Hydrogen Relief ofplated Parts
The restriction by QQ-P-416 [link by ed. to spec at Defense Logistics Agency, dla.mil] to the use of brighteners in cadmium plating baths is intendend to prevent hydrogen embrittlement. Bright cad plate is so non-porous that it does not allow for hydrogen contamination relief of the base metal during post plate bake operations.
However, would it not be possible to relieve hydrogen from the base metal if the part is not plated all over (i.e. the ID of a part is left unplated while the OD is bright cad plated)?
A recent test with TDC (thin dense chrome) plating on high tensile strength steel specimens showed that effective hydrogen embrittlement relief did not occur in specimens plated all over (baked for 23 hours at 375 F and into oven within one hour of removal from the plating tank). But specimens which were masked in certain areas - proper hydrogen embrittlement relief did occur.
Any one else with experience with this?Greg Haataja
helicopters - Fort Worth, Texas
Interesting comment. It goes opposite of every thing that I have read. It would make for an interesting paper.
I personally doubt if leaving the ID unplated would allow for the relief of a tube vs a tube with the ID plated. The H2 starts as a surface phenomanon. I doubt if it could migrate thru the metal enough to relieve. That obviously is a personal opinion based on experience, not research.James Watts
- Navarre, Florida
Hydrogen embrittlement is always a problem in plating on high strength steels. This problem arises from the diffusion of H atom produced on the surface into substrate material during metal deposition. Because H atom is very small, it can easily get into metal through displacement. The diffusion rate depends on the H atom density at the surface. It is believed that the brighteners in cadmium plating inhibit the combination of H atom into H2 gas, in turn raise the H atom density at the surface which promotes the diffusion of H atom into substrate material. Because of the large size of H2 gas, it is almost impossible for H2 to directly diffuse into the substrate material at ordinary conditions.
After H atoms get into the substrate material, they tend to assemble at the defects of the material and combine into H2 gases. Once H atoms convert to H2 gases, it is very hard to drive them out at usual baking temperature. This is why hydrogen relief generally follows plating immediately. Even though, it is almost impossible to remove the hydrogen in the substrate material completely. It can be done at elevated temperature with a long period of time,but this will damage the coating.
As James indicated, it is really hard for H atom to migrate through the thicker substrate material relative to coating layer. However, the phenomenon you observed is still possible providing the substrate material is very thin, say, foil, and has the lower diffusion resistance of hydrogen than the coating does, in a very long period of baking (you mentioned 23 hours).
- Grand Rapids, Michigan
January 15, 2008
This all sounds familiar. Seems I'm doing my homework in the same place as you.
- Hurst, Texas
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