Corrosion can happen. It did on mine, not with daily driving, but only because I needed to leave it in the long-stay carpark at Gatwick for a few weeks. The rear disc surface facing the prevailing wind was the worst and I needed to skim that. The other three surfaces just cleaned up on the lathe with an emery pad. The extra braking on the fronts during the drive home had cleaned them up, but three weeks later the rears were still bad, hence the skim. The improvement in ordinary braking with clean rear discs and new pads was astounding.
Some pictures here. (Forum Link updated 09/06/2021)
This coming winter I’ll be putting some el-cheapo wheel covers over them during the carpark stay. I’ve added some nylon string ties and curtain weights (to help with throwing the string ends around) to the little retaining hooks so I don’t need to reach behind the wheels…
https://www.amazon.co.uk/Protectors-Motorhome-Protector-Waterproof-Diameters/dp/B07C5D6RHB/