Originally Posted by
Panthers007
The mainly-alloy M-16 is parkerized - or whatever they choose to call it. Accurately or not. And you are right regards it going into the metal itself. It does go deep and is not a coating at all. But aside from minor chemical squabbling: My point is/was that alloy nipples would retain a black color. It wouldn't vanish with a small nick or scratch. Brass will easily scratch and remove the coating with it. Then you'd have some lovely green nipples down the road. Oh La La!
I've worked with some of the black nipples in the past and I don't think they are painted. They appear to be plated like the silver ones, however the black plating may be more brittle then the silver chrome plating.
Aluminum anodization is a surface coating also and can be removed with scratching, however it is somewhat tougher. It will fade with exposure to sunlight, however and eventually turn brown. Working with aluminum nipples has its own problems since they are more delicate.