Your right in saying fouling consists of salts.
I disagree with you though when you say salt attacks steel. Actually, the salts produced by real black powder and by the synthetic powders don't attack steel at all, until they absorb moisture. Then, it's the dissolved salts in the water that does the damage.
I'm not saying this to be argumentative. The reason I'm saying it is because if a shooter thoroughly cleans his gun within a few hours of shooting it, no damage will be done to the steel. I know I'm getting grumpy in my old age but, if the shooter "puts it off" his cleaning and doesn't clean their gun within a few hours after shooting, they will get what they deserve. Rusting and pitting. Unfortunetly, the innocent gun can end up ruined if the shooter puts off cleaning the gun for more than a few days.
Now, if the shooter really needs to put off cleaning the gun, if he thoroughly wets the fouling with oil and sets the gun aside, the oil will prevent much of the moisture from the air from getting into the fouling. If this is done, the gun might be able to go for several weeks before the rusting will start.
The problem with this is, often, the shooter will forget that he didn't clean the gun so the next time he wants to use it he will probably find the rusting has started.