Since a corrosion process is a nonlinear electrochemical phenomenon, a potential perturbation signal by one or more sine waves will generate current responses at more frequencies than the frequencies of the applied signal. Current responses can then be measured, for example, at zero, harmonic, and intermodulation frequencies. This simple principle offers various possibilities for corrosion rate measurements, like the intermodulation or electrochemical frequency modulation (EFM) technique in which the potential perturbation signal consists of two sine waves of different frequencies. With this novel EFM technique, the corrosion rate can be determined from the corrosion system responses at the intermodulation frequencies. With the EFM technique a corrosion rate can be obtained instantaneously, without prior knowledge of the so-called Tafel parameters. The EFM approach requires only a small polarizing signal, and measurements can be completed in a short period. A special advantage of the EFM technique is its capability of inherent data validation control using "causality factors" (parameters introduced for the first time in this paper). It is shown that the EFM technique can be used successfully for corrosion rate measurements under various corrosion conditions, such as mild steel in an acidic environment with and without inhibitors and mild steel in a neutral environment.