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My black oxide won't turn black

Yesterday my plating department could not get some sample rivets that were turned in house to turn black in our black ox line. We have been plating for many years using the hot black oxide method. The rivet material is a standard 12l14 bar stock turned on swiss style machines. My lab tech. suggested that this batch of bar stock may have other materials in their composition that was preventing the normal black oxide process from occurring. Can anyone tell me what is occurring here and offer a possible solution before this problem occurs at a production level.

Charles E teeter
Weber Knapp Co. - Jamestown, N.Y., U.S.A.
2006

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The free-machining 12L14 contains 0.3% sulfur and 0.25% lead. The lead smears over the surface and prevents blackening. Platers remove the lead with a fluoboric acid etch. However, this generates lead-containing wastewater. I suggest using a 'green' (non-leaded), free-machining steel.

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