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Cadmium blistering/adhesion problems

  

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We have two cyanide cadmium plating tanks. Recently a 15% draw off of the solution was conducted to maintain the carbonate levels. After this draw, one, and only one, of the tanks started having blistering/adhesion problems. The draw and chemistry make up was conducted in the same fashion.

Specifics:
Similarities
Both tanks use the same cleaner and HCl pickle.
These problems are limited to areas of the parts where the surface finish callout is 16 or smoother.
The chemistry of both tanks is very close, and are well towards the mid range of the specification.
Current densities of plated parts are set the same between tanks.
Size and agitation in both tanks are similar.
Temperatures are within 2 degrees.

Differences:
The tank without problems has measured rectifier ripple of 45% compared with 5% on the problem tank.
The tank without problems has ~60% of the anodic surface area compared with the troublesome tank.
Part to anode distance is 9" and 11" for the bad and good tank, respectively.
The bad tank has an FRP liner and the good tank has a Koroseal liner.

This has become a real puzzler, and any help or insight relating to troubleshooting the tank would be appreciated.

David Harvey
Aerospace - Portland, OR, USA

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I think this calls for Hull Cell testing. I think you can use a hanging Hull Cell in this case, which would further reduce the variables.

Ted Mooney, P.E. finishing.com Brick, New Jersey

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Has chromic acid from chromate carryover thru ragged racks entered the tank?

Have you mixed in titanium anode baskets, which alternately anodize and do not pass current causing voltage increase and then strip whjen the current is turned off?

Surface Prep? Are you plating onto leaded steel or 4000 high strength steel?

Switch the rectifier leads to opposite tanks and see what happens.

Dip out solution into a 5 gallon bucket, hang over an anode basket, add 4 oz/gal Sodium Cyanide, plate and see what happens.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como

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Thank you for all the input. Follow up to the suggestions are:

Hull cell plating between the tanks was inconclusive regarding adhesion.

Chrome levels in the tanks were measured and were quite low (< 2ppm).

We do use titanium baskets to hold our cadmium anode balls, but this has not changed in years.

We are plating 4000 series steels and use a strong alkaline soap clean followed by HCl pickle prior to plating.

Our tanks are not close enough together to allow for switching of the bussing to test differences in the ripple.


We did notice an improvement by putting reverse current to the anode baskets, but I'm not sure if this is coincidental or not. Any insight as to why this might improve the condition?

Also, I have always considered plating adhesion to happen on such a micro level that surface roughness should not have a real effect. Is this the case?

Thanks again for all the feedback.

David Harvey
- Portland, OR, USA

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You cannot use titanium baskets in a cyanide solution. When the current is turned on the titanium "anodizes" and becomes less conductive, the voltage goes up (if automatic) and the current goes down. Recommend change to steel baskets now.

A dip in HCl on 4000 is seldom enough activation, try a Nickel Chloride Strike: 2 Pounds per gallon Nickel Chloride with one quart per gallon HCl, cathodic. Do not use the S-rounds - use Rolled Depolarized Anodes and take then out when not in use.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como