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 phosphate coating. The minimum coating requirement is 11 g/m2 (1021 mg/ft.2) which is very high and typically not used as a pre-paint coating. It is often required as a 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 requirement. 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 discontinued. 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 specification 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 phosphate alone. A dip paint or similar coating may need to be applied first to insure coverage of all internal surfaces.
Originally published in the June 2017 issue.
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