Dear All,
How do you determine which potential range to sweep in a potentiodynamic Tafel scan?
How would you determine the corrosion potential for Titanium and its alloys?
Thank you Very much.
First of all, measure the open circuit potential (OCP).
The potentiodynamical polarization curve was obtained in the potential ranges of -250 to +250 mV with respect to OCP at a scan rate of 0.5-1 mV/s. Then use Tafel to get the current density, anodic and cathodic slopes and the corrosion potential.
Yes , the boundaries of the potential scan is very important parameter in the potentiodynamic Tafel scan, then to take full explanation about this boundaries, the bundle of practical knowledge presented in item 4.1.1. page 9 of attached file is very helpful. Please go through this technical report.
Best regards
Establish the Open Circuit Potential for an immersion period (varies with material - a minimum of two hours) and use that as the reference
Historically, we typically used three things: (1) OCP measurements (as discussed above), (2) electrochemical equilibrium or E-pH diagrams (aka, Pourbaix Diagrams), and (3) cyclic voltammograms (CVs, from below hydrogen evolution to above oxygen evolution at a high scan rate). However, it really depends on what you want to know (measure). If you want to do linear polarization resistance (LPR) to estimate corrosion rates via the Stern-Geary approximation, then only +/- 50mV wrt OCP should be sufficient. For estimation of Tafel kinetics, a large range is required, as discussed above (OCP +/- 250mV), if you want to know about the passive current density and passive film breakdown, then you want to scan to potentials above those that would cause pitting in a halide containing solution. However, if the pitting potential is greater then the oxygen evolution potential, you will not be able to measure it accurately. To measure the OCP, all you need is a reference electrode and a meter with an input impedance of greater than 1E10 ohms. Any electrochemical reaction with a fixed potential wrt the hydrogen evolution reaction can be used as a reference electrode, but they are commercially available and inexpensive. Of course, you can readily make your own (e.g. see Ives and Janz, 1961). Most commercial voltmeters have an input impedance of 1E6 ohms and require too much current for accurate measurements from a reference electrode. However, you can easily fabricate a unity gain buffer amp (aka voltage follower) from inexpensive op-amps. Any introductory book on op-amp circuit design will show you how. I believe that Sawyer (Sawyer DT, Sobkowiak A, Roberts JL (1995) Electrochemistry for Chemists (J. Wiley & Sons, Inc., New York), Vol. 2nd Ed.) provides details on how to set up an electrochemical experiment.
Thank you all. Sir M Al' Atia and Sir Richard, your answers were quite helpful especially the resources you mentioned.
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