In particular which part of the pipe, near diameter opening or more in the middle of the pipe or random? For the inside spots near the opening because it is the only part we usually check for outside in any part of the pipe but the outside spots are not a big problem.Are these black spots only present inside the pipe? Inside sometimes outside sometimes.What about your degreasing? Alkaline type heated from 60 -70 ☌.Are they pickled with strong acids of what type? HCl.What type of metal is the pipes made of? Steel.What pre-treatment process do you employ before taking the pipes to be immersed in hot dip zinc bath? Degreasing alkaline, Rinse, Pickle, Rinse, Prefluxing.Here are the replies to your request for clarification: Also, check your zinc bath temperature if consistent on whole length of pipes in the bath. 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. Are the black spots rough or smooth to touch? Do they look like burns or are simply spots that zinc didn't coat?.
what cooling system you use after you remove the pipes from the molten zinc baths?.how much time you use to remove the pipes from the molten zinc bath?.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?.what type of metal is the pipes made of? Are they pickled with strong acids of what type? What about your degreasing?.what pre-treatment process do you employ before taking the pipes to be immersed in hot dip zinc bath?.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 : My comments may need further investigation by others more knowledgeable than me so kindly bear with me. 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. We galvanize the pipes using the hot dip process. This happens more often in pipes of small diameter sizes (1/2" and 3/4").
From a long time ago we have had the problem called "black spots" or "black stains" usually in the inner part of pipes. But you will find many points to consider as you read these threads to enable you to sharpen up your query should you wish.Ĭlosely related Q&A's, oldest first: Black Spots on Hot Dip Galvanized Pipes 2001 But, as you will see from other postings and pictures, the phrase 'black spots' can mean different things to different people, so it is usually best to send photos to for posting here. The problem is obviously ubiquitous, and you can read problems & solutions on all those threads. We have many other threads on the same/similar problem, including thread 56867 "Spotting, Staining, Blackening of Hot Dip Galvanized Finish", 57163 "Galvanizing surface defect (looking like black spots)", 61293 "Chrome-free galvanizing gets black spots" and others. We appended your question to a thread about black spots on hot-dip galvanizing.
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