Page 133 - Corrosion Engineering Principles and Practice

108 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 109

e) An electrical connection can be made directly with the
working electrode, which will not be affected by the
solution.
f) The working electrode must be introduced into the solution
completely so as to eliminate any crevice at the solution
interface, unless this is a desired effect.
g) The test cell itself must be composed of a material that
will not corrode or deteriorate during the test, and which
will not contaminate the test solution, the volume of the
cell must be large enough to allow removal of the corroding
ions from the surface of the working electrode without
affecting the solution potential.
h) It may be necessary to include a mechanism for stirring the
solution, such as a stirring bar, or bubbling gas to ensure
uniformity of the solution chemistry.
DC Polarization Test Methods
DC polarization methods involve changing the potential of the
working electrode and monitoring the current that is produced as a
function of time or potential. For anodic polarization, the potential is
changed in the anodic (or more positive) direction causing the
working electrode to become the anode and forcing electrons to be
withdrawn from the metal being tested. For cathodic polarization,
the working electrode becomes more negative and electrons are
added to the metal, sometimes causing electrodeposition. For cyclic
polarization, both anodic and cathodic polarizations are performed
in a cyclic manner [6]. There are several accepted methods to carry
out DC polarization of specimens for corrosion testing.
Potentiodynamic Polarization. Potentiodynamic polarization refers to
a polarization technique in which the potential of the electrode is
varied over a relatively large potential domain at a selected rate by
the application of a current through the electrolyte. Figure 5.18 is an
example of a polarization plot obtained with a S43000 steel specimen
in a 0.05 M H SO solution.
4
2
A potentiodynamic polarization variant is cyclic voltammetry
which involves sweeping the potential in a positive direction until a
predetermined value of current or potential is reached, then the scan is
reversed toward more negative values until the original value of poten-
tial is reached. In some cases, this scan may be done repeatedly to deter-
mine changes in the current-potential curve produced with scanning.
Another variation of potentiodynamic polarization is the potentio-
staircase method which refers to a technique for polarizing 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 [6].