Black spots in hot dip zinc coated wire goods

A. Hi Abubakar. We appended your question to a thread about black spots on hot-dip galvanizing. 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.

The problem is obviously ubiquitous, and you can read problems & solutions on all those threads. 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 mooney@finishing.com for posting here. But you will find many points to consider as you read these threads to enable you to sharpen up your query should you wish.

Luck & Regards,

A. 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.

A. Dear Mr. Morales

As per my experience on the pipe galvanising line, these types 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.

A. Sir:
If you would post the zinc bath chemistry it could help to determine the cause of your problem. Also if you use a flux, then the composition of the flux solution could be helpful.
Regards,

A. Sir:

From your photographs it appears that the large spangle (crystals) are likely from alloyed elements in the zinc, for example lead, bismuth, tin, or perhaps antimony. In any case it is my impression that minimum spangle or no spangle is the usual for sheet galvanizing for forming into I presume automotive parts. The reason that low spangle is used is that it is normally a smoother coating and gives smoother finish when painted. The smaller white splotches seem to be "white rust" or sometimes called wet storage stain, which may interfere with painting. An assay of the zinc would be useful.

Regards,

A. See there are two reasons; one is please check your flux -- 95% time it is because of the flux. Second, the black material should be checked after pickling if the surface is free from any defect such as Colour, Varnish,Oil.

Regards,

A. I assume we are discussing continuous strand galvanizing. A Lead kettle or fluidized bed with a caustic dip will remove the lubricant from the wire before pickling. Lubricant that remains on the wire is the most common cause of black spotting on continuous strand wire galvanizing. The acid by itself will not completely removed the lubricant. Another cause of black spotting is poor pickling-rinse-fluxing before the kettle. There is no information given on any of these issues, so I am just identifying the most common causes.

A. If you fully test your flux solution, it may be possible to determine the source(s) of your black spots. For example: what is the pH of your flux solution, the density at room temperature, the % iron +2, the zinc chloride, the ammonium chloride, the non-traditional non-volatiles (e.g. NaCl, KCl, CaCl2, MgCl2, MnCl2). Do you use mono- double, triple, or quadra salt? From what country do you buy flux? What color is your working flux. How long have you used this same flux? Have you tried making up new flux? Is your drawing lubricant a mineral oil or an animal fat by-product? Is your caustic simply sodium hydroxide or a mixture of bases. Are you emulsifying or saponifying the drawing lubricant? If you post your test results perhaps solutions to your problems can be found.

Regards,

A. This looks like flux staining.
The material is expanded mesh, cut from sheet and stretched in the manufacturing, it has cut edges that are not smooth. Also being mesh in the HDG process when removing the mesh from the liquid zinc, some of the mesh "windows" get a thin flake of zinc forming a bubble like film which solidifies to something close to aluminium foil used in household cooking. To prevent these actions, galvanizers throw sold flux powder at the work as it emerges from the zinc. The flux acts like a surfactant, and lowers surface tension, preventing the flakey thin "feathers". But if done too much, it leaves stains that look like this. The stains could have other causes, but on such material as this, I suggest its likely flux stains.