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What you're showing is a pic of the brake rotor and the "hat" that covers the wheel hub. This is BMW's one piece rotor design.
Given the type of material BMW commonly uses, a little surface rust here is not unusual. Often acidic wheel cleaners will contribute to this problem so I highly recommend a pH neutral wheel cleaner and take extra time to rinse brake parts well after washing wheels.
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2023 840GC | 2021 740i | 2019 X3
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