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36 POWDER COATING, March 2016 I suggest you rethink your current process and make the changes necessary to improve your success. Check with your chemical suppliers. If they don't know how to improve the process, ask another supplier. Many good ones ad- vertise in this magazine. Good luck and keep trying. Applying a zinc primer over a zinc phosphate conversion coating Problem I recently read an interest- ing article about steel preparation for organic coatings. The article states that phosphating develops a nonconduct- ing layer between the base metal and coating. This is the first time I've ever heard this. If that is true, how could a zinc primer function over a zinc phos- phate conversion coating on cold rolled steel, since it would not have a conduction path to the steel substrate? Does the article really mean that the barrier can't conduct electricity (no electron flow) or the barrier is imper- meable to iron ions or something else altogether? E.B., Maple Valley, Wash. Solution What you're ultimately trying to get answered is this question: Do people use zinc primers over zinc phosphate conversion coatings? Why or why not? And if not, what is best for exterior, cosmetic applications in envi- ronments that range from coastal marine corrosion to desert ultraviolet-light- induced chalking to industrial acid rain? Whether zinc or iron, conversion coat- ings in nature function by neutralizing electrical differences on the substrate. They also provide a base for adhesion for additional organic coatings. This neu- tralization slows down underfilm corro- sion. Couple this with a well-wetted sur- face and a densely crosslinked coating, and you prevent or slow down water and oxygen permeation through the coating. Failure of a coated part often occurs f r o m a i r a n d w a t e r p e r m e a t i n g through to the substrate and rehy- drolyzing minute underfilm impuri- ties. Once this occurs, the impurities (often hard-water minerals) resolubi- lize, and set up a cell for corrosion. When corrosion begins, gases are liber- ated, and film separation occurs in the form of minute bubbles on the surface. Then, oxidation increases, and the base metal fails from the inside out. The reason zinc phosphate is generally more effective at preventing corrosion than iron phosphate is it has a crystalline structure. Iron phosphate is an amor- phous deposition. In simple terms, a zinc conversion coating has more pro- tection or density, generally 150 to 300 milligrams per square foot compared with 40 to 60 milligrams per square foot for iron phosphate. The reason you may want to apply a zinc primer over a zinc phosphate is that you increase the sacri- ficial layer or protective layer over the cold rolled steel substrate. In my opinion, the current state of the art for ultimate corrosion protection for a coated steel substrate is as follows: Start with a well-cleaned and rinsed substrate, apply a zinc phosphate, and then do a chrome-sealed final rinse in a range be- tween 150 to 350 parts per million per second of hexavalent chromium. (Many people today substitute a nonchrome, for the obvious reasons). Coat with a ca- thodic epoxy electrocoat film, partially cure, then apply a high-quality triglycidyl isocyanurate (TGIC) polyester powder coating, and fully cure. The other process option, which in my opinion is as good or better when you look into simplicity of process and envi- ronmental issues, would be like this: Six or seven stages of pretreatment with RO or DI water, then a chlorate-accelerated or highly proprietary organic accelerated iron phosphate, a final DI rinse, dry, a zinc-rich epoxy powder primer partially cured, and a TGIC polyester powder top coat fully cured. I have witnessed production settings in which the salt spray is from 2,000 to 3,000 hours ASTM B-117depositing enough iron phosphate. Pretreating aluminum the right way Problem We have spray pretreat- ment for steel with an iron phosphate conversion process. We have 3 minutes of alkaline degreasing, 1 minute of iron phosphatizing, and 30 seconds of chrome passivating. Can we treat alu- minum sections with a similar process? What is the right way to pretreat alu- minum? D.D., Mumbai, Maharashtra, India Solution Thank you for the ques- tion. High-quality pretreatment for Seeking an end to white streaks Problem My shop processes mildly soiled parts through a three-stage washer. We use an unheated cleaner and phos- phate in stage one, an overflowed water rinse in stage two, and a break-free rinse aid added to stage three. The shop processes light-gauge steel and alu- minum. At times, I see flash rust and streaks on the steel and white streaks on the aluminum after sheeting and drying. We usually don't powder coat the alu- minum. We do clean and charge the washer tanks regularly. What do you think could be causing the white streaks? R.S., Lincoln, Nebr. Solution Thanks for the question. We need to address your process. You indicate you're running steel and alu- minum. What is the percentage of each of these? You have two inherent problems in your system right now. The first one is your use of unheated cleaner and phos- phate in the first stage. The flash rust in the steel most likely comes from the in- ability to adequately phosphatize. You mention light soils, so you're probably getting a good enough cleaning, but without the heat, you're probably not What may be happening is that you clean and provide some pickling of the metal, but you're not depositing enough phosphate to passivate and pre- vent flash rust. By heating stage one, you'll accelerate the phosphate deposi- tion. Check with your pretreatment supplier to get ideal parameters for the particular phosphate you're using. With aluminum, you need to provide some deoxidation or etch; without heat, you're probably not doing that. Does the cleaner phosphate contain fluoride as well? Combination prod- ucts (for ferrous and nonferrous metals) generally incorporate fluoride into the formulation for deoxidation purposes. Your second problem originates in your third stage. My advice? Never use a wet- ting agent in the last stage. This only masks poor cleaning. Generally, any- thing that adds solids to that stage re- duces corrosion protection. This mater- ial may also be the cause of the white streaks on the aluminum. Remember, a part is no cleaner than the cleanliness of the last stage.