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PFonline.com/experts DAVID S. PETERSON Consultant David has been involved with the industrial parts cleaning industry for almost 30 years, and is author of "A Practical Guide to Industrial Metal Cleaning." How to Strip Zinc Phosphate Without Using Chromic Acid A. Chromic acid stripping of zinc phosphate has fallen out of use primarily due to the environmental and safety considerations associated with hexavalent chromium. 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 proce- dure 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. How to Use Pretreatment to Improve Salt Spray Hours Q. Our goal is to have our gas regulators pass ASTM B117 for 500 hours. We use an epoxy powder coat; parts are treated with zinc phosphate first, then powder coated. We are seeing the internal parts of the castings rusting prematurely. Which spec could I call out to increase corrosion protection, MIL-DTL-16232G Type Z, class? Or TT-C-490, Type I? A. The two specifications you bring up for phosphate coatings are targeted for different purposes and you do not mention details regarding your current zinc phosphate system or coating parameters. The MIL-DTL-16232G mentions that it is intended for applications requiring a heavy zinc phos- phate coating. The minimum coating requirement is 11 g/m 2 (1021 mg/ft. 2 ) which is very high and typically not used as a pre-paint coating. It is often required as a Q. I want to remove zinc phosphate from hardware parts aƞer 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? wear coating for bearings or subsequent forging of deep drawing applications. The various classes in this military standard (Classes 1 to 4) cover a variety of end use coatings to be applied to the component following the phosphate. In general, these are oils, rust preventatives or colorants. Additionally, the military standard specifies a chrome seal for all classes. Despite this very high coating weight and the chrome seal, the standard only specifies a 24-hour salt spray require- ment. The Class 2 coating with rust preventative or oil increases this to a 72-hour salt spray requirement. In the case of the TT-C-490 standard, there are five types called out, although only two are specific to zinc phosphate (Type 1 and 5). Type 2 is for iron phosphate, Type 3 for organic phosphate and Type 4 has been discon- tinued. The primary difference between Type 1 and 5 is the zinc coating weight thickness with Type 1, requiring 300-500 mg/ft. 2 for an immersion coating, while the Type 5 requirement is 500-1100 mg/ft. 2 . Anything higher than this would require the military standard. Another subtler difference is that Type 1 requires grain refinement, typically used for coating weights in this range. The smaller and denser zinc phosphate crystals provide a more adherent and corrosion-resistant base for paint. From the alternatives reviewed, the TT-C-490 Type 1 specifica- tion would be the preferred option for your powder coating operation. However, if you absolutely need to meet the 500-hour salt spray requirement on internal surfaces, you will need to consider something other than the zinc phos- phate alone. A dip paint or similar coating may need to be applied first to insure coverage of all internal surfaces. Intermittent Phosphate Line Usage Q. I inherited a cleaning and phosphate line and over the last two weeks, we have lost our phosphate solution twice. I am told it is because we have lost iron in the solution. Is there an additive that can be used to help maintain the solution as the tank is not in constant use? 38 JUNE 2017 — PFonline.com PARTS CLEANING C L I N I C Chromic acid stripping of zinc phosphate has fallen out of use primarily due to risks associated with hexavalent chromium.