The electropolishing system smoothens, polishes, deburrs, and cleans steel, stainless steel, copper alloys, and aluminum alloys in an electrolytic bath. The process selectively removes high points on metal surfaces, giving the surface a high luster.
How Electropolishing Chemicals Work
The metal part is immersed in a liquid media (electrolyte) and subjected to direct current. The metal part is made anodic (+) and a metal cathode (-), usually 316L stainless steel or copper, is used. The direct current then flows from the anode, which becomes polarized, allowing metal ions to diffuse through the film to the cathode, removing metal at a controlled rate. The amount of metal removed depends on the specific bath, temperature, current density, and the particular alloy being electropolished. Generally, on stainless steel, 0.0005 in. is removed in 1,500 amp-minutes per square foot. Current and time are two variables that can be controlled to reach the same surface finish. For example, 100 A/ft2 electropolished for 5 min is 500 amp-minutes; 200 A/ft2 for 21/2 min is 500 amp-minutes. Both pieces of metal would have about the same surface profile. Current densities of 90 to 800 A/ft2 are used in this process depending upon the part to be polished and other parameters. Electropolishing times vary from about 1-15 minutes.
Advantages of Electropolishing Chemicals
Conventional mechanical finishing systems tend to smear, bend, stress, and even fracture the crystalline metal surface to achieve smoothness or luster. Electropolishing offers the advantages of removing metal from the surface producing a unidirectional pattern that is both stress and occlusion-free, microscopically smooth, and often highly reflective. Additionally, improved corrosion resistance and passivity are achieved on many ferrous and nonferrous alloys. The process micro- and macro-polishes the metal part. Micro-polishing accounts for the brightness and macro-polishing accounts for the smoothness of the metal part.
Deburring is accomplished quickly because of the higher current density on the burr, and because oxygen shields the valleys, enabling the constant exposure of the tip of the burr. Because the metal part is bathed in oxygen, there is no hydrogen embrittlement to the part. In fact, electropolishing is like a stress-relieve anneal. It will remove hydrogen from the surface. This is important to parts placed under torque. Another benefit is that bacteria cannot successfully multiply on a surface devoid of hydrogen, therefore, electropolishing is ideal for medical, pharmaceutical, semiconductor, and food-processing equipment and parts. The combination of no directional lines due to mechanical finishing, plus a surface relatively devoid of hydrogen, results in a hygienically clean surface where no bacteria or dirt can multiply or accumulate.
Summary of Unique Qualities and Benefits of Electropolishing Chemicals
- Stress relief of surface
- Removes oxide
- Passivation of stainless steel, brass, and copper
- Superior corrosion resistance
- Hygienically clean surfaces
- Decarbonization of metals
- No hydrogen embrittlement
- No direction lines
- Low-resistance welding surface
- Reduces friction
- Both polishes and deburrs odd-shaped parts
- Radiuses or sharpens edges depending upon rack position
- Reduces annealing steps
Simplicity of the System
Practically speaking, three major process steps are necessary to electropolish most metal surfaces successfully:
1. Metal preparation and cleaning
2. Electropolish (electropolish drag-out rinse)
3. Post-treatment (rinse, 30% by volume of 42° Baumé nitric acid, rinse, deionized hot water rinse)
Equipment Required for Electropolishing
The electropolishing tank is generally constructed of 316L stainless steel, double welded inside and out. Stainless steel can withstand high temperatures, which are needed if too much water enters the electrolyte. Polypropylene usually 3/4 to 1-in. thick, is another tank choice. This tank can withstand temperatures of 180-190°F.
The direct current source is called a rectifier. The rectifier is generally matched to the size of the electropolish tank. If the tank is to be cooled by tap water through a plate coil, no more than 5.0 A/gal should be used, therefore, in a 500-gal tank, the capacity of the rectifier should not be more than 2,500 A. If 3,500 A are needed, then the tank size must be increased to compensate for the increased wattage going into the tank (amps volts = watts). Voltage is also determined by the number of amperes needed to electropolish the part. Generally, 600-3,000 A requires an 18-V DC output, and 3,500-10,000 requires a 24-V rectifier. Optimum running voltage is 9-13 V for stainless steel. Aluminum requires a 30-40 V rectifier. Aluminum is run by voltage rather than amperage.
Electropolish racks for most metals are made of copper spines and crosspieces, which have been pressed in a thin skin of titanium. Copper, phosphor-bronze, or titanium clips are used and can be bolted on with titanium nuts and bolts. Some racks are made of copper and copper spines and are coated with PVC. These racks are generally for electropolishing of aluminum, copper, brass, and bronze, although titanium can be used here instead. When building a rack, remember that 1 in 2 of copper carries 1,000 A.