That isn't corrosion, it's simply natural discoloration due to friction, this will happen with almost all airbrushes and because of where it is, it has no effect on the performance of your airbrush, this will however happen a lot quicker if you clean your airbrush regular and don't dry off parts like this thoroughly, water or cleaning agents still wet between the threads are the cause of this, the part you have in your hand is machined and chromed, and only after that are the screw threads machined, if they machined that first and then chromed it, it would never fit onto the body.
You should get yourself a Harder & Steenbeck Evolution Silverline 2 in 1 at twice the price and watch the serious discoloration that happens instantly over the whole body just from the sweat on your hands, then you start to really doubt the quality, I'd rather have what you have in your hand than the ugly monster that is usually in my hand, even my so called cheap rubbish airbrushes look better.
Look at this photo, the brush on top is a Black Bull rubbish cheapy, it looks like I bought it yesterday and has never been used, in fact it is a year older than the Evolution, has been used a lot more and even performs better, you can see the tarnishing on the back of the evolution as well as on the air valve, it has been like this since I first used it, the same brush has given me nothing but grief since the day I was possessed by it, I will never ever as long as I live by another H&S, I wouldn't even accept it free.
Note: the top brush was €50 and the bottom one €180
Believe me it isn't corroding, it's made of brass which doesn't corrode, it only turns black, it won't get any worse than it is now, you could clean it with brass polish and some fine scotch-brite and apply a little machine oil to protect it when done, then every time you clean after that it be sure it is dry before you reassemble, you should try to make sure all your airbrush parts are dry before rebuilding, any fluids in sealed or dark places will have the same effect as constant soaking, and a little lubing in the right places will keep it tip top.