Stainless Steel in Contact with Other Metallic Materials Electrolyte Metal 1 Metal 2 Anode Cathode e- ISBN 978-2-87997-263-3 Diamant Building · Bd. A. Reyers 80 · 1030 Brussels· Belgium ·Tel. +32 2 706 82-67 · Fax -69 · e-mail [email protected] · www.euro-inox.org Materials and Applications Series, Volume 10 C ONTACT WITH OTHER METALLIC MATERIALS Euro Inox Euro Inox is the European market development asso- Full members ciation for stainless steel. Members of Euro Inox include: • European stainless steel producers Acerinox www.acerinox.es • national stainless steel development associations • development associations of the alloying element Outokumpu www.outokumpu.com industries The prime objectives of Euro Inox are to create aware- ThyssenKrupp Acciai Speciali Terni www.acciaiterni.com ness of the unique properties of stainless steel and to fur- ther its use in existing applications and in new markets. ThyssenKrupp Nirosta www.nirosta.de To achieve these objectives, Euro Inox organizes confer- ences and seminars and issues guidance in printed and ArcelorMittal Stainless Belgium ArcelorMittal Stainless France electronic form to enable architects, designers, speci- www.arcelormittal.com fiers, fabricators and end users to become more familiar with the material. Euro Inox also supports technical and market research. Associate members 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 Institut de Développement de l’Inox (I.D.-Inox) www.idinox.com International Chromium Development Association (ICDA) www.icdachromium.com International Molybdenum Association (IMOA) www.imoa.info Nickel Institute www.nickelinstitute.org Paslanmaz Çelik Derneği (PASDER) www.turkpasder.com Polska Unia Dystrybutorów Stali (PUDS) www.puds.pl SWISS INOX www.swissinox.ch C ONTACT WITH OTHER METALLIC MATERIALS Table of contents Stainless Steel in Contact with Other Metallic Materials 1 Introduction 2 Materials and Applications Series, Volume 10 2 The principles of galvanic corrosion 3 ISBN 978-2-87997-263-3 3 Relevant factors and examples 5 © Euro Inox 2009 3.1 Electrolyte resistance 5 3.2 Wetting duration and environments 6 Translated and adapted from ARLT, N. / BURKERT, A / 3.3 The kinetics of electrode reactions 8 ISECKE, B., Edelstahl Rostfrei in Kontakt mit anderen 3.4 Cathode and anode area 8 Werkstoffen (Merkblatt 829), Düsseldorf, Informations- 4 Practical experience in different applications 10 stelle Edelstahl Rostfrei, 4th edition 2005 4.1 Water and sewage treatment 11 4.2 Components in atmospheric conditions 14 Publisher 4.3 Stainless steel in building and construction 15 Euro Inox 4.4 Stainless steel in transport applications 18 Diamant Building, Bd. A. Reyers 80, Frequently asked questions 20 1030 Brüssel, Belgium 5 Preventing galvanic corrosion 22 Phone +32 2 706 82 65 Fax +32 2 706 82 69 6 Literature 23 Disclaimer Euro Inox has made every effort to ensure that the infor- mation presented in this document is technically correct. However, the reader is advised that the material con- tained herein is for general information purposes only. Euro Inox, its members, specifically disclaim any liabili- ty or responsibility for loss, damage, or injury, resulting from the use of the information contained in this publi- cation. Photo credits: Copyright notice Atomium asbl / vzw, Brussels (B) This work is subject to copyright. Euro Inox reserves all Centro Inox, Milan (I) rights of translation in any language, reprinting, re-use of Bundesanstalt für Materialprüfung und -forschung, illustrations, recitation and broadcasting. No part of this Berlin (D) publication may be reproduced, stored in a retrieval David Cochrane, Sidcup (UK) system or transmitted in any form or by any means, Benoît Van Hecke, Hasselt (B) electronic, mechanical, photocopying, recording or Outokumpu, Tornio (FIN) otherwise, without the prior written permission of the Thomas Pauly, Brussels (B) copyright owner, Euro-Inox, Luxembourg. Violations may Christoph Seeberger, Munich (D) be subject to legal proceedings, involving monetary ThyssenKrupp Nirosta GmbH, Krefeld (D) damages as well as compensation for costs and legal Schöck Bauteile GmbH, Baden-Baden (D) fees, under Luxemburg copyright law and regulations Viega GmbH & Co. KG, Attendorn (D) within the European Union. 1 C ONTACT WITH OTHER METALLIC MATERIALS 1 Introduction Complex design requirements can make sion potential of the metals in contact; hence it necessary to combine different metallic there is usually a corrosion hazard for the materials within the same component. Also, partner material. chance combinations can often be found, The risk of galvanic corrosion occurring governed only by the availability of, for in- depends, however, on a multitude of factors. stance, fasteners or shims. In certain cir- Besides the materials used, environment cumstances, such mixed-material designs and design are crucial. It is therefore diffi- can lead to corrosion in one of the partner cult to make a priori judgments about materials. This phenomenon includes gal- the compatibility of materials. The present vanic corrosion1, in which two different publication describes the principles of galva- metals form a galvanic couple. nic corrosion and the main parameters As a result of the formation of galvanic that allow designers to estimate corrosion elements, accelerated corrosion of the less risk. noble material can occur. The latter may then suffer a corrosion rate far higher than that to be expected without any contact with the nobler partner metal. Corrosion-related dam- age such as unacceptable deterioration of appearance, leaking tubes or failing fasten- ers can drastically reduce the service life of a component and lead to premature re- placement. In most technical applications, 1Accelerated corrosion of a metal, due to the effect of a corrosion ele- ment. Other factors include concentration elements, aeration elements stainless steel has the more positive corro- and active/passive elements. 2 C ONTACT WITH OTHER METALLIC MATERIALS 2 The principles of galvanic corrosion For galvanic corrosion to occur, there naturally occur in the metal in isolation; how- must be: ever, the corrosive attack on the anode is • different corrosion potentials of greatly accelerated. In some cases, the for- the metals within a given system; mation of galvanic elements can lead to cor- • a conductive connection between the rosion in materials that would otherwise be two metals; corrosion resistant in the environment in • an electrically conductive humidity question. This can be the case for passive film (electrolyte) connecting both materials such as aluminium, which can be metals locally polarised in a certain environment. In Figure 1 shows the three prerequisites in such cases, localised corrosion phenomena graphic form. such as crevice corrosion or pitting corrosion If galvanic corrosion occurs, the less no- can be observed, which would not have ble material – the anode – is preferentially occurred without the shift in potential attacked whilst the more noble material – caused by the formation of galvanic ele- the cathode – is even protected against ments. corrosion. In fact, the principle of cathodic protection is based on sacrificial anodes providing protection from corrosion. The contact of two metals with different potentials in an electrically conductive solu- tion leads to a flow of electrons from the anode to the cathode. The electro-chemical reactions are the same as those that would Electrolyte Metal 1 Metal 2 e– Anode Cathode Figure 1 shows the three prerequi- sites in graphic form. 3 C ONTACT WITH OTHER METALLIC MATERIALS Contrary to widespread belief, the differ- ence of potential in an electrochemical cell Graphite Alloy 625/ C-276 alone is not a good indicator of the actual Superaustenitic stainless steel risk of galvanic corrosion. It only indicates Titanium Alloy 400 whether or not such a risk has to be taken Austenitic stainless steel grade 1.4404 (316 L), passive Nickel into account. In this context, it should be Ni-Al Bronze remembered that the numerous published 90/10 Cupro-Nickel Al-brass tables of the standard potentials of metals Copper only provide an approximation of differences Austenitic stainless steel casting Lead of potential. The decisive factor is not the Tin Carbon steel difference of potential observed under stan- Cast steel dardised experimental conditions but rather Al-2.7 Mg Zinc the actual difference of potential under real Aluminium Magnesium operating conditions. This is why empirical tables of galvanic series have been produced -2000 -1500 -1000 -500 0 500 Potential (mV SCE) for typical environments such as sea water. These position the potential of various met- als in a given environment (Figure 2). Figure 2: Awareness of the prerequisites of gal- The Galvanic Series in vanic corrosion and a proper understanding sea-water at 10 °C [11] of the examples in Figure 3 make it possible to determine preventive action, which will be discussed in section 5. Figure 3: Conditions in which galvanic corrosion cannot occur Galvanic corrosion cannot occur … … without electrically … in metals with no … without connection by an electrolyte conductive joints difference of potential Coating Electrolyte Electrolyte Electrolyte Electrolyte Metal 1 Metal 2 Metal 1 Metal 2 Metal 1 Metal 2 Metal 1 Metal 2 Insulator (Metal 1 = Anode, Metal 2 = Cathode) 4 C ONTACT WITH OTHER METALLIC MATERIALS