Cadmium plating low hydrogen embrittlement [Serbia]
June 29, 2010
We prepared Low hydrogen embrittlement cadmium plating bath volume
400 L with:
CdO 20g/L; NaCN 162g/L; NaOH 12.0 g/L.
We have two problems:
1. Lab analyses finds concentration of 90 g/L of cyanides, and in reality we made concentration of 162.5? I wondering how they cannot find real concentration by titration with HCl?
2.We had in bath stainless steel anodes, unfortunately it happened they start to dissolve under voltage of 2V, and solution become dark orange colored. It means that we have now Fe Cr Ni ions in electrolyte. Can we use this electroliyte for further Cadmium plating considering this fact?I'm afraid not, but I would like to check with some one.
Thanks in advance
engineer - Belgrade, Serbia
June 30, 2010
I 'm an engineer for plating.
We have prepared electrolyte (Volume 400L) for Low hydrogen embrittlement cadmium plating with 20kg CdO; 65kg NaCN; and 11 kg NaOH.
Expected concentration are: Cd 50 g/L; NaCN 162.5 g/L; NaOH 27.5 g/L
Found concentration by titration: Cd 45.45 g/L; NaCN 99.17 g/L; NaOH
38.92 g/L (expecting because of chemical reaction)
1. Question: How this titration cannot find the real concentration of NaCN?
We had stainless steel sheet anodes in this electrolyte, unfortunately it happened that they start to dissolve electrolytically because of contact with rectifier and solution turns to dark orange color. It seems that we have now Cr, Ni, Fe, etc ions in solution.
Trying to purify this solution we plate some metal sheets and cadmium plate looks visually good.
2. Question: Now I'm wondering is it possible to use this electrolyte in the future because of this metal ions of Cr, Ni, Fe etc?
engineer - Belgrade, Serbia
July 1, 2010
Vesna,
You seem to have some fundamental problems with understanding your process. This is going to make control of the process near impossible.
Firstly, unless there was a high cadmium content that needs reducing you should be using cadmium anodes, not steel.
You need to tell us your exact analysis method for cyanide. It sounds to me that you are probably only measuring the free cyanide content, or maybe the calculation is slightly wrong and your result is being expressed as CN rather than NaCN.
The foreign ion contamination is a little harder to deal with, it will really depend on how much contamination that you have in the bath. The standard method of removing contaminating ions is by a process called "dummying out", whereby you plate a very large area at very low amps over a period of time to reduce the contamination. If the contamination is very high then this method is impractical. I would suggest that you would have to discard the bath and start again if this is the case.
I would suggest you get hold of one or two books on plating. The Canning Handbook has a very good chapter on cadmium plating and its faults. The Electroplaters Engineering Handbook is also another useful publication and well worth having. Both texts also have analysis methods so that you verify that your analysis technique and calculation for the CN is correct.
aerospace - Yeovil, Somerset, United Kingdom
July 7, 2010
Dear Brian,
Thanks a lot for trying to answer my question.
Thanks for recommendation of books.
We have not started with using the bath because it's new and we made first controls. Bath is made up by Boeing prescription, and concentrations are much higher than usual for bright cadmium plating baths.
Steel anodes can be used for certain occasions in accordance with my books and Boeing procedure.We are buying Cd anodes.
In books I have found, for bright cadmium plating, that presence of Fe , Ni and Cr ions do not disturb the Cd plating. But considering the redox potential for Fe and Ni, I am afraid they will deposit in same time with Cd.
Methods for determination of concentrations:
1. NaOH determined by titration with 1N HCl and sulforange indicator
2. Total NaCN determined by titration with 0.1N AgNO3 in presence of KJ and NH3 until solution becomes cloudy.
3.Cd metal determined by titration with EDTA, indicator Erio Chrome in presence of formaldehyde and HCl.
- Belgrade, Serbia
First of two simultaneous responses -- July 7, 2010
Vesna-
The iron won't significantly coplate from the cadmium bath and it will accumulate in the bath over time especially if you use steel anodes. I wouldn't worry about it unless it get's over 1 g/L or so. Nickel will coplate somewhat and in fact it is a main ingredient in the common brightener systems used in bright cad baths. For an LHE bath, you shouldn't have too much or it will have a negative effect on the porosity of the plate. You should find the source of the nickel contamination and eliminate it from your system. It is important to note that in a cyanide solution the metals are present as anionic complexes, not as metal cations, so your potentials are different than you may have read. As far as you analyses go, I don't see anything wrong with what you have written. You may want to also provide us with your sample volume and your factor so that we could verify that there aren't any math errors.
|
Jon Barrows, MSF, EHSSC |
Second of two simultaneous responses -- July 8, 2010
There is a chemical reaction between the cyanide and cadmium to form a complex salt of cadmium cyanide. This type of cyanide complex can be analyze thru distillation method only.
to prove this, make a small equivalent solution of NaCN & NaOH, No cadmium Oxide and titrate for cyanide.
Good luck,
- Mays Landing, New Jersey, USA
July 12, 2010
Hamilton-
The cyanide is easily titratable with silver nitrate titrant. The silver displaces the cadmium from its complex as you titrate as the silver complex is thermodynamically preferred. The end of the titration is signaled by the cloudiness that forms when excess silver reacts with the iodide from the KI that was also added (along with ammonia). When there is excess silver, there is no more cyanide (free or complexed). This titration is the standard throughout the industry and it can be verified as you have suggested. A distillation procedure would also work, but it is too complicated and time-consuming for most shops.
|
|
Jon Barrows, MSF, EHSSC |
July 16, 2010
Vesna,
I am still interested in finding the missing "cyanide". I am confident in your math. Is it possible for you to run a simple test? Add & dissolve three grams of CdO in 100ml of the cadmium plating solution, titrate for cyanide. If the silver displaced all the cadmium in the cyanide complex, there should be no change in the cyanide concentration. However, if the silver did not displaced all the cadmium, then your cyanide analysis will be lower.
Good Luck,
- Mays Landing, New Jersey, USA
July 20, 2010
Thanks for help.
I will try this small experiment with CdO addition.
I got the concentration of foreign ions in this solution.
g/L=micrograms per liter ; g/L= grams per liter
Silver Ag.<0.1 g/L
Copper Cu.19.0 g/L
Potassium K...67.0 g/L
Calcium Ca.3.8 g/L
Magnesium Mg..0.1 g/L
Barium Ba.0.2 g/L
Strontium Sr.0.1 g/L
Cobalt Co..<0.1 g/L
Nickel Ni..6.2 g/L
Lead Pb.0.4 g/L
Zink Zn..6.8 g/L
Chromium Cr22.2 g/L
Iron Fe..112.6 g/L
Aluminum Al45.0 g/L
Manganese Mn 0.6 g/L
Cadmium Cd 24.87 g/L
Sodium Na 98.5 g/L ]
I do not know whether some of metal ions bothers Cd deposition and Cd plate quality.
- Belgrade, Serbia
July 22, 2010
Hello Vesna. Those contaminant levels will not be a problem for the plating quality. Actually, nothing there is anywhere near being a problem, so there is no need to look any further at the micro-level constituents. Just make sure that you are measuring the main ingredients properly- NaCN, Cd, NaCN:Cd ratio, and NaOH. The most important thing is the ratio. The ideal ratio will depend mainly on whether you are doing rack or barrel plating. I like about 4.5:1 NaCN:Cd for rack LHE plating.
|
|
Jon Barrows, MSF, EHSSC |
August 5, 2010
Dear Jon,
Thanks you have supported me with your suggestions.
I had to check with Boeing all results, and they said the same as you for this metal impurities.
I will try to manage this ratio NaCN:Cd within the concentration limits.
- Belgrade, Serbia
