Electropolishing with Special Reference to Two Commercial Types of Polishing Apparatus for Metallographic Purposes

In 19 10 S P I T A L S K Y ( 1 ) described, in a Germ an patent then granted, an electro­ lytic process which imparted to silver a highly increased lustre in the course of a few minutes. This, according to the description of the process, constitutes the first known exam ple of electrolytic polishing (electropolishing) that has come to the author’s notice. The silver was placed as anode in an alkaline bath containing silver cyanide, and the current density was about 1 amp/dm2— quite typical data. The next twenty years saw the appearance from time to time of works touching more or less consciously on the idea of electropolishing, mostly in connection with electrolytic pickling or cleaning. In 1930 the Societe Le M ateriel Telephonique (2) was granted a French patent for electrolytic polishing methods for nickel, copper, aluminium and molybdenum. The method was developed by F IG O U R and JA C O U E T , and, for the latter, this work became the introduction to an important career of research in this field. Oddly enough, however, the patents remained almost unnoticed in spite of the fact that they contained all such elements as were essential to the later development. Not till 19 35 did JA C Q U E T really appear in the lim elight with a work on electropolishing of copper in phosphoric acid solution (3), and, since then, a steadily increasing number of publica­ tions has appeared from all quarters and now totals more than 200 (4). In the middle of the thirties the Battelle M emorial Institute and some Am erican firms began a purely technical development, which has resulted in a number of industrially well-suited methods for metals (5). A t the present time it is possible to electropolish 15 different metals and a number of a lloys; of the alloys it holds good that a really brilliant lustre can only be achieved if they do not contain more than a single phase, as in the case of a lfa brass or austenitic steel; polyphase alloys exhibit a more or less matt appearance, which is due to relief polishing caused by the different dissolving rates of the various phases. No attempt should be made at m aking a too direct comparison between the appearance of a mechanically polished object and that of an electropolished. The special bright mirror finish which characterizes m echanically polished (short: mechanopolished) metals because of their quite plane surfaces cannot usually be achieved by means of electropolishing unless the object has previously been polished to almost the same brightness by mechanical m eans; this is due to the fact that electropolishing leaves the surface slightly w avy since scratches, etc. are only smoothed, not planed. But, on the other hand, electropolishing often imparts to metals an especially warm and beautiful lustre which cannot be achieved mechanically. Often mechanopolishing covers up small impurities, porosities and other defects simply because a metallic film is applied to the place by the mechanical action. The polishing does not reach to the bottom. The electrolytic method polishes even submicroscopic unevennesses and uncovers all impurities. For this reason an electropolish will often appear less clean in the microscope than a mechanopolish. It is especially important that electropolishing removes the uppermost layers of the surface; consequently the surface w ill possess no stresses and have no cold-worked