STATION 5. Electropolish
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 terminals are submerged in the solution, forming a complete electrical circuit. The current applied is direct (DC) current.
The principal chemical reaction occurring at the electrical anode, that is, at the part, is as follows:
The reaction states that metal is dissolved from the anodic electrode, passing into the solution to form a soluble salt of the metal. All of the components of stainless steel, namely the iron, the chromium, and the nickel, undergo this reaction simultaneously, producing the controlled smoothing of the surface. Several side reactions also occur, creating byproducts which must be controlled in order to produce the highest possible quality of electropolishing.
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 effect 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, consequently, rapidly dissolved. Low current density areas receive lesser amounts of current and may show a lower level of 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. Iron and nickel atoms are more easily extracted from the crystal lattice than are chromium atoms. For this reason, the electropolishing process removes the iron and nickel preferentially, leaving a surface rich in chromium and the chromium oxides. This phenomenon imparts the important property of “passivation” to electropolished surfaces.
The general relationship between applied current and voltage for a typical electropolishing system is illustrated in Figure 4. An understanding of the combined effects of current and voltage are key to the production of high quality electropolishing.
Electropolishing systems require rinse water to remove solution from the parts after each chemical operation. These rinses usually go to drain, and are subject to Federal, State, and Local regulations affecting discharge to public sewer treatment systems. Most modern electropolishing systems now incorporate evaporative recovery and/or multiple rinse technologies to minimize the amount of rinse water used. MCP can furnish simple waste treatment packages guaranteed to meet the current restrictions.
Electropolishing baths generate both hydrogen and oxygen gases, producing an acid mist which must be ventilated to meet OSHA requirements. Other chemicals in the line, such as cleaners and acid pickles, may also require ventilation to meet these regulations. MCP can furnish installed ventilation systems constructed entirely of PVC which meet or exceed such regulations.
High quality surface finishing also requires some analytical effort to ensure that solutions are chemically balanced. MCP will furnish analytical services free of charge to its chemical customers in good standing; however, each customer should also budget space, equipment, and reagents for the chemical controls needed to exercise local control for trouble-shooting.
Special equipment may be needed to ensure that quality criteria specified by the end-user are being met. Some end-uses require only visual examination of the parts to evaluate brightness, luster, or clarity of the finish. Others may require sophisticated instrumentation to determine surface profile, degree of passivation, corrosion resistance, reflectivity, oxide layer composition, or other specified performance characteristics.