Multi-metal Washer

Is a five-stage washer suitable for pretreating aluminum for coating it to AAMA 2604-13?


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Q. We have a five-stage washer that we use for both steel and aluminum parts. The stages include an alkaline cleaner followed by a rinse stage, an iron phosphate solution with fluoride added for the aluminum, a rinse and then a final rinse seal. We get good performance on both metals. Recently, we have been asked to coat some aluminum parts that must meet the American Aluminum Manufacturers Association (AAMA) 2604-13 specifications for outdoor durability. Will our current five-stage be good enough for that specification? If not, what can we do to make it better? –K.H.

A. Your washer arrangement has been used for many years by companies looking for a way to process both steel and aluminum in the same machine. The iron phosphate is time-tested way to treat steel for good bonding of the powder. The fluoride additive provides a decent etch to the aluminum and improves the dry adhesion of the powder to the aluminum substrate.

For indoor use, this is usually an effective pretreatment. However, when a product is used outside, it is subjected to much more aggressive corrosive factors that can cause failure if the pretreatment is not adequate. For outdoor durability, the aluminum must have some type of chrome or non-chrome conversion coating.

Chrome works very well but is not very desirable to use due to the obvious environmental challenges. Non-chrome treatments can be very effective when they are made exclusively for aluminum. Some coaters have a dry-in-place non-chrome seal for aluminum used only when they run aluminum through the washer. You may be able to add a stage for a non-chrome material for your aluminum. One other option may be to convert your iron phosphate washer to a zirconium oxide treatment or similar transitional metal treatment. These newer treatments can work very well on many different metals. However, they typically require stainless steel construction and very clean water. Talk to your pretreatment supplier about your options. I would not recommend that you try to meet AAMA 2604 with an iron phosphate and fluoride solution