Galvanic Corrosion: How to Stop Dissimilar Metals From Eating Each Other

Bolt a bright aluminum bracket to a stainless frame, add a little rain, and within a season the aluminum around the fastener turns to white powder. That is galvanic corrosion — and it is entirely predictable once you understand the electrochemistry behind it. This guide explains why it happens and gives you five field-proven ways to stop it.

The Three Ingredients of Galvanic Attack

Galvanic corrosion is a battery you did not mean to build. It needs three things present at once:

• Two different metals with different electrochemical potential.
• Electrical contact between them (they touch, or share a conductive fastener).
• An electrolyte bridging them — rain, condensation, salt spray, even humid air.

Remove any one of the three and the corrosion cell cannot run. That single fact is the basis of every prevention method below.

Reading the Galvanic Series

The galvanic series ranks metals from anodic (active) to cathodic (noble) in a given electrolyte, usually seawater. When two metals are coupled, the more anodic one becomes the sacrificial anode and corrodes faster; the more cathodic one is protected.

The wider the separation between two metals on this ladder, the stronger the driving voltage and the faster the anodic member disappears. A rule of thumb from marine and aerospace practice: keep coupled metals within about 0.15 V of one another, or isolate them.

The Area Ratio Trap

Position is only half the story; relative surface area decides how fast. A small anode feeding a large cathode is the worst case — all the corrosion current concentrates on a tiny area and eats through it quickly. This is why a carbon-steel fastener in a stainless plate fails fast (small anode, huge cathode), while a stainless fastener in a carbon-steel plate is comparatively benign. When you cannot avoid a couple, make the more anodic part the larger one, and never let a critical fastener be the anode.

Five Ways to Prevent It

1. Choose neighbors on the series

The cheapest fix is design intent: pair metals that sit close together. Aluminum with zinc-plated steel is far kinder than aluminum with bare copper or graphite.

2. Break the electrical path

Insulate the joint with dielectric washers, sleeves, and gaskets. Isolation hardware — nylon or phenolic shoulder washers and sleeves around a fastener — keeps the two metals electrically apart even though they are mechanically clamped.

3. Exclude the electrolyte

Paint, powder coat, sealant, or a bead of polysulfide at the interface stops water from bridging the couple. In marine work, bedding compound at every dissimilar joint is standard practice.

4. Use a sacrificial anode

Add a cheap, very-anodic metal (zinc or magnesium) that corrodes instead of your structure. This is why boats carry zinc anodes and buried pipe uses magnesium.

5. Match the fasteners up, not down

Fasteners should be equal to or slightly more noble than the parts they join, so the large parts protect the small fastener rather than the reverse.

Detailing tip: Where a coating is used, coat the cathode, not just the anode. A pinhole in a coating over the anode concentrates attack; a coating over the cathode reduces the driving current safely.

Three Failures You Have Probably Seen

The theory shows up constantly in the field:

• Aluminum on stainless in coastal air. Aluminum is anodic to passive stainless, so the aluminum pits and powders around every fastener. The fix is nylon isolation washers plus a sealed interface, or swapping to a coated fastener.
• Copper pipe draining onto galvanized steel. Water carries dissolved copper ions downstream onto the zinc coating, stripping it and rusting the steel. Plumbing codes require a dielectric union precisely to break this couple.
• Stainless fasteners in an aluminum plate — done right. Here the large aluminum plate is anodic but has a huge area, so attack spreads thin and slow. This is the favorable ratio, and why stainless hardware in aluminum is common and durable when the interface is sealed.

Environment Sets the Pace

The galvanic series is ranked in seawater, but the driving voltages and even the order shift with the electrolyte. Warm salt spray is the most aggressive common environment; clean, dry indoor air is nearly benign because there is no continuous electrolyte to complete the cell. Temperature, pH, and how long moisture lingers all change the corrosion rate. That is why a couple that survives for years in a dry plant fails in a single season on a coastal structure — the metals did not change, the electrolyte did. Always evaluate compatibility for the service environment, not the assembly bench.

Check a Pair Before You Build It

You do not have to memorize the ladder. Our Galvanic Compatibility Chart tells you instantly whether two metals are a safe couple, marginal, or a corrosion problem waiting to happen. For sealed assemblies where fluids meet elastomers, pair it with the seal compatibility checker, and browse related tools under Materials & Process.