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

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

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

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