Electropolishing is a cleaning process that allows electric current to pass through metal during acid bath immersion. Interchangeably referred to as reverse electroplating, the proper use of this process removes a fraction (generally from 0.0001" to 0.0004") of the surface of the material, leaving the chromium oxide and base metal content virtually untouched. When used on stainless steel, electropolishing removes iron, iron oxide, nickel and other nonmetallic materials from the surface, enriching the chromium content and corrosion resistance of the metal.
Regulating the voltage, current, time and temperature of the acid bath precisely controls the amount of surface material removed from the metal. Electropolishing first attacks the "high points" or "peaks" of the metal more aggresively, then gradually smooths the "valleys," resulting in a polished surface. This smoothing process reduces the microscopic crevices in which bacteria can propagate and tiny particles can accumulate, thus aiding in the sterilization and cleaning of metal products. Not surprisingly, the food service industry extensively uses electropolishing on their food handling machinery and surgical tools.
Commercial applications for electropolishing surfaced in the early 1950s. Most formal research on the process occurred at that time, motivated by the growth and sophistication in electroplating technology. Today, there are over 500 industrial installations and dozens of electropolishing job shops in the country.
Electropolishing Process
1. Prepare a bath or tub of combined rectified current and blended chemical electrolyte to remove flaws from the metal surface.
A power source converts AC current to DC at low voltages. A rubber-lined tank typically made of steel is used to hold the chemical bath. A series of lead, copper or stainless steel cathode plates are lowered into the bath and installed to the negative (-) side of the power source.
2. Mount stainless steel product on special copper or titanium racks.
A part or group of parts are fixtured to a rack made of titanium, copper or bronze. That rack in turn is fixtured to the positive (+) side of the power source.
3. Submerge the racks containing stainless steel product into an acid solution, maintained at a specific temperature.
4. Through the acid solution, the current passes from the anode (i.e., stainless steel product) to the cathode (i.e., lead), thus dissolving any impurities on the metal.
The metal part is charged positive (anodic) and immersed into the chemical bath. When current is applied, the electrolyte acts as a conductor to allow metal ions to be removed from the part. While the ions are drawn towards the cathode, the electrolyte maintains the dissolved metals in solution. Gassing in the form of oxygen occurs at the metal surface, furthering the cleaning process.
5. Remove the rack containing the electropolished product from the acid bath.
6. Air-dry the product.
7. Wipe off handprints and any clinging electrolytes from the metal with any glass cleaner and a soft lint-free material.
8. Inspect the electropolished products using the following optical examination guidelines:
- No visible damage to surface, contamination or residual machining marks.
- Minimal pitting.
- Bright surface, without hazy or milky appearance.
If using an electron microscopy, note the following:
- Average defect count <10; maximum <40.
- No contamination.
- No micropitting.
Following are other analytical methods of determining the quality of the electropolishing process:
- Surface roughness: Ra per specification.
- Solvent Extraction Analysis: Minimal or no detectable organic or ionic extractables.
- Energy Dispersive X-Ray Analysis: No elements detected other than those permitted in material composition specifications.
- Auger Electron Spectroscopy:
a) Oxide Layer Thickness: > 20 Angstroms
b) Maximum Cr/Fe Ratio: > 1.5
c) Iron Oxide Layer: < 5, preferably 0 Angstroms
d) Carbon Layer Thickness: < 8 Angstroms
e) Initial Carbon Concentration: <30 Atomic Percent
f) Minimal levels of other contaminants.
- Electron Spectroscopy for Chemical Analysis:
a) Total Cr/Fe Ratio: > 1.5
b) Total CrOx/FeOx Ratio: > 2.0
9. Pack the electropolished metal product in sealed plastic to avoid contact with dust and particulate.
Surface Condition Problems
- Contamination spots or water spots caused by: (a) contaminated water; (b) incomplete cleaning; and (c) improper handling.
- Visible Pitting caused by: (a) improper mechanical polishing (AFM process); (b) improper electropolishing; and (c) high inclusion content stainless steel.
- Dull Surface caused by fine scale surface roughness due to: (a) improper preparation of surface (polishing); and (b) improper electropolishing process.
- Hazy Surface caused by microscopic pitting due to: (a) improper mechanical polishing (AFM process); (b) improper preparation for electropolishing; and (c) surface contamination.
- High Surface Carbon Levels caused by: (a) contamination of cleaning and electropolishing fluids; (b) contamination of processing gases; (c) absorption from contaminated ambient atmosphere; and (d) contamination from packaging.
- Low Cr/Fe Ratios ESCA or Auger caused by: (a) improper electropolishing process; and (b) improper analytical technique.
Cleaning Methods
- Electropolishing 300 & 400 Series Stainless Steel
- Passivating
- Deburring
- Edgeforming
- Titanium, Chemical and Electropolishing of Nitinol
- Titanium Anodizing and Etching Service
Benefits of Electropolishing
- Provides a clean, bright and polished metal surface.
- Improves microfinish.
- Reduces impurities and crack-forming inclusions that can compromise weld strength.
- Allows superior surface preparation for welding.
- Newer electrolytes and advanced parts-handling techniques improve production yields on a wide range of metal products.
- Resists corrosion.
- Removes hydrogen, thus counteracting metal fatigue.
Factors Affecting Surface Finish
- Metal properties and composition
- Temperature
- Electrolytic bath agitation
- Anode (positive terminal) and cathode (negative terminal) reaction.
- Current
- Length of time of current flow
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