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Cosmetic Control on Black Oxide Surface

We are currently encountering several customer rejects relating to Black Oxide finishing. Rejects encountered like uneven patches, discoloration , bare metal exposed at the edges. The material used is Stainless steel class 301 #4 finish thickness 0.018". Our plating vendor insisted that the root cause is the raw material finish, that it is too shiny thus the oxide cannot adhere to the metal well and recommend pre plating process sandblasting to improve the adhesiveness thus eliminating the cosmetic issues encountered. In your opinion what will be the actual problem causing to the cosmetic failure. Can sandblasting eliminate the quality issues.
Thank you

Cynthia Teng
Manufacturing - Singapore
2006

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Your plating vendor isn't honest.

Black oxide is extremely adherent; it forms by oxidation of the metal surface rather than by deposition. Most likely, the 301 SS was given a black oxidation treatment used for common steels, e.g., (US) MIL-DTL-13924 [on DLA], Class 1. Instead, Class 2, 3 or 4 must be used for SS. I think Class 4 is most commonly used for 300-series SS. Note: a small amount of smut, removable by rubbing with a cotton cloth, is allowed, but there must be adherent black oxide beneath.

If your plating vendor cannot produce a suitable product, contact black oxide chemical suppliers (click 'Chemicals & Perishables' below) and ask for jobshop customers in your area.

Also, another blackening process produces a black CuSe film on ferrous metals (easier on common steel than on SS). This film is less adherent than the black oxide, so has limited applicability.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2006

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