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  for Metal Finishing since 1989


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Discoloration from Hydrogen Embrittlement Relief after Plating

Q. We have been cadmium plating steel parts of Landing Gear components. Recently we have acquired a new oven for de-embrittlement relief.
However after 23 hours at 375 °F baking the parts show local and severe discoloration as seen on the photos.

 

If you can guide us about the possible reasons or a way to identify root cause,we would be grateful.

Note: The issue has not been observed with the "old" oven, which suggests the problem is about the "new" oven.
Regards,

Ozgur Erdal
Engineer

Ozgur Erdal
Plating Shop Employee - Istanbul, TURKEY
February 23, 2011

A. Since it is an aircraft part, the new oven was supposed to have an oven temperature survey done before it was used. The part should have 1 or more certified thermocouples attached to the part. I would have another oven survey done, since you are having problems. James Watts - Navarre, Florida March 1, 2011 A. Hi Ozgur, As Trent has stated you need to check out for hot spots. The discolouration really does look like an overheating problem, something cadmium is quite sensitive to. Has your oven been calibrated at all? If not you need to get it calibrated in accordance with AMS2750 or BS 2M54, depending on who your customers are. As far as I am aware all aerospace companies require ovens to be calibrated in accordance with one of these specifications. Brian Terry Aerospace - Yeovil, Somerset, UK

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March 7, 2011

Q. Our specs also favors a reactivation agent such as sulfuric acid or sodium cyanide.

However we are not sure how serious the stains could be and try to avoid extra work (cause the "old" oven with ventilating duct do not have such complications).

As a summary,
- We have closed the flap of the "old" oven and finally observed similar discoloration, which strongly suggests the case could be about air quality/ventilation.

- For the local hot spots, I will use 3M temperature recorder tapes on the parts. (Thermocouple do not seem practical and cheap :))

I will inform once the "experiments" are finished.
Please inform if you will have additional comments/advices.

Regards,

Ozgur ERDAL [returning]
- Istanbul, Turkey

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De-embrittlement, alkaline zinc

Q. I am struggling to get a good aesthetic finish on zinc plated high-tensile parts. They look good after zinc plating, but after de-embrittlement (8 hrs at 200 °C) they come out of the oven blotchy.
I then colour passivate them but the blotches are still visible.
The zinc is not coming off and the problem is just a cosmetic one, but the customer is not happy.
How can this be rectified?

daniel hunt
- england
March 19, 2017

A. Hi Daniel, A better description of the blotchy finish would be helpful, maybe even a picture or two. If I was to take a guess I would suggest that the final rinse has been in tap water and the parts have been placed in the de-embrittlement oven wet. The temperature has then baked on the stains. Make sure the final rinse is de-mineralised or distilled and that the parts go into the oven dry, or at least mainly free of rinse water. Brian Terry Aerospace - Yeovil, Somerset, UK March 20, 2017 A. Hi Daniel, The staining after de-embrittlement is quite common. Often small amounts of plating residue coming out of the surface, or just an oxidation reaction with air. This can be removed by a few seconds dip in ~1m/l sulfuric acid solution. This is often done to reactivate the surface prior to chromate passivation, both in zinc and cadmium plating. I hope this is of use for you. Best regards Mark Lees Mark Lees - Cold rock in the Irish Sea March 20, 2017