Now that the days are getting longer and I can do critical service work without a flashlight in 9 degree weather, I can see a brown ring on my brake rotor. The caliper failed on this side.
I dont think my failed brake caliper siezed on the rotor and I dont think my brakes were stuck in a locked braking position and the brakes were not literally smoking.
I think the brake piston got stuck in the retracted postion and ceased to function and provide braking pressure.

Nevertheless my Rotor has a brown ring on it. Should this be indicative of a failed rotor and thermal discoloration? Or is the brown ring a sign of some less severe condition. I always though thermal discoloration of overheated high carbon steel was Blue and not brown...

Brown discoloration on Brake Disk Rotor 83 240D

Generally I agree with what you have said... Can you get a picture of it to share ?

The colour of thermal discolouration on carbon steel is dependant on the temperature reached. A light brown colour is achieved at relativly low temeratures - approximatly the temperature used to temper heat treated carbon steel. A deep blue colour indicates a high temerature - high enough to remove all of the hardness achieved in heat treatment.

Without a photograph, it's difficult to say weather or not the brown ring is a result of heat.

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1981 Mercedes 300D - 300 000 km
1980 Camaro Z28

If the brake pad hasn't been pusing on the disc on that side, that brown ring you're seeing could very well be rust. Given the type of use brake discs see, there's no reason to rust-proof them.

Just a thought...good luck!

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999

surface rust due to no friction with a brake pad and maybe some
water/rain.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

Unless they are blue I wouldn't worry to much about it. Brake rotors live a hellish life and can take a ton of abuse. I've seen 'em glow bright red on race cars and still work fine. If they are smooth and not warped I say use them. RT

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When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K

Carrameow,

All the brake rotors I've seen are cast iron which rusts easily and turns brown. If the rotor isn't shiny after use, the brake pad isn't touching the rotor when braking because of a stuck caliper.

P E H