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topic 52776

Pre-Galvanized vs. Post-Galvanized Sheet



A discussion started in 2002 & continuing through 2017

(2002)

Q. Sir,

This is regarding Pre-coated galvanized sheet vs. Post-coated galvanized sheet (CRCA). As precoated sheet is processed after coating, rust is forming along the cutting edge. Overall, which performs better with respect to corrosion, pre-coated or post coated galvanised sheet?

Gopinath
TVS-Electronics - Chennai, Tamilnadu, India


(2002)

A. Hi Gopinath. Well, yes, pre-coated sheet does that because there is no coverage on the sheared edges. The galvanizing on the rest of the sheet offers some protection from rusting, but it's not perfect. There would not be any post-coated products if pre-coating, which is cheaper and easier, performed as well as post-coating in all cases; it doesn't.

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



(2007)

Q. Do pre-galvanized steel sheets go through a hot dip process or is it some other galvanizing method? Apart from the edges having no corrosion protection is there any difference between pre-galvanized & post-galvanized steel

Thanks,

Jason Noronha
Engineer - Kuwait


(2007)

A. Components can be pre-galvanized via hot dipping, or preplated via zinc electroplating, (electrogalvanized), or post-galvanized or post electroplated, Jason. All four approaches are viable, with the hot dipping processes usually being significantly thicker (and more corrosion resistant) than the electroplating processes, and with post-galvanization via hot dipping being the thickest and most corrosion resistant. So, yes, the basic differences are whether the sheared edges have a coating, and the thickness of the coating.

If you can describe the parts and your needs we can possibly provide more help; or patient application of the search engine will produce dozens of Q&As about the differences between hot dip galvanizing and zinc electroplating. Good luck.

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



To minimize searching and offer multiple viewpoints, we combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition.



August 3, 2009

Q. Hi everybody,
I work as Research Engineer in Liège (Belgium), and I'm going to start a study comparing the performances of pre-galvanized steel vs. post-galvanized. Actually I'm mostly gathering information on this subject in order to better plan my activities; but, honestly, I'm not finding a lot of information about it.
Probably someone of you have great experience on this subject that can be useful and can allow me to save time and energies.
I just wanted to ask you if you can tell me some differences (durability in particular conditions, pro & con, price difference, etc.) or if you can address me to some specific bibliographical reference in order to speed up my gathering of information and to improve my "culture" on this topic pretty fast.

Thank you for your answer and for the time you're kindly spending for me.

Best regards,

Luca Felappi
Researcher - Liège, Belgium


August 4, 2009

A. Sir:

My publication, "Characteristics of Hot Galvanized, Metalized and Bare Edged Galvanized Sheet Products," METAL FINISHING,pages 63-68, May 1985, may be of interest to you. You can obtain a copy through interlibrary loan from any large library for the copying cost.

There was a major white rust problem when a cooling coil exchange unit changed from post galvanizing frames to galvanized sheet formed before bending. My research showed that the much higher aluminum content (e.g. 0.15% in the sheet) was the cause. Galvanizing after forming the product normally has only about 0.002% aluminum in the zinc coating.

Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA

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


August 5, 2009

A. What do you mean by "pre galvanized" and "post galvanized" steels?

Steel before it is galvanized and steel after it is galvanized?

Or items that are galvanized after fabrication, compared with materials made of steel galvanized before fabrication?

Clarifying your question will help obtaining answers.

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



August 17, 2009

thumbs up signDr Cook: Many thanks for your interest and your kind answer, I'm trying to get the article you've mentioned, and think it can be useful

Q. Mr. Crowley: Thanks for your interest, maybe I've used the wrong words:
With "pre-galvanized" I mean steel galvanized in continuous process (so, galvanized before bending or cutting).
With "post-galvanized", on the other hand, I mean the "classical" galvanizing process, where the Zinc is layed onto the finished pieces.
I hope I've clarified the question.

Best regards,

Luca Felappi [returning]
- Liège, Belgium


August 20, 2009

A. Pre-galvanized usually is in sheet form, though sometimes pipe. It has a much thinner zinc coating than items fabricated first then hot dip galvanized.
The thickness of the zinc is very carefully controlled in pregalvanized sheet, etc., but has little control in post fabrication galvanizing. "Post galv" has a wide variability of coating thickness as well as a much higher average thickness.
The life expectancy of a galvanized finish is directly proportional to the thickness of the zinc coating, so in general, post galv will last longer, perhaps by as much as 5-10 times, but as thickness of coating is also dependent on immersion time which in turn is proportional to thickness of the steel, then the difference in life expectancy is hard to compare.
Very few items of thin sheet metal are fabricated first then galvanized as distortion becomes an issue. Pregalv is typically stretched and leveled after galv, making it very smooth, flat finish. That's not possible with post galv.

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



July 7, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear Sir,

Actually we are Designing the LV Drive Sheet Boxes, in which we were using the material previously as C.R.C.A sheet with Zinc finish and then powder coating.

Now we want to go for Galvanized steel (Pre-Galvanized) sheet with powder coating for Box.

What Material notes should I write in Drawing so that supplier can easily understand the Steel? Is there any ISO or ASTM Standards available for the Pre-galvanized sheet?

From your posts I want to know if Electro galvanized steel is the same as pre galvanized steel, please confirm ASAP.

Thanks in advance.

Regards

Asit Rathod
Researcher - Ahmedabad,Gujarat,India


July 2014

A. Hi Asit. We appended your inquiry to a thread which previously answered that not all pre-galvanized sheet is electrogalvanized -- sheet can also be pre hot dip coated.

I think one of your issues will be how thick you want the coating to be. You might review letter 7666 "Hot Dipped Galvanized Coatings: G90 vs. G60" as a starting point for that, and for a discussion of your question about standards. Good luck.

Regards,

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



Coatings for bare edges of zinc plated steel

February 9, 2015

Q. We are using cut to length sections of 35mm DIN rail in zinc plated steel materials. The cut edges on each end are left unprotected by the zinc plating. I was thinking about specifying coating with a clear acrylic sealer. Is there a better option?

Thanks,
John

John Arnold
- Bend, Oregon USA


Cold Galvanizing Spray

January 2016

A. Hi John. I would conjecture that a cold galvanizing compound (zinc-rich paint) would be a better way to treat exposed edges than a clear acrylic because of the galvanic protection, but I can't quote a study to prove it.

Regards,

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


September 4, 2016

Q. Hi All, many thanks for all the information to date. But is there any other way of treating the cut edges on pre-galvanised sheet after production of components that will give better edge corrosion performance?
Is there a brush on product that is compatible with powder coating?
We will powder coat but would feel a little more confident if we had a gauge on life expectancy
Many thanks
Adam Canty

Adam Canty
HUXT Ltd - Hull, East Yorkshire, UK


November 2016

Hi Adam. If you are going to powder coat the galvanized parts, you should zinc phosphatize the items after galvanizing them anyway -- and this should provide fine corrosion resistance. If you are talking only about edges, rather than holes in the items, you might also be able to roll the edges. Good luck.

Regards,

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



April 26, 2016

Q. Hi Everyone,
I am working in Fire fighting systems and need to submit for approval the pipe. Mentioned in the drawing is: ASTM A-53 Grade-B Carbon Steel Pipe.

ACRONYMS:

ERW = Electro-Resistance Welded

Now we need approval for ERW type and galvanizing after fabrication, not before due to cost impact.
As per my understanding ASTM A-53 Grade-B Carbon Steel Pipe covers ERW, but not sure about the galvanizing after fabrication.
I need to know which standard should I mention in my approval request which will indicate ERW and galvanized after fabrication of pipe.

Yours assistance will be very helpful for me.

Thanks

Danish Sibtain
- Jubail, Saudi Arabia



Corrosion Resistance of Pregalvanized Steel: Edges Laser vs Shear/Punch

October 26, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am trying to find research related to a comparison of cutting processes in pre-galvanized G90 sheet steel. There is an opinion being floated, that laser cutting destroys the "self healing" properties of G90 steel. We typically process .059" up to .168" thicknesses on shears and turret punches. Can anyone point me to any studies comparing the "self healing" action between these two processes, or is this even a viable question to be asking?

Thank you, in advance for anyone who may provide some insight.

Mike Adams
- Champaign, Illinois, USA

----
Ed. note: We appended your inquiry to a thread where Dr. Cook mentions his article which attempted to determine the value of 'smeared' zinc from shearing/punching.


October 29, 2017

A. Hello,
Once you have punched a hole via laser or press you have penetrated the coating. So both the pieces are subject to corrosion. The difference is insignificant laser vs press. Ideally, fabrication is done first and then the steel is Hot dip Galvanised or Coated as per process requirement.

Vishal Agarwal
INDANA STEEL PVT.LTD - Kolkata,West bengal,India



November 9, 2017

Q. Hi,

Can anyone let me know that is there any self-healing process there for Pre Galvanised sheet. We are using Pre galvanised steel for Solar structures and they are having rust on the cut edges. We want to know if there is any study reference which can give us an idea whether the rust will spread from the cutting edge or not? Also we want to know whether there any chance that the properties of the steel will stop the rust from spreading further.

Harshith Vadnala
- Hyderabad, Telangana, India


November 11, 2017

A. The main differences between pregalvanized sheet and post fabrication galvanized are that pregalv is very flat sheet (having been stretch leveled during the process) and with thin coating. Post galv more likely to have some distortion, not be float, and have much thicker coating.
The thicker the coating the more the so called self healing effect. Actually this is just the zinc acting as a local anode, and it's effective to about 3-4 mm away from the zinc. Cut edges should be protected by the zinc either side, but the thicker that coating the better it will be. The chemistry of the steel doesn't normal have any direct effect on speed of rusting, but could have an indirect one by way of affecting the steel reactivity with zinc, causing either thinner or thicker coating. (I'm not allowing for alloyed steels or Corten, etc).

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




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