|
|
|
|
Polarization methods such as potentiodynamic polarization, potentiostaircase, and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarization methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential.
Anodic polarization: the potential is changed in the anodic (or more positive direction) causing the working electrode to become the anode and causing electrons to be withdrawn from it.
Cathodic polarization: the working electrode becomes more negative and electrons are added to the surface, in some cases causing electrodeposition.
Cyclic polarization: both anodic and cathodic polarization are performed in a cyclic manner.
Several methods may be used in polarization of specimens for corrosion testing. Potentiodynamic polarization is a technique where the potential of the electrode is varied at a selected rate by application of a current through the electrolyte. It is probably the most commonly used polarization testing method for measuring corrosion resistance and is used for a wide variety of functions: (reference)
Cyclic polarization tests: These tests are often used to evaluate pitting susceptibility. The potential is swept in a single cycle (or slightly less than one cycle), and the size of the hysteresis is examined along with the differences between the values of the starting open circuit corrosion potential and the return passivation potential. The existence of the hysteresis is usually indicative of pitting, while the size of the loop is often related to the amount of pitting.
Cyclic voltammetry: Cyclic voltammetry involves sweeping the potential in a positive direction until a predetermined value of current or potential is reached, then the scan is immediately reversed toward more negative values until the original value of potential is reached. In some cases, this scan is done repeatedly to determine changes in the current-potential curve produced with scanning.
Potentiostaircase method: This technique polarizes an electrode in a series of potential steps where the time spent at each potential is constant, while the current is often allowed to stabilize prior to changing the potential to the next step. The step increase may be small, in which case, the technique resembles a potentiodynamic curve, or it may be large.
Electrochemical potentiodynamic reactivation (EPR): This technique allows to measure the degree of sensitization of stainless steels such as S30400 and S30403 steels. This method uses a potentiodynamic sweep over a range of potentials from passive to active (called reactivation).
Linear polarization resistance (LPR): A widely used technique, the polarization resistance of a material is defined as the slope of the potential-current density curve at the free corrosion potential, yielding the polarization resistance, as described in the next section.
