⚡ Key takeaways
- Two dissimilar metals plus an electrolyte (moisture, salt water) equals galvanic corrosion. The more active metal becomes the anode and corrodes faster.
- The galvanic series decides who corrodes: aluminium is more active than steel, steel more active than stainless. A steel screw in an aluminium part corrodes the aluminium.
- Area ratio is critical. A small anode against a large cathode corrodes very fast. A steel screw (small) in an aluminium plate (large) is the worst case.
- Three countermeasures: isolate with an insulating washer, choose metals closer in potential, or use a sacrificial coating.
- Sealant and heat-shrink also insulate. They keep water out and break the electrical path.
- Indoor dry environments corrode slowly. Worry outdoors, on the coast and in wet service.
- A stainless screw in aluminium must be insulated, or the aluminium can perforate within one to two years.
1. A steel screw in an aluminium tray
2. How the cell works
3. The galvanic series
4. The area ratio effect
5. Common combinations
6. Three countermeasures
7. Decision tree and FAQ
1. A steel screw in an aluminium tray
In 2019 an EV supplier built battery trays from 6061 aluminium plate fastened with zinc-plated steel screws. A year later coastal customers reported deep pits around each screw and trays leaking.
The zinc-plated screw (cathode) was coupled to the aluminium plate (anode) with coastal salt mist as the electrolyte. The geometry was a small anode around each screw against a large cathode, so the aluminium corroded into deep pits.
The fix was stainless screws with insulating nylon washers plus sealant. Three years on there have been no further reports.
2. How the cell works
Two metals in an electrolyte form a galvanic cell:
- Anode (the active metal): loses electrons and corrodes.
- Cathode (the noble metal): gains electrons and is protected.
- Electrolyte: moisture, rain, salt mist.
The rule of thumb: a potential difference above 0.1 V carries galvanic risk.
3. The galvanic series
| Metal | Potential (V vs SCE) | Activity |
|---|---|---|
| Magnesium | -1.6 | Most active |
| Zinc | -1.0 | |
| Aluminium, high purity | -0.8 | |
| Carbon steel | -0.6 | |
| 304 stainless, active | -0.5 | |
| 304 stainless, passive | +0.1 | |
| Copper | -0.2 | |
| Titanium | +0.2 | Most noble |
The larger the difference, the more aggressive the corrosion.
4. The area ratio effect
This is the part engineers most often miss:
- Small anode, large cathode: high anodic current density, fast corrosion. Dangerous.
- Large anode, small cathode: low current density, slow corrosion. Safer.
A steel screw (small) in an aluminium plate (large) is small anode against large cathode, so the plate corrodes fast. Reverse it — an aluminium screw in a steel plate — and the screw corrodes, which is cheap to replace.
5. Common combinations
| Combination | Risk | Treatment |
|---|---|---|
| Aluminium part, steel screw | High | Insulating washer plus sealant |
| Aluminium part, stainless screw | High | Insulating washer |
| Steel part, stainless screw | Moderate | Usually acceptable |
| Steel part, zinc-plated screw | Low | Zinc acts as a sacrificial anode |
| Copper part, aluminium screw | High | Insulate |
| Stainless to stainless | None | Same potential |
6. Three countermeasures
6.1 Electrical isolation
- Nylon or PTFE washers.
- An insulating sleeve over the screw shank.
- Sealant, which also keeps water out.
6.2 Choose metals closer in potential
Aluminium screws in aluminium parts, though strength is limited; zinc-plated steel screws in steel parts.
6.3 Sacrificial coating
A zinc coating on a steel screw is more active than aluminium, so the zinc corrodes first and protects the aluminium. Once the coating wears through, however, the couple is back.
7. Decision tree and FAQ
Step 1 — Dry indoors or wet outdoors?
Dry indoors → low risk. Outdoors, coastal or wet → insulation is mandatory.
Step 2 — What is the potential difference?
Under 0.1 V → safe. Over 0.2 V → insulate.
Step 3 — Which side is the small anode?
If the small anode is the expensive part, such as the plate → insulate. If it is the cheap part, such as the screw → acceptable.
FAQ
Q. Can a stainless screw really not go into aluminium? Indoors it can; outdoors it must be insulated.
Q. Does an insulating washer reduce clamping force? Yes. Nylon creeps and preload falls over time, so critical joints need metal-backed insulating washers.
Q. Does anodised aluminium still corrode galvanically? The film insulates the surface, but the bore of the tapped hole is bare and still corrodes.
Q. What about zinc-plated steel screws in aluminium? Zinc is more active than aluminium so it protects the aluminium, which is better than bare steel — until the coating wears through.
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