The checker
Runs in your browser. Pick the two metals in contact and the environment.
How the check works
AC 43-4B, Corrosion Control for Aircraft (2018), paragraph 4.12.2.5: “The further apart the potential voltages of the metals listed in Table 4-1 are, the greater the tendency for galvanic corrosion. The metals in Table 4-2, Grouping of Metals and Alloys, have small differences in electrical potential, so they are relatively safe to use in contact with one another. However, the coupling of metals from different groups will result in corrosion of the group having a lower potential voltage.”
Table 4-2. Group I: magnesium and its alloys. Group II: aluminium and its alloys, zinc, cadmium and cadmium-titanium plate. Group III: iron and steels except stainless, lead, tin. Group IV: copper and its alloys, chromium, nickel and its alloys, cobalt alloys, carbon graphite, stainless steels, titanium and its alloys.
Table 4-1 gives electrode potentials from Alcoa Research Laboratories, from magnesium at −1.73 V to nickel at −0.07 V. The AC warns that these are lab values for pure materials and shift in service, which can, in a few cases, invert the order of two metals. The calculator therefore decides by group and shows the volts for context.
Worked example
A 300-series stainless screw in bare 2024-T3, outside the pressure hull, in rain.
- 2024-T3: Group II, about −0.69 V. Stainless steel: Group IV, −0.09 V.
- Different groups: dissimilar. The lower potential, the aluminium, corrodes.
- Gap: about 0.60 V.
Isolate it: wet-install the screw in primer or sealant, or use the plated or aluminium fastener the SRM calls for.
Limits — when NOT to use this
- Manufacturer's data governs. AC 43-4B applies where the manufacturer has not published corrosion instructions.
- Not a risk rating. The AC gives groups and potentials, not a scale of how fast. Area ratio (a small anode on a large cathode is worst) and the electrolyte matter a lot.
- Coatings change the couple. A cadmium-plated steel bolt presents cadmium (Group II) to an aluminium skin while the plating is intact.
Questions people actually ask
Why do Alclad and bare 2024 sit so far apart?
The cladding is pure aluminium or 7072, more anodic than the 2024 core: it corrodes first and protects the core. That is the point of Alclad.
Is carbon fibre a metal here?
No, but AC 43-4B puts carbon graphite in Group IV. Carbon composite against aluminium is a dissimilar couple and needs isolation, usually a glass ply or sealant.
Sources
- FAA AC 43-4B, Corrosion Control for Aircraft (9/11/18), par. 4.12.2.5, Tables 4-1 and 4-2 — the galvanic series in volts, the four groups, and the rule that the lower-potential group corrodes.
- FAA AC 43.13-1B CHG 1, par. 11-189 — a noncorrosive sealant such as MIL-S-8802 to seal dissimilar metals exposed to salt spray.