Inside of discs badly corroded

[Mike22]

I knew I had plenty of life left in my front brake pads, but every time I braked it sounded like my pads were worn down to the metal. When I stripped the brakes down I found my pads were hardly worn but the discs were knackered. The front showed some corrosion but the rear was very badly corroded. It looked like it had been under the sea. My passat diesel is four years old and has only done 23,000 miles. I changed my job a year ago and now travel to work by train. The car only gets used at weekends, so I think lack of use may be the problem why the discs have corroded so badly. Anyone else had a similar experience?