How to calculate/measure the thickness of zinc coatings

A. Hi Brian.

I'm not fully confident that I exactly understand the question, and Elcometer makes a few different types of instruments and probes, but ...
A gauge with a magnetic force probe, and which is calibrated, should give accurate thickness readings for any non-magnetic coating, including the ones you mentioned -- but an eddy current device (designed for coatings on non-magnetic substrates) does not sound like a good choice.
Luck & Regards,


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

A. Yes. You could use an Elcometer (that's a brand of many instruments) that is typically used in a galvanizing plant to measure zinc thickness on steel, to measure the thickness of the named other coatings.

Most galvanizers use a magnetic force instrument (Elcometer is one brand of several), to measure the non ferrous coating on a ferrous substrate.
If you want to measure thickness of say paint on top of galvanizing on top of steel, then there are instruments that use both eddy current and magnetic technique and can give the thickness of each coating.

Geoff Crowley, galvanizing consultant
Crithwood Ltd.

Bathgate, Scotland, UK

June 21, 2024

A. My friend Mike Rometer says he can measure the thickness of plating on rod stock or flat stock without using any fancy instruments.

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.



July 18, 2024

A. Hi, Vinod. Usually electroplating is expressed in thickness but phosphatizing is expressed in weight because the meaning and usefulness of the term 'thickness' becomes compromised when you are speaking of a conversion coating which includes some dissolution of the original substrate, transition areas, various different compounds, and a spikey surface.

But if this is zinc electroplating and the density of zinc metal is about 7.14 g/cm³, then you have 610/7.14 = 85.4 cm³ or .0000854 m³ of zinc spread across 1 m².

So the thickness is .0000854 meters or 85.4 microns or 3.3 thousandths of an inch. That would be too thick for zinc electroplating and sounds more like galvanizing. Typically, and per spec, zinc electroplating would be 5, 8, 13, or 25 microns thick based on whether the service condition was mild, moderate, severe, or very severe.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. 610 g/m² is the often used equivalent of 85 microns used for hot dip galvanizing.

Can't imagine electroplating that thick!

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

A. Hi John. If you multiply the thickness of the coating (100 microns) by the surface area you are coating, you will have the volume of zinc you are using. You can, if you wish, then multiply this by the density of zinc to convert the zinc usage to weight instead of volume.

Your unknown at this point, though, is how much surface area you are coating. Telling us the weight of steel you are coating doesn't help us because if we don't know the thickness of the steel we cannot convert its weight to its area.

As for how to reduce the thickness from 100 microns to 85 microns, we have many threads which address that issue, including:
thread 55032, "How can we minimize galvanize coating thickness",
thread 57597, "Reducing variance in galvanizing coating thickness"
and thread 45551, "Reducing coating thickness by adding nickel salts". Good luck.

Regards,

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

A. Hi Siva. I assume you are saying that you are supposed to offer a coating of 500 grams / meter² but don't know whether this is per side or total? Most specs mean total weight of zinc on both sides put together (although it's always dangerous to ask a 3rd party to guess what your customer wanted).

It's okay to estimate for your own purposes the grams / meter² from the thickness, or vice versa, per the examples worked out on this page -- such that, yes, 60 microns average total thickness should be about 430 grams / meter² -- but I think you are expected to verify the coating weight by stripping and weighing, not by taking a thickness reading at a couple of points and assuming that the thickness will be the same everywhere. Good luck.

Regards,

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

A. That's an easy question. Even easier than 'What is the meaning of life?' If your wire is 2.45 mm in diameter and you want to build it up to 2.50 mm, then you need 0.025 mm of plating, since you plate all around the wire. That's 25 micrometers. And the meaning of life? That's 42. At least according to Douglas Adams.

Tom Rochester
Plating Systems & Technologies, Inc.
Jackson, Michigan, USA

Tom is the author of
"Surface Area Formulas & Tables
- A Resource for Surface Finishers"

You'd think Adams would have learned something from Hitchhiking the whole damn Galaxy! The meaning of life is to be conscious and grateful, to strive to be happy, and to help others be happy. (That's 21 words, not 42, so it's probably only half-baked). Ted Mooney, P.E. RET Striving to live Aloha finishing.com - Pine Beach, New Jersey

A. Elcometer is a brand name of an instrument maker. They make many different instruments.
Your question isn't specific enough.
Which meter do you wish to use?
What do you want to measure?

If you wish to measure the length of the coated item, then a tape measure might do.
If (as I suspect, but you didn't say), you wish to measure the galvanizing thickness, then an instrument is available for this purpose, and ISO 1461, the International standard for galvanizing allows for such an instrument to be used. There are many manufacturers of such instruments, Elcometer is one of many reputable brands.
The standard dictates how many places you should measure, and how to decide where etc.

Geoff Crowley, galvanizing consultant
Crithwood Ltd.

Bathgate, Scotland, UK

A. Sorry Rahul, that one is unanswerable.

Regards,

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

A. Hi Nilesh. Several people have said 610GSM is 85 microns, and 430GSM is 60 microns, so 350GSM would be 350/610 x 85 = 48 microns; or 350/430 x 60 = 48 microns average thickness. But again, the math does not guarantee even thickness.

Regards,

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

A. Hi Mun. ZF100 means the coating weight is 100 grams per square meter (and that usually includes both sides). I think you need to consult a manufacturer's data sheet for the hardness.

Regards,

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

A. Hi Bappu.
Weight = Density X Volume, and
Volume = Surface Area X Thickness

You're making me guess :-)
Do you know the exact surface area you are trying to cover?
Did you know the math, but are actually asking for the density of sprayed zinc to plug into the formula?

Regards,

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

A. Dear friend,
After pickling the weight will be less. For example if your material's weight was 11.5 kg before galvanizing, but after pickling the weight will be 10.5 kg (supposed). Also the density of Zinc is not exactly 7.2, So here are some errors. That's why you observed the deviation.

Md. Shariful Islam
- Mymensingh, Bangladesh
September 27, 2023

A. There are always arguments about zinc consumption, actual zinc on an item, and about where the zinc goes to.
Your approach coming from thickness measurement of 150µ, sounds like a single thickness measurement? On an item of your stated 2.485m2 you would likely find thickness measurements ranging from about 80 to 2000µ. It will vary across the dimensions, with the longest immersion area (first in, last out) parts thickest coating, and opposite parts (as hung) will be thinner. At the last drip point there could be 2mm thickness. You'd need to have a carefully devised plan of many measurements to get an average thickness.
Taking weight into the process and weight out also has its problems. The pretreatment process will remove some weight of the original unprocessed steel item. Then the zinc adds weight. So there's 3 stages of weighing could be considered.
Your stated weight difference of 8.7% is excessive. If it's correct, then your process needs urgent attention. A 5mm thick item should gain about 5% weight.
How did you weigh this piece. You mentioned a weighbridge. Typically these are made for weighing a vehicle and its load, perhaps 40 tonnes. Even a small one for 10 tonnes has an accuracy of +/- several kg (say 2 kg in 10,000, or 0.02%). So to measure 2 kg is almost impossible.

So your question was "why is there a difference between two systems of measurement?". My answer is that both methods have such a level of inaccuracy that both answers are a bit meaningless, and a comparison between inaccuracy and inaccuracy is also meaningless. Sorry, but repeat the experiment with more accuracy and design of the errors involved.

Geoff Crowley, galvanizing consultant
Crithwood Ltd.

Bathgate, Scotland, UK

September 28, 2023

A. Hi Katakam. As you can see from the above question from Rukesh, it can be a bit difficult in practice, but in theory it's very easy ...

Weight = Volume X Density, and
Volume = Surface Area X Thickness, so
Weight = Surface Area X Thickness X Density.

Because there are 10,000 cm2 per m2 of Surface Area, and 1/10,000 of a cm in a micron, the numbers work out nicely, with 1 micron thickness over a square meter being equal to 1 cm3 of metal. So the gsm per micron thickness is, happily, simply the density of the metal per cm3.

Regards,

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

A. Hi Mahendra. If you don't know the substrate material, this problem is unsolvable because, without the length of the wire we have too many unknowns so we can't calculate anything. But if this is a galvanizing situation so we can assume that the substrate is steel, then --

1. Since you know the substrate material, you can estimate its density.
2. Now knowing its weight and density, you can solve for the volume of wire you have.
3. Now knowing the volume and the diameter, you can solve for the length.
4. Now knowing the length and diameter, you can solve for the surface area.
5. Finally, now knowing the added weight of the zinc coating and the surface area of the wire, you can solve for the gm/m2. Good luck!

Regards,

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

A. Hi Swapnil. If you ask a question you don't really understand, the answer isn't of much value; are you sure you understand the question? (The reason I ask is because you gave five pieces of info and four of them are extraneous: neither the dimensions of the plate nor its weight matter).
If a coating is 67.5 millionths of a meter thick, the volume of the coating on one side of a square meter of surface is 67.5 millionths of a cubic meter, and zinc weighs about 7140 kg per cubic meter, so the weight of the coating on that square meter is 0.482 kg or 482 grams per side. Good luck!

Regards,

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

A. Hi Dinesh. I'm not a galvanizer and can't answer your questions. Sorry! But as site curator I think you'll improve the chances of readers spending their time by realizing that people would like to learn something from you ... and they want to help an actual individual ... and they seek the satisfaction of helping to solve an actual problem ... and most of them don't relish just looking stuff up in books and quoting it.

So if you don't get quick responses, please humor me and try to add to your question in one or more of those ways. Thanks!

Oh! And one more thing people sometimes like to do: if they feel that someone is "lecturing" someone else, they like to prove them wrong. So maybe my posting will help after all :-)

Regards,

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

A. Hi Anu. I wish I could find a plumber who would charge me by how many washers he replaces :-)
I doubt that you are actually charged by the volume of zinc consumed because labor, handling, maintenance, and overhead are actually involved.
But the density of zinc is 7.133 g/cm^3 or 7133 kg/m3. So if the coating is 100 microns thick on each side, the volume of zinc deposited on a square meter of sheet would be more or less: 2 sides x (100 x 10^-6 m) thickness x 1 m^2 area = 2 x 10^-4 m^3. And the weight would be roughly (2 x 10^-4 m^3) x 7133 kg/m^3 or 1.427 kg.

I've been pleased to work examples on this page just to make sure there have been no misunderstandings about concepts. Ask away about any concepts you're not following but, sorry, I'm not doing additional arithmetic problems :-)

Luck & Regards,

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

A. As a galvanizer, I can confirm what Ted has said. Most galvanizers don't charge on zinc consumed. They might charge extra for something that consumes a lot of zinc, but the variability in zinc consumption from one job to another isn't enough to compensate for the variability in factory space it occupies.
You will typically get charged a lot for something that...
* is lightweight
* Has high surface to weight ratio
* Has a high handling element (lots of small items)
* is hollow (and so gets galvanized inside as well as outside)
* Takes a long time to dip (holes too small)
* Is a trivially small order (where the administration costs more than the galvanizing)

Geoff Crowley, galvanizing consultant
Crithwood Ltd.

Bathgate, Scotland, UK

A. Sounds like fairly simple arithmetical question to me. You have stated all the parameters, and Good will give you those you don't have, so you can calculate this easily yourself.
There's something wrong if you need this spelled out.

Geoff Crowley, galvanizing consultant
Crithwood Ltd.

Bathgate, Scotland, UK

A. Hi Vikash. I'm not really understanding your first question. If you are saying you are losing only 1.6%, it would seem that zinc pickup would have to account for the other 98.4% of consumption ... what am I missing?

Luck & Regards,

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

A. Numbers to aim for with the thickness of steel you quote...
Dross: 0.3 to 0.9% of the weight of steel galvanized (white weight)
Ash: 0.2 to 0.5% of the weight of steel galvanized (white weight)
Total zinc pickup should be about 4-5%

There are usually easily identifiable reasons for having greater losses than this.

Geoff Crowley, galvanizing consultant
Crithwood Ltd.

Bathgate, Scotland, UK

A. Hi Yassar. If you know the total surface area of something and the thickness of the coating, you simply multiply them together to get the volume of the coating. And then you simply multiply by the density of the coating to get its weight. But the tricky part is figuring out the surface area. For a hollow cylinder, the formula is:

This may be a bit hard to follow, but the Quora author is saying on his first line that the total surface area equals the area of the top surface and bottom surfaces (the ends of the tube), each equal to π*(R^2-r^2), plus the area of the outside cylinder, plus the area of the inside cylinder. His third line is his final simplified formula. His second line looks more like a complication than a simplification, but he knows where his third line was headed and this was the 'simplification' road to get there :-)

You might be interested in Tom Rochester's book, "Surface Area Formulas and Tables", which explains a lot about estimating surface areas. For example, when the wall of the tubing is thin enough, you can ignore the tiny area of the tube ends, and the difference between the inside and outside diameter and estimate the surface area as approximately 2 * π * OD * length. In your case the simplified formula is 99.38% of the actual.

Luck & Regards,

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

A. Not sure what you are asking, as you have stated the thickness of the galvanizing to be 40 µ

If you want to calculate the weight of zinc on a single pipe, then calculate the volume of zinc and multiply by its density (SG. Zinc is about 7.1 as a solid, 6.9 as a liquid)

Geoff Crowley, galvanizing consultant
Crithwood Ltd.

Bathgate, Scotland, UK

A. Hi Manish. You used the nonsense term "gsm/sq meter" three times in your short question, so I don't think it's a typo -- but rather that you may be attempting to ask for the answer without quite understanding the question :-)

That approach, mathematically solving for the answer without truly understanding the question, is reserved to quantum physicists, not we mere mortals :-)

"GSM" is an abbreviation/acronym for "grams per square meter", so "gsm/sq meter" would mean "grams per square meter per square meter" and makes no sense. I think that might be why the simple arithmetic of your question is puzzling you. Use either "grams per square meter" or its acronym "GSM".

You know the area of your coil in square meters: The area of the top surface is 850 mm x 681000 mm; the area of the bottom surface is the same; the area of the left edge is 0.22 mm x 681000 mm; the area of the right edge is the same. But 0.22 is SO much smaller than 850 that the area of the edges can be safely ignored. So figuring just the top and bottom, the area is 2 x 850 mm x 681000 mm, i.e., 2 x 0.850 m x 681 m, or 1158 square meters.

Multiply that number of square meters of area times your grams per square meter to arrive at your grams.

Luck & Regards,

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