Electrode polarization

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

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.

Thank you all for the explanation.

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 .

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,

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,

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,

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

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,

Hi 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 ;)

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

Regards,

? Hi Jessica!

Please tell me where is the measure taken, so I can understand the circuit. Maybe with more eyes we can get an idea (or a theory) of what is happening there.

Best of luck!

 Hi Thank you for the reply! As it turns out, I misunderstood the circuit myself, but a colleague explained it to me. There is no bulk transfer of protons through the membrane so there is no buildup of electrons at the anode. The current (I) is sent through the outer 2 electrodes so that the user specified voltage between the counter sense and reference electrodes is reached. There is a feedback loop from the voltage measuring electrodes (inner pair) to the potentiometer to send enough current to produce the desired voltage. So the problem I thought would occur will not. Sorry for confusing anyone!

Hi!

Sure, no problem! Sounds like an interesting experiment you are conducting there.

Good luck with your results then!

A. Hi Bernard, although polarization has an actual definition related to depletion or surfeit of electrons at the electrode, and there are several types/causes, people in different disciplines sometimes dismiss the original meaning and use the word to describe an effect. To most electroplaters most of the time, 'polarization' is an effect whereby the circuit doesn't work the way we'd like it to (plating isn't occurring or is too slow, or anodes aren't dissolving).

According to my very limited understanding of Cathodic Protection, polarization of the structure is a good thing because it means the driving voltage between anode and cathode is reduced to about 0.05 V which, in turn, means that the current flow is small, which means the anodes don't rapidly dissolve away. It seems to me that this could take a little while, although I have no experience from which to estimate a time frame ... but exactly what do you mean by "CP readings seem too low"?

Regards,

A. Hi again. Although I am not fluent in Cathodic Protection, I did know that "CP readings seem too low" was shorthand for "Cathodic Protection Voltage readings seem too low" :-)

What I meant was: what were your readings and why do you consider those readings too low?

Regards,