Heat, after years of it, the chrome plating will start to chip off and it will look like a crazy surface rusted header. The Rx-7s generate a LOOOOOOT of heat so the headers turn blue within the first 2 or 3 days, and at flaking by the first year, after2 or 3 years, it looks like this (the orange/brown pipe in the mid-bottom):
It's harmless for you lower exhaust gas temperature piston guys, and it's not even really a problem for us rotary guys unless the header is 15 or 20 years old.
