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Black oxide is salmon colored. Is copper the problem?

Q. Given the black oxide formula below, if copper makes its way into the bath and a salmon colored film develops on the steel part, what exactly happened? Is the copper solubilized by the nitrate/nitrite and then at pH of 13-14 bath as Cu2O or CuO2(-2) and it ppts out onto the steel surface where it is reduced? Note at zero potential the copper hydroxide on a Porbaix diagram shows no hydroxide above pH 10?

Weight Formula 0012 4.45-gal
NaOH 36 27
NaNO2 8 6
NaNO3 6 4.5
TEA 0 0
Water 49.3 37.0

Michael Edward
- Harvey, North Dakota
September 13, 2022

A. Hi Michael. When substantial copper is present in a conductive electrolyte it will immersion deposit on steel. We see this in high school science class demos of iron nails in copper sulphate and we see it all the time in various process steps in electroplating shops.

Although I probably haven't looked at Porbaix diagrams enough to claim that I understand them, the electromotive potential difference between iron and steel is probably sufficient to keep copper in the metallic state even at high pH.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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