-- The Home Page of the Finishing Industry
HomeFAQsBooksHelpWantedsForum letter 56867
Serious Education ... plus the most fun you can have in metal finishing.

Spotting, Staining, Blackening of Hot Dip Galvanized Finish

A discussion started in 2011 & continuing through 2017 -- add your Q to bring it back to the Hot Topics page.

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

July 11, 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

July 16, 2013

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
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland

simultaneous July 17, 2013

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
- Scarborough, UK

July 16, 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

July 24, 2013

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
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland

August 17, 2013

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

Galvanised Poles with Black Patches

March 23, 2015

Q. We recently received lighting poles where we expected a good galvanised finished look to the poles similar to poles purchased in the past. These poles appear to have a lot of black staining which stands out to the silvery finish. Is this as a result of contaminates like too much flux or what is it that we are experiencing?
Will this get worse with time or will the life of the pole be reduced?

56867-18c  56867-18b  56867-18a  56867-18d

Ed Kelly
- Dublin, Ireland

March 25, 2015

A. Ed,

The vertical pole looks like silicon semi-killed steed (between 0.04 and 0.14% silicon; e.g. the Sandlin Curve). Upon closer observation there seems to be a "cook-off" problem. My experience indicates:

1) The product may be improperly pickled in HCl. HCl temperature must be above 70 °F for proper pickling.

2) Cook-off time in the kettle may be insufficient. The bubbles must nearly stop rising to the surface of the molten zinc.

3) Your flux likely contains non-volatile impurities which are not releasing.

I have submitted a manuscript titled "Nonvolatile Impurities in Galvanizing Flux" and submitted it to the EGGA for possible presentation at the Intergalva conference in Liverpool June 7.

On a scale from "1" to "10" your photos show a "1."

Answers to the following questions would be helpful in solving your problems:

What is the temperature of your HCl and what is the density of the HCl?

What us the density of your flux solution. (e.g., g/ml, Tw, or baumé)? And what is the temp. of the flux?

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

March 26, 2015

I think there are several issues here.
1. There are what appear to be bare patches. This might be under-pickling (as Tom Cook says), which could be temperature of acid, (and its not warm yet in Ireland!), but that can be compensated for by longer pickle time in the acid. It's not warm in Scotland either, but we just pickle longer in winter.
This work does not meet the standard ISO 1461 [link is to spec at Amazon] if despatched with bare patches. It is not possible to be definitive from an image, but you could ask to have the thickness measured right on the black spot to see if it is really bare.
The standard does allow for repair of small bare patches by the use of zinc rich paint, but there are limits of size of repair, thickness of it, and the % of the total area that can be repaired.

2. The images indicate that elsewhere the coating is excessively thick. The second image with the "varicose veins" is one indication, and the porosity evident in another image (tiny pinholes even visible) is another example.
The most likely cause of this is the steel chemistry, which is an issue beyond the control of the galvanizer. It sounds like you purchased these, not being either the manufacturer nor the galvanizer. In that case you could ask the manufacturer, or perhaps the vendor for a certificate of conformity to the standard for such galvanizing (ISO 1461) and see what happens. If they refuse,be suspicious.

I think you have good grounds to ask for clarification that this galvanizing meets the standard. As a customer you cannot be expected to know the technical details above, you just want lamp posts that will last for maybe 50 years to first coating maintenance (depends on location and environment how long galvanizing lasts). That's why there are standards. So that the purchaser can have some assurance that why they buy is "right".
Ask for that assurance, and don't take any nonsense for replies.

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

January 2, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I want to ask you what is reason of blackening the galvanized sheets after some days of manufacturing. Or I can put it another way, what is the reason of premature darkening of the galvanized sheets. What is the remedy of that problem? Is it related with passivation (Chromic acid) or alloys like Lead, Sb & Al or some other reasons?

Kantilal Trivedi
- Baroda, Gujarat, India

May 20, 2016

A. Seems your problem is because of improper flux chemistry. Improper passivation problem can be seen as soon as the material cools down.

Nitesh Kumar Agarwal
indana steel - Kolkata, India

Galvanizing rejected for poor appearance

June 27, 2017

Q. Client has rejected the grating due to poor appearance. Request you to please advise is it a valid rejection as per ASTM A123 [link by ed. to spec at TechStreet]

56867-19b   56867-19a  


July 5, 2017

A. As per any specification the goods cannot be rejected on the grounds of its appearance. Galvanizing is not at all a cosmetic coating. But it should look reasonably good, Galvanized in Zn 98 Min.

U. C. Dalela
Galvanizers & Consultants. - NEW DELHI. India

July 9, 2017

A. You might ask the customer which clause of ASTM 123 they rejected the galvanizing under? It seems to be aesthetically poor, but the standard acknowledges that HDG isn't an aesthetic coating, it's a functional one.

The real issue in this sort of situation is often that the client reads a standard number off Wikipedia or some such place, then quotes it without having actually read the standard.
They have their own idea on what they expected, but didn't discuss and agree that expectation with the supplier (galvanizer), then blame the supplier when they didn't get what they thought they should have got.

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

July 2017

thumbs up sign I think you nailed it Geoff! We have a number of threads here where a buyer wants to know what spec or standard to put on his P.O., but has no intention of spending the $30 or $60 to obtain a copy :-)


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

ADD a Q or A to THIS thread START a NEW THREADView This Week's HOT TOPICS

Disclaimer: It is not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices

©1995-2017     -    Privacy Policy
How Google uses data when you visit this site.