I just had my plugs changed last month and when it was extracted, the heads had some rusting on it. Quite a fair bit actually. So, we got the plugs replaced, and in doing so, sprayed some anti-rust over it.
After reading your comments, you might be right that the temperature in there might just totally wipe of the anti-rust.
Now its left me thinking why were there rust in the first place? Is it common???