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Thin layer of zinc phosphate

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I am having a problem. I am in a pipe threading company. All this while, we are running the zinc phosphating process on pipe thread. The pipes came from L80 grade, K55 grade, P110 grade, and Q125 grade. Our zinc phosphate process are as follows;

Use kerosene to clean up the pipe thread (having grease on it)

Tank 1: Alkaline cleaner
Tank 2: Rinsing Water
Tank 3: Rinsing Water
Tank 4: Zinc Phosphate
Tank 5: Hot water rinsing
Tank 6: Oiling

The last process is to apply grease and install protector to protect the thread.

However, we always have problems of thin layer of zinc phosphate especially on the P110 and Q125 grade. After the process, you can see that some portion is not fully coated with zinc phosphate. We have to do rework.

I would like to know what are the causes? Why the L80 grade does not give us such problem as compared to P110 and Q125 grade? Iyt has been many years that we have no answer on that and hope someone can let me know the major causes and how to solve this problem? Thank you.

James Chong
Pipe Threading - Malaysia

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