Pretreatment Options for Cast Zinc or Aluminum Parts

What pretreatment do we need in order to powder coat cast parts? Expert Rodger Talbert explains that a three-stage washer might get those cast parts clean, but there are some considerations.


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Q. We have been using a three-stage iron phosphate washer for our steel parts, but now also need to clean cast zinc and cast aluminum. Can we use our current washer, or do we need to think about adding more stages? What do we need in order to handle the cast parts?

A. Your three-stage washer might get those cast parts clean, but you do have some considerations. As always, one of the first questions to clear up is whether the parts will be used indoors or outdoors. If indoors only, your three-stage washer could be adequate, but if the parts will be used outdoors, the three-stage will not do much to prevent moisture penetration under the film and failure of the coating. 

There are usually a few options that could get you outdoor performance. Good conversion coatings made for aluminum and zinc phosphate will work well on the cast zinc. Zinc phosphate is a problem from a cost standpoint and also requires waste treatment, so you may be reluctant to get into that type of treatment. Zirconium oxide, chrome or non-chrome treatments are very effective on aluminum if you have a stainless steel washer that can handle these kinds of products. Probably the best answer for adding outdoor castings to your business is to blast the surface, apply a layer of powder primer and then add the topcoat finish. The primer will add significantly to the corrosion protection and enhance adhesion. 

The best option depends partly on what is easiest for you to implement and most effective for the product being coated.


Q. We use silver and other metallic powders on aluminum extrusions that are sometimes used outdoors. Is there a way to get better mar resistance from these powders?

A. It is possible you could get more without paying more, but not likely. Powder coatings can be engineered to provide properties that are needed to meet specific commercial requirements. When a product must have improved performance in a certain property, it will often require that the manufacturer add a more costly ingredient to achieve the desired standard, and that ingredient that provides the improved performance will naturally raise the cost of the powder. You need to decide what the enhanced performance is worth to you in the market. Will it help you avoid warranty costs? Will it help you keep existing customers? Will it add new customers because of the better coating performance you will be able to offer? If it will add to your profitability, it is probably worth the added cost.


Q. What are the options for stripping powder from hooks and racks, and how do they compare to one another?

A. In a broad sense, powder can be removed chemically or thermally or by some combination of the two. The most popular method is by using a rack burn-off oven. This is a controlled thermal pyrolysis process in which the racks are placed in the oven and exposed to temperatures of around 800°F for four to six hours (the time depends on how much powder is present). This type of oven is reasonable in cost, and that is the main reason this option is so popular. It does involve a lot of handling to load racks, remove racks and clean up the resulting ash, and the high heat does wear on the racks over time.

Another system uses fluidized sand inside a tank to abrade and break down the powder. It also operates in a similar temperature range as the burn-off oven, but the fluidized sand can remove the powder much faster than just the heat of a burn-off oven. Typical cycle times are around 30 to 60 minutes, and hooks come out much cleaner. Although these systems are much more costly than the burn-off oven, the results are superior.

Molten salt systems are another option for coating removal. Hooks are immersed in a molten salt bath for a brief time, typically not more than two minutes, often less, and then conveyed into a rinse tank to remove any residual byproducts and cool the metal. Hooks come out very clean in this process, and the cycle time is faster than with any other method of stripping. Although the equipment is much more costly than the burn-off oven, the rapid stripping cycle and completely clean metal are huge benefits. Some byproducts do form sludge in the process tank that must be removed and treated or waste-hauled, however.

There also are a number of commercially available chemical products that can remove powder. The initial investment for the chemicals can be high, but the per-use cost is reasonable. Some of these products are quite toxic, but there are some that are fairly easy to use without causing major environmental issues. The coating material is a byproduct that must be removed from the tank and waste-hauled. 

If you have the capital, it is smart to consider the faster and more complete stripping of fluid-bed sand or molten salt. If you do not have the capital, you may take a harder look at the burn-off oven. If you have relatively small hooks and racks in lower volume and you are comfortable with the use of chemical strippers, that is always an option. 


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