ordinary stainless such as 304 is not particularly corrosion resistant. 316 is better, but neither is particularly strong, about 35,000 psi yield until it work hardens, then it starts getting brittle. A duplex allow like 2205 will have 2 to 3x the yield strength, and better crevice corrosion and stress cracking corrosion resistance than 300 series. There are some other duplex alloys that are nearly immune from any sort of chloride corrosion at any normal temperature.

I don't think magnafluxing is going to work on your stainless chainplates. This is a process of temporarily magnetizing a piece of steel and using magnetic powder to find cracks (the magnetic flux density is much higher at the sharp edges). Doesn't work on non-magnetic materials. Penetrant dye testing might be what they meant, that can find existing cracks. X-ray would be your best bet. If you wanted to save the existing chainplates I would send them out to have them normalized (a heat treating process that will relieve the internal stresses including work hardening fatigue, to some extent) then have them X-rayed by a testing lab. You do it in that order because latent cracks may open during normalization. It isn't all that expensive, maybe $80 for the stress relieving and $150 for the x-ray. They would have to be re-polished as the heat treating will turn them golden brown until polished again.