Letter 20004

Remove Di-Chromate... replace with Phosphate

am working on developing a relife process for 4130 Steel Tubes that have previously been Cadmium plated with a Di-Chromate seal and then painted. I would like to lightly plastic media blast the paint off and apply phosphate rather than chromate for paint adhesion purposes (ref QQ-P-416 [link is to spec at TechStreet] ). Question is... In what condition should the tubes be in before I send them out to be Phosphate coated? I hope not to have to Strip and Replate the Cad but to simply just replace the Chromate with Phosphate.

Any input is greatly appreciated.

Chad Parkhill
- Phoenix, AZ, USA

I think you'll find it difficult but not impossible to remove the paint and the chromate without removing the cadmium plating--see letter 9406. However, these days the industry tends to use cadmium for corrosion resistance, but only when a combination of its properties is required (lubricity and freedom from gummy corrosion products, for example).

So, as long as you're getting rid of the chromate, it may be a good time to substitute zinc or zinc alloy for the cadmium. Good luck.

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

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