Metal finishing Q&As since 1989
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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
- 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 [on
eBay or
Amazon]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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