1engineer
Moderator
Normally I wouldn't disagree with you 1e but you cited a specific alloy of steel. Chemist and engineers spend billions of dollars every year to develop coatings and paint systems to fight rust. If rust were a protective barrier for all steels then nothing would be painted.
You of course are right. I only bought up that specific example because is some cases rust IS a protective coating. On a vehicle, the type of rust on axles and yokes is absolutely nothing to worry about. Unless you live in the desert you will eventually get some form of surface rust on unpainted or primed surfaces which again is nothing to worry about. Don't tell anyone to look at their brake discs after a rain and been setting for a day or two lol or they will worry.