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topic 7651

Black Spots on Pipes Hot Dipped


From a long time ago we have had the problem called "black spots" or "black stains" usually in the inner part of pipes. This happens more often in pipes of small diameter sizes (1/2" and 3/4"). We galvanize the pipes using the hot dip process. When our people who do the final revision of the pipe and they see this defect they reject the pipes and we have to re-galvanize again. Some hints will be helpful.

Morales Ernualdo
- Apodaca n.l. Mexico


My comments may need further investigation by others more knowledgeable than me so kindly bear with me... My experience with hot dip pipes specially with small diameter 1/2 and 3/4 gives me several suggestions but I need further info from you, namely :

  1. what pre-treatment process do you employ before taking the pipes to be immersed in hot dip zinc bath?
  2. what type of metal is the pipes made of? Are they pickled with strong acids of what type? What about your degreasing?
  3. are these black spots only present inside the pipe? in particular which part of the pipe, near diameter opening or more in the middle of the pipe or random? How long are these pipes?
  4. how much time you use to remove the pipes from the molten zinc bath?
  5. what cooling system you use after you remove the pipes from the molten zinc baths?
  6. Are the black spots rough or smooth to touch? Do they look like burns or are simply spots that zinc didn't coat?

Larger diameter pipes are much have bigger openings to have the molten zinc flow out when the pipes are removed from the bath, so the molten zinc flow faster out unlike smaller diameter pipes. Also, check your zinc bath temperature if consistent on whole length of pipes in the bath.

Looking forward to your reply.

Anthony Austria
- Philippines


Dear Mr. Austria:

Here are the answers to your questions:

Thanks for your help.

Morales Ernualdo
- Apodaca NL Mexico


Dear Mr. Morales

As per my experience on the pipe galvanising line, these type of the black spot in inner side of the pipe may due to improper drying of pipe before entering into the zinc bath. The preheating must more efficient to avoid the same. My technical paper on it released in the Steel technology of Asia in the month of March' 99 edition, so, you may refer this article for solving the same problem.

Good luck.

- Mumbai, Maharastra, India



I have come across this thread and try to look the technical paper in "Steel technology of asia" but unable to track this edition any where on net, please let me know from where I can get it and other technical papers and related literature on pipe Galvanizing.

Qaiser Majid

Qaiser Majid
Lahore, Pakistan

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