Letter 11465

Phosphate coating 

Dear All,

We are using phosphate coating chemical from three different suppliers. Two of these, creating more sludge and one of these generates only minimum quantity of sludge. I think this may be due to composition of chemical.

But in this my question is sludge deposition on heating coil in more slude generating chemical is very very less in some places no sludge also. But in less sludge generating chemical, sludge deposition on heating coil is too high (even 2-3 inch). What is the actual reason for this problem. How to avoid this problem. How can we remove this sludge easily from S.S heating coil. Pls suggest me if any chemical method to remove this easily. Other bath parameters are same for same for all chemicals.

Mahendran
- U.A.E

Actually, this is a pretty well known problem, and there are three defenses. 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 from time to time for the exact purpose of minimized sludge accumulation on heating coils. Third, and probably better than paint, the coils should be electropolished so they have no "tooth" for the sludge to cling to.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

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.

George Gorecki
- Naperville, Illinois

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

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