Distinguish Between Electrodeposited Zinc and Cadmium Coating

A. Dear Mr Gho,

Strip the coating in 50%v/v (warm) nitric acid.
Add concentrated ammonia ⇦ this on eBay or Amazon [affil links] to the solution until red litmus paper turns blue(careful, splashing may occur!)
Then add a few drops 10% sodium sulfide solution.
A white precipitate indicates zinc, a yellow precipitate indicates cadmium.

Best regards,

A. Dear Mr Gho,

Sorry for my late response.
I'll try to give you the answers on your questions:

1a) You don't have to peel off the zinc or cadmium by scratching with a pen or something like that.
Just put the bolts in the nitric acid and wait till the zinc or cadmium has dissolved.

1b) You can dilute the nitric acid either with de-ionised or normal tap water.

1c) I should say about 40 °C. When the temperature is less it's no problem but it will take more time to strip (dissolve) the coating. A higher temperature can cause a too vigorous reaction.

2) By concentrated ammonia I mean 30 w/w %(commercial grade, density about 0.89 kg/l)
A little bit less concentrated is no problem but then do you have to add more.
4a) The accuracy of the test is, in my experience, very good.

4b) I don't know if there is a international standard that refers to this test.

Kind regards,

A. Hi Hary. As you see, we moved your question to a thread where it was already answered. But if anything about Sjamp's excellent postings was unclear please request specific clarification. Good luck.

Regards,

A. I would like to suggest a non invasive way of testing the difference between Zinc and Cadmium. Using a simple galvanic cell to test these metals natural standard reduction potential by calculating their electromotive force in an electrolyte of Magnesium sulphate(Epson Salt is much safer than sulfuric acid and less expensive than CuSO4, ZnSO4 and CdSO4)

Copper standard electrodes are available to test dissimilar metal coupling (testing for galvanic corrosion), but for testing bolts we can build simple single container galvanic cell.

The known Standard reduction potential of these metals are: Cadmium (-0.403V) and for Zinc is (-0.763V), the standard reduction potential of copper is (+0.34). in this case a strip of electrical wire (99.9% copper) will be the cathode in our simple galvanic cell. Place the two bolts separately from each other and from the copper wire, using a voltmeter(set up to 2 volt direct current settings) contact the red lead(+) to the copper wire. And the black lead(-) on one of the bolts, the Zinc coated volt will read close to 1.1 volts and the Cadmium coated volt will be 0.743 volts. You can also omit the Copper wire and do a simple bolt to bolt testing, in this simple galvanic cell, Zinc will be Anodic(-) to Cadmium and the voltmeter will read about 0.34 volts.

The Solution can be prepared by mixing about 10 grams of Epson salts with about 100 ml of tap water, the container can be a shallow one, the bolts can be partially submerged, after the test rinse the bolts with tap water and dry them so they can be used for their intended purpose.

 That's a pretty slick intelligent idea. Do you have experimental results that confirm it? Sometimes theory and practice diverge.

A. I have used this method many times before, but not to test Cadmium (it's nearly impossible to get my hands on, and zinc is a very good substitute, at least for what I do).

I use it to test Nickel/Cobalt, Nickel/iron alloys, Copper/Zinc alloys; while these and many alloys have very interesting physical properties, but their electromotive force or reduction potential will be the sum of their parts according to the alloy composition.

For more info on test procedures and background of the process please check PRACTICAL GALVANIC SERIES .