Put a stainless bracket against an aluminium tank and nothing happens. Leave it dry for years and still nothing happens. Add road spray, condensation and a crevice that holds moisture, and the aluminium starts to disappear around the fastener. The metals did not change. The circuit closed.
Galvanic corrosion is a three-part problem: two dissimilar metals, an electrical path between them, and an electrolyte bridging the gap. Remove any one of the three and the attack stops. Most of the useful design work is done on the third part, because the first two are usually fixed by the function of the part.
Every metal sits somewhere on a galvanic series, and the further apart two metals sit, the stronger the driving force when they are coupled in an electrolyte. Aluminium sits well away from stainless, which is why the pairing is such a common problem in engine bays and under vehicles.
But the series only tells you the tendency, not the rate. A large potential difference with no moisture present does nothing. A modest difference in a joint that stays wet and warm can do a great deal of damage. Treating the series as a verdict rather than a tendency is where a lot of design argument goes wrong.
The rate of attack depends heavily on the relative areas of the anode and the cathode. A small anode coupled to a large cathode concentrates all the corrosion current into a small area, and that area is consumed quickly. A large anode facing a small cathode spreads the same current over much more metal and the loss is barely visible.
On a vehicle the geometry usually makes this worse rather than better. A stainless fastener is small, the aluminium panel or tank wall around it is large in the joint but tiny in the local contact patch, and the current densities at the interface run high. This is why the damage appears as a neat ring of white powder and pitting around a bolt head.
Aluminium protects itself with an oxide layer, and that layer is unusual because it is attacked by both acids and alkalis. It is stable only in a fairly narrow band of conditions. Once corrosion starts in a pit, the local chemistry inside that pit drifts away from that band and the oxide stops protecting anything.
This is a self-sustaining process, and it explains why aluminium damage looks so localised and so deep compared with the broad surface staining you get on steel. It also explains why washing a joint matters more than coating it. Dilution and rinsing push the local chemistry back towards neutral, and the pit stops growing.
The obvious places are the ones where a stainless part is bolted to an aluminium one: intercooler end tanks to steel brackets, aluminium catch cans and water tanks to stainless straps, aluminium intake tubing to steel stays, stainless fasteners into aluminium castings. Each of those is a designed-in couple with a crevice around it.
The less obvious places matter as much. A stainless heat shield sitting against an aluminium bracket collects road film in the gap between them. An aluminium tank sitting on a steel cradle traps water along the contact line. The joint that looks structurally trivial is often the one that fails first, because it is the one nobody thought of as a joint.
A crevice is worse than an open surface for a simple reason: oxygen cannot get in, so the cathodic reaction cannot happen there, while the anodic reaction continues. The chemistry inside drifts, chlorides concentrate, and the environment becomes far more aggressive than the bulk fluid outside.
Any joint that holds water is therefore a corrosion site regardless of the metals involved. Adding a dissimilar metal couple simply accelerates what the crevice started. This is why the practical fixes are about geometry as much as materials: a drain path, a gap that air can move through, a contact line that does not sit flat against a horizontal surface collecting dirt.
Breaking the electrical path is the cleanest fix where the joint allows it. A polymer washer under a stainless fastener, a nylon sleeve in the hole, a rubber pad between a tank and its strap all interrupt the couple without changing either material. The catch is that the insulation has to survive the service temperature and the vibration, or it becomes a spacer that has quietly disappeared by the time corrosion begins.
Coating is the next option, and the order matters. Coating the aluminium anode is tempting and often counterproductive, because a scratch in the coating creates a tiny exposed anode facing a large coated cathode, which is the worst possible area ratio. Coating the cathode and leaving the anode exposed is more forgiving. Better still is to keep the electrolyte out and let the water leave.
Welding an aluminium assembly does not create a galvanic problem by itself, but it does change the microstructure around the weld, and some filler and parent combinations are more active than the plate they sit in. A weld bead that sits slightly anodic to the surrounding plate will preferentially corrode in a wet chloride environment, which is why the bead sometimes shows attack first.
Welding also creates geometry: overlaps, unfused laps, tight roots, all of which are crevices. A welded aluminium tank with a full penetration joint and a smooth external profile has far less trapped volume than one built from lapped sections. That is a fabrication decision with a corrosion consequence, and it is made at the drawing stage. You can see how we approach aluminium joints under TIG aluminum welding.
Mixed-material assemblies are normal in a performance build, so the aim is not to avoid them but to make the couple predictable. These are the questions that decide whether it stays predictable.
The practical conclusion is that the electrolyte, not the metal pair, is the variable a fabricator can influence most. Keep water moving, keep dissimilar surfaces from sharing a wet crevice, and the couple that looked alarming on paper becomes ordinary. Our titanium exhaust system and stainless steel exhaust system work both live or die on that detail, the material selection guidance sits under shop by material, and the same rules govern our titanium fabrication components and every TIG stainless steel welding assembly we build.
