This happens most often when a great stress or strain is put upon a corset or bodice. Uncoated steel bones will rust over time due to damp air, sweat, washings, etc. Coated steel bones are more protected and won't rust under normal wear. However, the coating can be chipped, scratched or otherwise damaged and the exposed steel will rust. If you find that a stay has been scratched or the steel for some reason is showing, use clear nail polish or something similar to coat the bare steel before any problem starts.
Stays are white plastic-coated steel with dipped, rounded ends.
All stays are .014mm thick. I have discovered I must implement a handling charge. I usually mail the stays priority class. From Texas west, I mail priority.