If two aluminium electrodes, in deionised water, were biased to 5V relative to one another, would we just observe electrolysis of the water? Let us assume that the water is perfectly pure, just containing H2O, OH- and H3O+. Or might the passivation thicken at the anode and be reduced at the cathode?
Also, how might the temperature influence these reactions? This is part of an accelerated aging setup, so the experiment is being performed at 67 °C, but we really want to understand performance at 37 °C (for medical implants).
for this question you should perform corrosion test especially potentiodynamic polarization and give information from tafel expolaration diagram if your voltage on cathodic curve you observe electrolysis of the water and if on anodic curve observe the readuction at the cathode and produce passivation thicken at the anode.
based on nernest equation potential revelent to temperature:
E=-RT/nF ln(productions/reactants)
1. Observe a Pourbaix diagram for Al, you have a good one in this paper (Fig. 2):
Article Polarization Curve and its Analysis of Aluminum in Condition...
or
2. Try directly a simple electrochemical test.
Based only on the Pourbaix diagram, I suggest that, at pH=7, you have water electrolysis with a low rate. You may expect a rate increase at a higher temperature.
In order to get electrolysis you need conductivity in the water. I expect you will get electrode polarization (distortion of the field) in your situation. At 67C, I would expect some dissolution or diffusion of ions into the system and perhaps some electrolysis.
Hi
In your case, the similar electrodes (Cahtode & anode) of aluminum will have supposedly a zero potential as an open circuit potential of the electrodes in the deionized water. However, when an anodic potential is applied between the two similar electrodes in such relatively a high resistance solution, i.e., deionized water, the anodic-aluminum electrode might slightly corrode with respect to the other aluminum electrode. In other words, a low current signal might be detected as a result of the applied potential between the similar electrodes of aluminum in the high resistance of the deionized water.
In these conditions, one should use the technique of electrochemical noise(EN) alongside to a zero resistance Ammeter (ZRA). In this technique, a new parameter called the corrosion admittance, Ac,. The Ac is defined as in a way to reflect localized corrosion, passivation or re-passivation and uniform corrosion of the anodic electrode in an electrochemical cell of two similar electrodes in a high resistance solution. The Ac is quantitatively described as follow:
Ac=dJ/dV Eq.(1)
where, dJ is the difference between the corrosion current density (J) of two subsequent values, normally measured by a zero resistance ammeter, an electronic method, because of the low current signal in an electrochemical cell of two similar electrodes in a high resistance solution. The dV is the difference between the applied potential (V) of two subsequent values, normally measured by a potentiometer, another electronic method. According to Eq. (1),
If Ac 0; this indicates that the tested sample (working electrode) is under uniform corrosion.
If Ac = 0; this indicates that the tested sample (working electrode) is under passivation or re-passivation.
In this way all the corrosion activities (either localized corrosion or uniform corrosion) are clearly revealed in the Ac spectra.
Refer to the attached Link:
As Khaled J. Habib. Also, you might be interested to have additional informations by measuring dV's[1], using a reference electrode (RE, Ag/AgCl[4]).
1. dV1WE for Ac1=dJ/dV1WE , as well as dV2CE for Ac2=dJ/dV2CE .
2. Voltage Effect on Electrochemical Anodization of Aluminum at Ambient Temperature Article Voltage effect on electrochemical anodization of aluminum at...
3. Resistance monitoring of aluminum plates to microbiologically influenced corrosion ... spectroscopy methods Article Resistance monitoring of aluminum plates to microbiologicall...
4. Study of colored anodized aluminum ... https://onlinelibrary.wiley.com/doi/pdf/10.1002/maco.200390052
Under the assumption of pure water, reference electrode cannot be used as it will be contaminated. The corrosion in such situation can be affected by the absoption of CO2 in air in water and the dissolved oxygen in the system. Care must be taken when doing such experiment.
If you are doing theorectical simulation or just want to know the general idea, a E-pH diagram should be fine though 5 V is high. However, this can be done as well.
If you need the diagram, you can find it online or I can help you with that.
best
chengsheng
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