It's often mentioned that ferro-rod sparkers are moisture sensitive and will corrode over time in a wet environment.

A TIP:
Along with protecting them in shrink-wrap tubing (as mentioned elsewhere), you can also just put a layer of nail varnish on them. The first couple of strikes remove the varnish & then it works just fine.

Obviouslly this isn't practical for an 'every-day' kit, but if you are worried about one stored in your BOB or whatever, then a coat of varnish is just the job to keep it moisture protected until you need it. (I paint mine with yellow varnish to make it more visable if dropped too.)