Aluminum Castings and PorosityRegarding Joe Powder’s answer about coating the aluminum castings on sailboats (March 2008,“Ask Joe Powder ”), I would like to add that the finisher should have the castings impregnated with resins to eliminate (fill) the porosity. As a materials and processes engineer for more than 30 years, I have been heavily involved with coatings, corrosion and castings. The U.S. Department of Defense mandates resin impregnation for all aluminum castings used in the military specifically for this reason - porosity in the castings causes coating failures and corrosion. Even when the castings are dried, the porosity traps air. I have seen it happen often. Impregnating castings with resins before powder coating can cause blistering in the bake oven. However, such problems can be avoided by telling the resin manufacturers the temperatures at which the coatings will be baked/fused, and making sure that the resin used is one that will withstand those temperatures.
Joe Powder responds: In my experience, there are only a few materials that can adequately fill imperfections in a casting or metal surface. One is called Lab-metal® (Alvin Products), the other All-Metal® (US Chemical). Both work, but I have had the best results with All-Metal.
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Chrome Wheel Solutions
After reading the question about chrome powderin the December 2007 issue ofFinishing Today(“Is it possible to achieve a chrome finish on wheels with powder?”), two suppliers wrote in to recommend products.
Spectrum Metalworks has a product called PermaChrome™ that uses a chrome/nickel chrome physical vapor deposition followed by a clearcoat painting process. The product is a chrome wheel, but it reportedly has three primary advantages over conventional chrome:
For more information about Perma-Chrome, call 888.PERMACHrome. More information about Durabright can be found at www.alcoa.com/alcoawheels
What is the best way to fluidize powder without fluidizing membranes/plates?
Here's some insight from Gary Bullard, technical service representative at Hentzen Coatings:
“Fluidizing membranes is recommended for fluidizing powder properly. Some coaters will drill small holes in PVC pipe that has an air fitting attached to the end of the pipe. The pipe is set in the bottom of the hopper, and the air is turned on. This solution typically works well. However, please be aware that if the steel air attachment is located inside the hopper, it could potentially create an unsafe environment. Some coaters ground the air fittings and others make the pipe long enough so that it is located outside of the hopper.”
About a year ago I powder coated some large aluminum gear housings for geared windlasses on a sailboat. The castings had become corroded, with several bare aluminum spots where the original coating was gone. For preparation, the castings were sandblasted to white and outgassed for two hours at 450°F. After cooling, the castings were acid etched and DI (deionized water) rinsed for any oxidation that may have occurred with the heating. They were then preheated and epoxy primered while hot so that the powder gelled on contact. After partial curing, they were then topcoated with polyester powder and fully cured. The total DFT (dry film thickness) was 5 mils or more. Now, after a little more than a year back in service (on salt water) some blisters are developing. If I were to start over is there anything that could be done to prevent coating failure?
On the surface it appears that everything was done right during the first powder job. Here are a few issues to ponder:
- Were the corroded spots completely cleaned - even inside the pits?
- What type of acid etch process did you use? Was it specifically formulated for aluminum?
- Did the DI rinse completely rinse all the acid away?
- 450°F seems high for aluminum. Did this affect the temper of the aluminum alloy?
- Were the powder coatings thoroughly cured? Undercured powders can blister in the field.
- Was the epoxy a high- or low-gloss product? Some low-gloss epoxies possess reduced corrosion resistance compared to higher-gloss formulas.
- Was the final coat a polyester/TGIC or polyester/urethane product? Polyester/urethane powders can sometimes develop voids when applied at thick films (>4.0 mils).
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