IDB-GAL-041
Corrosion · galvanic series · dissimilar metals
Galvanic & corrosion compatibility
Which metals can touch — the galvanic series and anodic-index rules — plus the other corrosion forms and how to design them out of an assembly.
Abstract
When two dissimilar metals touch in the presence of an electrolyte (even humidity), the more active one corrodes preferentially — galvanic corrosion. It's one of the most common and most avoidable field failures in mixed-material assemblies. The galvanic series and a few rules tell you which couples are safe.
Section 1 explains galvanic corrosion. Section 2 is the galvanic series / anodic index. Section 3 is the compatibility rules. Section 4 covers the other corrosion forms. Section 5 is mitigation. Section 6 is a checklist.
1.Galvanic corrosion
Galvanic corrosion needs three things at once — remove any one and it stops:
1. Two dissimilar metals (different nobility), 2. an electrolyte bridging them (seawater, condensation, even humid air), and 3. electrical contact between them.
The more active (anodic) metal becomes the anode and corrodes faster than it would alone; the nobler (cathodic) metal is protected. The bigger the nobility gap and the more conductive the electrolyte, the faster it goes.
2.Galvanic series (anodic index)
Ranked from noble (cathodic, protected) to active (anodic, corrodes) — the index difference between two coupled metals predicts the risk (MIL-STD-889 style):
| Metal | Anodic index (V) |
|---|---|
| Gold, platinum, graphite | 0.00 |
| Silver | 0.15 |
| Stainless (passive), titanium, nickel | 0.30 |
| Copper, brass, bronze | 0.40 |
| Tin, lead, solder | 0.65 |
| Stainless (active) | 0.50–0.85 |
| Carbon steel, cast iron | 0.85 |
| Aluminium alloys | 0.90–0.95 |
| Cadmium | 0.80 |
| Zinc (and galvanising) | 1.25 |
| Magnesium | 1.75 |
Zinc and aluminium are deliberately sacrificial — galvanising and zinc-rich primers protect steel by corroding first. Stainless is noble when passive but drops toward active in stagnant, oxygen-starved crevices.
3.Compatibility rules
Keep the anodic-index difference below the threshold for the environment:
| Environment | Max index difference |
|---|---|
| Harsh / marine / outdoor | ≤ 0.15 V |
| Normal indoor | ≤ 0.25 V |
| Dry / temperature- & humidity-controlled | ≤ 0.50 V |
And mind the area ratio: a small anode feeding a large cathode corrodes fast (e.g. a steel fastener in a big copper plate — bad; a copper fastener in a big steel plate — far less so). Fasteners should be at least as noble as the parts they join.
4.Other corrosion forms
| Form | Cause | Guard against it |
|---|---|---|
| Uniform | general oxidation | coatings, paint, nobler alloy, thickness allowance |
| Pitting | chloride attack on passive films (stainless) | 316/higher-Mo grades, avoid stagnation |
| Crevice | oxygen-starved gaps (under seals, laps) | seal/eliminate crevices, drainage |
| Stress-corrosion cracking | tensile stress + specific environment | lower stress, resistant alloy, shot-peen |
| Intergranular | sensitised stainless welds (Cr carbides) | low-carbon (304L/316L) or stabilised grades |
5.Mitigation
- Select compatible couplesthe cheapest fix is choosing metals within the allowed index gap.
- Isolate when you must mix: non-conductive gaskets, shoulder washers/sleeves on fasteners, coatings on one or both faces (break the electrical path).
- Favourable area ratiomake the anode large or the cathode small; never a tiny anodic fastener in a large cathodic part.
- Coat the cathode (counter-intuitive but right: a coating defect on the anode concentrates attack).
- Sacrificial anodes (zinc/magnesium) for large structures; inhibitors / sealants to exclude the electrolyte.
- Drainagedesign out water traps and crevices.
6.Checklist
- List every metal in contact (including fasteners, coatings, springs).
- Electrolyte? indoor dry, humid, splash, or immersed/marine.
- Index gaplook up each metal; check the difference against the environment limit.
- Area ratiois the anode small relative to the cathode? Fasteners at least as noble as the joint.
- If incompatible: isolate (gasket/sleeve/coating), reselect, or add a sacrificial elementand design out crevices and water traps. Pairs with the Heat & surface treatment and Materials references.