Electrochemical machining (ECM) is an unconventional technique used to shape steel and other conductive materials and to produce high quality products. The basic principle is that of a specific anodic dissolution at high current densities between 5 and 100 A cm−۲ in highly conductive electrolytes [1–۴]. A detailed knowledge of the process is essential in order to obtain an optimized, economically attractive technique, but the dissolution mechanism of ECM is not completely understood. Due to the transient and harsh conditions, the investigations of dissolution mechanism under ECM conditions are quite difficult. Some attempts to investigate and describe the dissolution process of certain metals are discussed in the literature [5, 6]. One main subject investigated for several metals such as iron, nickel, or copper is the distinction between active and transpassive dissolution mechanism owing to the close relation to the surface topography, machining accuracy, and current efficiency. The anodic dissolution of common base metals such as iron and nickel is strongly influenced by the electrolyte: electrolytes containing halides such as chloride or bromide support the active dissolution mechanism. Characteristics of active dissolution are as follows: & Direct passage of atoms into electrolyte as hydrated or complexed species & Formation of an etch pattern due to preferential dissolution of energetically favored planes, kinks, and grain boundaries & Kinetics expressed by Tafel behavior in the low potential region without control of mass transport & High current efficiency, no oxygen evolution. However, with oxidative or less aggressive electrolytes such as nitrates and chlorates, metal surfaces can passivate. This behavior is common in most electrolytes for valve metals such Al, Nb, Ta, and Ti. Hence, transpassive dissolution must be discussed. Characteristics of transpassive dissolution are as follows: & Formation of a surface layer, usually an oxide film of some nm & Oxygen evolution as side reaction & Lower current efficiency compared to active dissolution & Kinetics controlled by mass transport in the layer & Uniform dissolution, independent of the microstructure, which is a typical result of high-field oxide films. Oxygen evolution seems to be coupled with the presence of an oxide film which means the oxide acts similar to a catalyst. Hence, it can be used as an indicator of transpassive dissolution [7].