This is what I added to the thread:
When carbon dioxide dissolves in the water in air, the pH is about 5.6-5.7. Neutral is pH 7, so that means that rainwater is suprizingly acidic.
Take a Tennifer coated Glock, put it in and pull it out of a holster umpteen times, and you'll get scratches down to the metal in time. That's where the corrosion starts. Then it works its way under the coating.
Add road salt, plating facilities dumping wastes down the drain, and other chemicals making it down to the storm drain, and the rust is not the least bit surprizing. I'd be more suprized if it didn't rust badly.
Ron