It isn't exactly normal, but not unheard of. You need a lot of time on the highway (like 2 hours) in cold temperatures (like 20°)to evaporate all the condensation up there, correspondingly less time as the ambient temperatures increase. Many years ago I had smashed the little louver/vents on the valve cover shut in an attempt reduce dust intrusion. Well, they were put there for a reason. I eventually had chunks of rust falling off the valve cover. I ended up getting a nice shiny aluminum valve cover, which will get some corrosion I'm sure, but nowhere near the extent the steel cover did.
