How to stop hydrogen permeation during gold plating of a SS diaphragm? 3

[jasgeer]

We use gold plating to stop hydrogen permeation in a thin SS sensing diaphragm, but the process of applying the gold plating is causing hydrogen to go through the diaphragm and to be stored in it. How might we change the plating process or post-treat the sensor to avoid the hydrogen problem?
The diaphragm is .0015 bright annealed 316L SS. The first prepatory step is to strike nickel plate per APS 4.17 (QQ-N-290A), followed by Gold plate over the nickel per APS 4.4 (MIL-G-45204C, Type III, Gr A) with .000025 thick. It is my understanding that the surfaces are first cleaned with HCl acid (30-90 seconds) and that HCl is used in the plating processes. I believe HCl is the culprit.
Any suggestions to minimize the hydrogen or to post-treat? The maximum temperature that the diaphragm can be exposed to is less than 200°F.
Thanks,
Jim

 

[kenvlach]

The hydrogen is unavoidable,* it is inherent in electrolytic gold and nickel plating processes. Hence, post-treatment is necessary. A bit of a guess, but I would try 24 hours at 180[sup]o[/sup]F in vacuum.
Another method of hydrogen removal is anodic treatment, as by electropolishing the SS in a 160[sup]o[/sup]F acidic solution. Much quicker, but trickier, and your part seems rather delicate.

*unless you can sputter clean and then evaporate the gold in vacuum.

 

[unclesyd]

Can you plate from a cyanide bath?
Can you used a Copper strike in place of the nickel?
HCl and S/S don't go togather at all.

They are some other processe for Gold Plate that should be amendable to your part.

Take a look at this website. They were a great help to me.

http://www.technic.com/chm/gold.htm

 

[kenvlach]

Cyanide baths are (nearly) all electrolytic and produce hydrogen at the cathode. The Metal Finishing Guidebook 2003 shows that nearly all high purity gold (MIL-G-45204C, Type III -- 99.9%) is electroplated from either alkaline or neutral cyanide gold solutions. Non-cyanide, electrolytic processes are becoming more common, though.

A nickel diffusion barrier is required between copper and gold -- MIL-G-45204C, Paragraph 6.3.

Good plating adhesion to SS, which readily passivates in rinsewater and air, requires Ni strike plating from an HCl-containing Wood’s nickel bath – ASTM B254.

As for alternative processes, it is not too difficult to prepare gold films. The biggest difficulty seems to be maintaining an oxygen-free surface on the SS prior to depositing the gold. Hard to avoid both oxygen and hydrogen. Perhaps do a hydrogen bake-out of the Ni-plated SS within a vacuum (to avoid having to re-activate the surface with acid), then deposit the gold.
Some ideas on alternatives:

Most electroless plating solutions do not produce high purity gold due to incorporation of the reducing element, which also reduces flexibility. Uyemura Int. has an ‘AuBEL’ electroless process capable of 99.9+% deposits, although it operates at pH 14 and 140[sup]o[/sup]F, which rules out many substrates.
(

http://www.uyemura.com/productlist.htm)

Immersion gold plating processes are too thin (monolayers) and lack adhesion.

Years ago, I metallized some zirconia by brushing on a gold suspension and then burning off the carrier. The supplier was Johnson-Matthey. Suspect that the coating was rather porous, though.

Mechanical plating minimizes hydrogen incorporation, although I have never heard of it being used for gold except for traditional goldsmithing. Perhaps press, roll or hammer gold film onto the cleaned SS foil within a hard vacuum (or in a glovebox containing highly purified argon). Or use a PVD process, e.g., argon sputter the SS clean, then evaporate the gold.

 

[JimMetalsCeramics]

At such a thin dimension, the only I hope I can see is to sputter or evaporate. If you evaporate, you will need to ion mill or sputter etch the surfaces first.

Ideally, you will sputter the gold (also the Ni).

 

[LBlount]

I concur with Jim. Instead of chemical plating try thin film coating. No Hydrogen present! Evap will keep temps below 125 degrees. Use plasma to sputter the surface and vacuum will extract most of the excess gasses if you leave it in long enough. Quite a set up for one project.