This document provides a description of the electropolishing procedures performed by Terra Universal. It also specifies a series of criteria for evaluating the integrity and cleanliness of the electropolished surfaces. Because these electropolishing procedures are followed on all work performed by Terra Universal, the results may be understood as indicative of standard Terra Universal electropolishing, and the specified evaluation criteria may be used as acceptance standards for work performed by Terra Universal.
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 the anodic part is removed ion by ion, yielding a nickel and chromium-rich surface free of microscopic “peaks” or “valleys” that could lead to metal fatigue or contamination. Optimal results depend on careful control over the current density, the precise chemical composition of the electrolytic solution, and 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 offers excellent light reflection, yielding a bright, smooth and uniform polish.
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 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 then dried and buffed with a soft, non-particulating cloth.
C. Process Controls
Some of these variables are relatively easy to quantify and monitor, although some variation must be exercised in response to a given sample in order to produce the optimal results.
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.
Performance Evaluation Parameters
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 nonuniform 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 (such as a No. 8 mil finish) sample 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.