Pretreatment in Existing Line


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Q. Presently, we are using GI sheet components for powder coating. The pretreatment method used is iron phosphating spray method in the following sequence:

First stage: cleaner/coater
Second stage: water rinse
Third stage: non-chrome passivation.

Now we are introducing die-cast aluminum components that have to be powder coated. The question is, can we perform the pretreatment for these parts in the same line without changing any of the current parameters? J.F.


A. Many companies run aluminum and other nonferrous metals in an iron phosphate washer with three or more stages. You can clean aluminum castings in your present washer, and it will remove typical organic soils like oil and shop dust.
However, a cast part may have die-release material left on the surface that will not come off in your washer. You may need to sand or polish the surface to get good adhesion.

Also, the aluminum surface will be clean but it will not have a conversion coating. If the parts are used in a corrosive environment, the powder may fail. Any aluminum surface that will be used in an outdoor environment should have a true aluminum conversion coating to provide corrosion resistance. Chrome is often used, but there are many suitable non-chrome products that can provide excellent protection against moisture penetration and oxidation.

Another factor you should be aware of if you have not run castings before is that they may have trapped porosity near the surface. These small air pockets will expand when heated in the curing oven and may cause small holes to form in the molten film. This condition is referred to as out-gassing. You can help limit the problem by preheating the castings above the cure temperature before coating. The powder supplier may also be able to help with the formulation of the powder.

Last, try to cure at a lower temperature (350ºF), if possible.