What Does Polarization Curve Mean?
A polarization curve is a plot of current density (i) versus electrode potential (E) for a specific electrode-electrolyte combination. Plots of log |i| vs. E or vs. (E - Eo) are called polarization curves. The polarization curve is the basic kinetic law for any electrochemical reaction.
Polarization curves are valuable in quantifying the behaviors of metals under various conditions. Polarization curves for passive systems may show active/passive and/or passive/trans-passive transitions.
Polarization curves have many variants depending on conditions:
- Activation polarization curve
- Concentration polarization curve
- Ohmic polarization curve
- Anodic polarization curve
- Cathodic polarization curve
- Electrochemical polarization curve
- Potentiostatic polarization curve
Corrosionpedia Explains Polarization Curve
Polarization curves are plotted in various ways. Often, polarization curves are plotted in which log |i| is given along the abscissa, even though it is the electrode potential (E) and not the current which is the independent variable. This practice dates back to the very early days of corrosion science when polarization curves were determined galvanostatically—by applying a constant current and then observing the resultant electrode potential.
Polarization curves should be determined only after the open-circuit electrode potential of the test metal has attained a steady-state value. The length of time required to reach the steady state depends markedly upon the nature of the system.
Today most polarization curves are determined potentiostatically so that the electrode potential is the independent experimental variable. In addition, according to absolute reaction rate theory, it is by changing the electrode potential that the free energy barrier is either lowered or raised so that a concomitant current flow is observed. Thus, the electrode potential is properly the independent variable and should be plotted on the abscissa in a polarization curve.
The combined measurement of potential and current relationships for an operating corrosion cell over a wide range of oxidizing conditions results in polarization curves that describe the electrochemical reactions. The combined measurement of potential and current at potentials that are very close to the freely corroding potential of the system gives rise to linear polarization curves. Both of these combined potential and current measurements have useful applications in corrosion control.