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Corrosion of Black Oxide

Corrosion of Black Oxide

Corrosion of Black Oxide

(OP)
Sorry if this has been covered before...

I have two steel parts (Aut20 & 18MnCr5) that need to pass a 200 hr salt spray test (ASTM B117). I read that a black oxide finish can withstand 200 hrs of salt spray if the proper sealant is used.

http://www.pfonline.com/articles/pfd0025.html

Is it really possible to obtain this type of corrosion resistance using Black Oxide? If not, can anyone recommend an alternative black plating/coating that is cost effective?

Thank you very much for your help on this!

Securitech

RE: Corrosion of Black Oxide

As the article mentions, the black oxide does not provide the corrosion resistance, the "sealer" does.  In order to achieve several hundred hours of resistance to red rust in a salt spray test one would need to apply ~ 20-50 micrometers of an organic coating (paint) over black oxide or another pretreatment, such as zinc phosphate.  If you want something durable, skip the black oxide and specify a high quality zinc phosphate + ecoat or zinc phosphate + powder coat.

RE: Corrosion of Black Oxide

(OP)
The tolerances of the parts are critical. What type of dimensional change does the zinc phosphate + e-coat or powder coat cause?

RE: Corrosion of Black Oxide

~ 20 micrometers is the typical thickness of zinc phosphate + ecoat.  Powder coating is more variable, with thickness usually in the range of 35-75 micrometers.

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