What is the role of the open circuit test on the potentiodynamic corrosion test?
OCP is measured so that you know where the corrosion potential of the alloy is before you initiate the potentiodynamnic corrosion test. The measured corrosion potential will deviate from the OCP depending on your polarization procedure (e.g., rate of polarization, typically suggested to be kept below 0.2 mV/s).
The OCP is needed in order to know what value to set as the initial potential for a measurement. Starting with an potential different from the OCP would have an influence on your results.
It is good practice to monitor the OCP of a test prior to starting a potentiodynamic sweep so that you can verify that the sample has come to at least a pseudo equilibrium with the test solution. Failure to do this can produce results that are not reproducible.
Thanks Prof. Stephen, but What is the acceptable value for the slope of potential vs. time that I can say that I reach to the pseudo equilibrium?
Another question, If I perform the potentiodynamic test two times consecutively -after I reached to the steady state from OCP test- , Could I obtain the same result?
Perhaps someone else who is familiar with some of the current standards for potentiodynamic testing can quote you a specific value for the slope of the curve. However, part of the answer lies in the combination of the chemistry of the conditions that you are testing, how much reproducibility that you need and simply how long you can afford to wait.
As for running the test two times, remember that when you run a potentiodynamic sweep, you are altering the conditions on the sample surface. The extent of this alteration depends first upon the current density and second upon the potentials at the reversal points. Running a test that only deviates from OCP by a few millivolts will usually not produce enough current density to change in the near surface chemistry very much - but also probably will not tell you much about your system. Running to higher current densities will tell you more about you system, but also alters the environment on the surface. So you end up with a Heisenberg Uncertainty situation with respect to the test. Running the test to get the information you seek MAY alter the conditions sufficiently that running it again on that sample will not provide an entirely reproducible result, especially with respect to the OCP.
A perfect example of this would to be to run a sweep on a stainless steel, such as 316, in a NaCl solution to an anodic potential that is in the transpassive region. Running the sweep back towards OCP will result in a hysteresis loop that will not return to the same OCP that you started at. If you try to run the sweep again on the same sample, you will get a similar curve that has a similar shape, but it will not exactly reproduce the first curve due to corrosion changes on the metal surface that occurred on the first sweep.
Thank you so much prof. Stephen for this valuable information. and I want to inform your respect that I performed this test two times consecutively -I switched off the potentiostate for seconds then I began the same test again - and obtain the same results exactly and the same shape at the same time, I was using Ti alloys!
It is essential to measure the OCP before potentiodynamic tests because It tells you at what value of potential should be set as initial potential.
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