Galvanic Corrosion Prevention for Mixed Metal Signs: A Factory Floor Guide

I got a call last March from a sign shop in Tampa. They'd hung a gorgeous set of 304 stainless steel letters on an aluminum raceway six months earlier. The letters were fine. The raceway looked like someone took a cheese grater to it. White powder, pitting, flaking paint. They thought it was a bad batch of aluminum. Nope. Galvanic corrosion. And it was entirely preventable.

The ugly truth is half the sign industry still treats mixed metal like a non-issue. "Stainless and aluminum? They're both metal. It's fine." I've heard that from installers who've been doing this longer than I've been alive.

It's not fine. Put two different metals together with an electrolyte—rainwater, condensation, even humidity—and you've built a battery. The less noble metal corrodes. Aluminum is less noble than stainless steel. So the aluminum eats itself while the stainless sits there looking pretty.

What Galvanic Corrosion Actually Is (And Why Your Aluminum Side Keeps Dying)

Here's what nobody tells you: the galvanic series isn't some abstract chemistry chart. It's a ranking of metals by electrical potential. The farther apart two metals are on that chart, the faster the less noble one sacrifices itself.

Aluminum sits way down on the anodic end. Stainless steel—304 with its 18% chromium and 8% nickel—sits way up on the cathodic end. The difference is about 0.5 to 0.6 volts in saltwater. That's enough to chew through 1.6mm aluminum sheet in under two years if you're near the coast.

I've watched this exact failure on painted aluminum lightbox frames with stainless mounting clips. The paint didn't fail. The stainless clips just sawed through it. That's the brutal part: galvanic corrosion happens even under intact coatings because the electrical connection is still there. The coating only delays the inevitable if you don't isolate the metals.

The most common materials we mix? Aluminum raceways with stainless steel fasteners. Aluminum sign panels with steel support brackets. Stainless letters mounted on galvanized steel backplates. Cast aluminum plaques with bronze inserts. Each one is a battery waiting for