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Thick Galvanized Coatings, How to Get, What is Maximum


An ongoing discussion from 1999 through 2016 . . .

(1999)

CLOSE BUT NO CIGAR?

You might instead be interested in:

letter 55032, "How can we minimize galvanize coating thickness?" or

letter 7666, "Hot Dipped Galvanized Coatings: G140 vs. G90 vs. G60" or

letter 45732, "Thickness of hot dip galvanizing vs. salt spray test results"

Q. I am trying to find information on the relationship between the quality of the hot-dipped zinc coating on light gauge non-structural sheet steel and the thickness of the steel.

We fabricate roofing and venting products for the construction industry.

A rather non-specific standard call out is the term "corrosion resistant" galvanized steel and sometimes an actual thickness (i.e., .019" or 26 Ga) will be included. Little else is given in terms of what specifically it it meant by "corrosion resistant" in relation to the use of the metal. Our contention is that the thickness of the steel has less to do with it's ability to withstand galvanic corrosion than the type, quality, and thickness of the zinc coating.

As of present, we have been unable to find any information to assist in better understanding this relationship. Any assistance would be greatly appreciated.

Neil Chapman
metal products - Long Beach California


(1999)

A. You are correct,

"the protection against corrosion... is essentially determined by the thickness of the (zinc) coating"

from the Metals Handbook, American Society for Metals. So a specification as to thickness of coating versus service conditions, similar to some electroplating standards against corrosion, is probably available. When I was in the coil coating business, my boss was trying to coil coat galvanized steel so that it would last longer in use for pig shelters. I don't know how far he got. See books on galvanizing available from this site.

Look up specification ASTM A123 [link by ed. to spec at TechStreet] and ASTM A153 [link by ed. to spec at TechStreet] for coating weight requirements:

A123/A123M-97ae1 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products Copyright 1999 AMERICAN SOCIETY FOR TESTING AND MATERIALS, West Conshohocken, PA. All rights reserved.

1. Scope 1.1 This specification covers the requirements for zinc coating (galvanizing) by the hot-dip process on iron and steel products made from rolled pressed and forged shapes, castings, plates, bars, and strips.

or

A153/A153M-98 Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware Copyright 1999 AMERICAN SOCIETY FOR TESTING AND MATERIALS, West Conshohocken, PA. All rights reserved. 1. Scope 1.1 This specification covers zinc coatings applied by the hot-dip process on iron and steel hardware.

Best regards, Tom

tom pullizzi monitor
Tom Pullizzi
Falls Township,
   Pennsylvania 



(1999)

A. I agree with Tom's recommendation. However, there is still the matter of the bonding strength of the zinc coating (no matter how thick) to the substrate. Factors prior to zinc coating such as cleaning, degreasing, and other surface preparation makes it worth this condition.


Q. Speaking of thickness, our technical staff have been thinking whether a zinc electroplated material can attain the same thickness (assuming same bonding strength) of a hot dip galvanized either by plating in much longer time or accelerating the time by adjusting the electrical supply. Is this feasible, technical and economy wise? Your inputs would greatly be appreciated.

Anthony Austria
- Philippines


Zinc Plating
by Geduld





Hot Dip Galvanizing

A. No, Anthony, it's probably not feasible. Normal zinc electroplating thicknesses are from about 0.0002" to about 0.0010" thick (8 µm to 25 µm). Beyond that thickness it not only takes too long and consumes too much energy, but the properties start becoming different (too much porosity, stresses too high, lack of brightness, etc.)

When you want thin coatings, zinc electroplating is usually better than galvanizing or any other method of applying the zinc; when you want the thickest coatings, galvanizing (or flame spray for field work) usually wins; in between, mechanical plating, dip-spin-coatings, or sherardizing might be applicable.

Regards,

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.



June 16, 2011

Q. Hi,

Please I have to know that if Galvanize coating thickness is 85 microns, what should be maximum admissible thickness of Galvanize coating.

Highly appreciate your prompt reply. Thanks

Regards

Rizwan Ahmed
Employee - UAE


June 17, 2011

A. Most standards state a minimum thickness for galvanizing, and no maximum.

But there is a practical maximum, defined by physical things that happen when coating is thick, or by the cost of obtaining a thick coating.

To get a very thick coating (say >1000µ ), long immersion times are required. For plain mild steel, some hours might be required. But for processes to make money, they normally want immersion times of 5-10 minutes, or less. So there's a cost in getting thick coatings.

But very thick coatings tend to separate along the lines of the various alloys involved in the coating. In simple terms this means some of the coating falls off, and other parts become apparently brittle. (while not technically brittle, they delaminate easily with impact).

So, please expand your question. Why do you want thick coatings?

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



August 1, 2011

A. That is absolutely correct: all specifications of Hot Dip Galvanizing demand minimum Zinc Coatings.

It is also equally important what thickness of work you are Galvanizing. It is very difficult to give 100 µ Zinc coating on 1.5 mm thick sheet; on the other hand it is also again very difficult to give 40 µ zinc coating on a 25 mm thick plate.

Normally in fabricated steel structures, customers define the plating thickness.

Umesh Dalela
- Mumbai, Maharashtra, India


August 9, 2011

Q. I will not add some comments or answers to your queries but I will ask questions that could help also my problem on galvanization. Here it is: What is the standard minimum thickness of galvanization for metal hardware? I have 1 mm thickness hardware that is used for pet gates. Is there any standard galvanization materials that will guarantee best results? Thanks to let us know.

Regards,
Luichito

Luichito Durban
- Ho Chi Minh City, Vietnam


August 9, 2011

A. Dear Luichito,

The standard minimum coating thickness depends on the item's steel thickness. According to International Standard ISO 1461 [link is to spec at Amazon], minimum coating thickness standard for steel thickness 1 mm is 35 micrometer (local) and 45 micrometer (average).

Regards,

CS Leong
- Selangor, Malaysia



Is 250 µm galvanizing too thick?

August 21, 2012

Q. We have a case of galvanising mild steel (E 250, Fe 410, C quality Weldable as per IS 2062) to a thickness of 250 micron.

Would like to get comments on the appropriateness of the coating thickness of 250 micron, from de-lamination point of view.

Ram Navas
engineering consultants - Gurgaon, Haryana, India



How to get required galvanizing thickness in shorter time?

January 21, 2016

Q. sir,

We are using 2.74 thick grade 50 steel for galvanizing, and our customer requirement is 150 microns. But to provide that we have to give 40+ minutes dipping time. That is the thing which we want to overcome; please suggest something so that we can get 150 microns in less time period.

shahzeb mohammed
engineer - oman
  ^- Privately contact this inquirer -^


January 22, 2016

A. Shotblast the steel to at least SA2.5 prior to galvanizing and you will reduce this excessively long immersion time.

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



March 17, 2016

A. The thickness of galvanized coatings is influenced by:
1- Surface condition of steel: Grit blasting steel before galvanizing roughens the surface and increases its surface area, producing thicker coatings
2- Composition of the steel: Both silicon and phosphorous contents can have major effects on the properties of galvanized coatings.
3- Material temperature prior to galvanizing: more dried and heated materials delivered to the kettle will lead to a shorter galvanizing time. You could check the dryer prior to galvanizing.

Mohamed Saad
Energya Steel Solution - Jeddah, Saudi Arabia


March 17, 2016

A. Sir:

Turn "Up" the molten zinc temperature. For example, from 835 °F to 850 °F.

Regards,

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


March 19, 2016

A. Turning up the temp in a zinc kettle requires caution.
With increasing temp (up to something like 500? °C) there's increased kettle wall erosion. A 50 mm wall kettle should last 10 years. At 500 °C it is measured in hours.
For anything much over 470 °C, use a ceramic kettle.

Small temp increases (say from 450 °C to 460 °C) don't show any increased galv thickness that is statistically different.

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




Want 200 µm galvanizing -- what thickness steel is needed?

January 10, 2017

Q. Regarding the zinc thickness in HDG procedure, I have a question: If there is a project which requires a minimum thickness of 200 microns, what thickness of steel should be chosen, considering a normal maintaining time in bath of 5 to 10 minutes. Non-blasted material and blasted material.
Thanks in advance.

Virgiliu Vitan
- Timișoara, Romania


January 13, 2017

A. Perhaps the biggest influence on galv thickness is the steel chemistry, not immersion time and not steel thickness (though both these also influence coating thickness).

It seems wrong to choose a thickness arbitrarily then decide what thickness of steel to achieve it.
Surely you wish to decide what size steel to use (strength, load, purpose, etc), and then decide how to protect it, by knowing the environment the steel will live in, and the desired longevity.

Then you can arrive at a desired thickness of galvanizing.

But you'll perhaps also know that over-thick coating lead to all sorts of problems, like delamination, cracking, susceptibility to damage, etc.

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



simultaneous January 18, 2017

A. Considering steel what you are Galvanizing normally, If you will take a 10 mm thick plate, shot Blast it both side, Galvanize it 445 °C; I'd hope to get desired 200 microns Zinc coating.

Umesh Dalela
Galvanizers & Consultants - Delhi, India


January 16, 2017

First, many thanks for your answer.
Yes, indeed we will take care at all the information contained in the received answer, along with the 1461 Standard observation.
Could we offer more information if the basic material is steel S235 (OL37) with a proper Si and P (in accordance with the ISO 1461 and 10025 Standards).
Thanks in advance.
Sincerely yours,

Virgiliu Vitan [returning]
- Timișoara, Romania



April 1, 2017

Q. Dear sir,
We have requirement for 140 micron coating thickness (Hot dip galvanizing)
Material used: Mild steel ASTM A 572 GR. 50
Thickness of Steel: 5 mm
Item: Round Tapered Street lighting Pole .

Requested Coating is possible? Could you explain the procedure to achieve the thickness?

JOSHY DAVIS
ALMEER INDUSTRIES - Ahmadi, KUWAIT


April 3, 2017

A. This steel should achieve about 100µ, but if your customer really wants 140µ (and many don't really know why they want it), then shotblasting the steel first will get about 140µ

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



April 2017

thumbs up sign We love authoritative and confident replies, Geoff!

Regards,

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


April 6, 2017

Actually speaking the thickness of Zinc coating is well defined in respective specification. Even then sometimes a client asks for a higher Zinc coating, then: Shot Blast the entire job properly then Galvanize. Might be you get desired result.
Carry my regards.

U C DALELA
- Delhi, India

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