Prior to electropolishing, parts are mechanically prepared to ensure optimal results. All welds are ground, deburred, and inspected to ensure that all seams are free of pockets or gaps. Finally, selected surfaces are mechanically buffed to a smooth finish.
Next, the part is fitted with electrodes, immersed in an electrolyte solution, and subjected to a direct electrical current. During this electrolytic process, the metallic surface of the anodic part (in this case, stainless steel) is preferentially dissolved ion by ion, yielding a nickel- and chromium-rich surface that protects against metal fatigue or contamination. Optimal results depend on careful control over the current density, the precise chemical composition of the electrolytic solution, the temperature and agitation of the bath, and the duration of current exposure.
Unlike mechanically finished stainless steel, electropolished surfaces feature no fine directional lines and hence offer less friction and surface drag. The chromium-rich surface results in excellent light reflection, yielding a bright, smooth and uniform polish. The images below illustrate the visual difference between EP and non-EP stainless steel surfaces:
A Few Words About Stainless Steel
The amount of chromium present differentiates “regular” steel from “stainless” steel. Chromium that is exposed to oxygen forms a strongly-bonded film of chromium oxide on the stainless steel, blocking the oxygen from damaging the surface, as well as infiltrating the internal structure. Electropolishing increases the surface chromium, thereby promoting formation of this protective film. Non-stainless-steel, with lower levels of chromium, will eventually rust when exposed to moisture and air.
The difference between 304 and 316 stainless steels, the two types that Terra uses, is the ratio of some of the elements present, such as chromium and nickel. 316 stainless steel is a “cleaner” grade of steel, even less susceptible to corrosion than 304.
Lastly, steel comes in mill finishes. Terra uses types 2B or 4, which simply refers to polishing or brushing steps that give the steel a polished aesthetic “finish.” Electropolishing is the last treatment step after the 304 or 316 steel has been manufactured into its final functional form.
A. Material Selection
B. Precleaning and Postcleaning
Stainless steel parts intended for electropolishing are designed with these requirements in mind. All welds are carefully inspected to ensure continuous seams, free of pits or gaps where the solution could collect, and all hollow members are drilled to permit effective flushing of the chemical solution after electropolishing.
Because the electropolishing process removes only a very thin surface layer (typically between 0.001" and 0.0001"), selected surfaces are mechanically buffed, using progressively finer grits to produce the smoothest possible finish.
Following electropolishing, all traces of the electrolyte solution are thoroughly removed from the part, and any hollow cavities are flushed to eliminate the chance of subsequent outgassing. Surfaces are dried and buffed with a soft, non-particulating cloth. Parts are then immediately wrapped in non-shedding material to guard against fingerprints and other surface contaminants while on their journey to the end-user.
C. Process Controls
For other variables, effective control depends on significant experience. Attention to these considerations, combined with close adherence to the procedures mentioned above, results in a truly superior electropolished finish that combines artistry and technology.
Performance Evaluation Parameters
A. Visual Inspection
Unlike mechanical buffing, which distorts the surface of the metal and may conceal the material's true characteristics, electropolishing reveals the imperfections in the structure of the stainless steel. Electropolishing will accentuate any welding flaws, and a non-uniform appearance indicates a high volume of inclusions or a large-grained grade of stainless steel.B. Micrographs
A better test of the integrity of the surface is provided by photomicrographs of the surface. Although a highly buffed sample (such as a No. 8 mill mirror finish,) and an electropolished sample may appear equally brilliant to the unaided eye, the differences between the two are apparent when they are viewed under very high magnifications. The sample micrographs below, taken at 1,000X, dramatically illustrate the smooth, featureless surface that results from electropolishing.