In the early 1960s, application of electrochemistry to corrosion was limited to the open-circuit potential measurements, the Stern and Geary methods, and the plot of current-voltage curves with the recently invented potentiostats. These techniques were totally unable to provide a reliable description of the intricate electrochemical processes involved in the corrosion of pure metals and alloys. It was established that the interesting information lies most often in the low and very low frequency ranges of the complex impedance. In the 1970s, the development of highly improved equipment, especially digital frequency response analyzers, provided access to more accurate and faster measurements extending, when necessary, to the millihertz or below. The most significant contribution of these techniques to corrosion studies will be illustrated. The contribution to the theory and measurement of space-resolved impedance, which can match the uneven distribution of reactions on a corroding surface, is also presented. Electrochemical measurements, including, but not limited to, alternating current (AC) impedance, were also applied to more practical problems of corrosion rate and corrosion mechanisms. Among them, corrosion protection by inorganic and organic coatings prevailed. These works were often based on instrumented samples designed for separating the contributions of the individual components of a protecting system (sacrificial coating, multi-layer paints, etc.).