Letter 32017

Does black oxide require baking?  

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I HAVE BEEN KICKING THIS AROUND FOR QUITE A LONG TIME NOW AND CAN NOT SEEM TO GET A VERY CLEAR ANSWER TO MY QUESTION. WHEN BLACK OXIDE COATING HIGHER ALLOY STRENGTH FASTENERS THAT ARE PLAIN AND ALSO ZINC PLATED IS A BAKE REQUIRED TO RELIEVE HYDROGEN?

IHOR RAJCA
SHELDON ENTERPRISES LL/FASTENER DISTRIBUTOR - MIDDLEBURG HTS, OHIO, USA

If the black oxide is formed by the hot alkaline process there is no hydrogen entrapment. Hydrogen is formed in an electrolytic deposition system at the cathode in acid solutions eg Nickel etc so you have no worries with the black oxide.

Allan Bennett
- Hobart, Tasmania, Australia

Hydrogen embrittlement perhaps not, but most steel must see some sort of pretreatment prior to black oxide. If the pretreat includes cathodic cleaning or acid pickling, hydrogen embrittlement might be a concern.

A couple of other interesting thoughts on this are (1) the new Mil-DTL-13924 [link is to spec at TechStreet] for mil spec black oxide requires post coating baking presumably for hydrogen embrittlement relief. Black oxide has such poor corrosion resistance by itself, we often end up with rusted parts right out of the oven! Oiling the parts prior to baking is a stinky, smoky mess that I wouldn't suggest. (2) Helicopter manufacturer Sikorsky no longer uses black oxide due to what they claim was CAUSTIC EMBRITTLEMENT of high strength steel. I'm not positive that the two had anything to do with the other but it's interesting that the new mil spec and Sikorsky's "no black oxide" edict coincided within months of one another! I think there's a conspiracy here and it begs the question: is CAUSTIC EMBRITTLEMENT removed by baking like HYDROGEN? Any insight into either the mil spec rationale for baking or the phenomenon of caustic embrittlement would be appreciated.

Milt Stevenson, Jr. Anoplate Corp Syracuse, NY, USA

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