Galvanic series
A 304 stainless screw in a 6061 aluminium part is 0.55 V apart on the anodic index, over the 0.25 V allowed outdoors: the aluminium is the anode and corrodes round the screw. Pick the fastener and the part it goes in for which metal corrodes, whether the pair is allowed where it will live, and what to do about it if not.
The two metals are marked on the anodic index, with the band the limit allows, and the joint is drawn with the anode pitted.
Not compatible: the part corrodes.
- Difference
- Limit
- 0.25V
- Anode, corrodes
- Aluminium, wrought (6061, 5083, 7075), silicon cast alloys (0.90 V)
- Cathode, protected
- Copper, low brass and bronze, 300 series stainless (0.35 V)
† Calculated from a non-standard input.
Stainless fasteners in aluminium are the right way round, the aluminium being the large anode, but the couple still needs protection: MIL-STD-889D rates aluminium against 316 stainless well into its incompatible classes.
- Make the anode the big part. A small anode against a large cathode corrodes fast; the cathode should be the smaller part. Stainless screws in an aluminium sheet, never aluminium screws in stainless.
- Break the circuit. Where the joint need not conduct, put an inert, non-absorbing barrier between the metals: an insulating washer, sleeve or gasket, or sealant.
- Keep the water out. Seal the faying surfaces and their edges, and put corrosion-inhibiting compound under fastener heads.
- Coat it. Paint or coat both metals (the cathode above all), or plate the cathode with a metal close to the anode in the table.
How it’s calculated
| Group | Metals | Anodic index, V |
|---|---|---|
| 1 | Gold, platinum | 0.00 |
| 2 | Rhodium plate | 0.10 |
| 3 | Silver | 0.15 |
| 4 | Nickel, Monel | 0.30 |
| 5 | Copper, low brass and bronze, 300 series stainless | 0.35 |
| 6 | Yellow brass and bronze | 0.40 |
| 7 | High brass, naval brass, Muntz metal | 0.45 |
| 8 | 18 % chromium stainless (430) | 0.50 |
| 9 | Chromium plate, tin plate, 12 % chromium stainless (410, 416) | 0.60 |
| 10 | Tin-plate, terneplate, tin-lead solder | 0.65 |
| 11 | Lead | 0.70 |
| 12 | Aluminium, 2000 series (2024) | 0.75 |
| 13 | Carbon and low-alloy steel, cast iron | 0.85 |
| 14 | Aluminium, wrought (6061, 5083, 7075), silicon cast alloys | 0.90 |
| 15 | Aluminium, other cast alloys; cadmium plate | 0.95 |
| 16 | Galvanised steel (hot-dip zinc) | 1.20 |
| 17 | Zinc, zinc plate, zinc die castings | 1.25 |
| 18 | Magnesium | 1.75 |
Why do stainless screws in aluminium get away with it?
How fast the anode corrodes depends on how much cathode it works against. Stainless screws in an aluminium sheet put a small cathode against a large anode, so the attack is spread thin; aluminium screws in stainless sheet put a large cathode against a small anode and the screws go quickly. The pair is still over the limit, so the joint is sealed, isolated or coated as well.