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POWDER COATING, March 2016 37 powder coating aluminum alloys is a long subject. I'll give you the short answer here but encourage you to read the two-part article entitled "Alu- minum pretreatment: Old issues and new developments," published in October and November 2003. The articles are in the online Article Index at www.pcoating.com. The three nonnegotiable standards for nonferrous pretreatment are as fol- lows: 1. Remove all organic soils. 2. Etch or deoxidize the part. 3. Final rinse in high-quality water. Should you meet that criteria, then move on to a conversion coating, which could be iron phosphate, zinc phosphate, nonchrome conversion dedicated to aluminum only, chrome phosphate, or chrome conversion. You mention that you have 3 minutes of cleaning for steel substrates in your first stage. Should this product contain sodium or potassium hydroxide in a low-to-moderate level, you can run aluminum through it to partially chemically deoxidize or etch the alu- minum. Carefully pretest the alloys you have to see if you actually etch without producing smut, the gray- black residue left after dissolving the pure aluminum. Smut in excess re- quires a desmuting stage. Various al- loys are more or less prone to smut de- velopment, depending on the chemi- cal makeup of the cleaner in use. You don't mention rinses between each stage, so I hope you have them. Double rinses are more effective than single, and the closer you get to the final stage, the more important it is to have a reverse-osmosis (RO) or a deionized (DI) freshwater halo rinse at least. Many of the new nonchrome conversion coatings dedicated for non- ferrous materials require high-quality rinsing and RO or DI as the makeup water for these conversion coatings. In your particular case, you say you have 1 minute of iron phosphatizing. Check with your chemical supplier. Generally, iron phosphates are the least effective, especially when you are trying to get ac- celerated salt-spray performance. The best phosphates for multi-metal lines are those that contain a fluoride source to assist in deoxidation and alkaline car- ryover from stage one. Finally, you mention that you have a chrome passivation stage. Chrome is still the very best for aluminum when considering all alloys and all coatings. It will depend on the chrome concen- tration. A passivation stage for steel may be too concentrated for alu- minum. Most aluminum chromate conversion coatings operate in a spe- cific range. Again, check with your supplier to see if it's possible. You could also pretest sample parts and vi- sually inspect and coat with a salt-spray verification of the results. I hope this helps you to get on track with a high- quality line. PC Editor's note For further reading, see articles under the Surface preparation heading in the "Index to Articles and Authors 1990- 2015" Reference and Buyer's Resource Issue, Powder Coating, vol. 26, no. 6 (December 2015), or click on the Article Index at www.pcoating.com. Articles can be bought online. Have a question? Click on Problem Solving to submit one.