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topic 41928

Zinc Phosphate Sludge Buildup Problems

A discussion started in 206 but continuing through 2018


Q. What is the best heating method for a zinc phosphate immersion tank? I currently use a plate and frame heat exchanger. The phosphate temperature should be at 115F. I am using 150F water in the HX.

The HX gets fouled with sludge quickly. The frequency of cleaning of the HX with acid flush is every 3 days or so. Disassembly and hand cleaning is once a month.

We have thought about using a shell and tube HX or using plate coils inside the tank.

Any shared experience would be appreciated,

David MacDonald
Pretreatment chemicals - Rocky Hill, Connecticut, USA


A. Something is not right with the bath itself. Unless you're using it 24 hours per day you should not generate that much sludge. Sludge in a Zinc phosphate bath comes from three sources - one is the water being used to make up the bath, anything in the water will form sludge when the water evaporates, (hardness ions like Calcium and Magnesium make a hard, whitish scale). Another source is the metallic phosphates that naturally form during the processing of metals, being a conversion coating process there are some by-products that will form sludge. The third can be a byproduct of the accelerator.

OK, there's a fourth... excessive drag-in of dirty rinse water or cleaning materials WILL cause excess chemical usage and sludge.

In short, first get the bath under control, then be sure the water going into it is good, then keep the rinse ahead clean. If drag-in is a problem you'll have to address it.

My money is on process control and water issues. Take care of those two things and the savings will pay for the effort.

Jeff Watson
Jeff Watson
- Pearland, Texas


Q. David,

We have the exact same problem, and are on a similar if not more frequent schedule as you are. We currently use city water rather than RO water for water additions/makeup... Is RO water an absolute must to prevent excessive sludge?

Devon DuBose
- Redmond, Washington, USA

January 26, 2011

A. If the temperature is more than 15 or 20 degrees differential, you form zinc phosphate flakes inside the heat exchanger. The temperature must be modulated. That's what makes the flakes. Use a step down heat exchanger and an automatic johnson valve to control the temperature. It works. Good luck.

DENNY [last name deleted for privacy by Editor]
- Peru, Indiana, USA

June 14, 2018

A. I have seen zinc phosphate baths work with very little sludging. The tanks were heated with steam coils that were electropolished with quick release couplings to switch coils in and out quickly. The electropolish allows the dried sludge to fall off with one hit. One tip is the tank size should allow you to keep parts 6 inches off the bottom so as not to disturb the sludge. Every week let the tank sit overnight and draw off the liquid -- clean the walls and bottom of the tank and put the solution back in your tank. This keeps the tank broken-in, keeps your ratios in line.
If you idle the tank without running work thru it the ratio changes so it is best to have a basket of steel to run to keep the ratio up if you only have the occasional parts.

Robin Tiede
Hubbard Hall Inc. - Waterbury, Connecticut, USA

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