On June 24, 2004, approximately 1510 mountain daylight time, a Cessna 185F, N4585F, ground-looped during the landing roll at Gallatin Field, Bozeman, Montana. The private pilot and his passenger were not injured, but the aircraft, which is owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which departed from an unknown location, was being operated in visual meteorological conditions. No flight plan had been filed. According to the pilot, during the landing roll he was applying normal heavy braking in order to turn off the runway at taxiway Echo, when the aircraft suddenly and rapidly ground-looped to the left. An initial post-accident inspection of the aircraft revealed that the right main gear brake rotor assembly had torn lose from the wheel itself, rendering the right wheel brake totally inoperative. The wheel hub and brake disk assembly were shipped to the NTSB Materials Laboratory, where it was determined that the brake disk attachment flange failed in fatigue in the bend around the attachment flange. The brake disk showed a substantial amount of general corrosion, and although the attachment flange met the minimum thickness requirements, most of its surface was covered by corrosion and corrosion pitting. The fatigue crack propagation that eventually lead to the failure had originated in one of these corrosion pits. The pilot did not return a completed NTSB Form 6120.1/2, so the total flight time information for this report was taken from data recorded on his last FAA medical examination.