Mine did this too, the trim rubs the paint off. Spent about £600 having the lower drivers door, bumper and rear passenger quarter sprayed. Then had to buy new door trims as I took them all off the check the other doors and they fell apart (screws rip out - replaced with stainless on the new ones). They cost about £160 on their own!
Audi didn't want to know about the corrosion warranty - tried two dealers. One was Maidstone who said as the car had bodywork done previously the warranty was invalid, even though the paint they refered to was on the tailgate. The other was Bexley who were very proud in telling me I was wrong, it wasn't covered because i'd said the trim caused the issue and that it was only only rusting from the inside out that was covered and that 'because of what you've said it's not covered'. Ah well.