Last month we wrote about electropolishing (EP) —the science and some problems. We also introduced the idea that removing a surface metal layer, and everything attached to it, is actually a step of final cleaning. In this column we’ll solve those problems, consider costs, and examine some cleaning applications.

The Problems

Last month we saw that the EP process is described by the following equation:

The metal is the surface film being removed. For stable operation, the acid must be replenished and the salt and the gas must be disposed. Fortunately, there is little hydrogen produced (usually less than 1 CF/hr - SF metal). Usually it can be easily vented without breaking safety or environmental rules.

Take a Bath

The EP bath contains a liquid mixture of several strong acids, soluble and insoluble salts, and perhaps some other chemicals. Strong acids are necessary to dissolve the metal salts liberated from the metal surface

All of these chemicals are extremely hazardous. Many, such as acids, present several hazards. An EP bath is not as relatively benign as is an aqueous cleaning bath. Avoidance of these hazards is one reason why most EP work is completed in job shops.

Most users doing EP work, and most job shops, allow the metal salt to accumulate and precipitate. It is periodically removed as a sludge. Similarly, fresh acid is added to make up that consumed. This is a brute force approach. It probably isn’t adequate for the level of process control needed in critical cleaning.

New Technology

Fortunately, some have developed better ideas. One method (US Pat. No. 5,882,500) is to purge from the EP bath to a second solution tank in which the reverse of EP (electroplating) is conducted. Here the metal salt is converted to acid and the metal is plated on another surface. The acid is returned to the EP bath. A US commercial firm (Global Stainless Technology) has a variant of this approach. They claim years of life without replenishing the EP bath.

Costs of Cleaning vs Electropolishing

I don’t know what EP costs. Operating costs are a closely-held secret because so much EP work is done in competitive job shops.

The two major components are labor and management of waste products. Labor includes racking and unracking of parts, selection and creation of the appropriate cathodes, and learning time to develop the proper conditions of voltage, time, and bath composition.

The best estimates I could obtain from several sources are $0.10 to $0.50 per “part,” with the “part” specifications being undefined. If you don’t have a specific “part,” I suggest you use the value for EP of a nickel per square inch. In my experience, that is cheaper than costs of critical cleaning for a part not specifically defined.

Applications

Stainless steel and copper are the most common metals to which electropolishing is applied. Some work is now being done with aluminum. Industries where these metals are nearly always electropolished are: food and beverage; medical and pharmaceutical; nuclear; electronic; and where work is done in a cleanroom.

The high conductivity of these metals makes the process more efficient and predictable. Electropolishing is being done with silicon—an inefficient conductor—in the production of semiconductor wafers.

The tendency of EP to preferentially remove material above the surface was recognized by many, and found to be an advantage in an uncommon application—sharpening of edges (U.S. Pat. Nos. 3,492,178: 4,406,759: 4,710,279: and 5,616,255). Other unique examples are sharpening of wires (U.S. Pat. Nos. 4,375,396: 4,935,865: 3,697,403: and 2,434,286) and needle electrodes (U.S. Pat. Nos. 4,587,202: 4,777,096: 5,693,454: and 5,762,811).

 Cleaning Up

Cleaning is normally thought to be the removal of undesirables from surfaces. Another way to view cleaning is as removal of the entire surface — including all the undesirables adhering to it. That’s