Issue link: https://www.e-digitaleditions.com/i/280102
32 POWDER COATING, March 2014 soil and can partially delaminate during the cure of the powder due to the two different masses, that is, scale has less mass than the thicker iron, and when heat is applied, the two shift causing fracturing of the scale and often delamination. Finishers I've dealt with claim to receive 200 to 300 hours salt spray when using HRS. This is about the maximum. Another problem with HRS is that the surface has imper- fections and isn't really a smooth fin- ish. Anything you put on top of the scale will also show those surface dis- crepancies, especially with higher- gloss materials. To answer your question directly— no. You typically won't receive much more than a couple hundred hours of salt spray when using HRS. If you want to achieve highly consistent cor- rosion resistance, you must re move the scale, preferably, mechanically or purchase HPO. You could cer- tainly try to apply a zinc-rich or epoxy primer before the top coat. This may prove satisfactory for your customer. I guess I would test it both ways and offer it to the cus- tomer for their selection after the two different approaches have been tested against the current standard. Really, in the end you would be bet- ter off going to HPO. Meeting RoHS requirements when prepping aluminum for powder coating Problem Recently, we started using a flat (5 percent gloss), self- texturizing, mica-filled epoxy pow- der coating. Previously, we had used a hexavalent chromate pretreat- ment with a liquid polyurethane. To meet European RoHS (Restriction of Hazardous Substances) require- ments, we changed to a nonchrome passivation process with fluoride for aluminum. There has been an adhe- sion problem with the powder coat- ing. In some areas, the coating sticks; in other areas, it doesn't. Would a trivalent chromate pre- treatment, MIL-DTL-5541F, Type II, provide better paint adhesion? I've also read about coatings substitut- ing chromium (Cr) for other metals such as tungsten (W) in Group 6 of the periodic table. I also have con- cerns that the paint is too high in pigment solids, and there isn't enough binder to promote adhesion. Areas failing adhesion have voids at 100X magnification possibly from outgassing of the pretreatment or substrate, or incomplete flow and wetting due to the homogeneity of the surface energy of the substrate. L.T., Irving, Tex. Solution Thanks for the ques- tion. The original process you had that included the hexavalent chromium conversion is the overall best for aluminum substrates. To meet RoHS compliance, I don't believe you can go to a trivalent chromium material, but you should verify this. A chrome-phosphate would indeed improve adhesion, but again will this meet your RoHS requirements? Probably not. Aluminum requires a clean or oil-free surface, a mechanical or chemical deoxidation of the surface, and a high-quality rinse, like de-ionized (DI) e e- Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER By popular demand, we're bringing you a series of past pretreatment problem columns, representing the best of Brad B. Gruss, our columnist for more than 20 years, who recently retired as president of the consulting firm, Pretreatment & Pro - cess Consulting LLC. Brad specialized in training, troubleshooting, and independent line audits for pretreatment pro cesses. In his more than 30 years in the industry, he made numerous presentations on pretreat- ment for powder coating. Consistent salt-spray results with HRS Problem We have a five-stage pretreatment system that gets con- sistent 1,000-hour salt-spray results on cold roll steel (CRS). But one of our customers uses hot rolled steel (HRS), not hot rolled pickled and oiled (HPO), on their outdoor gener- ator enclosures, and they warranty their product for a year. Is there any way to get consistent results on HRS? J.K., Milwaukee, Wis. Solution To achieve 1,000 hours salt spray ASTM B-117 on any sub- strate requires a sound process, good control, and continual maintenance. Because of its nature, HRS is very difficult to prepare for accelerated corrosion and good adhesion. Scale, the dark gray or dark bluish surface found on HRS, is very inert and con- tains very little reactive iron neces- sary for iron phosphatizing. Scale is also attached to the virgin iron very well in some places and very poorly in other areas. This depends on the incoming HRS quality you purchase. Scale will often entrap moisture or l-PretreatProb-32-34_Masters 3/11/14 12:27 PM Page 32