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Spotting, Staining, Blackening of Hot Dip Galvanized Finish

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Q. I am a QC in charge of an organisation.
We have a hot dip galvanizing plant of fasteners. After hot dip galvanizing we saw that there are black spots on all fasteners.

56867-10a   56867-10b  

We can't understand the reason.
Materials are perfectly cleaned and pre-flux, and we are using dichromate solution afterwards.

Sumanta Adhikari
Employee - Howrah, India
July 16, 2022

The black spots could be from many reasons. To narrow down the possibilities, some questions to use.

• Is all work being affected or just some batches, or even some specific orders? (process issue or materials issue)

• Do the spots appear immediately, or develop after some time? (galvanizing issue or post galv treatment / storage / contamination)

• Is there zinc under the black spot? Can it be removed by scraping with a knife or sharp tool? (issue in pretreatment or in the zinc?)

Geoff Crowley
Crithwood Ltd.
supporting advertiser
Bathgate, Scotland, UK
crithwood logo

Closely related Q&A's, oldest first:

Q. Hello, my name is Charles Stewart. I am a QC Inspector for a trailer mfgr. in Lebanon, Mo. The business offers powder coat and galvanized coating. Recently the galvanized finish has had dark spots that look like rain drops. Our customers do not like this. The spots are tough to remove.Scotts Brite Pads and water leave small scratches in the finish. It also speeds up the oxidation process. I tried using a weak solution of water and Clorox and then wiping dry. This turns the galvanizing black over a period of days. My question is four part:
1 Other than debris or carbon, what is causing the spots in the finish?
2 By using the Clorox (stopped when trailer turned black,about a week)Have I opened pores in the finish?
3 If so will they close?
4 If the problem cannot be resolved at the galvanizing plant, is there a product that will remove the black spots without damaging the finish?

Any suggestions would be greatly appreciated and thank you for your time.

56867-1a   56867-1b   56867-1c   56867-1d   56867-1e   56867-1f   56867-1g   56867-1h   56867-1i  

Charles Stewart
employee - Lebanon, Missouri USA

March 1, 2011

A. Staining or colour variation in galvanizing falls into two main categories.
1. Alloying variation. (a couple of your pictures might be this, but hard to tell).
Galvanizing is an alloying reaction between zinc and steel. The underlying part of the coating is alloy, which is dull grey, and the outer part of the coating is pure zinc which is initially bright silver. If allowed to, the whole coating will turn to alloy through its full thickness. By "allowed to", I mean that the steel remains at temperatures of about 450 or more. This is common in thicker steel sections, where residual heat allows the upper layer to also alloy. None of this is really avoidable, and is purely aesthetic, having no impact on corrosion protection.

2. Flux staining.
The majority of your images look like this.
An item to be galvanized is firstly cleaned chemically, typically in alkalis to degrease, then acid (hydrochloric or sulphuric) to remove surface oxides of iron. As the coating is an alloying one, it only happens with immersion in zinc of chemically clean steel.
Just before immersion in zinc, the item is immersed in a flux solution. This has similar effect to flux in other processes such as soldering, welding etc.
The flux is usually a mix of ammonium chloride and zinc chloride, and it is dissolved in hot water. Immersion of the steel and subsequent drying in air leaves a thin layer of these crystals on the surface.
During immersion this flux layer helps further deoxidize the steel, and the zinc at the interface between steel and molten zinc, and also acts as a "wetting agent".
But the flux can be to excess. Either wrong chemistry in the flux tank, or by the use of solid (undissolved) pure flux thrown by hand onto the zinc. This melts on the surface of the zinc into a black tar looking substance, and stains the work on withdrawal from the zinc.
ISO1461 is the international standard for galvanizing, and flux staining is a cause for rejection of the galvanizing. Flux deposits, ash deposits etc are not permissible. You might use a different standard, but its likely to say the same thing.
Ask your galvanizer which standard he works to, and ask to see that standard (its not a long document), and look for this, and suggest that this work doesn't comply, and ask him/her what should be done to resolve this.
If they don't know, or will not work with you to solve this problem, change supplier.

There is a solution to fix the problem afterward. Several proprietary brand citric acid based solutions are available that will wash this off, without causing the gross discolouration you have seen by using scouring pads.
Your scouring pad will not have caused "porosity" or similar, in fact its quite difficult to damage galvanizing, (except chemically).

Good luck

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

Q. Dear sirs,

I'm working in a company that galvanizes steels for power transmission towers. We're having a constant defect in our galvanized steel. It looks like black spots, but I believe it's something different and I don't know what it is.

We use these steps:
Acid degrease (10%-12%)
Chloridric acid (16% initial)
A tank for water rinse
Flux (29 °Bé)
Zinc (Zn 99,8%, Al <0,005%, Cu 0,12% I don't know why we have this much copper)

I really need some help because we're having this problem in all the big pieces.


Daniel Athayde
Chemical Engineer - Belo Horizonte, Brazil
May 14, 2013

A. Hi Daniel. I'm not a galvanizing expert, but we have many threads on line about black spots in hot dip galvanize, and people may describe different situations as 'black spots'' -- so a picture would probably help. Your email is bouncing, so I hope you see this.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Dear Athayde,

Your problem is probably about flux baumé. Hold this baumé about 22-25 distance. Also it depends on product. Because other chemicals and foreign items aren't wanted in process. So they must remain in acid tank. Prolong the product's standstill.

- Bursa, TURKEY

A. Hard to say without the suggested pictures, but could be excess aluminium.
Your 0.005% sounds a bit high to me. I suggest <0.002 max.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

Here are photographs:

Black spots in galvanizing 1   Black spots in galvanizing 2   Black spots in galvanizing 3   Black spots in galvanizing 4   Black spots in galvanizing 5   Black spots in galvanizing 6  


Daniel Athayde [returning]
- Belo Horizonte, Brazil
May 21, 2013

A. Looks like flux staining to me.
Have you measured the zinc thickness in the middle of a spot?
If it's normal, then I suggest flux stains.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

Q. Dear Geoff,

For this particular kind of piece, the zinc thickness needs to be over 100 µm (microns). In those black spots, the zinc thickness is about 30 µm and, after brushing with a wire brush, we could see the steel.

We found out that the inhibitor we were using may present a problem to the process because we needed to spend more time in the pickling solution. We stopped using the inhibitor and the black spots appeared less but it's still a constant problem in our line. We're facing about 1% of rework everyday because of this problem!

I hope you can help me with your advice.


Daniel Athayde [returning]
- Belo Horizonte, Brazil
May 28, 2013

A. Given that these are now established as being bare patches with little or no zinc cover, then I suggest this is a problem with too short an immersion time in acid.
Do you use hydrochloric or sulphuric?

Not every part cleans at the same rate, and when the operator is too enthusiastic, and gets the job out of the acid too quickly, some parts will be still covered with oxides.

You do a trial by extending the time in acid by say 25%, for identified parts / jigs, and see what happens. If no improvement try +50%.
This simple test will check if this theory is right. If wrong, then we need to think again.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

A. I believe you have a pickling problem. This could be from a multitude of things such as weak acid, high iron, too much inhibitor, contaminants on steel (such as oil, paint), too short dwell time in acid, poor racking,etc. I would take a batch aside in between acid rinse and flux that your pickle crew thinks is ready and look for spots. I suspect there will be areas un-pickled very similar to the post galvanizing problems you are having. You may very well have a flux issue as well, but I suspect pickling to be the bigger issue for this. always remember that the steel won't galvanize if it isn't pickled.

Angle iron like the ones in the photo should be able to pickle easily in 30 minutes. A good check-up of chemistries throughout your entire pickling/fluxing process should be done.

David Jaye
Houston, Texas

Q. Dear Geoff and David,

We solved the problem. It was the inhibitor that caused the problem, because the pickling rate was too slow. Only by stop using the inhibitor this kind of problem stopped.

My goal now is to make a research of the best inhibitor concentration in a way that we don't compromise the process and still maintain the inhibitor advantages.

One question about pickling time. I set up an experiment where I measured the speed of the reaction between the steel and the acid, but I don't believe the reaction speed is the same of the reaction between the acid and the rust!

So, is there a way to measure the reaction rate between the acid and the rust or, in other words, the speed that the acid clean the surface of the steel? Or the only way is visually?

Thanks for your help!

Daniel Athayde [returning]
- Belo Horizonte, Brazil
July 8, 2013

Q. Hi,

We hot dip galvanise our own steel products and whenever we use 'weathering steel', we always see issues with black spot. We have tried variations in the time that the steel spends in the cleaner and pickling tanks, but this has not solved the problem. Our mild steel products process through the same galvanizing tanks without issue. It is just the weathering steel that gives us this problem.


Has anyone any theories in this?

Phil Rudland
- Scarborough, North Yorkshire, UK

A. Once saw this and found that light sweep blasting before processing eliminated the issue, but we never did understand the mechanism to know why.
(Assume you mean Corten, when you say "weathering steel"?)

But we've often wondered why people specify weathering steel, and then galvanize it?

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo


Q. Hi Geoff, thanks for your comments. Yes we call it Corten steel. What do you mean by 'Light sweep blasting'? I've not come across this term before?

Phil Rudland [returning]
- Scarborough, UK
July 17, 2013

A. Sirs:

The original name for weathering steel was cor-ten. I just finished galvanizing (using SHG zinc at 835F and withdrawal speed of 3 feet/minute)a sample of cor-ten with the results:

three minutes: 4.86 mils (123 microns)
six minutes: 7.95 mils (202 microns)
nine minutes: 10.43 mils (265 microns)

The zinc coatings were very smooth and without problems.
I do not like to galvanize cor-ten because I believe that some copper gets into the zinc in this way. Also some secondary brightener bar is contaminated with copper. I do not like to see copper above 0.05% in the kettle zinc.


Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

A. "Sweep Blasting"

In shotblasting there are a variety of purposes / desired outcomes.
A standard for blasting often used is the one rating the degree of blast as SA2, SA2.5, SA3 etc. This refers to the amount of re-profiling of the steel and is altered by air pressure, grit size, length of time/exposure of any area to the moving grit etc.
SA2.5 is perhaps most common and is often described as having all traces of rust and scale removed, surface re-proofed/re-profiled, but not quite 100% white metal.

Sweep blasting is a light touch, where the blasting operator just sweeps the grit stream over the surface. It removes little, and re-profiles little.
But it seems to remove enough of something (I'm not sure what), that alters galvanizing effects.
For example in reactive steels (sometimes called Sandelin steels, and involving undesirable Si% composition), sweep blasting before galvanizing can markedly reduce the coating thickness from the galvanizing. I don't claim to understand the mechanism here, only experienced that it works.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

A. Phil,

If you are unable to sweep blast, here are some old-school and brutal methods for you:

1- Dip the material into zinc bath without doing any pretreatment, skim all the zinc, dip it into acid while it is hot, continue your normal process


2- Do the normal process, dip and withdraw the material after 1 min or so, sprinkle ammonium chloride on the material while on the kettle (especially to uncoated places) dip it again (don't forget, very nasty white fume will rise)

These methods are costly and unhealthy but sometimes useful when you are in a hurry.

- Doha, Qatar

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