Bolt a stainless bracket to an aluminum frame, leave it outside, and come back in a year to find the aluminum chalky, pitted, and weeping white powder. The stainless looks perfect. That’s not bad luck or bad material — it’s a battery you built by accident. Two dissimilar metals plus an electrolyte equals galvanic corrosion, and it will quietly eat the cheaper, more “active” metal to protect the noble one.
The good news: it’s entirely predictable. Once you understand the galvanic series and the three conditions corrosion needs, you can pair almost any two metals safely. Check any combination instantly with the galvanic compatibility checker.
The three ingredients of a galvanic cell
Galvanic corrosion only happens when all three of these are present at once. Remove any one and it stops:
- Two dissimilar metals — different positions on the galvanic series.
- Electrical contact — a conductive path between them (the bolted joint itself).
- An electrolyte — moisture, salt spray, condensation, even humid air bridging the two.
That last point is why galvanic corrosion is a coastal and marine nightmare but a relative non-issue in a heated, dry interior. Salt water is a ferocious electrolyte; dry desert air barely conducts at all.
The galvanic series: who eats whom
Every metal has an electrical potential in a given electrolyte. Rank them and you get the galvanic series, from anodic (active, gets sacrificed) to cathodic (noble, gets protected). When two are coupled, the more anodic one corrodes faster and the more cathodic one corrodes slower.
| End | Metals (seawater) | Behavior |
|---|---|---|
| Anodic | Magnesium, zinc, aluminum | Sacrificial — corrode to protect others |
| ↓ | Carbon steel, cast iron | Middle — common structural metals |
| ↓ | Lead, tin, brass, copper | Fairly noble |
| Cathodic | Stainless (passive), titanium, gold, graphite | Protected — force their neighbors to corrode |
A rough rule of thumb: keep the potential difference between coupled metals under about 0.15–0.25 V and galvanic attack stays mild. Aluminum (anodic) bolted with graphite or stainless (cathodic) is a worst-case pairing — a wide gap and a brutal electrolyte. The galvanic compatibility chart shows the full ranking at a glance.
The area ratio trap
This is the detail that catches even experienced designers. Galvanic corrosion concentrates on the anode, so the ratio of cathode area to anode area matters enormously. A small anode feeding a large cathode is a disaster — all that corrosion current funnels into a tiny area and chews through it fast.
Disaster: aluminum rivets (small anode) in a large stainless panel — the rivets dissolve.
The takeaway: make your fasteners the noble metal, not the anode. A small cathodic bolt is fine; a small anodic bolt gets sacrificed to the whole structure.
Five ways to stop it
- Isolate — break the electrical path with nylon washers, sleeves, or gaskets at the joint.
- Coat — paint, plate, or anodize so the electrolyte never touches bare dissimilar metal.
- Seal the interface — a non-conductive grease keeps moisture out of the joint. A film of dielectric grease or marine anti-seize on the threads is cheap insurance, and quality tubes of it are a few dollars on Amazon — far cheaper than replacing a corroded assembly.
- Choose closer neighbors — pick metals near each other on the series.
- Add a sacrificial anode — a block of zinc or magnesium that corrodes first (how boat hulls and water heaters survive).
Whatever you do at the joint, get the clamp load right too — the bolt torque calculator keeps your isolated fasteners properly preloaded, and the rest of the materials & process tools cover the surrounding decisions. For deeper background, see the corrosion resistance reference.
A field example: the galvanized-to-copper trap
Plumbers learn this one the hard way. Connect a galvanized (zinc-coated) steel pipe directly to a copper line and the water itself becomes the electrolyte. Zinc is strongly anodic to copper, so the galvanized side corrodes from the inside out, building up rust scale that chokes the flow and eventually leaks — often within a few years. The standard fix is a dielectric union, a fitting with a plastic sleeve and gasket that breaks the electrical path while still carrying water. It’s the same isolate-the-joint logic that protects a stainless-on-aluminum bracket, just plumbed. Whenever two metals far apart on the series have to meet, assume you need a deliberate break between them rather than a bare metal-to-metal joint.
Frequently asked questions
Can I bolt stainless steel directly to aluminum?
Only with protection. Stainless is cathodic and aluminum is anodic, so in any moist environment the aluminum corrodes. Isolate the joint with nylon washers or sleeves, coat the surfaces, and apply dielectric grease — or in dry indoor conditions the risk is low.
Why does the surface area ratio matter so much?
Corrosion current concentrates on the anode. A small anode coupled to a large cathode focuses all the damage into a tiny area and fails fast. Always make fasteners the more noble (cathodic) metal so a large anode spreads the attack thin.
What voltage difference is considered safe?
As a rule of thumb, keeping the galvanic potential difference between coupled metals under about 0.15 to 0.25 volts keeps attack mild. Larger gaps demand isolation, coatings, or sacrificial anodes.