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Electropolishing Stainless Steels Electropolishing Stainless Steels Materials and Applications Series, Volume 11 E l ect r o P o l is h ing stain l ess stee l S Euro Inox Euro Inox is the European market development asso- Full members ciation for stainless steel. Acerinox Members of Euro Inox include: www.acerinox.es • European stainless steel producers ArcelorMittal Stainless Belgium • national stainless steel development associations ArcelorMittal Stainless France • development associations of the alloying element www.arcelormittal.com industries Outokumpu The prime objectives of Euro Inox are to create aware- www.outokumpu.com ness of the unique properties of stainless steel and to further its use in existing applications and in new mar- ThyssenKrupp Acciai Speciali Terni kets. To achieve these objectives, Euro Inox organises www.acciaiterni.it conferences and seminars, issues guidance in printed ThyssenKrupp Nirosta and electronic form to enable architects, designers, www.nirosta.de specifiers, fabricators and end users to become more familiar with the material. Euro Inox also supports Associate members technical and market research. Acroni www.acroni.si British Stainless Steel Association (BSSA) www.bssa.org.uk Cedinox www.cedinox.es Centro Inox www.centroinox.it Informationsstelle Edelstahl Rostfrei www.edelstahl-rostfrei.de International Chromium Development Association (ICDA) www.icdachromium.com ISBN 978-2-87997-310-4 International Molybdenum Association (IMOA) 978-2-87997-311-1 German version www.imoa.info 978-2-87997-312-8 Finnish version Nickel Institute 978-2-87997-313-5 French version www.nickelinstitute.org 978-2-87997-314-2 Italian version 978-2-87997-315-9 Dutch version Paslanmaz Çelik Derneği (PASDER) www.turkpasder.com 978-2-87997-316-6 Polish version 978-2-87997-317-3 Spanish version Polska Unia Dystrybutorów Stali (PUDS) 978-2-87997-318-0 Swedish version www.puds.pl 978-2-87997-319-7 Czech version SWISS INOX 978-2-87997-320-3 Turkish version www.swissinox.ch E l ect r o P o l is h ing stain l ess stee l S Contents Electropolishing Stainless Steels 1. Introduction 2 First Edition 2010 2. The principle 4 (Materials and Applications Series, Volume 11) 3. Process steps 6 © Euro Inox 2010 3.1 Metal preparation 7 3.2 Electropolishing 7 3.3 Post-treatment 8 4. Electropolishing vs. other surface-finishing techniques 9 4.1 Mechanical polishing 9 Editor 4.2 Electroplating 10 Euro Inox 5. Specifying electropolished finishes 11 Diamant Building, Bd. A. reyers 80 6. Typical applications 12 1030 Brussels, Belgium 7. Glossary 15 Tel.: +32 2 706 82 67 8. References 17 Fax: +32 2 706 82 69 E-mail: [email protected] Internet: www.euro-inox.org Author Alenka Kosmač, Brussels (B) Acknowledgements Euro Inox wishes to thank Mr. Siegfried Pießlinger– Schweiger, Poligrat (D) and Mr. John Swain, Anopol (UK) for their contributions and their critical reading of the draft publication. Photos Copyright notice Cover page photograph: Packo Surface Treatment, This work is subject to copyright. Euro Inox reserves Diksmuide (B) all rights of translation in any language, reprinting, re-use of illustrations, recitation and broadcasting. No Disclaimer part of this publication may be reproduced, stored in Euro Inox has made every effort to ensure that the a retrieval system or transmitted in any form or by any information presented in this document is techni- means, electronic, mechanical, photocopying, record- cally correct. however, the reader is advised that the ing or otherwise, without the prior written permission material contained herein is for general information of the copyright owner, Euro-Inox. Violations may be purposes only. Euro Inox, its members, specifically subject to legal proceedings, involving monetary dam- disclaim any liability or responsibility for loss, dam- ages as well as compensation for costs and legal fees, age, or injury, resulting from the use of the informa- under luxembourg copyright law and regulations tion contained in this publication. within the European Union. 1 E l ect r o P o l is h ing stain l ess stee l S 1 Introduction Electropolishing is a chemical surface- electropolishing has become a common finishing technique (see box), by which treatment for stainless steel in industries in metal is electrolytically removed, ion by which corrosion-resistance and cleanability ion, from the surface of a metal object [1]. requirements are particularly high. Typical The primary objective is to minimize micro- applications are found in the pharmaceu- roughness, thus dramatically reducing the tical, biochemical and food-processing risk of dirt or product residues adhering industries. and improving the cleanability of surfaces. Electropolishing is also used for deburring, Since electropolishing involves no mechan- brightening and passivating. ical, thermal or chemical impact, small and mechanically fragile parts can be treated. The process exposes an undisturbed, met- Electropolishing can be applied to parts of allurgically clean surface. Possible unwant- almost any shape or size. ed effects of mechanical surface treatment – mechanical and thermal stress, embed- ding of particles and surface roughening – are avoided or reversed. The inherent corrosion-resistance of a given stainless steel grade is fully used. For these reasons, 2 E l ect r o P o l is h ing stain l ess stee l S Chemical surface treatments: when to consider what and why There is a range of different chemical surface the formation of the passive layer, ensuring treatments. Although these all contribute to that it builds up quickly to its full strength, producing a clean metallic surface and have under controlled conditions. Passivation is overlapping effects, they have distinctly dif- performed in diluted nitric acid, the treat- ferent purposes: ment time varying between 15 minutes and 1 hour. Pickling Decontamination or acid cleaning Pickling removes oxides specifically heat This removes ferrous particles which would tint (from welding) and other discolourations corrode if left on the stainless steel surface – or corrosion products. It creates a clean for example when stainless steel parts have metallic surface that provides the conditions been exposed to ferrous contaminants (grind- necessary for the natural self-passivation of ing dust, rust particles from adjacent work on stainless steel to take place. Typical compo- carbon steel, abrasion from tools, etc.)1 sitions involve nitric acid and hydrofluoric acid. Treatment time depends on the tem- Electropolishing perature of the solution and the degree of contamination. Electropolishing is applied to clean metallic surfaces, to reduce micro-roughness and Passivating produce a number of other highly desirable effects, described in this publication. The The self-passivation of stainless steel nor- process depends on the principle of elec- mally takes place spontaneously in the trolysis, involving a flow of current and a presence of oxygen from air or water. solution of mostly sulphuric acid and ortho- however, it can take a couple of days before phosphoric acid as an electrolyte. Treatment the passive layer develops its full thickness. times are usually between 2 and 20 minutes. Chemical passivation treatment speeds up 1 cf. CROOKES, roger, Pickling and Passivating Stainless Steel, luxembourg: Euro Inox, Second edition 2007 (Materials and Applications Series, Volume 4) – http://www.euro-inox.org/pdf/map/Passivating_Pickling_EN.pdf 3 E l ect r o P o l is h ing stain l ess stee l S 2 The principle Electropolishing removes metal from a (cathodic) terminal is connected to a suit- workpiece by passing an electric current able conductor. Both positive and negative while the workpiece is submerged in an terminals are submerged in the electrolyte, electrolyte of a specific composition. The forming a complete electrical circuit. The process is essentially the reverse of elec- current applied is direct (DC) current. troplating. In a plating system, metal ions are deposited from the solution onto the As the adjoining illustration shows, the workpiece. In an electropolishing system, metal part is charged positive (anodic) and the workpiece itself is eroded, adding metal immersed in the chemical bath. When cur- ions to the solution. rent is applied, the electrolyte acts as a conductor (“tool”) to allow metal ions to be A typical electropolishing installation is removed from the part. While the ions are similar in appearance to an electroplating drawn toward the cathode, the majority of line. A power source converts AC current to the dissolved metals remain in solution. DC at low voltages. A tank fabricated from Some ions are deposited in sludge form on plastic or lead-lined tanks is commonly the cathodes, which require regular clean- used to hold the chemical bath. A series ing for efficiency. Gassing, in the form of of lead, copper or stainless steel cathode oxygen, occurs at the metal surface, fur- plates are lowered into the bath, to the thering the electrolytic action. negative (-) side of the power source. A part or group of parts is fixed to a rack made The quantity of metal removed from the of titanium, copper or bronze. That rack in workpiece is proportional to the current Electropolishing is an turn is coupled to the positive (+) side of the applied, the efficiency of the electrolyte efficient method of deburring – even in power source. and the exposure time. In the course of parts that would be dif- electropolishing, burrs and other projec- ficult to treat mechani- The workpiece is thus connected to the pos- tions become areas of very high current cally. Photo: Poligrat, itive (anodic) terminal, while the negative density and are preferentially eroded. The Munich (D) workpiece process parameters are set to control the amount of metal removed, so dimensional tolerances are maintained. 4 ElECTroPolIShING STAINlESS STEElS rectifi er rack Tank Cathode Cathode Workpiece The workpiece being the (Anode) anode of the electro- chemical process, ma- terial is removed from heater the surface [3]. In the case of stainless steel, the various ele- ments, which make up the alloy, are removed at different rates.
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