galvanic corrosion occurs when two different metals are in electrical contact in the presence of an electrolyte such as moisture or salt water. The less noble metal (for example zinc, aluminium or carbon steel) corrodes faster. It is prevented by choosing compatible metals, avoiding small anodes next to large cathodes, isolating with non-conductive washers or sleeves, and keeping joints dry.
Steel structures often combine carbon steel, galvanized steel, stainless steel, aluminium and copper. In the wrong combination and conditions, one metal can corrode rapidly.
How it works
Three conditions are needed:
- Two dissimilar metals
- Electrical contact between them
- An electrolyte (rain, condensation, sea spray, wet soil)
The metal that is more "active" in the galvanic series becomes the anode and corrodes faster; the more "noble" metal is protected.
Simplified galvanic series (seawater)
More active (corrodes) → Magnesium → Zinc → Aluminium → Carbon steel → Cast iron → Lead → Brass/Copper → Stainless steel (passive) → Titanium → Graphite → more noble
Area ratio matters
- Small anode, large cathode = bad: carbon steel bolts in a stainless sheet will corrode fast
- Large anode, small cathode = usually acceptable: stainless bolts in a large galvanized or aluminium structure in dry conditions
Common problem combinations
| Combination | Risk |
|---|---|
| Carbon steel fasteners on stainless | High (small anode) |
| Copper pipes touching galvanized steel | High in wet conditions |
| Aluminium cladding on steel without isolation | Moderate–high near coast |
| Stainless bolts on galvanized steel (marine) | Moderate |
Prevention
- Choose metals close in the galvanic series
- Make fasteners more noble than the parts they join
- Use isolating washers, sleeves and gaskets
- Paint or seal joints (paint the cathode too, or at least not only the anode)
- Design for drainage — keep joints dry
- Avoid copper-rich water runoff onto zinc or aluminium
In Saudi conditions
Coastal humidity and salt deposits provide the electrolyte, so detailing matters more in Jeddah, Dammam, Jubail and Yanbu than in Riyadh. See corrosion protection in Saudi Arabia and why stainless rusts.