For obtain E(couple) and i(couple) in galvanic corrosion, in theorical should Tafel lines sum together. But if measurment is by device, polarization curves should sum thogether. Which result is correct?Why?
Dear Ehsan,
To obtain the individual polarization curves you have presented, you do not need the assembly with 4 electrodes as you have described. So, in a regular 3 electrodes arrangement you can get the polarization curves of each metal that can in the future be put in a galvanic contact. The interception of the polarization curves is just a way to predict the real Ecouple or icouple. The real value for icouple can be found using a ZRA or a potentiostat in a potentiostatic mode applying 0,0 volts between the metals (one connected to working electrode output and the other connected to the counter electrode and the reference electrode in the normal connection) in the galvanic couple (it is the way to be sure of the galvanic contact or short circuiting between both metals); the current registered with time is the galvanic couple current density. The real Ecouple is easily measured making the immersion of both metals (in a known exposed area relation such as 1:1 or any other relation) and a reference electrode and a potentiostat or simply using a high impedance voltmeter. Maybe there is some misunderstanding. I hope I have helped you.
Working electrode 1 connect to metal "1" (example iron) and other metal (copper)connected to working electrode 2. Refrence electrode is calomel. auxilary electrode is Pt. Sweep rate is 60 mv/min. Results are shown in the following figure
Red is Cu and blue is Fe.
Why IntersectionTafel lines is different with this result?
Mr. Ehsan both of the two approaches is correct for interpretation. I think that the results you got are not easy to explain for E(couple) and I (couple) by Tafel lines intercept, but can by explain based on the concept of corrosion mixed potential theory.
The intercept point between the cathodic polarization line of Cu and the anodic polarization line of Fe represent the galvanic couple potential value and the total corrosion current but not the galvanic current. The galvanic current approximate the corrosion current of the more active metal (in your example I think is Fe) since the contribution of the more noble metal (Cu) is very small.
Dear Ehsan,
To obtain the individual polarization curves you have presented, you do not need the assembly with 4 electrodes as you have described. So, in a regular 3 electrodes arrangement you can get the polarization curves of each metal that can in the future be put in a galvanic contact. The interception of the polarization curves is just a way to predict the real Ecouple or icouple. The real value for icouple can be found using a ZRA or a potentiostat in a potentiostatic mode applying 0,0 volts between the metals (one connected to working electrode output and the other connected to the counter electrode and the reference electrode in the normal connection) in the galvanic couple (it is the way to be sure of the galvanic contact or short circuiting between both metals); the current registered with time is the galvanic couple current density. The real Ecouple is easily measured making the immersion of both metals (in a known exposed area relation such as 1:1 or any other relation) and a reference electrode and a potentiostat or simply using a high impedance voltmeter. Maybe there is some misunderstanding. I hope I have helped you.
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