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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Electro-purification (dummy) methods for electroplating tanks
Q. Good day
Recently I had left a bath idle for a while with no work, before showing no issues. After resuming work dullness was present. Fellow platers have told me I may have built up contaminants.
Some searching of a few documents I have mostly relating to Hull cell tests mention electro-purification to get rid of contaminants in the solution.
I may say here I have heard of both HCD Dummying and LCD Dummying, where I'm confused and unsure which type of dummying filters impurities from the solution the best.
I attach a picture I came across in regards to electro-purification in a Hull Cell. [ed. note: we rec'd no picture]
My Question here is - In a working bath, what would be the method used to plate out impurities, anode/cathode sizing , shape and angles and distance in relation to each other.
I can only assume that in order to purify, there must be current running through as much plating solution as possible, as such anode and cathode must be as far away as possible at extreme ends of the tank. I assume that surface area must be as much as possible, so it is firing through more areas of the solution throughout the entire tank. Though practice often trumps theory so any advice is appreciated.
Is a certain bath temperature, whether ambient, or as elevated as possible more encouraging to plating out impurities.
My bath solution is non cyanide alkaline zinc
Regards
Boris
- Perth, Australia
September 1, 2022
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A. Hi Boris. The only high current density electro-purification I have heard of personally is for removing chloride from chromium plating tanks. There is also standard current density dummying.
Otherwise, it's been low current density dummying for the purpose of removing tramp metals, or possibly standard plating level current density for reduction of organics. The simplified theory of LCD dummying is that if you keep the current density very low, for example 2 ASF, you will preferentially/disproportionately plate out the more noble metal -- for example, dummying of nickel baths for the removal of copper has long been an industry staple. The dummying would be done at the usual plating tank temperature because you don't want to mess around with temperature changes which might cause ingredients to precipitate out or organics to oil out.
Your assumption about the cathodes is correct except that simplicity usually rules: it's typical to use a large piece of corrugated sheet metal and just put it in the center of the tank where parts would usually hang (an exception would be for continuous dummying, which used to be commonplace but now is rare, and in which case the plating tank is equipped with a side compartment specifically for continuous dummying).
I am not personally familiar with dummying being used in alkaline zinc to remove tramp metals, but some suggest it to deal with excess organics (although carbon treatment might be a better approach).
However, the actual crux of your problem seems to be that you don't yet know what is wrong with the bath, so it's hard to suggest whether dummying, carbon treatment, zinc dust, freezing out of carbonates -- or none of the above -- is called for. Assuming 'general contamination' will not be not enough. What must be done is to gather any historical info about what may have happened, carefully review the defective parts to ascertain in what way they are unsatisfactory, run hull cell tests to gain further insight into what may be the problem, chemically analyze what you can analyze, and then make your best guess as to what specifically is most likely to be the problem. My first guess, based on only the reported defect, is that the brighteners deteriorated; and carbon treatment is needed to remove them, followed by replacing them.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. Boris,
If you believe the contamination of alkaline zinc bath, run LCD dummy, run hull cell panel before dummy and after. When you do the dummy pull out brightener from your bath, add brightener to hull cell and run the panel then make a calculation to add brightener in alkaline zinc bath.
Electro-purification (dummy) methods for electroplating tanks: Need LCD dummy to your non-cyanide alkaline zinc bath, Dummy at 2-4 amps per square feet area, check with your chemical supplier for purifier to add, skim the oil from zinc bath or install oil skimmer to your cleaner bath.
Popatbhai B. Patel
electroplating consultant - Roseville, Michigan
May 6, 2024
Q. Thank you Ted and Popatbhai for your suggestions and advice. I have had some information to share recently on what I had found :
I have since found that in that specific case, the cause was low caustic, it had dropped to about 70 g/l, not due to dragout, but by depleting of NaOH due to gradual formation of carbonates, lesson learnt, keep an eye on the NaOH even if low dragout.
However still, when Zinc and Sodium Hydroxide were on range, I would at times get passable but dull results at every current density, I can only assume dullness at every current density is not an impurity issue?. What I have since been advised, and realized is the tank responds very well to some stimulation, that is initially running at low CD (4ASF) for an hour or 2, followed by an hour or 2 of full amps, in my case 20 ASF. Generally after this, the brightness and throw is the best, what actually happens on the macro level by doing this, I have no idea - but it works and my results have improved greatly
May I ask, in Alkaline Non-Cyanide baths does brightener gradually get spent (rendered useless) on standing, e.g., without plating? I've heard yes and no to this.
Would carbon treatment still filter out spent brightener or only fresh brightener.
Popatbhai, I may ask on your point about the purifier agent, I am very careful in only using a small amount, as I do not yet know how an excess may effect plating results. The only issue I have read about on various tech documents is that it can cause a non-conductive film on the anodes, which is no issue as I clean our anodes periodically.
For now, things are running well, but it's always good to be armed with information
Thank you
- Perth, Australia
May 7, 2024
A. Hi again, Boris.
I have never personally operated such a plating line, so my understanding is only from book knowledge and hanging around the plating industry for 50+ years, but continuous carbon filtration is intended to try to remove only exhausted brighteners and other organics. Hot carbon treatment is intended to destroy and remove everything; so after completion it requires re-adding all organic addition agents.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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