Letter 531

Spec for black oxide?

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I've been searching for black oxide specifications, particularly relating to coating of fasteners. I've found a general AMS black oxide spec but no others. My fastener supplier isn't much help either.

My company is moving away from Zinc plating due to repeated Hydrogen embrittlement problems and we need to specify the appropriate black oxide processes.

Do you have any suggestions?

Thank you
Mike

Mike S [name deleted for privacy] -
fluid management

Hi, Mike. Here are some spec numbers: ASTM D769 [link is to spec at TechStreet], AMS2485 [link is to spec at TechStreet], Mil-DTL-13924 [link is to spec at TechStreet], Mil-HNDBK-205 [link is to spec at TechStreet] -- there are others too.

Switching to black oxide should be a helpful step toward avoiding hydrogen embrittlement (acid causes hydrogen embrittlement,too), but note that the corrosion resistance of black oxide does not compare to zinc plating!

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Mike - there is still the potential for hydrogen embrittlement. Some guidelines we have adopted.
- Use Mil-C-13924 specification instead of AMS2485, there are better controls in the Military specification and it provides details on correct processing for different materials.
- Do not use acid cleaning for high strength steels, vapor blast is effective for cleaning and surface preparation.
- Require a magnetic particle inspection (MPI) after the Black Oxide process.
- Oiling of the surface is required after Black Oxide, this oil must be removed to allow effective MPI and then the oil reapplied after inspection to assure corrosion protection.
- Acid stripping and re-coating cycles will compound a hydrogen absorption problem since the part is exposed to the acid longer.
- Use hooks and fixtures made of iron or steel, and mask parts of assemblies that are not ferrous material to prevent corrosion contamination of the bath.
- The Black Oxide processing time must be closely controlled to prevent intergranular attack (IGA) and alkaline embrittlement.

Melissa Parks
GE Aircraft Engines

Mike, would you be willing to look at a new PVD plating process for hydrogen-embrittlement-free zinc? No post bake is required. Perfect for high strength steels. Ted is correct, black oxide will not provide sacrificial properties.

Mandar Sunthankar
- Fort Collins, CO

Hi, Mike,

I believe that the zinc black oxide process is still good choice for your applications because of its most convenience and cost-efficiency.

Hydrogen embrittlement is caused by zinc plating and acid pickling not black treatment. Actually, zinc plating has the least risk of hydrogen embrittlement relative to other metal platings because the electrochemical resistance of hydrogen evolution is quite high on zinc. If zinc plating process is properly controlled with right pH range, current density, temperature, time, and additives, hydrogen embrittlement can be minimized. You may notice that zinc plating has almost 100% current efficiency, which means almost no hydrogen evolution on parts during plating.

Baking should be processed immediately after zinc plating at >200 C for at least 4 hours. This is able to eliminate hydrogen in parts completely. Then the baked parts should be activated in either dilute sulfuric acid or nitric acid, depending on black formulations, and thoroughly rinsed in tape water and DI water respectively immediately before blackening treatment.

If you have further concerns on this issue, please feel free to contact me.

L. Hao

Mr. Hao, Your comments are correct, except you didn't mention that the 100% efficiency only applies to Acid Zinc.I think this is important, since Cyanide zinc is quite different in respect of hydrogen embrittlement.

Carlos Cielak
- Mexico

Hi,
I am khemandas chute working as process engineer desire to start the blackodising process for our tools,jigs ,fixtures.I don't have any information about blackodising process.I want to know the chemicals used & process details,so that i can be able to start this process.

Khemandas Chute
Sr.Engineer - Aurangabad,Maharashtra,India.

Hi, Khemandas. In addition to the specs, you can get an intro via our FAQ on Black Oxide & Cold Blackening; and there is a good chapter in the Metal Finishing Guidebook. Many people use proprietary chemistry rather than having to try to independently develop an optimum formulation, but expired patents should point the way to workable mixes. Good luck.

Regards,

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Please give the detailed process of blackodising & chemicals used for this process. I need this information for my knowledge & study. Many of our components are blacodised but nobody is thoroughly knoweledgable about the process.

Ravindra Jawale
Part of the mechanical company - Pune, India

Hi, Ravindra. Can you please try to express your question in terms of what has already been mentioned here? In other words, what specific questions do you have that weren't covered by the referenced specs, our FAQ on the subject, the MFG, and the discussions on this page. Thanks!

Regards,

Ted Mooney, P.E. 
finishing.com
Brick, New Jersey

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