Hi,

I will be do comsol simulation about change electric field during anodization in near future, so then I will be able to help you. 

Hi,

I will be do comsol simulation about change electric field during anodization in near future, so then I will be able to help you. 

 Most of the potential drop is on/near the electrodes. In the bulk solution the potential drop is iR and is commonly very low. By definition E=U/d.In the bulk solution the electric field strength is iR/d (d= distance) and correspondingly also low. For anodisation , the highest field strength is in the oxide layer . 

The actual potential  applied between anode and cathode is not constant in both regimes (galvanostatic or potentiostatic) and you can't use this voltage value to calculate E.

I may suppose a decrease of E in the oxide layer during layer growth, in potentiostatic mode, and a constant E during growth in galvanostatic mode, but why is this important?

Dear Marta, 

Very nice you trying to use Consol to solve this question. Please let me now when you have the first results.

Dear Prof. Cornel, 

Thanks for your help. I think you get my point. It is not ease to solve this problem if we consider a moving frontier problem during the barrier layer growth under both regimes. A lot complex and simultaneous phenomena should be considered.

I think that several authors already wrote the following idea:

“In potentiostatic regime, the potential is kept constant during the process, and the observed response is the current variation as a function of time. In this case, the system energy is kept constant such that the processes tend to be limited by the applied potential. The current through the system tends to decrease up to a stationary value. This current decrease is related to the formation of a compact oxide layer over the electrode that increases the local electrical field intensity. Thus, the kinetic energy of the process is not held constant. Eventually, the observed growth rate of the oxide is so small that the film thickness can be considered to have reached a limit value. The total current with this stage may or may not be small, given that it corresponds to the sum of the ionic and electronic currents. Therefore, the limit thickness hypothesis is acceptable in most cases.”

Also, “In the galvanostatic method, the oxide is grown by application of a constant current density, and the observed response is the potential as a function of time. In this method, it is possible to monitor oxide growth by observing the different processes occurring during synthesis. The growth rate is kept constant and the system is free to reach a stationary state characterized by a plateau in the potential versus time curve. As mentioned previously, the metallic substrate always presents an inhomogeneous thin film of native oxide, formed as soon as the metal is exposed to an oxygen-rich atmosphere. In this context, the heterogeneities of the oxide film play an important role because the electric field is inversely proportional to the film thickness. In other words, the oxidation reaction of the metal occurs locally, and the number of active areas is proportional to the applied current. Hence, in this constant current method, oxidation processes are more difficult to control and prevent local disruption of the formed oxide layer over the electrode.”

With these ideas in mind, that I think is correct, we believe that we might consider a change in the electrical field strength inside the oxide during the growth of the barrier layer and would be different for both regimes. But, at what extent? What is the transient profile of the electrical field intensity during the first seconds of anodization?

Answer your question; we have some results related to the presence of impurities from the electrolyte inside the first layer formed during anodization under galvanostatically regime being different from those described in literature for potentiostatic regime. We are trying to bring up some hypotheses to explain this behavior. One might be this different kinetics for both anodizing regimes. I will appreciate if you could deliver to us your ideas for that.

Thanks for your explanation. I will follow this topic. Recently I have prepared Ti nanotubes in an organic solvent and I was curious to know how the potential drop is distributed between electrodes. For a 40 V applied voltage I found the cathode to be close to -1 V vs REF and the anode close to +39 V vs. REF. Thus a high electric field may be supposed to exist within the oxide layer. It is also known that the diameter of the nanotubes  is proportional to the applied voltage and after the mentioned observation I may suppose that the tube diameter is correlated with the local/anodic electric field strength  .