no metal? what are the strap buckles made out of?
anyhow, it would seem to me that any metal buckle is going to be subject to rusting (or some other sort of oxidation) over time, especially when the surface has been scratched, etc. it is just the way things go.
to prevent, try to keep your buckles dry (obviously). you can ride with it in garbage-ass weather all you want, but wipe water off so that the buckles aren't sitting around covered in water for a long time. also, if you store the bag in a particularly humid place for some reason, that'll speed up the process. pretty common sense, eh?