Metal finishing Q&As since 1989

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How to do phosphating on Cadmium and Zinc

December 9, 2009

Greetings All,

I have a spec dilemma. There are some steel parts that we must procure, one with a cadmium plating and one with zinc plating, but my final top coat needs to be either zinc or manganese phosphate. My options are to strip of the cadmium or zinc then put phosphate on the steel or to put phosphate directly on the cadmium or zinc. In understand that it is possible to put phosphate on both of these platings. What chemistry should be used for the phosphate (Mn or Zn)? Is there any special pre-treatment required for the cadmium or zinc?

I've tried our phosphate supplier, but they recently bought this line from someone else and have no technical expertise with phosphate.

Thanks in advance for your help.

Michael Costello
Engineer - Grand Junction, CO USA

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Michael,

It is possible to phosphate over zinc or cadmium with either zinc or manganese phosphate.

Ensure the plating is not dichromate passivated as this inhibits the formation of the coating.

Talk to your local phosphating suppliers, they will be able to advise you of the most suitable product. I don't think I would self-formulate if I was you.

Your coating will not look like normal phosphating, the zinc phosphate coating will be very pale, the manganese phosphate slightly darker.

Brian Terry
Aerospace - Yeovil, Somerset, UK
December 11, 2009

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Thanks Brian,

If the platings have been chromium passivated, would a brief grit-blasting activate the surface?

Michael Costello
- Grand Junction, CO USA
December 14, 2009

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December 17, 2009

Michael,

I wouldn't recommend blasting. Cadmium is a very soft metal and you are likely to damage the plating. I suppose you may get away with it at very low pressure, but I would be extremely cautious about using this method.

Personally I have used a 10% caustic solution at room temperature. If the passivation is new it will be leached out in just a few seconds, if it is older it will take a few minutes. This will not affect the cadmium.

Alternatively, you can try lightly abrading the surface, but then this will create hazards such as hexavalent chrome and cadmium in a fine dust, so must only be done using appropriate PPE.

Brian Terry
Aerospace - Yeovil, Somerset, UK

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Q. Please what kind of Manganese phosphate can I use over cadmium plating and if I need to activate the part before phosphate.

Thanks

Carlos Carrera
- Miami, Florida, USA
November 6, 2012