Dear Tasneem

According to the ASTM standard, it should take an hour to reach a stable OCP value. Also, you could work with an acceptable stable range of OCP, i.e. a few +/- mV, if OCP hasn't reached a steady state value at all. In any case, the OCP is a potential difference value rather than an absolute value.

Thank you for responding to my initial query. I ran the OCP task twice, once on copper and once on low alloy steel. I measured the OCP against time for over an hour but it kept decreasingly rapidly. Is it possible that this is because I have the counter electrode connected too? Should I be measuring the OCP of the working electrode with reference to the SCE electrode without connecting the counter electrode to the potentiostat?

Your help is much appreciated.

The amount of time it takes to reach a stable OCP is highly variable. I have seen some researchers wait months to get stable OCP values when dealing with solutions sitting in tanks for years. It really depends on how long it takes to get a passive layer that is stable and that resembles the scenario you are trying to test. There are some potentiostats (e.g. BioLogic) that have selections to correct for drift in the OCP during EIS. This takes more cycles and more time, but it is a potential work around. One question to ask yourself is how low in frequency are you really wanting to go with your scans. If you are studying fast phenomena and are not interested in data below 1-2 Hz, you might be ok with the drift in the OCP. If you are trying to get a corrosion rate similar to an LPR test, I typically go down to ~ 1-2mHz in which case that will not work. But, I just wanted to fast scans as an option if that would work for your application.

Yes, the open circuit potential (OCP)of a working electrode, i.e., copper or steel, in a particular aqueous solution can be measured with respect to only a reference electrode ,i.e., SCE, as in a two electrodes-electrochemical cell. In contrast, one needs a counter electrode, i.e., platinum or graphite, in the electrochemical cell during a potentiostatic/ a potentiodynamic polarization test, as in a three electrodes-electrochemical cell.

The counter electrode will function as an electron dump from the working electrode.

Khaled,

Thank your response. I ran another OCP vs Time test, although I had the same issue. To make it clearer I have attached the results. After 3 hours, the OCP was still reducing, reaching a value around -7V. I would expect for HSLA steel, the OCP would measure around -0.2 to -0.4V. The reference electrode used is an SCE. The image of the cell is also provided in the excel file. Many thanks for your input so far. I am a little confused at the equipment gave me a final OCP value of 5.607V?

Hello Tasneem

Indeed the potential values that you are measuring are not realistic for the system you described. To rule out possible problems with the potentiostat/software I would suggest to make a simple measurement with a digital voltmeter. If the first readings are still in the order of volts, then it might be possible that there is something wrong with your reference electrode. Try with another oner. I would also suggest to test the electrical continuity in your working electrode. I hope this is useful.

Hello Maximo,

Thank you for you response. Would it be possible to guide me through what you mean by measuring with a voltmeter please? Do you mean connecting the working and references electrode to an external voltmeter, whilst they are immersed in the solution? Also, should I wait for an hour before doing the measurement?

Yes, I meant using an external DVM (see schematic illustration). As I said before, the first readings should give you an indication if there is a problem. If these values are still abnormal, there is no need to wait one hour: do the procedure I indicated (change the RE and/or test continuity of the working electrode). Once you get normal values of OCP you can go back and connect the cell to the equipment to record data during 1 h.

Hi Maximo,

Thank you for the suggestion. We measured the voltage using a multimeter and obtained a value of -0.45V and this remained the same. I measured this immediately after connecting it to the potentiostat and obtained the results attached to the excel file provided. Do you reckon it is the potentiostat that is the problem?

After 40 mins the multimeter gave the same reading of -0.45V.

Thanks for all the advice so far.

Hello Tasneem

You really have a problem with your connections to the potentiostat or with the potentiostat itself. The values that you show are not realistic and the multimeter reading should be ok.

Is it your first time with this potentiostat or have you already got good results before?

Regards

Joao Salvador Fernandes

Hi Tasneem,

As Professor Fernandes says it appears that the problem is with the connections (i.e. the "cell cable" that comes with the equipment) or with the potentiostat hardware/software. If you have a spare set of cables try with those. If the problem persists then I would suggest to contact the supplier/vendor of your equipment and seek technical advice from them. I believe you are using a VersaSTAT but I am not familiar with such potentiostats, so I can not give you further advice on its operation.

J.C.S. and Maximo,

I managed to sort out the OCP stabilisation issue; it was a cabling/connection issue from my end. This is all very new to me so I'm still learning from my mistakes.

Thank you to you both the helpful advice.

You are wellcome Tasneem.

Best regards,

Max Pech-Canul