cold Zinc Phosphate

Q. I am a finishing engineer in a sheet metal fabricating factory. In light of increasing energy costs, we have become very energy-conscious. At a recent staff meeting, our boss asked us to look into reducing every energy-using operation. We have lowered the temperature of our iron phosphate pretreatment system. Can we do the same with the zinc phosphate pretreatment? One of the chemical salesmen said he had a product that we could use with city water, straight out of the tap, without any heating at all. Is a cold zinc phosphate possible? H.A.

A. I suppose anything is possible. In theory, a cold zinc phosphate is possible. In practice, there may be difficulties. For example, in the cleaner stage the solution temperature must be high enough to mobilize the oily soils. I don’t want to sound like a broken record (I’ll bet some of the young folks have never even heard one), because I think I’ve said this before: you can’t remove soils unless you can move them. This “moving” of oily soils occurs at 100°F or higher. City water straight out of the tap may be 60°F or colder.

Although you can deposit a zinc phosphate coating at fairly low temperatures, as with any chemical reaction, deposition rate is temperature-dependent. According to the “Ten Degree Rule,” the rate of reaction doubles with every 10° increase in temperature. Therefore, the higher the temperature, the higher the zinc phosphate deposition rate. On the other hand, sludge formation, the other phenomenon in the zinc phosphate reaction, increases with decreasing temperatures. It is also important to remember that hot and cold are relative terms.