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topic 4456

Cadmium/Chromate on Nickel


(2000)

We have a 105-85 steel casting (40" x 24" cylinder), the part is nickel plated and then Cadmium Chromate. We see a few worn or scratched areas on the surface. No rust evident on the surfaces. Is there some way to tell how much corrosion protection (sacrificial) is left in the Cad/Chromate.

We are overhauling this part and would prefer to redo the Cad/Chromate. Can we "dip" this part again without removing the Cad. If we do remove the Cad, I am told that the Nickel needs to be activated. Does this involve dipping in a Ni tank. Once the Ni is activated how long does it stay activated. We are probably going to settle for some brush plating touchup. Can someone recommend reading material on this subject?

Materials Engineer with no finishing experience.

Thanks for your help

NancyP

Nancy G. Pace
- Clearfield, Utah USA


(2000)

I think you have to find the plating spec for this part, Nancy, because:

When I hear a description of nickel followed by cadmium, I am not confident that separate layers are desired. Boeing developed many decades ago a nickel-cadmium diffused plating which, as I understand it, offers high temperature corrosion resistance which is not available from a regular cadmium surface. This is achieved by nickel plating, then cadmium plating, then baking for diffusion.

Further, steel is subject to "cadmium embrittlement" and it is necessary that the cadmium not actually touch the steel (which it could if you were to cadmium plate a scratch).

I would suggest reading AMS2416 [link by ed. to spec at TechStreet].

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(2000)

hi Nancy

I hope my response is not too late for you.

I agree with Ted that cadmium should not touch steel.

I think brush plating touch up is not a suitable remedy of this problem because of chances of cadmium to touch steel.

But I did not get any idea about your 'scratches'. Is steel is quite visible through these scratches.Why don't you do a simple test to verify this. Copper sulfate should give you bluish colour when sprinkled on this bare surface i.e., scratches.

Yes you do need activation of nickel layer if you strip off cadmium chromate.

How long does the activation last? Good question?

My advise is the sooner you coat cadmium afterwards the better it will be. For best adhesion nickel plated part should be plated for the next layer within 1-5 minute.

But if you have bare steel showing up at those scratches then it would not be a good idea to cadmium chromate after nickel activation.

You may get peeling off problem at those points after some time or immediately.

For this probably you need a complete stripping of nickel layer as well. After this you may need to grind or polish the uneven areas on those spots.

Then follow preparation of cast steel steps and go ahead

Regards

Hemant Kumar
Hemant Kumar
- Florida, USA



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