Electropolishing of aluminum takes place in a stainless steel or ¾ inch PVC tank filled with the electropolishing solution. On either side of the rack of parts there are 316L stainless steel, round cathodes. The racks of parts are made anodic. A rectifier is used to provide direct current to the production tank. The rectifier should be rated at 30 to 40 V DC. The current is determined by how many square feet of work is to be processed in the cycle time of 3 to 10 minutes. Generally, 10 to 15 A/ft of work processed is used to determine the amperage of the rectifier. Good agitation is required for a bright finish. Agitation used is generally clean, dry compressed air along with rod agitation. Operating temperature is 150 to 170° F.

Since these solutions are environmentally friendly the first rinse following electropolishing is a running rinse tank, kept fairly fresh. Rinse tanks can be counter-flowed in the system. After good rinsing is accomplished the parts may either go to a hot DI rinse or to a commercial desmut tank, rinse, and anodize. If the parts are kept wet before anodizing no vail exists; therefore, a brighter and clearing anodic coating is produced. Waste treatment consists of neutralizing with 50% liquid caustic soda.

Historically, aluminum has been and is still brightened using the aluminum bright dip bath. This bath consists of about 71.5% phosphoric acid, 10% aluminum phosphate, 10% water, and 2.5% nitric acid. Sometimes sulfuric acid is added, about 6 to 10%. The bath runs at 225 to 240° F. Nitric is a volatile acid and must be added each hour to the bath. Nitric acid also forms nitrogen dioxide, which is a yellow fume that is toxic; therefore, a fume suppressant, such as diammonium phosphate, is also added to the bath to break the nitrogen dioxide down to nitrogen and oxygen. This bath requires a shrouded hood with slots going up to the height of the racks being pulled out of the tank. The parts give off nitrogen dioxide in the air and the fumes must be sucked off into the exhaust hood and into the fume scrubber. There are about 17 variables that have to be controlled each hour to produce bright, pit-free aluminum. Plant corrosion is very bad.

The advantage of the bright dip is there is no rectifier involved and small parts can be done in bulk because wherever the solution touches the part it will brighten. Very little leveling or deburring is accomplished.

Electropolishing has been around for a long time, but the only solution that worked was one requiring several pounds of chromic acid mixed in phosphoric acid. Chromic acid is more costly to waste treat and is also toxic.

Today, electropolishing solutions have been developed without chromic acid and are much more environmentally friendly. These solutions brighten, level and deburr aluminum. (See figs. 1 –3) There are no toxic fumes; hence, a simple lip exhaust is all that is required. A typical line would include:

1. Non-etch cleaner

2. Rinse

3. Rinse

4. Electropolish

5. Rinse

6. Rinse

7. Commercial desmut or 30% nitric acid at room temperature

8. Rinse

9. Hot DI Rinse

10. Anodize, etc.

Reflective tests have shown that electropolished aluminum will be 10 to 12% brighter after anodizing. It will also produce a sharper reflected image because no vail is produced as with bright dipping.

Parts are racked on titanium racks or copper encapsulated with titanium. In some cases, PVC-coated racks are also used. The coated racks are generally used for critical electropolishing where no current can be conducted by the rack spines, or where a large part is too close to the rack spine and shadowing could occur. The parts on the rack are hung on the anode bar of the electropolishing tank for about 20 to 30 seconds before the current is turned on. This helps remove uneven oxides on the surface of the part before electropolishing takes place.

Any alloy that bright dips can be electropolished. Many newer alloys that cannot be bright dipped can also be electropolished. For example, new alloys for reflectors have been developed especially for electropolishing. With lower energy light bulbs, reflectors previously bright dipped did not produce the same intensity of light. Higher reflectivity was needed. The only way was by electropolishing.

Today’s new electropolishing baths can replace bright dipping, eliminating toxic fumes, expensive hooding, extensive plant corrosion, expensive chemical costs, hourly additions, extensive bath analysis, and air makeup costs in cold climates. In addition, electropolishing will not only brighten but also will level and deburr aluminum parts. Some of the aluminum baths will also brighten various alloys of stainless steel, copper, and bronze.

Kenneth B. Hensel is President of Electro Polish Systems, Inc., a supplier of electropolishing processes and equipment.         E-mail, khensel@ep-systems.com.