New Rotors - vented or unvented - Page 2

rab60bit · 23rd August 2021, 16:11

Brake discs are made of centrifugally cast iron so will start to corrode immediately any air (the air we breath contains some degree of moisture) comes into contact; machined cast iron will show brown rust marks within hours of being exposed to outdoor air. OEM discs are partially machined but otherwise unprotected (except for a temporary coating of moisture inhibitor grease whilst in storage). There are chemically based decorative colour finishes (which look nice but don't offer much corrosion protection but some aftermarket discs are coated with a thin coat of epoxy paint (like the black jobs on post #7) so once the pads have bedded in and the coating has abraded only the working surface of cast iron is exposed to the elements and the rest should retain it's protected coating and remain pristine.
Initially corrosion of cast iron is very rapid but once there is a macro 'surface' of oxide the rate of corrosion declines and eventually becomes a thicker/loose fine crust; unlike rusted steel (which flakes unevenly in layers), the strength of rusted cast iron is only marginally affected by rust unless it's over many decades and is in high-tension load bearing application. Fortunately our brake discs will wear out with use long before they weaken with corrosion.

23rd August 2021, 16:11	#11
rab60bit Posted a thing or two Rover 75 Saloon & Tourer Join Date: Mar 2013 Location: Wilmslow Posts: 1,515 Thanks: 433 Thanked 301 Times in 212 Posts	Brake discs are made of centrifugally cast iron so will start to corrode immediately any air (the air we breath contains some degree of moisture) comes into contact; machined cast iron will show brown rust marks within hours of being exposed to outdoor air. OEM discs are partially machined but otherwise unprotected (except for a temporary coating of moisture inhibitor grease whilst in storage). There are chemically based decorative colour finishes (which look nice but don't offer much corrosion protection but some aftermarket discs are coated with a thin coat of epoxy paint (like the black jobs on post #7) so once the pads have bedded in and the coating has abraded only the working surface of cast iron is exposed to the elements and the rest should retain it's protected coating and remain pristine. Initially corrosion of cast iron is very rapid but once there is a macro 'surface' of oxide the rate of corrosion declines and eventually becomes a thicker/loose fine crust; unlike rusted steel (which flakes unevenly in layers), the strength of rusted cast iron is only marginally affected by rust unless it's over many decades and is in high-tension load bearing application. Fortunately our brake discs will wear out with use long before they weaken with corrosion.