Galvanizing defects: black spots / white haze

A. You didn't have any way to identify each image, so hard to reference them. But...
The two on the right and one at bottom left [ed note: B, D, & H?] are from poor skimming technique, nothing to do with bath chemistry.
The ash (scum on surface) that continually forms as a result of the zinc oxidizing with exposure to air, has been swept up onto the steel and that's what those lines and folds of surface zinc oxide are. That's avoidable with more frequent and closer skimming.
Some, those with black marks could be either excess aluminium (Al) or under pickling. (not in acid long enough and some oxides are left of the steel before it goes to zinc).
Excess Al (> 0.008%) can cause these. Al target is around 0.002-0.004% in the molten Zn.
But whosever told you about sampling technique has a good point. Al sits near the surface and if your Zn sample sent for analysis was taken deep it will be very different to taken from the surface. I'm amazed sometimes how little thought goes into taking a sample when we can get three decimal points or more of a % accuracy in the analysis, but no thought on how representative that sample was of the whole.

Your Ni looks low, but not so low to say you were not adding some. Did you add some? If you want Ni then 0.05 - 0.06% target. Its purpose is to modify the reaction between zinc and reactive steel (whose chemistry has awkward Si and P levels). The images with pimply finish could be reactive steel, and there's not enough Ni to stop that.

You can tell more about these black spots by scratching them to see if they are a deposit on top of zinc (like flux residues) or an actual gap in the coating - a genuine bare patch. If zinc under, then just remove the stain. If no zinc then look to acid pickling being under done, and or Al% being too high.

A. Pictures D, B, H are from lack of skimming the surface during withdrawal of the steel from the zinc. This folds are zinc oxide. Once the steel is immersed in the zinc, and while they are still submerged, and when all turbulence is stopped, the surface of the zinc should be Swept / wiped / skimmed. A scum of oxide is continually forming, it must be swept back with a suitable tool to present clean shiny zinc, and then the steel raised. Target seed of raise is 1m/min. Then when fully out of the zinc, wipe the steel of drips before they solidify.

Shotblasting is never a very successful method of pretreatment in galvanizing. Chemical treatment using usually hydrochloric acid sometimes sulfuric, and rinses is far superior.

With the black spots: To test if its zinc under, use a coating thickness meter and measure the coating. All galvanizers should have one anyway, there are s=many manufacturers making them. Scrape away the black and measure the zinc underneath. If there is coating under the black spot, then its likely a flux residue, and could wash off with warm water. If no zinc under, then its a bare patch from unclean steel before dipping in zinc.

Take zinc samples from about 5cm under surface if possible.

If you're adding Ni, there's not enough. Some methods of addition are far more effective than others, and in each method some brands far more effective that other brands of same method. Powder the least effective, pellets not much better. Alloy and wax tablets good, via flux not great.
Target 0.05 - 0.06%

Al does not last long in zinc and needs " a little and often" additions. Many do 2 x / shift.

A. Wipe the steel - yes remove the drips. Usually not by knocking but smoothly touching the skimming tool against the drips and wiping them off while still molten zinc.

Flux residue and bare spots are different. Bare spot is exposing raw steel, no zinc coating. Flux residue is flux on the surface of the zinc coating.

Nickel reduces the coating thickness in the cases of reactive steel whose %Si and %P cause it to be in excess. The result of no Ni in these cases leaves excess pimpling or coating thickness or both, sometimes so thick that the coating delaminates.

The Al when no production reports to the ash.

Ash inclusions as a true "inclusion" is rare. More often its a deposit on top of the zinc coating.

Sparks on the surface are hydrogen being evolved from flux reacting with zinc.
This is one of several reactions taking place...
2NH4Cl + Zn = H2 + NH3 + ZnCl2 :NH3
That H2 burns, but in the presence of ammonia ⇦this on eBay or Amazon [affil links] can sometimes appear as a spark. The chemistry happening on the surface of the zinc is quite complex with several simultaneous reactions happening. It's normal and not a concern.

A. Dear Nitin,
The problem you're facing is common in Hot rolled material like angles & channels.
Sometimes while rolling, some undercuts and laminations are developed in the material, which gets filled with thick Mill scale.These mill scales are found very hard to remove in pickling.
All I can say that there is nothing wrong with your process, the defect is in your steel.
I would suggest you consult your material supplier.

Regards.

A. I agree, this looks like either slag inclusions in hot rolling, or laminations in rolling. Not a galvanizing fault, a steel rolling fault. Geoff Crowley Crithwood Ltd. Westfield, Scotland, UK A. Sir: On the one hand, Samrat may be correct. On the other hand, a stronger flux and altering the zinc chemistry may well prevent these black spots (bare areas). Since only last summer I have learned to overcome some problems like these by altering the zinc chemistry. Although I have learned much in the last 40 years about galvanizing, since last summer I have at least doubled what I previously have learned. Regards, Dr. Thomas H. Cook Galvanizing Consultant - Hot Springs, South Dakota, USA

A. Dear Mr. Mohammad Reza Ourang,
The dew point and Aluminum are not the cause of uncoated spots. It is possible for leaking radiant tubes to cause uncoated spots. The combustion mix in the radiant tube should be lean (4% oxygen in the exhaust flue gas) and this will oxidize the strip in the radiant tube section. An easy way to confirm this is to manually shut down the burners of the leaking tubes by shutting off both the air and gas valves. If the uncoated spots disappear, then the leaking radiant tubes are the culprit. Please remember that there are a lot more possible causes for such defects.

Happy New Year
Voo

A. This looks like "White rust", and your mention of the change in passivation sounds like the likely cause.

A. Those marks are NOT a cause for rejection of galvanizing if ISO 1461 is used as the standard.
The marks are caused by tension on the surface of the oxide payer as the steel emerges from the liquid zinc in the process. The zinc pool is continually oxidizing, forming a zinc oxide "scum" on the surface of the molten zinc.
The steel item being taken from that zinc vertically attracts some of that thin layer of zinc oxide and it sticks to the surface. But the rate of withdrawal of the steel exceeds the rate of deposition of the oxide, so the layer of oxide tears, leaving a gap where there's no oxide.
In time this mark will disappear, after perhaps 6 months outdoors exposure.
You could measure the thickness of the zinc at each point and find no difference.

A. Hot dip galvanizing should not be selected if an aesthetic look is aimed at. It's a corrosion protection coating, a functional coating.

The standard is already mentioned in recent post.

If you want the look of electroplating, then select that, but be aware that there's a huge difference in the corrosion protection afforded by these two quite different coatings.

A. Dear Mr. Pires,
For a defect that looks like dross and only appears on one side, it is usually zinc oxide (zinc dust ⇦this on eBay or Amazon [affil links] falling on the moving strip.
Another possible cause is the accumulation of top dross floating inside the snout. Use nitrogen bubbling method to agitate the dross floating inside the snout. If there is an increased amount of the defect, it is due to the top dross inside the snout.
You mention you have been operating for some years now. It is possible that there is an accumulation of bottom dross plus scum at the bottom of the pot. Bottom dross / scum is mainly due to unstable Al control, unstable molten zinc temperature control or contaminated zinc ingots.
As for the pitting of the sink roll, have a discussion with your sink roll supplier. most probably the problem will disappear with the new rolls.

Best regards

Voo

A. @ Joao Pires I think you should check some problems:
1- some alloy elements: Mn, Cr and Si that is present in zinc pot
2- Please check temperature of Top Roll at cooling Tower
3- I think you use some little Mg
4- protective gas Hydrogen and Nitrogen in Furnace.