I've noticed this on my kitchen sink plumbing brass:
It looks like as if something has dried up on it, however, I have no clue what this is. I tried scratching it off, and it looks like the metal underneath is not damaged.
Any idea what this might be ? Thanks.
While brass is made to be corrosion/oxidation resistant it can oxidize.
However the color you have indicates more of a calc deposits (build up) from a very small leak somewhere above that spot.
First test would be to use calc remover (like vinegar) to see if it goes away. Please do not use ammonia.
Once cleaned to shiny metal, seal it in a lacquer, to protect the brass.
Needless to say, brass while decorative, is not recommended to use as drain pipe.
