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Help on plating Pd on aluminum alloys

2000

I am using a Devanathan cell to measure hydrogen permeation through aluminum alloys (6061, for example). This is a very sensitive and dynamic method to measure hydrogen permeation in metals. It consists of two electrochemical cells with a metal foil sealed between them. One cell is used to cathodically inject hydrogen in the foil, and the other cell holds the metal foil anodic so that the atomic hydrogen is oxidized. The chemical environment is such that the metal does not significantly corrode; so the oxidation of the atomic hydrogen is a major electrochemical reaction. I use 0.1 M K2CrO4 in this side of the cell because aluminum does not undergo localized attack. Measuring the current due to hydrogen as function of time gives the diffusion coefficient, trapping densities, etc.

Here is my problem-I have to have a thin (porous) layer of Pd that is in close, metallic contact with the aluminum because it makes the oxidation of the atomic hydrogen kinetically very fast, but I cannot reliably plate it on the aluminum. The times I have been successful is when I add a pinch of PdCl2 to the 0.1 M K2CrO4 and then cathodically plate for 15 minutes at about 1 mA/cm2, which gives a thin, brown coating. After that, I can make that side anodic and it works like a champ for measuring hydrogen permeation. I believe PdCl2 is unstable in the 0.1 M K2CrO4 because I get a powder in the bottom of the contain which probably means there is a reaction that reduces the palladium salt to the metal state. The next time I add a pinch of PdCl2, it does not work. I cannot get an adherent plate by using a 3.5% NaCl solution with 0.001 PdCl2 + 0.01 M NaNO3-this gives a thin, black coating that just rubs off, and the method does not work for oxidizing hydrogen when the 0.1 M ! K2CrO4 is reintroduced into the cell.

I hypothesize that if an aluminum oxide forms between the aluminum metal and the Pd metal, the aluminum oxide acts as a hydrogen barrier. Any ideas of what I need to do? regards Mike Danielson Battelle

Mike Danielson
Battelle Pacific Northwest Laboratory - Richland, Washington, USA

Hey Mike, You might try either of these two methods.

One, Immersion plating using the acid chloride solution. This was done over copper but I would try it over a zincate on the aluminum. This may be too thin so,

Two, plate the aluminum outside of the cell. First, double zincate the aluminum then plate using Pd(NH3)2(NO2)2 in an ammoniacal medium with a supporting electrolyte of ammonium phosphate. Pd = 5 to 10 g/l and temp 40 to 50 degrees C. Keep the Current Density low. Palladium is a effective diffusion barrier so not to thick. Regards, Fred Mueller, CEF

Fred Mueller, CEF
- Royersford, Pennsylvania
2000