So I have bunch of 9mm and 556 spent casings from last year. They have been in a open container in the garage for a year.

Some have white/greenish bubbles and others (specially 9mm) look like complete rust (brown).

 

When I scratch these white/green powdery bubbles, they expose copper like metal area.  Are casings made of copper with brass coating ?

 

I thought brass casings dont go bad.  Is something wrong with the casings or the way they were stored open in garage ? 

Everything oxidizes when exposed to some level of moisture.  Brass will show white and green like the Statue of Liberty.  Toss them in the tumbler and clean them up GTG.  Brown rust-like on brass is tarnish.  Same rules as oxidation.  Rarely will the brass be no good after exposure for a year in the garage.  Brass is made of copper and zinc.

Cool. Will do. thx

Brass is copper and zinc. If you have serious brown spots, and they remain after tumbling, it is likely a spot where the zinc was leeched out and the cartridge is no longer as strong. It is one of the reasons in not fond of wet tumbling with steel media and lemishine. It can hide such issues.