How to measure plating & galvanizing thickness

A. Hi CHANDRASEKARAN,
It seems to me that microsection (destructive) or XRF (non-destructive) would be the most appropriate methods.
Luck & Regards,


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

A. There are two additional ways to measure the coating thickness. One is to use a micrometer which would be accurate to one tenth of a mil (as platers call 'a tenth'). Keep in mind that you would be measuring two deposits - the top and the bottom. Another procedure is gravimetric - weighing equal quantities of plated and unplated springs. The surface area of the coil springs is easily calculated as 1.5563 square feet per pound. (You can use google to change this number into any other system of measurement.)

A. Hi Hendri. Readers will offer their suggestions based on their experiences, but as a preface we should add that even though you would like to simplify, a single device may not be best for all applications: galvanizing is conductive whereas anodizing is not; galvanizing is done on steel, which allows magnetic thickness testing, not so with anodizing which is done on non-magnetic aluminum. Galvanizing and hardcoat anodizing thicknesses may be 10X higher than zinc plating thicknesses or more. Checking coating thickness on rounded surfaces, IDs, threads, and other applications may require different testing equipment.

Regards,

Defelsko Positector 6000
combo magnetic & eddy-current coating thickness tester

on eBay or

Amazon

(affil links)

A. Luke Engineering specializes in hardcoating and anodizing of aluminum. We have been very pleased with the Positectors. They performed the best in our R&R testings at a third of the cost of the more expensive eddy current units. Each of our operators has one to check every load for coating thickness. Unfortunately, they can be a little too convenient. In the years we have been using them, we have dropped more than one into a tank.

A. Dear Mr. Chandra:

I have used one of the PosiPens now for several years. It is extremely handy on a plating line, since it looks just like a pencil. I obtained it from DeFelsko, and many platers have decided to get them after seeing my tester work. For a person up on a plating line, who wants to test over metallic surfaces like steel, they are great. Obviously for Aluminum they are useless. Suggest requesting literature from several vendors. Try those who advertise here on Finishing.Com first.

Good luck in your search.


Ed Budman [deceased]
- Pennsylvania
With deep sadness we advise that our good friend Ed passed away Nov. 24, 2018

A. Hi, Mr. Chandra,

I suggest you try some models made by Elektro-Physik Koln. I have purchased many thickness gauges from them and found all of them quite satisfying. Actually they even surprised me that they could manufacture such a vast range of products. Galvanized coatings, anodized coatings, paints, anticorrosive coatings, and many others.

Good luck.

----
Ed. note: Thanks Chis, Ed, Alex! We appreciate everyone's input, but to the extent possible, please try to speak in terms of technology rather than brands (huh? why?).

A. I would try the Kocour dropping test first. You drip acid onto the part, counting the drops until the zinc layer is dissolved.

Crude, but effective.

A. Depending on the thickness of the zinc and the accuracy that you require, you may be able to use a magnetic method. Various alternatives exist, from very inexpensive pen-type devices (not very accurate or precise or repeatable) to more precision bench-top units (more expensive). The latter have ASTM standard practices and NIST traceable standards for calibration.

Other methods, such a x-ray fluorescence, metallographic cross sections, etc. are standard methods, but require more $$ to set up and expertise to perform.

I have assumed that you are measuring the zinc plating thickness. The chromate layer is much thinner, of course, and thickness measurement is a different story.

A. Why have you not contacted Electro-Physik (the manufacturer of the gauge)?

A. Theoretically, the non-conductive coating method (eddy current) can be used, but magnetic gauges are easier to calibrate and more accurate (or very inexpensive with less accuracy). Other suitable methods, e.g., coulometric & XRF, are more costly As the Metal Finishing Guidebook article on Control, Analysis, & Testing points out, the conductivity of zinc plating varies with the cyanide, chloride, or alkaline plating solution; hence, the eddy current method requires solution-specific standards.

Magnetic gauges are available as inexpensive 'pencil' (± 15%) and 'banana' (+-5%) style mechanical gauges or as digital meters (± 1 to 3 %).

A. Hello, Warren,
You may analyze the possibility of taking impressions or casts from the inside of a bare and a plated pipe with something like an epoxy or Bondo ⇦this on eBay or Amazon [affil links] , then take them to an optic profile comparator. You may also try by dissolving the plating with a little muriatic acid ⇦this on eBay or Amazon [affil links] . You will see how long it takes and where the plate was thinner. Remember that electroplating follows the laws of electricity and never plates uniformly over threads or anything that has corners. Expect large variations in thickness.

A. Hi Warren -

I think that cutting up a statistically valid number of specimens for met lab thickness analysis is your best choice unfortunately.

I know that thickness varies greatly dependent on exactly where on the thread that you are measuring (peaks versus valleys). Also, I believe that the Eddy Current devices give different readings depending on how you orient them.

A. Have you considered simple Go/No-Go gauges? They will not tell you what the thickness of plate is but will tell you whether you meet the dimensional tolerances as described on your design drawing.

A. Hi Pathak. Because the steel substrate is magnetic and the zinc plating is not, a simple magnetic gauge should indicate that there is plating and how thick it is. Sorry if I am misunderstanding, but no need to remove the chromate conversion coating for magnetic thickness testing.

You will also see on this page now Tom Pullizzi's suggestion of Kocour testing. Good luck.

Regards,

A. 8 microns is not much zinc, so you are not going to get much protection.
The main problem is that you will not get any plate at the contact area of the spring wire to each other. No plate = rust.
Preplated wire might help, but I will bet a cup of tea that it will scrape off in many places in the winding operation.
There is a wild outside possibility that a thin electroless nickel might work after winding.
You may have to go to a heat treatable CRES.

A. The past processing sequences & procedures aren't clear to me.

Winding precoated wire may result in problems.
To what specification is the precoated wire purchased?
Is this electroplated or hot dip galvanized wire?
If electroplated, the cyanide zinc process generally gives more ductile coatings than do acid chloride or alkaline non-cyanide processes.
Galvanized coatings may be brittle or ductile depending upon processing. Galvanized wire meeting ASTM A641 requirements can be tightly wound: A 1 mm diameter wire with a 6.3 micron coating can be closely wound around a 1 mm diameter mandrel; with a 21.4 micron or thicker coating, it can be wound around a 3 mm mandrel. A relatively slow winding rate (not more than 15 turns/minute) is used in the testing, however.

Presumably trivalent means trivalent chromium passivation. It almost certainly needs a supplemental organic coating (e.g., wax) for lubrication during winding and to improve corrosion resistance.
What is the salt test specification and its duration?

Re. thickness (for uniform coatings or for average value): Measure the diameter by micrometer, strip the zinc using inhibited hydrochloric acid, then remeasure. A more accurate method involving weighing of a measured length is given in ASTM A90 (which also gives the stripping solution). Density of zinc is 7.14 grams/cm2.
For thickness of non-uniform coatings, microscopy on cut sections is probably best.
For detecting bare spots (exposed iron), ASTM A239 gives an immersion test method (although not normally used for wire products).

Investigate the process & specify as necessary. You don't get what you don't pay for.

A. Hi Abdul. I have personally never heard of measuring the thickness/viscosity of the chromate, and I'm not quite sure if we are simply having language difficulties. I think pH, concentration, and temperature (if you're mildly warming it) are better control factors. If you purchase this, get the technical data sheet; if you make-it in-house, get the specs.

Luck & Regards,

A. Hi Kashi. We appended your inquiry to a similar thread which may help answer it.

Cross-sectioning is labor intensive and can be done poorly, but it is still the "referee" method that tells us what the "real" thickness is. X-ray fluorescence is very expensive but probably the most versatile and reliable non-destructive method. But let's not keep it abstract, boring, and possibly misleading: instead please tell us the details of your situation :-)

Regards,

A. Hi Muhammad. Both devices must be calibrated. You can use microscopic cross sectioning as the referee device if you cannot resolve the discrepancy. Good luck.

Regards,

A. Muhammed,
It seems to me you have your answer on the thickness discrepancies. Is it possible that there are other magnetic sources that are interfering with the PosiPen? It could be just a matter of technique using the PosiPen?

Main conditions that affect plating thicknesses are:
Current Density, time, metal concentration in the bath, bath temperature and pH comes into play most of the time.

A. Hi again, Muhammad. You should probably also track amp-hours and do a Faraday's Law calculation of anticipated plating thickness. It may not give you the precise thickness but it is a great reasonableness check, and will probably tell you that your PosiPen readings are impossible.

If the readings are wrong the "operator plus instrument" are, NOT calibrated, despite any calibration efforts you have made :-)
Good luck.

Regards,

A. I have done a lot of film thickness measurements over the years and have yet to find a method that will pass a measurement study (aka gauge reliability and repeatability study). Fortunately in my circumstance (measuring paint coating thickness) ways to evade this issue are available.

A. Hi Muhammad,

There a few things that can influence magnetic and eddy current type equipment for measuring thickness. Two of the commonest errors are to measure too close to an edge and to measure on a curved surface when the equipment has not been calibrated on a curved surface.

A. Hi Muhammad,
I have just one line suggestion for you.
TRY DEMAGNETIZING YOUR PARTS before you use a POSI PEN.
All the best.

Thanks, Vikram. I'll bet that's exactly the answer!

Regards,

Usually available on eBay;

sometimes on
AbeBooks or Amazon

(affil links)
free pdf is currently available from academia.edu

A. Hi Rajendra. For a discussion of the possible destructive & non-destructive thickness testing methods, please review the thickness testing chapter of the Metal Finishing Guidebook .
which Ken Vlach suggested.

If you want suggestions for methods most appropriate for your particular application, please tell us your substrate material (magnetic/non-magnetic?, conductive/non-conductive?), the typical thickness (different methods have min. and max. thicknesses which they are appropriate for), and info about the parts and the area to be tested (do you want the minimum thickness on a pinhead sized area, or the average thickness on a flat shape).

Ken has offered the best general answer: try a magnetic gauge if the substrate is magnetic, the thickness is in the 5 to 25 micron range, you can't afford to destroy parts with destructive testing, reasonably sized flat areas will be tested, you want a simple inexpensive device, and a good statistical average thickness will suffice. Good luck!

Regards,

A. Hi Vikas!

I don't use coulometry, but removing passivation/conversion coating without removing zinc is really difficult.

I would put the part in caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] 10% until it de-colors and begins to make some bubbles (hydrogen). You rinse it and measure. This method will not remove much zinc and will indeed remove passivation from the plated part.

Regards,

A. Years ago (actually decades ago) when I ran the Kocour Drop Test I used to remove yellow chromates with an eraser. For clear chromates I used about the same time and pressure. And long before that, platers used to estimate the coating thickness by putting a plated part in a pickle tank and seeing how long it would take until it stopped generating hydrogen. The Kocour Test is just a more sophisticated version of that test.

A. Tom's eraser method might not remove 100% of the chromate, I've never done an analysis, but it certainly removes the majority of it quickly & easily while removing no zinc at all. If you must remove all of the chromate, maybe getting most of it off with an eraser before doing Daniel's method is the best approach?

Regards,

A. Hi Cody. I have only book knowledge of this, not hands-on experience, but we appended your inquiry to a thread where Ken Vlach told us that the eddy current conductivity of the different zinc platings do differ (he refers you to the Metal Finishing Guidebook); since you are plating on a magnetic substrate, his advice that a magnetic gauge would probably be a better approach sounds on the money to me :-)

Regards,

A. A reference standard will give you a close approximation. But if you want to be more accurate use an unplated article for zeroing your instrument. Most platers keep a few unplated articles for this purpose. If those aren't available, you can strip the articles to obtain a reference. BTW be sure there is no residual magnetism in the article.

A. There are expensive ways to do it. But a simple, pragmatic method is to see it there is enough residual magnetism to move a common paper clip. Hold the article contacting the paper clip and see if you can move it.

A. Hi Nad. It's probably just typos, but chrome plating and anodizing have nothing to do with your situation. Chromate conversion coating is nothing like chrome plating. Defelsko will tell you how to calibrate their devices but two good things to have on hand for it are an unplated part and a standard.

Luck & Regards,

A. Hi Eddy. You have to know what specs you are plating in accord with, and to consult those specs to determine what the thickness requirement and other requirements are, and any info about required testing. But if the specs are silent on your issues, the chromate will not interfere with your measurement; and I would guess that sthe nickel strike, being both magnetic and very thin, should not interfere with it either.

Luck & Regards,

Usually available on eBay;

sometimes on
AbeBooks or Amazon

(affil links)
free pdf is currently available from academia.edu

A. Hi Stephanie. There are many different methods and different types of measurement equipment available to measure plating thicknesses including:

• using a micrometer before and after,
• magnetic thickness gauges (measure a magnetic coating on a non-magnetic substrate and vice versa,
• hall effect thickness gauges (which look rather like electronic magnetic gauges but can measure thickness when neither substrate nor plating are magnetic),
• coulometric (destructive) measurements where you time how much acid or how much time is needed to eat through the plating,
• x-ray fluorescence devices which emit radiation which excites the plating and the fluorescence pattern is measured (these are much more expensive and sophisticated than the previous methods),
• beta backscatter devices which are less costly and simpler that x-ray fluorescence but can have the limitation of backscatter interferences depending on the substrate and the platings,
• cutting through the component and doing a microsection that you look at through a microscope.

Probably the first thing you will want to learn about this is what type of thickness gauge or measurement device you are using ... then it may be possible to determine if/why you must measure the nickel coating thickness first. You can download The Metal Finishing Guidebook which has three chapters on thickness testing to give you a thorough introduction.

Luck & Regards,