Polarization methods such as potentiodynamic polarization, potentiostaircase, and cyclic voltammetry are often used for laboratory corrosion testing. They can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Although these methods are well established, the results which they provide are not always clear and occasionally can be misleading. Real life examples will be provided where polarization methods were used and the results analyzed. The examples will be from several sources including industrial applications and will illustrate the possible problems and the eventual solutions which can come from using polarization methods. Comparisons will be made, where appropriate with results from similar materials or from various techniques. Examples of both successes and failures will be shown and potential pitfalls and difficulties will be highlighted.
Introduction and Background
Polarization methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential, ASTM defines polarization as: the change from the open-circuit electrode potential as a result of the passage of current.l For 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the anodic (or more positive direction) causing the working electrode to become the anode and causing electrons to be withdrawn from it. The anode is the electrode where corrosion occurs. For cathodic polarization, the working electrode becomes more negative and electrons are added to the surface, in some cases causing electrode position. For cyclic polarization, both anodic and cathodic polarization will be done. Historically, polarization methods were developed in the eighteenth and nineteenth century by electrochemists whose names are well known such as Tafel, Faraday, Helmholtz and Gibbs. More recently, (within the last 75 years) applications of the electrochemical methods have been applied to corrosion processes by other well known researchers such as Stern, Geary, Pourbaix, Fontana, and others. The fundamental equations for this development can be found in many standard reference texts2?3?4?5. The polarization methods have become well enough developed for standard equipment to become available for making measurements, software for taking and analyzing the data and, of particular importance to those new to the field, standard methods have been developed for testing materials*?7?8. Polarization measurements pushed the development in the 1930s of mixed potential theory, in order for the experimental results to be explained. Figure 1, after a similar figure in Jones? and ASTM1l, shows a schematic representation of an anodic and cathodic reaction, typical of a corroding sample (a) and the experimental polarization curve which could be derived from the reactions in (a) shown as Figure 1(b). This figure shows both the anodic and cathodic regions, the region of Ecorr, the projected value for ic~, the Tafel regions for the anodic and cathodic branches, and a typical way of plotting polarization curves.
Polarization Test Methods
The proper method for polarization testing of materials is facilitated by the
development of standard test methods such as ASTM G5 Standard Reference
Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization
Measurements. As is mentioned in the Scope of this method, it is designed to
be used to check one?s experimental technique and instrumentation. If
followed, this test method will provide repeatable potentiostatic and
potentiodynamic anodic polarization measurements... 6. Other descriptions of