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Materials of construction for zinc phosphating tank and accessories



A discussion started in 2000 & continuing through 2017 -- add your Q to bring it back to the Hot Topics page.

(2000)

Q. 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° C). 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 aluminum in zinc I need to control? Thank you

Richard [last name deleted for privacy by Editor]
- Malaysia


Phosphating Metal Pretreatment
by Freeman





Phosphating of Metals
by Rausch

(2000)

A. 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 de-ruster (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 svc. w/ chemical supplier - Grand Rapids, Michigan


(2001)

A. 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 de-sludge 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 e.g., or HDG with old versions; you need some special activators inside.

Anyway better to consult with specialist in there.

Regards,

Hossein Asgari
- Tehran, Iran


January 19, 2012

Q. Why use stainless steel 316 for heating coils?

Carlos Abdala
- Venezuela


January 19, 2012

A. Hi, cousin Carlos. Bill and Hossein have already answered. Can you please re-phrase your question in terms of the answers already offered if you are not quite understanding the reasons they mentioned?

I would add that 316 stainless steel can be electropolished to an extremely smooth surface with no "tooth" at all, and that can make for a more easily cleanable heating coil. Some additional info on coil design and cleaning for phosphating solutions can be found in letter 11465 "Sludge buildup on heating coils in phosphating tank". Good luck!

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



To minimize searching and offer multiple viewpoints, we combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition.



Is there a special coating for Steam Coils in Zinc Phosphate System to help remove build-up

May 7, 2010

Q. We have been experience this issue for years and I know many face the same thing. We would like to find a way to reduce hammering the zinc phosphate build-up off the coils without implementing a whole new tank. Is there some kind of coating we can put on the coils that will either resist build-up or make removal easy? I appreciate any feedback or suggestions. I'm open to anything that can help.

Brian S. [last name deleted for privacy by Editor]
Chemical Manager - Clyde, Ohio, United States


May 7, 2010

A. Hi, Brian. Many many years ago I heard of a paint for this purpose, but haven't heard of it in decades. My best advice is to oversize the coils so they operate at lower temperatures (which means you'll also be able to go longer between cleaning because you have extra coil area, and sludge buildup will be slower because they are cooler). Also make them of electropolished stainless steel because this eliminates the "tooth" as much as is possible, making them easier to clean.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


May 10, 2010

A. Using double derated heaters will help as they do not get as hot. I have used teflon coils in small tanks and they still get coated, so I doubt if there is any coating.

James Watts
- Navarre, Florida


May 17, 2010

thumbs up signThanks for the feedback. I greatly appreciate it!

Brian S. [last name deleted for privacy by Editor]
Chemical Manager - Clyde, Ohio, United States



September 12, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi,

I would like to know the suitable heater for phosphate tank. The sheath of the heater? SS316, titanium, or any suggestion? Thanks.

noya azema
- johor bahru, johor, malaysia


September 2014

A. Hi Noya. We appended your question to a similar thread. Electropolished 316SS is ideal, but the heaters should be oversized. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


September 15, 2014

Q. Thanks,

But, may I know what you means by over-sized the heater? I'm not very clear about that.

noya azema [returning]
- johor bahru , johor. malaysia


September 2014

A. Hello again, Noya. The heaters used in phosphatizing tanks should be "Oversized" or "Derated", meaning that they should have a surface area significantly larger than calculated to be necessary when heating other solutions. In this way, the surface of the heater does not get as hot, and therefore it causes less sludge to "dry" or "burn" onto it. Also, it does not need to be removed for cleaning as often because it has extra capacity. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



October 20, 2014

Q. I have a zinc phosphate line which I am rebuilding. We are planning to heat it with a boiler. Who would be the best person to tell me the size of boiler I need to heat the tanks?

Ronnie Verzi
- Calgary, Alberta


October 2014

A. Hi Ronnie. Most people who bill themselves as metal finishing consultants will have had wide and varied experience, and will be perfectly capable of sizing the boiler. But if you are looking for free help, calculation guidelines are available from all of the suppliers of heating coils, and some zinc phosphate process suppliers have tech service people who can probably be a lot of help. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


November 5, 2014

Q. Ted,

Thanks for the response, I have another question on tanks for the zinc phosphate solution. How do you feel about lining the tanks in plastic, fiberglass?

Thanks

Ronnie

Ronnie Verzi [returning]
- Calgary, AB, Canada


November 2014

A. Hi again. My generally favorite material of construction for metal finishing equipment is vinyl ester fiberglass. But I think my first choice for a zinc phosphate tank specifically would be stainless steel, electropolished if practical, because I think it minimizes sludge accumulation issues. Liners, whether cemented in place or drop-in, might present some durability issues if the phosphating tank needs frequent de-sludging and cleaning.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



Insulating a zinc phosphating tank

June 15, 2015

Q. I am looking for some guidance on the type of insulation that should be used when constructing a zinc phosphating tank.

Elinor Jones
Purchasing Manager - Scotland


July 23, 2015

A. I've insulated tanks like this with polystyrene foam on the outside of a steel tank.
It can be glued to the outside, is low cost and easily handled, but has a flammability issue to be taken into account.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland



June 14, 2017

Q. Dear All,

Is it possible use SS 316 L heater instead of SS 316 heater?

If we use it, will it create any impact on the zinc phosphate solution or coating quality?

Thanks in Advance for your responses.

P. Karthikeyan
- Bangalore, Karnataka, India


June 2017

A. Hi P. It will be fine. 316L is identical to 316 except for having slightly less carbon; that carbon can be an issue in welded designs because of reduced corrosion resistance due to carbide precipitation.

I see that some vendors are selling 316L-316 stainless steel, apparently on the basis that the difference in cost isn't enough to justify the cost of stocking the two different grades. I believe that 316L probably fulfills every requirement of 316.

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



June 16, 2017

Q. Can you use stainless steel tanks in the powder coating pretreatment process.

Mobius Shumba
- Harare, Zimbabwe


June 2017

A. Hi Mobius. Certainly.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"

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