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by Ron Joseph

November, 2005

Adhesion of Zinc Phosphate

Q. Recently my company was awarded business to produce a power steerring cooler for the auto industry with a very tight internal cleanliness specification (90 x 90 micron particle size). The internal portion of the cooler has a .047" steel wire dia. turbulator(spring) that mixes the fluid as it passes through the cooler. The turbulator has a zinc phosphate coating, working as a rust inhibitor and lubricant during the forming process. My question is can this coating (zinc phosphate) breakaway from the substrate material inside the power steering cooler and thus cause failure by not meeting the cleanlinness spec.

We have done millipore test on these springs and cannot get them to pass the spec., could the zinc phosphate be the culprit?

A. Wow! What an interesting question. My first reaction to your question is that it is unlikely that the zinc phosphate will spall off from the steel wire. The coating is chemically bonded to the substrate and I have never seen zinc phosphate "break away". However, it is possible that if the process is not properly performed or if contaminants are present, you might have the problem you are experiencing. Another alternative is that individual crystals of the phosphate might be breaking off. You are probably aware that a zinc phosphate as applied to metals is crystalline, and the size of the crystals depends on the chemicals being used and the parameters of the phosphating process. Who applies the zinc phosphate? Do you have a vendor who does this, or do you apply it yourself?

As you know, zinc phosphate comprises several chemical baths and rinse stages. If you believe that the contamination is due to the wire you should thoroughly investigate the phosphating process to see if there are any weaknesses in the system. If you are not able to do so, I am willing to assist you on a consulting basis.

Best wishes,

Ron Joseph

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