If you are working with metal or alloy in aqueous solution, you might need to check the pH of the distilled or bidistilled water. I do not know which electrolyte you are using, can you tell more about the electrodes and electrolyte. for some of corrosion tests, you need to start from E_equilibrium, that is why you need to have a stable OCP before start.

Sometimes, the bidistilled water pH going to be more acidic, and this can making delay in attaining the stable OCP. I hope this helps, Cheers.

Hi Adel,

I'll check it for sure. thank you for the advice. my electrolyte is phosphate buffer saline with 10.5mg/ml of bovine serum albumin. my alloy is CoCrMo.

How long you have your PBS?, if it is more than a month, then might be you need to have a new one. You can prepare a new one to be sure that it is not contaminated. Normally most of the Cr-MO alloys have the pitting corrosion susceptibility not for a general corrosion. Since you are working in a buffer, then I doubt the distilled water pH will change any thing, however, if your distiller have a scales which is not cleaned for long time, it can come into the water which can affect the result.

I have it since a week. I am gonna prepare one just to make sure. I am really confused my results were fine and suddenly there is a big change. I repeated the test three times just to make sure but I still get high OCP results

Is the OCP going to negative values or positive values compared to the normal experiments, going more to negative, this will indicate an oxidation (corrosion), and if it is going to positive, this is means that it is passivating (protection). Check the surface of your electrode for any abnormal scratch, or pitting which means you will need to replace this electrode with a new one to the same surface finish you have in your first experiment. Cheers.

it is going to more negative values. what I do usually is after finishing my experiment I polish my sample to the same finish and reuse it. I already changed my samples and I am still getting the same results. Thank you so much Adel. I really appreciate your help

You are welcome. How you making your surface finish? if it is by sand paper pad, try to use a new one, it might be contaminated, if sure, have a new fresh buffer. hope this helps.

Fogot to say, check the electrode by magnifying lens, or under optical microscope to see that the surface is not damged, you might need to have completely new electrode with same surface finish.

I use silicone carbide paper for finishing. I think I'll try a fresh buffer. hopefully it works this time

That's a great idea. Thank you so much

There is a difference between equilibrium potential and mixed potential. Generally, if you have only one redox-couple in the system (for example, like in reference electrode), the equilibrium potential is established and it is not moving. If you have several redox-couples, the mixed potential is established, corresponding to total zero current of all redox-couples in the system. Typical example is corrosion. In this case the system is continiously evolving, and the value of mixed potential (measured as OCP) is constantly shifting. The drift may be very slow, if the processes involved are slow, but it may be quite significant for fast processes.

Dear Sebastian,

I am using epoxy for my sample. i don't use the sample holder. so I don't have ring.

thank you so much for your reply.

Hi Sergey,

I am wondering how can I know if my test have one equilibrium potential or mixed potential?? my background solution is PBS and I am adding bovine serum albumin BSA. My sample is CoCrMo.

Thank you so much

Unless you are sure you only have one possible redox reaction (which usually requires that you are using gold, platinum or carbon as your electrode), you should assume that you have a mixed potential. Also epoxy-mounted specimens are very susceptible to the formation of a crevice between the epoxy and the metal due to shrinkage effects. This can be avoided by very careful pretreatment of the metal before coating and subsequent curing of the resin, but it is a difficult problem. Try a search for "epoxy" and "crevice corrosion".

Thank you Robert. I will search. I am using CoCrMo. I am wondering how can I know if there is a crevice corrosion or not?? can I look to my sample under the optical microscope??

Dear Rana....

There is no problem in your system....Generally OCP will not stabilize untill you can by longer immersion period. You can see the current potential curves, if they are looks like sinusoidal wave, you can proceed with your experiment viz. impedance and polarization studies....

thank you so much Chaitanya. It looks like sinusoidal .

Hi Sebastian,

I 'll check it. Thank you so much. I really learned a lot from you

From my experience the OCP always shows an evolution, especially if you use a complex system like yours.

There are a lot of factors which may have an influence. Starting with your equipement, your handling of all samples, preparation steps, which includes the used fluids here as well and ending with your microstructure.

I read the thread quite fast, so i may have missed if you already refered to the following:

Did you check on your equipment? I mean a test with the usually available reference circuit? Make shure you don't have any fractures within your cable and that your potantiostat works well.

Did you maybe accidentaly change something in your prepration procedure, or do different people work in your lab using the same polishing pads (the already mentioned contamination topic). Did you buy new fluids => polishing media? Maybe just the time between sample preparation and conduction of your test chaged?

What about the temperature and relative humidity conditions in your lab? How do you store your PBS? Do you buy it, or do you prepare it yourself? If second, where do you get your Water from? It may even be (worst case scenario) some of your colleagues installed new equipment in a lab next to yours ...

However, do you use cast or wrought samples? Even your microstructure may have influence on the result.

What steps belong to your electrochemical procedure? Do you perform some kind of electrochemical cleaning of your sample surface before starting the OCP measurement? Do you rinse your PBS with some gas?

From now 10 years experience in corrosion testing the ones mentioned above are only the tip of the iceberg ;)

You could define a slope (rate of change in potential) which would allow you for considering the system to be stabilized.

Kindly

Michael

Hi Michel

Regarding to the first question, Yes I have checked my experiment and before one experiment there was a detached cable and I fixed it but even after fixation I had the same issue which finally I was able to solve it.

for the preparation, Actually I am using the same polishing pads with my colleague, usually I change the polishing paper before my polishing except for the one micron and the six micron. For the solution, I made it. However, I have not paid attention to the time between the polishing and conducting the test.

for the humidity condition I do not monitor that however, I connect my cell to a water path to keep 37 degree. I prepare the PBS. The water we have deionized water in our lap and I got the water from the lap. That’s right my colleague installed a new equipment next to my but I have not notice a change in my results. I use wrought samples

for the cleaning I use the ultrasonic cleaning which is basically immersion my sample in the water with soap for 5 min then I immerse it in deionized water for 5 min more. After that, I raise it with ethanol and dry it with hot air.

I am wondering how can I define the slope? is it by using the versa studio?? I have noticed that my issue got solved because I let the sample to stabilize for about 4-6 h. I was using my colleague results as a reference because she worked in the same lap with the same samples but different finishing. She used 1 h OCP and for her experiment that was not enough for my experiment. I am assuming the finishing is the reason, she polished until 6m and I do polish until 1 micron.

Thank you so much Micheal, I really appreciated your comments