Distinguish Between Electrodeposited Zinc and Cadmium Coating

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Would anyone be able to advise, on how we identify the coating deposits of the studbolts by a simple spot test to distinguish cadmium from zinc plating. I have read from a previously posted method, where we may use dropping bottles, a watch glass, perhaps a white and black spot plate, 50% acids, sodium sulfide, safety glasses, and a careful person who is interested. Unfortunately, i am not familiar on the actual testing method itself, could someone advise the whole testing method ?

Note : XRF is not available to use.

Your response is highly appreciated.

Hary Gho
studbolts manufacturer - Jakarta, Indonesia

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Dear Mr Gho,

Strip the coating in 50%v/v (warm) nitric acid.
Add concentrated ammonia 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,

Sjamp van Esch
- Eindhoven, The Netherlands

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Dear Mr. Sjamp van Esch,

Thank you very much for your answers. However since I have very little knowledge to any chemical testing method, I hope you could a little bit more elaborate on your previous explanation.

Your previously wrote :

1) Strip the coating in 50%v/v (warm) nitric acid. >>>
a) Strip will refer to peeling the cadmium or zinc from the coated studbolts ? For i.e by scratch using a pen ?
b) 50%v/v nitric acid meaning dissolving the nitric acid with 50% de-ionised water or normal tap water ?
c) Warm , please advise the approximate temperature.

2) Add concentrated ammonia to the solution until red litmus paper turns blue(careful, splashing may occur!!)
a) concentrated ammonia meaning pure ammonia solution i.e 99% v/v ?

3) Then add a few drops 10% sodium sulfide solution.

4) white precipitate indicates zinc, a yellow precipitate indicates cadmium
a) How accurate and reliable is this test method ?
b) Is there a international standard that refer to this test method as per above ?

Once again, I thank you for your kind attention and hope to hear your response again.

Harry Gho
Studbolts Manufacturer - Jakarta, Indonesia

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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,

Sjamp van Esch
- Eindhoven, The Netherlands

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Dear Mr. Sjamp van Esch,

We went through your procedure and the resultant is good. We look for a spot test for Trivalent Chromium identification on a plated component. Please suggest a spot test to identify the same.

Narasimhun Parthasarathy
Automotive Electricals - CHENNAI, TAMILNADU, INDIA