LHE Cadmium plating -- Problems & Solutions

A. At United Airlines, low embrittlement cad is critical. We use a low embrittlement chemistry (non-bright), check for contaminants that would influence the hydrogen entrapment weekly (Lawrence Gauge) and bake after plating. There are few cad platers left and fewer still that could comply with your embrittlement request.

A. You cannot use titanium anode baskets in an alkaline solution. The titanium will "anodize" while the current is on and as the coating forms you will see the amps fall off (unless the voltage is automatically increased). Then when the current is turned off the oxide will fall off as a white powder; then when the current is turned back on the anodizing will slowly begin again and continue until no current passes at the voltage that is impressed.

A. Mild Steel.

A. Hi Marilyn,

I am afraid you can only use steel baskets; however they are not the problem. We have 20 year old cadmium solutions and still no problem. Have the supplier analyse and adjust basic chemistry and check for any porosity in the base metal.

All the best,

A. Many experienced and generous platers, technicians and scientists participate here but you are asking for mind reading, Carla. What is your process and chemistry? What are your parameters? What is your substrate? What type of pre-plating process are you using? What have you tried to solve the problem?
Guillermo Marrufo-Mexico

A. Hi Carla. Please see letter 2114, "Cadmium Plating: Brightness = Embrittlement?" for some good insights. Remember that most dull plating can be easily burnished/buffed to brightness. Is there a chance that these 'coupons' are exposed to any sort of mechanical motion that could be burnishing those areas?

Regards,

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A. 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.

A. Vesna-
The iron won't significantly co-plate 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 co-plate 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.

A. 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 analyzed thru distillation method only.

To prove this, make a small equivalent solution of NaCN & NaOH, no cadmium Oxide and titrate for cyanide. Good luck,

A. 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.

A. 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 100 ml 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,

A. 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.

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 these metal impurities.
I will try to manage this NaCN:Cd ratio within the concentration limits.

A. You cannot use titanium anode baskets in a cyanide solution.

A. Correction, Robert. Titanium baskets have been used in Boeing's Ti-Cad cyanide process for decades. Still, if the bath is LHE and not Ti-Cad, I agree that titanium baskets are not appropriate.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

A. What specification are you plating to? At first glance, your Cd metal is a little too low, but your NaCN is way too high. The ratio between NaCN to Cd is more important than the concentration and should not be more than 5 parts NaCN to 1 part Cd metal (not CdO) for LHE plating. Your hydroxide looks a little bit low. The carbonate is good where it is. For starters, I would add more CdO. This will do lots of good things at the same time. It will lower your free NaCN, get your ratio a little bit back into line, and increase the NaOH content. If you are following a particular specification, which I am sure that you must be, you would of course have to defer to whatever it says.

A. Good day Greg.
From your data, it seems the NaCN is a little high, I would think the efficiency is a little low. Why not take an as is solution sample and plate a test panel, cut this solution 30% and add CdO to 28 g/l, NaOH 17.25,(NaCN @115 g/l) in the lab and try adding some carbon and filter then plate another panel and compare? Keep in mind Na2CO3 will rise on its own as this is normal during electrolysis. This 30 % reduction will give you a comfortable Na2CO3 concentration also.
What is your bath temperature? I would try 75 °F. Hope this helps.
Regards,

A. Your NaCN is very high - our upper limit is 150 g/L, with a target of 118.5 g/L. Try dropping that and check the results.

Thank you for all the suggestions. I just have one more question ... I read that the anode to cathode ratio should be 2:1. Is this true?

A. Syed,

Your post is rather abstract so it will be impossible to give you an answer, but here are a couple of bits of info:

Ti-Cd plating can be a right royal pain in the behind to control properly, needs investment in some good plant and needs constant analysis. This makes the coating considerably more expensive than conventional cadmium.

LHE cadmium doesn't require any specialist equipment and I do know of companies that use the same solution for conventional and LHE cadmium, it's all in the way it is applied.

Both Ti-Cd and LHE cadmium are used for the ability to diffuse the hydrogen out much easier than conventional cadmium.

Without knowing more about what you want the coating to do I certainly can't advise anything more.