Every time you bolt a steel bracket to an aluminum frame, you're building a battery. Not a useful one — one that slowly eats your structure. This is galvanic corrosion, and understanding it separates designs that last decades from ones that fail in months.

How it works: When two dissimilar metals are in electrical contact and exposed to an electrolyte (water, humidity, salt spray), the more anodic (less noble) metal corrodes preferentially, sacrificing itself to protect the more cathodic (noble) metal. The driving force is the voltage difference between them on the galvanic series — a ranking of metals by their electrode potential in seawater.

A simplified galvanic series, from most anodic (corrodes first) to most cathodic (protected):

• Magnesium, Zinc, Aluminum
• Carbon Steel, Cast Iron
• Stainless Steel (passive), Copper, Bronze
• Titanium, Graphite, Gold

The rule of thumb: Keep dissimilar metals within 0.25V of each other on the galvanic series. Beyond that, you're asking for trouble — especially in wet or salt-laden environments. Aluminum to copper (~0.6V difference) is a classic failure combination seen in HVAC systems where copper refrigerant lines contact aluminum fins.

Real-world example: Marine engineers learned this the hard way. A bronze propeller bolted to a steel shaft in saltwater will destroy the shaft. The fix? A sacrificial zinc anode — a block of zinc bolted to the hull near the prop. Zinc is more anodic than both bronze and steel, so it corrodes instead. Boat owners replace these cheap zinc blocks annually rather than replacing a shaft every few years. This is cathodic protection, and it's the same principle protecting buried steel pipelines worldwide.

Three ways to prevent galvanic corrosion in your designs:

• Isolate the metals: Use nylon washers, rubber gaskets, or paint between dissimilar metals to break electrical contact. This is why aluminum window frames use plastic thermal breaks where they meet steel structure.
• Choose compatible metals: Stainless steel fasteners in stainless steel components. Aluminum rivets in aluminum sheet. Keep pairs close on the series.
• Mind the area ratio: A small anode connected to a large cathode corrodes extremely fast. A steel bolt in a large copper plate will dissolve quickly. Flip it — a copper bolt in a steel plate — and corrosion is spread over the large steel surface, slowing the rate dramatically. If you must mix metals, make the more noble metal the smaller component.

The area ratio calculation: Corrosion rate scales roughly with the cathode-to-anode area ratio. A 10:1 cathode-to-anode ratio means the anode corrodes roughly 10× faster than if the areas were equal. This is why a single steel screw in a copper sheet is far worse than a copper screw in a steel sheet.