I'm repairing something that contains a sandwich of 2mm thick alloy plates (unknown spec.), which was originally held together by M2 brass countersunk screws, tapped into one plate. Moisture exposure has caused the alloy plates to develop surface corrosion, looking like filiform corrosion, although the alloy was not coated in any way. Not surprisingly, galvanic corrosion caused the threads to seize and then be damaged or destroyed when the brass screws were removed.

I considered Helicoil-type inserts, but assume that these might also cause galvanic corrosion, as would the use of metallic screws (perhaps Zn-plated would be OK), so we'd be back where we started pretty soon.

Plan B (which seems better) is to make some flanged, square-section, tapped plastic inserts, locating in square-section holes (anti-rotation), and use nylon screws, to avoid dissimilar metal contact. The square would be about 4mm AF. There is no access from the back of the assembly, when it's fixed in place, so conventional nuts are a no-go. Please can anyone suggest a suitable plastic, other than perspex, which can be turned and milled, and into which a M2 (coarse) thread could be tapped successfully?

Thanks everyone for the thoughts and advice – keep 'em coming!

This is one of those jobs that gets worse, the more you think about it. I may now be at the stage of over-thinking, but better safe than sorry. Unfortunately, I'm stuck with a countersunk screw head, which can't be of larger diameter than a M2 csk head. The M2 female threads are gone, so some sort of insert is needed. The thread will be 2mm max in length, so 5 threads engaged max for M2. M3 would be 4 threads, but the csk head is deeper, and it sits in 1mm thick plate, so engagement would be less. Also, the Italian who made the thing fettled the threaded holes with quite deep countersinking, so the original threads were a skin-of-the-teeth variety. A self-tapper into a plastic insert is attractive, but there's the risk of dissimilar metal corrosion at the head end. Thus, I feel the safest option is a plastic M2 csk screw. In NZ, Helicoil or Recoil kits seem amusingly to cost (in real terms) about double the UK price, so, even though irritating to make, plastic inserts have a huge cost advantage.

I'm aware that Zn-plated , Cd-plated, and stainless fasteners are used a lot in alloy threads, often without any precautionary addition of a chromate treatment, such as Duralac (of which I've had a tube for years, for similar awkward applications). Also, Helicoil-type inserts are used a lot, but I believe these should ideally not be the standard, non-coated type. Perehaps, as suggested, a good dose of Duralac would be OK.

Murphy's law will operate. If I were to take no precautions, and use brass screws in alloy threads, as originally manufactured, there will never again be any corrosion. But, if I'm aware of this, and take the chance, all hell will break loose. Sometimes it's best to accept that you just can't win, and try to design yourself out of the problem…