Galvanic corrosion: the material-pair trap

Carbon fiber is conductive and electrochemically noble. Couple it to a less noble metal in the presence of an electrolyte and the metal becomes the anode.

Why carbon fiber matters

MIL-STD-889C lists graphite as the most noble entry in its galvanic series (Table II) [1]. Its compatibility table (Table I, graphite column) rates aluminum and carbon / low-alloy steel incompatible with graphite in marine atmosphere, sea water and industrial atmosphere alike [1]. The composites handbook puts it plainly: galvanic corrosion between carbon and aluminum "has all but eliminated the use of aluminum fasteners" in carbon structure (MIL-HDBK-17-3F §6.3.1) [2].

It's the pair, not the side

Galvanic attack depends on the material couple and the electrolyte, not on which side of the joint is blind. That's why the selector picks the fastener alloy from the material pair, separately from the blind-bulb question.

The compatible alloys

In MIL-STD-889C Table I, titanium, the austenitic / PH / heat-resistant stainless steels (which include A-286), Monel and nickel alloys are rated compatible with graphite in marine and industrial atmospheres and incompatible only in sea-water immersion [1]. For blind attachments to composite substructure, MIL-HDBK-17-3F calls for titanium or A286 (C-32) [2]. Titanium's thermal expansion (~8.8 µm/m·°C) is also far closer to carbon fiber (about -0.6 axial) than 2024 aluminum's (~22.5) [3][4].

Breaking the path

Where an incompatible pair is unavoidable, MIL-STD-889C calls for an inert barrier such as sealant between the faying surfaces and sealed exposed edges (§5.1.2) [1]. MIL-HDBK-17-3F adds: stainless fasteners in contact with carbon should be permanent and installed wet with sealant (C-12), and carbon panels joined to aluminum need a fiberglass or Kevlar layer of at least 0.005 in on the faying surface (C-13) [2]. The selector raises a sealant flag whenever it sees a dissimilar pair in moisture or salt.