metal - Brass - is this corrosion or oxidation and why did it

I have bought a new brass Y-split water valve which is plain brass but inside is a more polished one, it was packed nicely and I opened it 2 days ago, washed it off with water for some reason, then

Machined Products: Corrosion Tests Prove Free-Cutting Brass Outlasts Plated Steel

I have seen some brass oxidize to green but I have also seen brass oxidize to brown. What is causing the difference? How much copper is in some brass? - Quora

WHY DO BRASS, BRONZE AND COPPER TURN GREEN?, by Adam Markon

A thin layer of alles by oxidation. aluminium aI COROSION: PREVENTION OF..

Preventing Oxidation: The Purpose of Metal Polishing

Is this corrosion? Anything to worry about? How to fix? : r/ammo

Rust, oxidation, acid bleed

metal - Brass - is this corrosion or oxidation and why did it happen? - Chemistry Stack Exchange

Galvanic and Electrolytic Corrosion: Reducing Damage with Zinc Anodes - Belmont Metals