Metal finishing Q&As 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 ^
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
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