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Is zirconium phosphate OK for mixed metals?

January 30, 2008

If you read my answers to some powder coating questions, you might agree that the answers are helpful to the questionnaire. Now it is my turn to ask the question.

We have a potential powder coating client who processes aluminium and mild steel components. Local authorities prevent the use of chromate and there is the question of a treatment that supports the processing of two opposing metal substrates.
Consequently, the substrate is cleaned prior to being placed on a monorail that takes the items through a spray system of zirconium phosphate, water wash, drying oven, powder coating and final bake. This customer is now experiencing the occasional problems of delamination of the polyester powder when applied to the aluminium components. The aluminium components have a nice crystalline finish upon leaving the pre-treatment.

The chemical supplier has informed him that his testing of the pre-treatment solution indicates that all are within the acceptable parameters and, the problem arises because the powder is being under-baked.

My visit included running a 'Grant Meter' through the oven, which indicated 9 minutes at 220-240°C (metal temperature). I would suggest this is more than adequate.
Unfortunately, I am not an expert at diagnosing faults in pre-treatment, but the pre-treatment representative states:
a) You can mix ferrous and non-ferrous metals on the same spray line.
b) The flash rusting on the mild steel is normal. Note: No rust prior to entering the zirconium phosphate, but covered on emerging from the water wash cabinet (phosphate and water wash processing, consists of one long tunnel). Although no adhesion problems have been reported, I did point out the seriousness of flash rust to the long-term integrity of the powder coated mild steel component. Should an anti-corrosive additive be incorporated into the water wash to prevent flash rusting?

Any advice on the subject would be appreciated, especially concerning the aluminium defect.

Terry Hickling
Birmingham, United Kingdom

February 2, 2008

I have just become aware that the zirconium phosphate solution is very acidic (Ph 2.5). Naturally, the mild steel items will flash rust. Since my first visit, it became obvious that the pre-treatment representative was unaware that the client was processing the amount of steel items that I saw going through the plant, hence is comment, "normal". Maybe, this proves a point, that in such circumstances, all the companies involved in attempting to resolve a problem, should all be present, on site, at the same time.
However, there is still the problem of delaminating.

Terry Hickling
Birmingham, United Kingdom

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