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Q. We have a relatively new product line that has steel support beams and aluminum benches. We installed a new powder coating line and we have been coating these benches for the past year with a polyester urethane powder material. The steel parts are blasted and then washed in a five-stage washer with an alkaline cleaner, rinse, iron phosphate, rinse and rinse steel. The aluminum parts are run on the same line but not blasted. We use a fluoride additive in the phosphate solution to treat the aluminum. The benches are installed outside in locations all over the world.

Now that we have had some in the field for a while, we are getting complaints about rust on the steel parts and peeling on the aluminum benches. Do you know what the cause is or how we can prevent these problems? A.D.


A. It is difficult to say exactly what is causing the failures but there are some very likely possibilities. Let’s talk about the steel first.

What is the depth of the blast profile from the high points to the low points? You can measure this with a profile gauge that can be purchased from a test instrument company. The depth of the profile is important to help determine how much film thickness you need to be sure you will get good corrosion protection. You will need to apply enough powder to cover the peaks of the blast profile plus around 3 mils of powder to ensure good performance in an outdoor application. In addition to the right thickness you also need to make sure that the coverage is complete in any recessed area an on all edges. If the coating is thin or there are any voids, the part will begin to rust in a relatively short period of time.

How are you measuring and controlling the pretreatment solutions? You need to have a good program in place to make sure the solutions are the right temperature and concentration and that the coating weight from the iron phosphate is within the manufacturer’s recommended range.

Are you getting a full cure of the powder? You can test the cure with a solvent rub test as described by the Powder Coating Institute in Test Method #8. When the solvent is rubbed over the surface of the powder film, it may cause the powder to turn dull or even remove some of the color onto the cloth.

The test reveals how resistant the powder film is to the solvent. Compare this with a known cured sample of your powder to see if it is the same. If it is the same, you have cure. If you are able to remove more color than the known sample, your parts are under-cured. A film that is not as hard as it should be or has a higher gloss is also an indication of under-cure. If the film is not fully cured, the powder will not deliver full performance properties.

You also need to be aware of potential damage in handling. If chipped through to the steel during handling and installation, the spot will begin to rust. Installation damage should be touched up with high quality liquid polyurethane coating in the field.
Now let’s consider the aluminum. The primary challenge when coating aluminum is the oxide layer always present on the surface. The current process you describe has limited ability to provide good adhesion and long-term performance on the aluminum surface.

The fluoride will provide some etch for initial adhesion, but it does not prevent oxidation. The iron phosphate doesn’t treat the aluminum surface. The important fact to remember is that iron phosphate doesn’t react on the aluminum surface and fluoride is marginally useful for improvement of adhesion but it does not improve corrosion resistance. In order to achieve long-term corrosion resistance in an outdoor environment, the aluminum must be treated with some type of conversion coating.
Yellow chrome has been used for many years by aluminum coaters, and it provides excellent corrosion resistance under a quality topcoat. There are also many non-chrome products that have been used successfully by many coaters in North America and Europe.

A good way to test for aluminum adhesion is a wet adhesion test as described in American Architectural Manufacturers Association (AAMA) 2605-02 specifications. The test panel is scribed in a crosshatch pattern and placed in boiling, demineralized water for 20 min. The panel is then dried, and a tape pull test is performed the same as a dry crosshatch test as described in ASTM D 3359.

The use of boiling water is much more aggressive than a dry test. If the coating passes the AAMA test, you can have a reasonable amount of confidence in the pretreatment process. Now the problem confronting you is how to treat both steel and aluminum in the same washer and obtain reliable quality on the aluminum.

You have a couple of options. One is to blast the steel and blow off the dust from the blast operation. Turn the washer off and powder coat over the blast-only surface. Set up the washer with an aluminum treatment process and use it for aluminum only. This will greatly improve corrosion resistance on the aluminum benches, but you are at risk with the steel. Be sure the film thickness and coverage are good, and you may be able to produce acceptable quality on both substrates.

A second possible option is to convert the washer to a new product like a zirconium oxide. Some of the newer nano-scale technologies work well on both aluminum and steel, and have some other advantages because they don’t need heat and they don’t sludge like iron phosphate. Be sure to test the product thoroughly before using it in production. Also be aware that you may need a stainless steel washer, very good water quality and exceptional rinsing, especially if you stick with an alkaline cleaner. 


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