Intermittent Phosphate Line Usage

Q. I inherited a cleaning and phosphate line and over the last two weeks, we have lost our phosphate solution twice. I am told it is because we have lost iron in the solution. Is there an additive that can be used to help maintain the solution as the tank is not in constant use?

A. In my experience, it is unusual to completely shut down and start up a phosphating line multiple times in a week. I would think this would be problematic for several of the ingredients in the phosphating stage. Some of the ingredients may be soluble at the elevated operating temperatures, but insoluble as the bath is allowed to cool on the off days. This precipitation, along with a natural tendency to form sludge in this tank, will be exacerbated by the frequent line cool downs. One of the ingredients that could precipitate would be the iron phosphate or free iron.

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The first thing I would do is insure that the tank is fully up to its specified temperature range before starting any production. Then perform any required titrations from your supplier, typically total acid, free acid and pH. Some suppliers also have procedures available to check the accelerator and grain refiner concentrations (if formulated) as well as free iron.

An old method to rapidly increase free iron would be to add some steel wool to the phosphate tank to provide it with a quick supply to dissolve. However, this should only be done if you confirm the tank is low in iron. I would also recommend contacting your supplier of the phosphating chemicals. They should be familiar with the chemistry of the bath and would be able to tell you more specifics regarding what is going wrong. 

Originially published in the June 2017 issue.