Letter 5142

Materials for zinc phosphating tank 

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Hello, Could anyone tell me what kind of material is suitable to make a tank for zinc phosphating solutions and I found out that the zinc phosphate coated on the surface of the stainless steel steam heater which prevent the heater from functioning well.

Is zinc phosphating only react with hot material(in my case the steam heater is around 70 degrees). Can anyone let me know that is Al can cause a failure ( black Colour) on a galvanized steel after salt spray test? and what is the percentage of aluminium in zinc i need to control? Thank you

Richard
- Malaysia

Richard the equipment to contain the zinc phosphate can be mid steel. For much longer life the tank and heating coil should be 316 stainless steel. You must clean the build-up from the coil periodically, either in a strong alkaline deruster(hot) solution or remove it manually. The temperature of the zinc phosphate should be a minimum of 71 C. Works best at 82 C. As the temperature decreases, the coating weight decreases. The best corrosion protection and uniformity of the coating is obtained in approximately 30 minutes at 82 C. The bath itself is controlled by total and free acid points and is based on total iron, not aluminum. You MUST titrate this bath to control it. Hopefully you are using a proprietary bath. If so, contact the chemical vendor for help. Do not know how you thought aluminum is involved.

Richard the zinc phosphate coating must be sealed as the final step such as water soluble oil. You will not have good salt spray results unless the coating is sealed with something.

Tip: When the coil has to be cleaned have a second coil already clean ready to install.

Bill Hemp-Tech Service
- Grand Rapids, Michigan

Dear Richard,

There are many things to consider: The best choice for Zn Phos. bath material & heating coils is SS 316 because of low working pH~2.5-3.3 of solution. What you see on the coils is not actually a phosphate layer but by-products of phosphate reaction, so-called SLUDGE which is mainly insoluble Fe3PO4 and it reduces the heat-transfer coefficient, but you can not avoid it in any event just to reduce it, normally with increasing temperature zinc will tend to precipitate and you lose a lot of cations as sludge instead of using them as coating; you should switch to newer versions of chemicals which can also work at lower temp. 50-60 C(as paint base) and replace the coils on tank WALLS instead of BOTTOM to lessen the settlement, or better to use shell+tube or plate+frame heat exchanger out of bath because there you'll have just max. 10 C temp difference but high efficiency, also the tank should be conical at bottom in order to desludge the tank periodically.

The last part of your question is not so clear for me but if you mean phosphating of galvanized steel TAKE CARE: Al is a poison in Zn Phos bath and if you do not use new versions of chemicals, the bath must be dumped; you must not use Al equipment NEVER. Also you can not coat galvanized steel-either EG or HDG with old versions; you need some special activators inside.

Anyway better to consult with specialist in there.

Regards,

Hossein Asgari
- Tehran, Iran

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Ed. note: Some additional info on coil design and cleaning for phosphating solutions can be found on letter 11465.

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