Here are my observations in 51 years on this earth. I've noticed when shooting a shotgun in humid weather that the chamber does get very moist. I've always warned my shooting partners when the conditions are right and that they had better not neglect their guns or chamber rust would result. I believe it is from the sudden release of the pressure like what occurs in a refrigeration system.
Some people seem to have more sweaty and acidic hands than others. A friend of mine's guns always seemed to rust on him. I don't have much of a problem with this.
We double gun fans have all seen those nice old doubles with no finnish and some of the nice Remington 1894's in the double gun journal. I have an 1894 trap gun. It seems that Remington used a type of steel that was more prone to rusting and pitting. Look at the bottoms of the frames on those nice 1894's where they were carried and you will see that real fine pitting eating through the pollish and case hardening. Mine has this also. Look at some of the old Parkers and Fox guns (and some others). The steel is white but there is no corrosion. I'm not saying that they won't rust but they seem to be more resistant to it. Some of these guns have seen alot of action on the marshes and the rusting is minamal. You know many didn't get tip top care either if any.
Some years ago in the 1990's Remington was sued in a class action lawsuite over the steel that they used in their 1100 barrels. Many of us with 1100's were party to this and recieved a settlement from them after they lost the case. The adds were run in the American Rifleman for those who owned these guns. Many of us sent that money back to the NRA to fight gun control. They were accused of using weak and /or cheap steel in the barrels. I'm satisfied with the steel that they used but after the case they did change the steel to a different grade. Steels that are made from scrap metal and have to have elements added to bring the steel up to the specified spec are called Modified. The tramp metals in the steel will cause the steel to corrode faster like the japanese car bodies do.
I like Cody's answer the best though.