Electropolishing

Introduction to Electropolishing:

Electropolishing is a reverse electrolysis process, it is an electrochemical process which produces smoothing and brightening to a metal surface by removing surface metal ion by ion. It provides a chemically and physically clean surface, the surface becomes smoother and any mechanical surface asperities, contaminants and free irons will be removed, which may be detrimental to the future performance and appearance of a Stainless Steel product. This will improve the corrosion resistance, reduce the product adhesion and contamination buildup, de-burr machined edges, corners and holes as well as removes any imbedded iron from the manufacturing process and create a lustrous finish. Electropolishing permanently removes heat discoloration and flux marks from welding. Electropolishing is commonly done on Stainless Steel as a final finish. Electropolishing eliminates Hydrogen Embrittlement and removes stress from springs and stampings by removing microscopic nicks and scratches from the surface. Electropolishing is used as an inspection tool to detect surface cracks or imperfections in castings and forgings. In addition, Electropolishing passivates the surface of the metals as per the ASTM A-967 specification. It is the most effective commercial method for passivating stainless steel. Electropolished Tools steels offers greatly enhanced protection from corrosion and tarnishing. Two important advantages of electropolishing are the speed and uniformity of the process, readily translated into important savings, particularly when treating complicated shapes.

Benefits of Electropolishing:

Better Physical Appearance:

• No fine directional lines from abrasive polishing
• Excellent light reflection and depth of clarity
• Bright, smooth polish; uniform luster of shaped parts

Enhanced Mechanical Properties:

• Less friction and surface drag
• Increased production and duty cycles in process equipment. Electropolishing greatly reduces fouling, plugging, scaling and product build-up
• Surface retains the true grain structure and properties of the bulk metal
• Fatigue strength is not reduced. Electropolishing allows the true fatigue strength of a part to be accurately determined
• Higher fatigue strength can be promoted by particle-blasting the surface to reintroduce compressive stress without losing electropolishing's advantages
• Stress-relieving of the surface
• Reduces galling of threads on stainless and carbon steel and other alloys

Better Corrosion Protection:

Electropolishing yields maximum tarnish and corrosion resistance in many metals and alloys. Stainless steel contains metallic and non-metallic inclusions, which are unavoidably included during manufacture. Mechanical polishing not only fails to remove inclusions, but also tends to push them further into the surface and even increase them by further pick-up of abrasive materials. These inclusions eventually can become points of corrosion.

Ease of Cleaning:

• Substantially reduces product contamination and adhesion due to the microscopic smoothness of an electropolished surface (much like a glass surface)
• Decreases cleaning time. Electropolished surfaces can be effectively hydroblasted in less time and with less pressure. Some companies report that electropolished process equipment surfaces have reduced cleaning time by more than 50 percent.
• Improves sterilization and maintenance of hygienically clean surfaces for food, drug, beverage and chemical processing equipment
• Provides best passivation of stainless steel. Unipotentializes stainless steel with the oxygen absorbed by the surface, creating a monomolecular oxide film
• Decarburizes metals
• Removes cold-worked metal oxides

Surface Roughness:

Surface roughness is commonly measured or classified as Ra (Roughness average) or Rq (the equivalent of RMS -- Root Mean Square). Both are measured in microinches and denote the smoothness of ground or machined surfaces. For comparison, a Ra reading is approximately 87.5 percent of an Rq (RMS) reading. Roughness measurements have no real relationship to how easily an electropolished surface can be cleaned after use or to its non- contaminating, non-particulating or non-stick properties. Surface roughness is usually measured with a profilometer. This instrument cannot accurately read the distances between the "peaks" and "valleys." Electropolishing may reduce the peaks from substantial points to insignificant mounds without changing the peak-to-peak distance at the same ratio. However, microscopic examination of the surface will show up to a 90 percent reduction in surface area and up to a 50 percent improvement in profilometer readings. Smoothness is not an independent variable in surface definition. It is one factor of an important subject that is referred to as "surface metallurgy." Smoothness specifications, based on gages, can be achieved by both electropolishing and mechanical abrasive finishing techniques.

Surface Chemical Analysis:

One of electropolishing's primary benefits is the chromium enrichment of the surface resulting from properly controlled processing. A consistent chromium-rich oxide layer only is attained when the atomic concentration of chrome exceeds the iron in the surface layer as demonstrated by the Auger Electron Spectroscopy (AES). AES analysis also measures depth and extent of surface passivation. Electropolishing maximizes surface passivation because the surface contains very low levels of iron (Fe) in zero oxidation states. Other surface chemistry analysis can be made for sulfide inclusions, precipitated carbides and other similar impurities, all of which affect the final appearance of electropolished surfaces. The end-grain surfaces of free-machining stainless grades such as Types 303 and 416 will appear frosty after electropolishing due to the removal of the sulfide inclusions. Type 302 stainless steel will show pitting if the annealing process failed to redissolve the precipitated carbides.

Friction Reduction:

Electropolishing reduces the coefficient of friction of metals. The process removes or rounds off the small surface asperities, yielding a coefficient of friction that measures approximately one-fourth of the coefficient registered by a mechanically finished surface.