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Black Oxide on stainless finish WITHOUT sulfur

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My company currently uses black chromium plated stainless (type 304 and 316) parts for experiments, but would like to replace this with a less expensive and more robust black oxide process. The blackening is for optical absorption, such as laser dumps, laser baffles, and spectrometer view backgrounds. The parts are in a ultra-high vacuum and often get up to 350 deg C. The problem is that tests have shown that the black oxide parts contain traces of sulfur that "outgas" in the vacuum and sulfur is extremely problematic to our experiments. It seems that the "activators" that are used prior to the blackening salts use sodium bisulfate. Is there a black oxide process for stainless steel that is sulfur free? What happens if the activator is not used prior to using the salts?

Thank you.

Doug Taussig
General Atomics - San Diego, California

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