Sludge buildup on heating coils in phosphating tank

A. Hi Raveesh. I'm not certain what you mean by 'choked pipes', but I'm guessing you mean the heating coils in the phosphate tank? If they are 316SS, which is ideal, they can be removed and immersed in a mild acid (but not hydrochloric/muriatic).

I have some confidence that they can be pressure washed in situ if you are patient enough to let the outer surface slowly erode off, and if you have some way of removing and recycling the large volume of accumulating water, such as a centrifuge or filter press.

The coils should be oversize (derated) and you should probably be speaking to your chemical supplier about additives to slow the sludge buildup.

Luck & Regards,

A. Hi again. I would try recycling water through a pump, catching any loosened sludge in a filter. Water can sometimes gradually loosen and abrade such sludges. If only clear water comes out despite the recycling, then you will have to add a mild (not HCl!) acid to the recycled water.

Luck & Regards,

A. At a previous job I used spent anodizing solution (30% sulfuric acid) with great success for soak cleaning parts. I also had good success with as low as 10% sulfuric acid, but generally speaking the higher the acid content the faster things cleaned up. This was on zinc phosphate. We found that as long as we got the pH sufficiently low the sludge would soften up and slough off easily.

A. Actually, this is a very well known problem, and there are some defenses (in addition, of course, to the chemistry). First, the heating coils should be oversized ('derated') to allow them to continue to provide enough heat even when covered with sludge. Second, special paints have been advertised for the purpose of minimized sludge accumulation on heating coils; or probably better than paint, the coils should be electropolished so they have no "tooth" for the sludge to cling to.

A. Phosphate sludge can be removed from your heating coils using either a mineral acid or a strong caustic solution. The hotter the bath, the more rapid the removal. As for your question about the deposition of the sludge, you will have to describe your process in greater detail because this can be caused for a variety of reasons.

---
Ed. note: letter 5142 has some additional tips on this problem.

A. Hi Carl. It's not an immediate answer, but the stainless steel should be electropolished so that it has no "tooth" for the sludge to stick to.

Regards,

A. Hi Bernard. We appended your inquiry to an earlier thread which contains 4 points on this subject --
1. Different formulations create different sludge build-up volumes. Be sure to work with your supplier towards understanding and minimizing the sludging.
2. If possible, everything should be made of electropolished type 316 stainless steel so there is no "tooth". This slows the buildup and makes sludge removal easier.
3. Coils should be oversize so they operate at lower temperature, slowing the buildup, and so they can continue to provide sufficient heat even when sludged up pretty badly.
4. The sludge can be dissolved with either hot mineral acid or alkali per George's reply; sorry, I personally don't know which is more promising.

Good luck.

Regards,

A. Hi Sree. Actually derating of heaters will always "work" -- it's a matter of what you mean. If you derate heaters, using them at lower temperature, the sludge will build up slower; further, with twice the surface area, you can maintain tank temperature with them longer. But apparently it hasn't worked significantly towards solving the overall sludging problem in the tank and elsewhere.

Did you "work with your supplier towards understanding and minimizing the sludging"? If not, at least spend some time with our search engine and you will find perspectives on the subject.

Sorry, I haven't seen that paint advertised for many years now, but possibly your process supplier is familiar with it. Boats use ablative paint to minimize fouling, and there might theoretically be an ablative paint for phosphating coils. However, using electropolished stainless steel means less "tooth" for the sludge to adhere to, and easier cleaning. Good luck.

Regards,

A. Is it possible that the build-up is calcium phosphate? If so, then soft water for additions to the bath may help.

A. Hi Samuel,
We run a manganese phosphate tank with a titanium coil.
It does build up with the typical sand coloured iron/manganese phosphate concretion.
Once the heating efficiency goes down we take the coil out of the tank and bang the deposit off it.
We see it as part and parcel of running a phosphate tank.
We have never been inclined to see it as something to prevent.
Once a year or so we empty the tank and dig out the rest of the deposit that has formed at the bottom of the tank.
With this regimen we are able to produce good phosphating on our parts.