A smooth, highly reflective electropolished surface is determined to a large extent by the surface conditions of the metal and the process controls.
Base metal conditions that can result in less than optimum electropolishing finishing characteristics include the presence of non-metallic inclusions, improper annealing, overpickling, heat scale, large grain size, directional roll marks, insufficient cold reduction or excessive cold working. These conditions may be inherent in the metal as it comes from the mill. During electropolishing, metal is removed, revealing these flawed characteristics.
The degree to which the electropolishing process is controlled determines the final quality and consistency of the finish. Many electropolishing shops employ, at best, a minimum number of controls, or no controls at all. A lack of process controls produces inconsistent and unpredictable quality. While some variables are functions of technology, others fall under what many call "the art of electropolishing."
It is imperative that the proper electrolyte be used, that its temperature be maintained precisely (heated or cooled as necessary) and that its chemistry be monitored constantly. Critical factors in the chemistry are the specific gravity (an indicator of water content and a major consideration in hygroscopic electrolytes), the acid concentration and the metals content. A supply of clean, ripple-free DC power must be available to drive the process, as well as appropriately sized cables and connectors to the anodes and cathodes. The DC power must be applied at the correct voltage and current density (amperes/square foot).
The "art" of electropolishing is the ability of a technician with many years of experience to configure a cathode for optimum polishing in inaccessible areas, corners, and areas of low current density. Equally important is the knowledge of where, when and how to agitate either the electrolyte or the part in order to prevent gassing streaks, flow marks and similar undesirable markings.
For many products, electropolishing's mirror-like luster is the goal. Many others, however, require additional functional or technical advantages of electropolishing not attainable by mechanical means. Because there is no mechanical disturbance of the surface during the metal removal process, the electropolished surface possesses the true grain structure and properties of the bulk metal.
In contrast, all mechanical polishing procedures leave a layer of disturbed structure. The mechanically finished surface will not have the properties reported for the bulk metal, regardless of the thickness of the disturbed surface or the mechanical process employed.
The surface of a metal is often described as that place where the metal ceases to exist. This certainly can be said of an electropolished surface, but not for a surface that has been mechanically finished by cutting, smearing, skin rolling, buffing, drilling, boring, reaming, broaching or grinding.
Basic Mill Plate Finishes for Stainless Steel
There are eight basic stainless steel mill plate finishes available. Finishes 3, 4, 6, 7 and 8 are produced mechanically by using abrasive compositions and buffing wheels.
Unpolished No. 1. This is a very dull finish produced by hot-rolling the steel on hand sheet mills to specified thicknesses, followed by annealing and descaling. The surface is microscopically very porous and uneven, allowing a level of contamination pick-up that is many times its geometric surface area. Used in industrial applications where resistance to heat or corrosion, not a smooth finish, is desired.
Unpolished No. 2D. This is a dull finish produced on hand sheet mills or continuous mills by cold rolling the metal to the specified thickness, followed by annealing and descaling. The dull finish may result from the descaling operations or may be developed by a final light cold roll pass on dull rolls. This finish is favorable to the surface retention of lubricants in deep drawing operations, and it is generally used in forming deep-drawn articles that may be polished after fabrication.
Unpolished No. 2B. This is a bright, cold-rolled finish produced similarly to the No. 2D finish, except the annealed and descaled sheet receives a final light cold roll pass on polished rolls. The metal grains are flattened, which facilitates removal of smearable contamination. However, the etched boundaries between the grains are only partially sealed, resulting in a network of sub-surface crevices. Contaminants lodging in these crevices are protected from contact with cleaning agents, leading to possible subsequent migration of trapped contaminants onto the cleaned surface (bleeding). No. 2B is a general purpose finish used for all but the most difficult deep-drawing applications, and it is more readily polished than No. 1 or No. 2D.
Polished No. 3. This is an intermediate polished finish for use where a semi-finished polished surface is required for subsequent finishing operations following fabrication, or as a final finish with a 50 or 80 grit abrasive compound. (In the case of metal sheets or articles that will not be subject to additional finishing and polishing, a No. 4 finish is ideal.)
Polished No. 4. This is a general purpose mechanically polished stainless steel finish that is widely used for architectural panels and trim as well as for restaurant, dairy and kitchen equipment. Initial grinding is followed by coarser abrasives, with sheets eventually finished out with 100-150 mesh abrasives. Although microscopically flat, the grain of Polished No. 4 stainless steel still contains deep grooves and other microscopic cavities that entrap and retain contaminants.
Buffed No. 6. A dull satin finish possessing low reflectivity, a Buffed No. 6 finish is produced with a greaseless compound and 200-grit abrasive, and is top-dressed with chrome rouge.
Buffed No. 7. This very reflective finish is produced by buffing a surface that first was refined with 220-grit abrasives to approximate a No. 6 finish. It is then buffed lightly with a white chrome rouge without removing the satin finish lines.
Buffed No. 8. This is the most reflective mechanical finish. It is obtained by polishing with progressively finer abrasives (320-grit and finer), followed by extensive buffing with very fine white chrome bar buffing compounds. To the unaided eye, the surface appears free of grit lines from previous grinding operations.
Electropolished Surfaces. Electropolished surfaces are extremely smooth, macro-scopically flat, microscopically featureless, and exhibit high high luster, reflectivity and brightness. Electropolishing process substantially reduces the surface area available for contamination pick-up and eliminates all micro-cracks and internal crevices.
Differences Between Polished and Buffed Milled Finishes, Abrasive Grit Numbers and Surface Roughness (Before and After Electropolishing)
|Mill No.||Grit No.||Before Electropolishing|
Surface Roughness, Ra
Surface Roughness, Ra
|3||60||3.56 max||140 max||1.78-2.25||70-90|
|4||120||1.14 max||45 max||0.57-0.75||23-30|
|4||180||0.64 max||25 max||0.32-0.40||13-16|
*Values are approximate. Electropolishing generally reduces surface roughness readings of a non-electropolished surface by 50 percent.
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 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, an 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.
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.
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.
Various alloys are used in most castings, making this particular product less well suited to electropolishing. A major exception to this rule is stainless steel.
Electropolished stainless steel castings will be brightened but will not be as smooth as strip stock nor obtain the same mirror finish. The process decontaminates and passivates the metal surface. Note: During electro-polishing, the surface of the casting will be removed and may expose sub-surface porosity.
Investment castings are better candidates for electropolishing than sand castings, primarily because of the inherently smoother surface of investment grade castings. However, sand castings will electropolish to a clean and bright surface.
Electropolishing cannot smear over or otherwise conceal defects such as seams and non-metallic inclusions in metals. In addition, heavy orange peel, mold-surface texture and rough scratches are not removed by a practical amount of electropolishing and thus require an initial "cutdown" with abrasives. Multiphase alloys in which one phase is relatively resistant to anodic dissolution usually are not well suited to an electropolishing treatment.