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Need help with GM 4347M Specification


We need to apply a Black Oxide Finish per GM 4347M Specification on a Client part.
We have years of experience in Black Oxide processes, but mainly with Oil or Wax as a final finish.
In some instances we have applied a Chromate Rinse, diluting chromic Acid in the final rinse, according to Mil standards in lieu of Oil or Wax.

The part we are confused on this GM spec is the final finish to use. The spec calls in this case for a Black oxide finish without oil, but it doesn't say clearly if we need to use a Chromate rinse or not.
In 1.2 on the spec, Material Description, it states that suppliers using this spec provide coatings with hexavalent chromate and some without hexavalent chromate. All suppliers who are providing hex free coating must submit the name of chemicals being used and a signed document that the system is Cr6 free, with no Cr6 in final product effective July 2005.
So, our question is; Do we need to apply a Chromate (Cr6 or Cr3) or not? since it doesn't say; apply Chromate after black oxide ....

If we need to apply a Chromate, should this be Cr3 or can we use any other Top Coat as long as it is Cr6 free? Some Top Coats don't have either one of the Chromates (3 or 6)

We use Trivalent chromates for zinc plating, but these cannot be used for this application, mainly for their low pH. What should we use?

Thanks again for your help,

Sergio Hernandez


Chromate finishes are not required as part of this standard, therefore you do not need to apply one. If you choose to use a chromate, it must be trivalent not hexavalent. Only oil or wax is allowed to be used over the black oxide according to the standard, but you may be able to request a deviation to use some other type of organic topcoat.

Toby Padfield


Thanks for your help.
If the spec calls for a Black oxide finish without oil, but it doesn't say clearly if we need to use a Chromate rinse or not. (Why the mention abour Chrome free process?)

What would you use? The client wants their parts dry, but no oil. We are familiar with dry-to-touch oils but I think this is not what the client wants.

Thanks again for your help.

Sergio Hernandez


The first thing I would do is confirm whether or not a dry-to-touch oil is acceptable. If it is acceptable, then I would use this option. If this is not acceptable, then I would discuss with your chemical vendor the availability of a trivalent chromate rinse that would replace the normally used chromic acid (hexavalent) rinse.

Toby Padfield



Also, I forgot to ask, will this process be compliant with the requirements of GMW3059 Restricted and reportableSubstances for parts ?

Considering the Chrome 3 post rinse.

Thanks for your help.

Sergio Hernandez

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