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topic 56867p2

Spotting, Staining, Blackening of Hot Dip Galvanized Finish



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A discussion started in 2011 & continuing through 2017

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.

56867-17

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?
Thanks

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.

Regards,

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

or

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.

Ozge SARACOGLU
- 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
- 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  

Dipu kOMALAN
METAL MANUFACTURING - Dubai UAE


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 have a copy of the spec :-)

Regards,

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


October 29, 2017

A. Mr. Dipu: the answer is your tank has too much of dross in it.

Vishal Agarwal
INDANA STEEL PVT.LTD - kolkata,Westbengal,India



Galvanize Discoloration

October 16, 2017

Q. what are the common causes of discoloration of a galvanized metal?
Please see my photo:

56867-20

Bredix Duman
- Rosario, Cavite, Philippines


October 19, 2017

A. Common causes of discolouration (like that in your image) include:
Variable steel chemistry
Variable dip withdrawal speed
Contamination on withdrawal from the zinc
Variable cooling after withdrawal from the zinc
Various different types of quench or passivate
Flux staining
Variable zinc chemistry (eg Ni, Pb, Al, Bi, Sn)

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



October 29, 2017

A. See in Asia this a common problem.Two primary reasons associated with this is:
1.High Zinc bath temperature.
2.High silicone content

Ideally bath temperature should not be above 450 °C.
Withdrawal rate has more to do with the zinc coating thickness.

Vishal Agarwal
- kolkata, Westbengal, India


October 30, 2017

A. Sirs:

The photo of the galvanized steel shows a result of zero aluminum in the molten zinc (as shown by the blue color). And the grey-black thick zinc is likely from reactive steel.

Regards,

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


November 17, 2017

thumbs up sign Thank you guys for giving your ideas :)

Bredix Duman [returning]
- Rosario, Cavite, Philippines


November 18, 2017

A. Dear
Along with the given issue I wish to add:
Remove the Galvanized goods from Galvanizing Plant as early as possible. Normally the Galvanizing Plant atmosphere is somewhat corrosive. And it affects the physical appearance of Galvanized goods.
And please mind it: Galvanizing Coating is not at all a cosmetic coating.

U. C. Dalela
- NEW DELHI, India



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