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Hydrogen embrittlement relief by oil bath instead of air


Q. We would like to subcontract hard chromium plating to a company which does embrittlement relief using an oil bath, but we noted that the applicable AMS spec requires the use of an air atmosphere. Other chrome specs with which we are familiar do not have this limitation. Can you think of a specific reason for this limitation? Is it likely that our customer can be convinced to waive this point and accept the use of the oil bath?

Aryeh Asher
- Ashkelon, Israel


A. Aryeh,
Specifications are written by people based on their knowledge, experience and literature. Most people would not want to use oil since it is a mess to clean up afterwards. I can not think of any other reason why it is not a listed option. Your customer is the only one that can tell you if he will allow it. Literature is not going to be much of a help since nearly everyone uses a furnace.
My experience with hydrogen embrittlement is keep the acid dip as short as possible, get it into the plating tank immediately and bake the part as soon as possible after it comes out of the tank.
We used 20 minutes as a goal on cad and 1 hour for chrome that had been waxed, (they were large parts). No failures.
If your customer balks, you might try doing notch bars at higher than required pull or for longer as a technical "push".
Finally, I do not like oil for a bake because he will undoubtedly go in hot or let parts set a long time. Heavy builds on large parts will expand differently as they heat up differently and quite rapidly. I think you will get failures from time to time. Compare the coefficients of expansion of the plate and the parent metal. Do not forget to include dimensions, as the bigger the part, the worse the problem. There are compensations that can be made to reduce this like a very slow immersion.

James Watts
- Navarre, Florida


Q. Some more details are provided here:

We subcontract hard chromium plating as per SAE-AMS2406 [link by ed. to spec at TechStreet]H and would like to use a subcontractor who built his line to conform to FED QQ-C-320 [link by ed. to spec at TechStreet]. This facility is equipped with an oil bath for performing hydrogen embrittlement relief. Unlike the QQ spec, the AMS spec specifically indicates (para. 3.2.2) that parts must be heated in air.

Can you think of a reason for this limitation? It is hard for me to believe that oil can impede the diffusion of hydrogen from the parts. If there is no technical reason for it, do you think that we have grounds for trying to convince our customer (aerospace) to waive this requirement?

Thank you.


Aryeh Asher
- Ashkelon, Israel

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