Evidence grade: A2 — Institutional literature / B1 — Trade practice Effort: Easy · 1–2 hours · nylon washers and sleeves, dielectric paste or sealant, basic hand tools
Yes — aluminum bolted to steel in any damp environment becomes a battery, and the aluminum is the part that dissolves. The fix is to break the electrical circuit: insulate every contact point (nylon washers plus a sleeve on the bolt shank, not just the washer), and keep water out with paint or sealant.
What is actually happening
Galvanic corrosion is electrochemistry, not ordinary rusting. It needs exactly three things: two dissimilar metals, electrical contact between them, and an electrolyte — usually just moisture. Put aluminum and steel together with water, and a galvanic cell forms: the aluminum becomes the anode and corrodes preferentially, sacrificing itself to protect the steel, which acts as the cathode (source 1).
Three things make it worse:
- The area ratio. A small anode next to a large cathode corrodes fast. A steel bolt head through an aluminum panel is nearly the worst case: the tiny aluminum contact ring does all the dissolving (source 1).
- Salt. Seawater spray, road salt, and de-icer make the electrolyte far more conductive, so the corrosion current — and the damage rate — climbs.
- Passive films don't save you. Stainless steel's protective oxide film makes it an even stronger cathode relative to aluminum, so stainless fasteners in aluminum are a classic failure combination (source 1).
The white crusty powder around the bolt is aluminum oxide — the aluminum literally turning into corrosion product. Once it starts, the joint loosens as the metal around the hole thins, and the cycle accelerates.
The fix: break the circuit
Every effective solution removes at least one of the three conditions. The standard trade practice combines all of them (source 1):
1. Electrically isolate the metals. Use nylon or plastic washers under the bolt head and nut, plus an insulating sleeve around the bolt shank. The sleeve matters: washers alone fail if the steel shank still touches the aluminum inside the hole — the circuit must be broken continuously, not just at the faces. Rubber gaskets or insulating plates serve the same purpose between larger surfaces (source 1).
2. Keep the electrolyte out. Seal the assembled joint with paint, caulk, or a dielectric (non-conductive) barrier paste around the fastener. If water can't reach the contact, the cell can't run (source 1).
3. Use compatible fasteners where you can. Metals closer together in the galvanic series drive less current. Aluminum fasteners in aluminum, or 300-series stainless in stainless, are far kinder pairings. When you must mix, isolation (step 1) is doing the real work (source 1).
⚠️ Stop rules
- Below-waterline boat metal or through-hull fittings — STOP. Underwater galvanic corrosion eats fasteners from the inside out, and the failure mode is the boat sinking. Hand this to a marine professional — zinc sizing and bonding systems are not DIY judgment calls.
- Dissimilar-metal joints in pressure piping, fuel lines, or gas lines — STOP. A corroded fitting on a pressurized line is a leak or a burst waiting for a date. Licensed trade, full stop.
- The fastener head or the aluminum around the hole is already eaten away — STOP trying to save it. No isolation scheme restores lost metal. Replace the fastener (and upsize the hole with a bushing if it's wallowed — see Repair Lab A10), then isolate the new joint.
PPE and handling
- Safety glasses and a dust mask are mandatory when grinding or sanding corroded aluminum — the oxide dust is an irritant, and old coatings may contain lead (treat unknown paint as suspect).
- Wear gloves; corroded aluminum edges are sharp, and stainless hardware under galvanic attack can pit into knife edges.
- Ventilation: ordinary shop ventilation is enough for hand assembly and painting. Grinding or wire-wheeling — do it with dust extraction or outdoors.
Honest limits
- Isolation only works if it is complete. One steel shank touching aluminum through a cracked sleeve restarts the whole cell — inspect sleeves for damage during assembly.
- Sealants age. A caulked joint that looked watertight five years ago may be letting water in today. Re-inspect mixed-metal joints annually in salt environments.
- This article covers aluminum/steel, the most common pair. Exotic couples (carbon fiber to aluminum is notoriously aggressive) follow the same three rules but need specialist material selection.
Sources
- Senmit — "Prevent Galvanic Corrosion in Mixed Metals" — https://senmit.com/blogs/academy/galvanic-corrosion-aluminum-stainless-steel — galvanic cell's three requirements; aluminum anodic to passive stainless; small-anode/large-cathode severity; continuous electrical isolation (sleeves, not just washers); nylon washers, insulating sleeves, rubber gaskets, insulating plates, dielectric barriers.
Repair Lab · Release 1 · Evidence last checked September 30, 2026 · A project of AOJEL