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Problems in electroforming

 

Dears Sirs:

We operate a small (50 liters) Ni sulfamate bath. Normally we grow small electroformed pieces with excellent results.

Now we tried a 28 cm x 28 cm piece. They had developed internal stresses and a corner "buckled out".

Test was performed in a Hull cell with "StressTabs cathode", and no stress were found up to 2.4 A/dm2 (our working current density is 0.6 A/dm2).

What can we do?

The bath conditions are:

Ni concentration = 70 g/l
Boric Acid= 35 g/l pH = 3,8 - 4,0
Anti-pitting
Temp = 42 °C
Continuous filtration.
Cathodic movement

Irene L. Alanis
- Argentina

The amperage at the corners is far above the average amperage that you are plating at. On a piece that large, I would guess that you would be closer to 10 times the current density of the average density. As a sulfamate plater, but not an electroformer, I would guess that you need shields or robbers or both to avoid the corner problem.

James Watts
- Navarre, Florida

Irene:

If your bath runs properly in the Hull cell then it must be a problem in your bath arrangement. How is your current distribution? Are you placing the anodes properly, robers or shields?.

Please advise.

Guillermo Marrufo
Monterrey, NL, Mexico

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1. Have a solution sample distilled to determine ammonium ion content. ammonia ⇦ this on eBay or Amazon [affil links] (age, temp, anode current density) causes brittleness (not necessarily stress) and if marginal it would show up in the HCD areas first. ammonia should be under 5000 ppm for your application.

2. You did not mention a halogen to dissolve the anode and you did not mention what kind of anodes. S-Rounds will almost dissolve without a halogen but all other anodes require some chloride or bromide, too much raises the tensile stress, too little polarizes the anodes, and that causes ammonia to form, and that causes brittleness, first on the HCD, later all over. There is NO WAY TO REMOVE AMMONIA.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina