After disassembling my forks which have a total of 5 hours of use I've noticed that left fork midvalve shims are corroded badly. They were covered in a black substance which made their surface rough, some shims were bonded together. I used a sandpaper to clean them up and it turned out pretty good, they are smooth now and thicknesses are within specs. But my question is what causes shims to corrode? Cartridges were full of clean factory oil. I'm confused.