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#125 Surface Adjustments - Electrolytic Polishing

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August 3, 2012

Electrolytic polishing is a process of providing flat and glossy surface on the subject material by applying electrical current between the material as an anode and another appropriate cathode submerged in an ca. The Fig.1 shows the electrolytic polishing system principle. The system is comprised of a DC power supply and its control device, an electrolysis tank and agitating device (the subject material is often rocked within the electrolyte solution), and a bath solution temperature control system (pre-heating the bath solution prior to the process, and cooling during the process).

There are various theories regarding the electrolytic polishing principle. Although the process is not yet precisely clarified, one of the theories explains that a highly viscous oxidation layer is formed on the surface by a reaction when the subject metal becomes anodic within the electrolyte solution. Boundary of this oxidation surface layer and the electrolyte is flat, so the oxidation layer on concave section of the substrate metal to be polished will be thicker, and the convex section would be less thick. The electrical current will be concentrated on the convex sections since electrolyte has small electrical resistance, but the oxidation layer has larger electrical resistance. Therefore, it is thought that the surface flattening occurs due to preferential dissolution of the minute convex sections.

[Fig.1]Electrolytic Polishing Device

It is difficult for the electrolytic polishing to achieve macro level surface flattening as shown in Fig.2, but is more suitable for micro level flat surfacing, in other words mirror gloss surfacing.

Mechanical polishing can flatten the surfaces by removing concaves and convexes, and macro and micro level flattening are possible by selecting appropriate abrasives. However, with mechanical polishing, surface layer crystals are destroyed and altered layers and hardened layers are formed. This causes microscopic visual blemishes due to micro polishing traces and embedded micro crystalline structures remaining on the surfaces. The electrolytic polishing, on the other hand, the surfaces are dissolved, not causing such altered layers on surfaces, leaving chemically clean surfaces with little blemishes.

[Fig.2]Micro Level - Macro Level Polished Surfaces

However, there are relatively few types of metals that electrolytic polishing is applied, limited to Aluminum, Stainless Steel, Copper and its alloys. The reason for this is that the electrolytic polishing process is largely affected by the subject specimen's material purity, composition, construction, and heat treatment histories, as well as difficulties in the selection of electrolyte solutions and application conditions. The electrolyte baths are chosen based on the subject metal types, surface reflectance required. For Aluminum which is an amphoteric metal, either acidic or alkaline baths are used. In general, a bath solution mainly composed of phosphoric acid with sulfuric acid and chromic acid added were in common use, but a bath solution without phosphoric acid and chromic acid are now in use.

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