14. AC Impedance

The electrochemical techniques which have been considered so far in this text have been direct current techniques. This chapter describes a powerful technique [1, 2] which uses alternating current (AC). AC impedance techniques are not new, but prior to about 1980, AC measurements were generally made with a Schering bridge which requires manual balancing to null the bridge in order to measure the equivalent parallel resistance and capacitance. This method is tedious and time-consuming. With the advent of improved instrumentation under computer control and acquisition, AC impedance techniques have become an important method of making electrochemical measurements. AC impedance has been used to study the metal/solution interface, oxide films and surface treatments, and the corrosion behavior of organic coatings on metals. Each of these three applications is considered in this chapter.