Thick Galvanized Coatings, How to Get, What is Maximum

A. You are correct,

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 and ASTM A153 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

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.

A. No, Anthony, it's probably not feasible. Normal zinc electroplating thicknesses are from about 0.0002" to about 0.0010" thick (5 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,

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?

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.

A. Dear Luichito,

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

Regards,

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

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.

A. Sir:

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

Regards,

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.

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.

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

We love authoritative and confident replies, Geoff!

Regards,

A. 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.

A. Hi Peter. I'm not a galvanizer and don't have the answer. But to minimize lost motion when a knowledgable galvanizer gets here, I think they are going to say that it depends on the type of steel, so you might want to get back to us with the grade or composition of your steel.

Regards,

A. Peter, your galvanizer (of whom you asked the question), is probably right. Getting that thickness of galv on 2.5mm thick steel is very difficult. While immersion time is a variable influencing thickness of coating, it's a pretty impractical one, and one that few galvanizers will even consider.
Silicon and phosphorous content of the steel are by far the biggest factors to be considered, and a lower influencing factor is surface profile (blasting). Most fairly plain carbon steels of 2.5 mm thickness will get about 45-55µ thickness. Blast that steel to SA2.5 (standard) and you might get 80µ. I don't think there's a practical way to get 150-170µ.
Where this often comes from is specifiers that don't know what they're talking about! They read something that says for example that 80µ will last 40 years. They also read that the life expectancy is proportional to thickness, so they use these two factors to reason that 160µ will last 80 years. Then they just specify that, without knowing that it might be impossible to achieve it. We know this from specifications for all sorts of weird numbers - 125µ, 135µ, 115µ, 155µ etc.
So your galvanizer that said "no, can't be done" is probably right. If design life for the steel fabrication exceeds the life deliverable by galvanizing, then there are other ways to get there, such as an organic coating (paint) over HDG. Or there are means such as metal spraying to get thicker coatings. But start at the spec. Likely it starts with someone making wrong assumptions that could be sorted out on paper before you go spending the money in failing something.

A. Sanjay,
You didn't state what might be more important, a MINIMUM zinc thickness, nor why you want spangles?
Without any minimum thickness stated, you probably could achieve this on your product by taking it to any galvanizer.
Spangle is an aesthetic issue, not a functional one. Generally the desire to have it is misguided, thinking that galvanizing is a beauty coating. It isn't. It might be best described as the ugliest coating available, but the one that delivers the most cost effective corrosion protection. So if you want to prevent rust - Galvanize. If you want great looks and beauty, do something else!