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topic 56867

Spotting, Staining, Blackening of Hot Dip Galvanized Finish

A discussion started in 2011 & continuing through 2017

March 1, 2011

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.

56867-1 56867-2 56867-3 56867-4 56867-5 56867-6 56867-7 56867-8 56867-9

Charles Stewart
employee - Lebanon, Missouri USA

March 4, 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
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland

May 14, 2013

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, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

May 17, 2013

A. Dear Athayde,

Your problem is probably about flux baume. Hold this baume 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

May 20, 2013

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
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland

May 21, 2013

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 22, 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
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland

May 28, 2013

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

June 4, 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
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland

June 14, 2013

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

July 8, 2013

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

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