I believe nickel is more susceptible to oxidation than steel (just a hunch); and frankly I'm okay with the term "rust" as generally meaning ANY metal oxide even though John is 100% right, it is actually ferric iron oxide as you would find in the redox cycling of many cellular enzymes. I'm not an inorganic chemist which makes me a lot sloppier when I dabble in this field.
Mark suggested some strings may resist oxidation more than others. I believe this is highly likely to be true. Would be a fun study.
I am actually making some string cleaning tubes with denatured
ethanol (okay Gary and John?

), and now I'm wondering about making some more to test NaOH and NaBH4 aqueous solutions to see if I can, respectively, oxidize and reduce or age and "revitalize" strings successfully. Could be a fun little side endeavor.
The interesting thing about metal oxides in strings is that they don't really travel around in a fluid system the way the electrons themselves can, so it's quite different from what I'm used to, dealing with the oxides as a part of enzymes that are used to biotransform toxins (drugs), protoxins (prodrugs), and naturally synthesized hormones. In that case the oxide rarely remains an oxide for very long, a few minutes at best. In a semi-isolated system like a string, I'd imagine it is difficult for an oxide to rearrange itself through any other reaction without being suspended in a solution (sweat or cleaning fluids).