Dear Yuelin Wu

congratulations to that question.

Due to the quite low current densities at corrosion measurements it does not mean, that one can neglect the ohmic drop. In battery and fuel cell research one has introduced some well constructed test cells.

You can help yourself by measuring the ohmic drop with your test equipement (1000 Hz AC impedance or current interruption method. But make sure that you are not creating passive layers on you electrode with that test procedure.

Another possibility would be using a test cell with a well constructed Haber Luggin capillary (done in several cells) and a parallel field line distribution (not done in the most cells) http://www.gaskatel.de/eng/downloads/20131108_Manual_Half_Cell.pdf

Good luck

Hans

Dear Kohnke,

Thank you so much for your instruction. I also need to investigate the corrosion rate of Cu-Al alloy, some of the samples are spontaneously passive at Ecorr, So I am wondering if that will affect the accuracy of solution resistance I measured using current interruption.

Now I obtain the polarization curve without solution resistance compensation and find that polarization resistance is smaller than solution resistance (polarization resistance is supposed to be equal to the sum of charge transfer resistance, solution resistance and joint contact resistance and other resistances). I am wondering if there is any reasonable explanation for that.

Thanks again.

Yuelin

You may be experiencing some classic uncertainties in attempting to measure extreme values of resistance:  the act of observing may be altering that which you desire to measure. 

My colleagues and I have observed in both AC and DC measurements that the important behaviors are different from what is commonly described in the literature.  For example, in DC measurements, it is the passivation behavior of the total cell that controls the corrosion behavior of the metal at the anode, and that behavior is a strong function of the electrolyte composition and the applied potential.  It is also dependent on pre-cleaning conditions.

But the greatest variable turned out to be the solution chemistry in the electrolyte itself.  It turns out that one of the stronger passivators of steel is bicarbonate ion, and that the primary source comes from atmospheric CO2.  After struggling to "stabilize" various electrolyte compositions, we learned that we couldn't even stabilize our deionized water -- it rapidly absorbed CO2 from the air, which then strongly passivated the steel that we were trying to study.  Analyzing for bicarbonate in our electrolytes (and DI water) was difficult, but not impossible.

While the obvious solution would have been to construct elaborate controlled environmental chambers to control for this variable, the nature of our grants did not facilitate this.  Fortunately, we were not attempting to operate at extreme ends of purity, so we were able to measure and live with the variation. 

So before you become too preoccupied with trying to fit various theoretical models, you might want to take some time to analyze your materials.  The first hint is pH drift over time in your DI water samples.. 

Regards,

Lou Floyd

Dear Yuelin Wu

The phenomena you described and the comments of Lou Floyd do exactly match the point. Try to measure the ohmic drop of the electrolyte will influence the passivation layer of the electrode.

Therefore you would need a test cell where the ohmic drop of the electrolyte is almost zero.

Or you will take a test cell with a very precice distance between HaBer Luggin capillary and working electrode. As you can measure the conductivity of the electrolyte outside with a simple conductometer you will be able to calculate the data without the ohmic drop.

Good Luck again yours

Hans

Lou Floyd would galvanic coupling also impact these measurements if so more than the passivation ? you might also want to look at a cleanliness tester "Resistivity of Solvent Extract (ROSE)" these instruments measure down to 0.005ug/NaCl