Letter 20008

Dissimilar metals problem [Florida] 

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We have rf components that have silver plating on their flanges which in turn match against aluminum alloy waveguide. These waveguides are exposed to salt fog and need to last 10 years. Obviously the silver to Al alloy galvanic match is less than workable. What would be the best approach to removing the dissimilar metals problem here. We are pumping 2.5 kw of rf power thru these waveguides so I can not have any arcing going on due to corrosion or gaps in the waveguide flange faces due to an uneven coating at the corners. would electroless nickel on the aluminum flange be the best choice here for the money and fabrication cost incurred due to the selective plating?

Bob Smithy
- Palm Bay

Leaving the galvanic phenomena to other responders, I want to address the problem of silver "migration".

Most likely the aluminum received an immersion zinc(ate) coating before the silver was applied. Silver will march right through zinc, aluminum, gold, and almost anything.

Find a precious metal supplier listed on finishing.com and talk to him about using nickel as a diffusion barrier, or if the galvanic thing is turned around, consider Palladium as a diffusion barrier.

Be aware that proprietary silver brighteners usually contain selenium or antimony both of which co-deposit and lower the RF. You should use the carbon disulfide type brightener.

And I do not believe you will ever get 10 years with bare silver on top.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

Aluminum has an electronegativity of 1.5, silver is 1.9 and nickel is 1.8. Nickel offers a slight improvement over silver.

Todd Osmolski Charlotte Plating, Inc. Charlotte, NC USA

I experienced a similar problem between aluminium parts (6061T6) and silver-coated C-ring (used for hermetic seal in a non-saline humid environment, expected life: a few decades).

I attempted electroless Nickel of the highest quality (high-phosphor type, thickness 30 microns, no cracks). Absolute failure! After only a limited time in hot wet chamber the Ni turned brown,pits formed, starting a wild reaction with the substrate. I have little explanation,and would welcome some advice.

The next step (in my case) is anodising, as electrical insulation is both possible and desirable. Question: any experience around on highly protective anodising compatible with perfect hermeticity? (insulating with polymers is not an option).

Guillaume FIAULT
- Limeil-brevannes, FRANCE

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