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Electrode polarization

 

Q. What does electrode polarization mean?

Rahul Jairath
- San Jose, California


 

A. I read your question about polarized anode means I believe some kind of layer or film on anode to make anode to inactive.

Popat Patel
    plating shop
Roseville, Michigan


 

A. What the term means depends on the context. In one context--something wrong preventing proper plating--Mr. Patel's definition is correct.

In another context, though, anodes are always polarized,i.e., there is always an anode polarization which the plating voltage must overcome. In this context, you account where the rectifier voltage went: some went to anode polarization, some to cathode polarization, some to boundary layer ion depletion polarization, some to solution gradient polarization, etc.

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


 

thumbsup2Thank you all for the explanation.

Rahul Jairath
- San Jose, California


 

A. Hi,

There are two important types of polarization.1)Activation polarization and 2)Concentration polarization.

Activation polarization is caused by inhibition of the passage of potential determining ions through phase boundary i.e. the electrolyte double layer. With many electrodes a large polarization is observed at low current density,mainly due to activation polarization.

The inhibition of the transport of an ion through a layer contiguous to the electrode causes resistance polarization. The presence of foreign substances on the cathode surface may consist of electrolyte anions and cations,oxides or hydroxides, or other organic or inorganic components of the electrolyte. They are adsorbed at the electrolyte surface and when the electrolyte is completely covered by foreign substance ,it is passive. This gives rise to resistance polarization.Resistance polarization is also a type of activation polarization.

Concentration polarization is due to the concentration difference which develop at the anode and cathode during electrolysis and are caused by the inhibition of the transport process.

An increase in the concentration of the dissolved metal ions in the anodic diffusion layer causes diffusion polarization. Diffusion polarization is a type of concentration polarization.

Polarization can under certain circumstances be greatly increased by the addition of specific substances. Polarization increases with increasing c.d ,but decreases with increasing temperature and increasing agitation of the bath.

If we sees some examples Nickel bath has low concentration polarization and high activation polarization . Cyanide baths often have high concentration polarization .

Venkat Raja
- Kitchener, Ontario, Canada



 

Q. Hi,

A brief introduction to what I am trying to do..

In my experiment to measure zeta potential, by streaming potential technique, I pump aqueous surfactant solution (anionic or cationic) through a capillary tube at increasing pressure and try to measure the potential difference(which is called streaming potential) across the capillary due to the flow. For this I place an Ag/AgCl electrode at the beginning of the capillary(Lets call this electrode E1) and one at the end(and lets call this electrode E2). These are connected to a data acquisition system with high internal resistance(the potential developed is of the order of 100mV DC).

The problem......

The electrode E1 is always placed at the beginning of the capillary and electrode E2 is always at the end of the capillary. After using the electrodes this way for few days, the electrodes started showing higher potential than expected. I.e., for example, if 1000 wppm concentration of the anionic surfactant generates a voltage of -100 mV DC and 1500 wppm generates -150mV DC, I expect 100 wppm to generate a voltage which is less than -100mV DC. But the electrodes actually are showing a value which is more than expected. The question.....

Is this because of electrode polarization? or is there some kind of ion accumulation taking place on these electrodes? I am not really sure what is going on. After each set of readings, I clean the electrodes and store them in 3M KCl solution as suggested by the manufacturer. Can some one throw some light on this issue....

Thanks,

Satish C Vishnubhatla
- Cincinnati, Ohio



thrashing

sidebar (1996)

Q. I am doing some numerical research on the mass transfer to a cylinder. The application of this work is the electroplating of cylindrical electric contacts (plugs), and in particular the gold plating of these cylinders. I am looking for some information about industrial situations, in particular : - concentration of gold solutions ? - current intensities (is it a limiting-current state ?) - what is the velocity of the pieces in the baths (laminar, turbulent, very turbulent) ? If anybody can tell me where I can find this information (and more; the more information I have, the better I can turn my research to possible applications) Thanks for the help.

Pedro Olivas
Roy. Inst. Technology/dpt. Mechanics - Stockholm


A. Hello, Pedro. Many different gold plating solutions are used. Per the Metal Finishing Guidebook, they typically contain 8 to 20 grams of gold per litre, with the higher concentrations used for higher speeds. Current densities range from 1 to 5 amps per square foot with no agitation, on up to 100 to 400 amps per square foot with violent agitation. Some proprietary gold solutions claim up to 2000 amps per square foot. Numerous technical papers have been published; a literature search incorporating Plating & Surface Finishing, Metal Finishing magazine, and Transactions of the IMF should give you plenty of background.

Good luck with it.

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

July 29, 2010

Q. I would like to know the relation between the current Density and the Cathodic current Efficiency. In my undergraduate Laboratory of Copper Plating from Copper Sulphate solution, I am finding an Inverse Relation. Can anybody explain?

LAKSHMI NARAYANA
- Hyderabad, Andhra Pradesh, India


July 29, 2010

A. Hi, Lakshmi. Yes, the general direction that you perceive is correct, but it's not quite that simple or exact. Think of it this way --

Your power supply forces electrons to move from the anode to the cathode through the external wiring or bussing, and that causes things to happen at the anode, cathode, and within the solution to maintain an electrical balance. Ideally, at 100 percent efficiency, what would happen is that the metal of the anode would give up those electrons and dissolve into the solution as positively charged metal ions. Those ions would be attracted to, and migrate towards the cathode; and when they reach it, the surfeit of electrons there would cause the metallic ions to be reduced to metal.

But as you raise the current density, it becomes increasingly difficult for the migration of the metal ions to keep up. At the cathode, those electrons are demanding to be balanced, and if there are insufficient metal ions arriving fast enough, they start liberating hydrogen out of the aqueous solution.

Regards,

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

March 6, 2012

What is polarization ? What is polarization of electrode?

Gunjan Jadhav
- Mumbai, Maharashtra, India


March 6, 2012

A. Hi, Gunjan.

The answer to your question is already on this page repeatedly; please do your best to phrase your question in terms of what you understood and still fail to understand. Thanks.

Regards,

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


Is polarization a good thing or a bad thing?

May 22, 2014

Q. Hi everybody.
Can someone explain to me whether polarization of electrode is a positive thing or negative? Or I will ask like this, do we want to have low polarization or high polarization?

Bojan Koren
- tolmin slovenia


May 2014

A. Hi Bojan. That is an excellent question. In general, polarization is a bad thing because it consumes electricity (by adding resistance), and it severely limits the plating speed.

But sometimes it's a good thing: if you read Venkat's excellent explanation you'll see that he mentions the fact that cyanide baths often have high "concentration polarization". The benefit is that, as the metal ions in the high current density area are plated out, it is somewhat difficult to restore available metal ions to that area of the solution, which steers the electrical power towards plating out the available metal ions in the lower current density areas. The end result is that you get more even plating distribution than you would with lower concentration polarization.

Regards,

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

May 22, 2014

Q. I read somewhere that in nickel plating is desirable to have high cathode polarization for good nickel deposition? Polarization causes a formation of hydrogen gas on the cathode and this is not a good thing, so how can a cathode polarization be a positive thing? Can someone explain me? Thanks

Bojan Koren [returning]
- tolmin slovenia


May 2014

A. Hi again Bojan. I don't want to either support or argue with an assertion that you "read somewhere" because that's somewhat vague and may lead to confusion and misunderstanding ... but I will just offer again that polarization can help with the thickness distribution. Nickel brighteners are polarizers, actually -- they shield the surface in such a way as to deter plating at a high current density spot, thereby encouraging formation of a new adatom for a smaller, tighter, less porous deposit. So yes, polarization can be beneficial.

Regards,

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

May 26, 2014

thumbsup2Hi Ted, thank you for your answer! If i understand this correctly, when we add brighteners to the nickel bath we make cathode more polarized? And so thickness distribution is better?
i just started to work in company where we do electroplating ( Nickel, Chrome). I'm new and I'm still learning. Thanks for your help ;)

Bojan Koren
- tolmin slovenia


May 2014

Yes, that's right. I'm old and I'm still learning :-)

Regards,

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
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