Corrosion Behavior of Duplex Stainless Steels in Acidic Chloride Solutions Studied with Micrometer Resolution Marc Femenia i Nobell Doctoral Thesis Department of Materials Science and Engineering Division of Corrosion Science Royal Institute of Technology SE 100 44 Stockholm Sweden Stockholm 2003 ISRN KTH MSE 03 08 SE KORR AVH ISBN 91 7283 459 5 Doktorsavhandling som med tillst nd av Kungliga Tekniska H gskolan i Stockholm framl gges till offentlig granskning f r avl ggande av teknisk doktorsexamen fredagen den 28 mars 2003 kl 13 00 i Kollegiesalen Valhallav gen 79 Kungliga Tekniska H gskolan Fakultetsopponent r Dr Roland Oltra Laboratoire de Recherches sur la R activit des Solides UMR 5613 BP 47 870 21078 Dijon Cedex France Marc Femenia i Nobell 2003 Corrosion Behavior of Duplex Stainless Steels in Acidic Chloride Solutions Studied with Micrometer Resolution Department of Materials Science and Engineering Division of Corrosion Science Royal Institute of Technology SE 100 44 Stockholm Sweden Abstract The local corrosion behavior of duplex stainless steel DSS is affected by a wide variety of factors Localized corrosion of DSS frequently starts at micrometer scale inclusions or precipitates which are often segregated in the austenite ferrite boundary regions Moreover due to the partitioning of the key alloying elements of ferrite Cr and Mo and austenite N and Ni the local interactions between the phases must also be considered The aim of this doctoral study was to increase the knowledge about the local dissolution behavior of DSS in acidic chloride environments The recent developments of new local probing techniques have opened a new frontier in corrosion science providing valuable local information not accessible in the past The local techniques used include electrochemical scanning tunneling microscopy EC STM scanning probe force microscopy SKPFM magnetic force microscopy MFM and scanning Auger electron Spectroscopy SAES all with micrometer or sub micrometer resolution With EC STM it was possible to monitor local dissolution processes on DSS in situ and in real time MFM was capable of imaging the phase distribution in DSS without the need of the traditional surface etching while SKPFM revealed that the Volta potential difference between the two phases was measurable and significant SAES showed that the composition gradient at the phase boundaries is narrower than 2 m Different types of DSSs have been studied from low alloyed DSS to superduplex Higher contents of Cr Mo and N strengthened both phases as well as the phase boundaries resulting in phases having similar corrosion resistance that showed a more uniform dissolution behavior However the Volta potential difference between the phases proved to be of the same order for all the DSSs studied Austenite was in general associated to regions displaying a more noble Volta potential than ferrite resulting in a higher dissolution rate of the ferrite next to the austenite phase Key words In situ local dissolution electrochemical STM SKPFM MFM SAES duplex stainless steel acidic chloride solution PREFACE The thesis includes the following papers referred to in the text by their roman numerals I In Situ Study of Selective Dissolution of Duplex Stainless Steel 2205 by Electrochemical Scanning Tunnelling Microscopy M Femenia J Pan C Leygraf and P Luukkonen Corrosion Science 43 1939 2001 II Corrosion Behavior of a Duplex Stainless Steel Studied by STM AFM Based Scanning Electrochemical Microscopy J Pan M Femenia and C Leygraf Localized In Situ Methods for Investigating Electrochemical Interfaces S R Taylor A C Hillier and M Seo Editors PV 99 28 p 131 The Electrochemical Society Proceedings Series Pennington NJ 1999 III In Situ Local Dissolution of Duplex Stainless Steels in 1M H2SO4 1M NaCl by Electrochemical Scanning Tunneling Microscopy M Femenia J Pan and C Leygraf Journal of The Electrochemical Society 149 B187 2002 IV Scanning Kelvin Probe Force Microscopy and Magnetic Force Microscopy for Characterization of Duplex Stainless Steels M Femenia C Canalias J Pan and C Leygraf Journal of The Electrochemical Society in press V Characterization of Ferrite Austenite Boundary Region of Duplex Stainless Steels by Scanning Auger Electron Spectroscopy M Femenia J Pan and C Leygraf Manuscript to be submitted In addition there is a conference contribution that has not been included in the thesis In Situ Electrochemical STM Study of Dissolution Behavior of Duplex Stainless Steels in Aggressive Solutions M Femenia P Luukkonen J Pan and C Leygraf Proceedings of Duplex Stainless 2000 Venice Italy pp 261 270 Associazione Italiana di Metallurgia 2000 Table of contents 1 Introduction 1 2 Experimental5 2 1 Materials and Solutions5 2 2 Conventional Electrochemical Techniques6 2 3 Electrochemical Scanning Tunneling Microscopy and Scanning Electrochemical Microsocopy7 2 4 Ex situ Microstructure Characterization10 2 5 Magnetic Force Microscopy and Scanning Kelvin Probe Force Microscopy11 2