How to Strip Zinc Phosphate Without Using Chromic Acid

Q. I want to remove zinc phosphate from hardware parts after cold heading. Typically to remove zinc phosphate, I would soak in chromic acid at a high temperature, but this is not a viable or safe method for bulk treatment. Can I just hit the parts with a higher concentration of potassium hydroxide at a higher temperature to remove the zinc phosphate?

A. Chromic acid stripping of zinc phosphate has fallen out of use primarily due to the environmental and safety considerations associated with hexavalent chromium.

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For that reason, alternative methods are necessary in the removal of some of the coatings previously using chromic acid. For zinc phosphate, one recognized substitute is to use a combination of sodium hydroxide and tetrasodium ethylenediaminetetraacetic acid (abbreviated EDTA). Adding 125 grams of each compound to 1 liter of water is recommended by military standard MIL-DTL-16232G. A similar recommendation can be found in federal standard TT-C-490E.  

While this is significantly less hazardous than the chromic acid solution, care should still be exercised with the handling of the sodium hydroxide and the resultant high pH solution generated. The mixing procedure will also generate a considerable amount of heat. It can be used at ambient temperatures with about a 15- to 30-minute soak time, depending on the coating weight. Higher coating weights will require more time or possibly a higher temperature. The parts should be thoroughly rinsed following this treatment. 

Originally published in the June 2017 issue.