Electropolishing is a process by which metal is removed from a work piece by passage of electric current while the work is submerged in a specially-designed solution. The process is essentially the reverse of electroplating. In a plating system, metal ions are deposited from the solution onto the work piece; in an electropolishing system, the work piece itself is dissolved, adding metal ions to the solution.
Figure 1 is a schematic illustration of a typical electropolishing cell. The work piece is connected to the positive (or anodic) terminal, while the negative (cathodic) terminal is connected to a suitable conductor. Both positive and negative terminals are submerged in the solution, forming a complete electrical circuit. The current applied is direct (DC) current.
The quantity of metal removed from the work piece is proportional to the amount of current applied and the time. Other factors, such as the geometry of the work piece, affect the distribution of the current, and, consequently, have an important bearing upon the amount of metal removed in local areas. Figure 2 illustrates both high and low current density areas of the same part and notes the relative effects of electropolishing in these two areas.
The principle of differential rates of metal removal is important to the concept of deburring accomplished by electropolishing. Fine burrs become very high current density areas and are, subsequently, rapidly dissolved. Low current density areas receive lesser amounts of current and may show negligible metal removal.
In the course of electropolishing, the work piece is manipulated to control the amount of metal removal so that polishing is accomplished and, at the same time, dimensional tolerances are maintained. Electropolishing literally dissects the metal crystal atom by atom, with rapid attack on the high current density areas and lesser attack on the low current density areas. The result is an overall reduction of the surface profile with a simultaneous smoothing and brightening of the metal surface.
In the case of stainless steel alloys, an important effect is caused by differences in the rates of removal of the components of the alloy. For example, iron and nickel atoms are more easily extracted from the crystal lattice than are chromium atoms. The electropolishing process removes the iron and nickel preferentially, leaving an enhanced surface layer consisting of corrosion-resistant chromium oxide. This phenomenon imparts the important property of “passivation” to electropolished surfaces.