1、Corrosion Behavior of Duplex StainlessSteels in Acidic-Chloride Solutions Studiedwith Micrometer ResolutionMarc Femenia i NobellDoctoral ThesisDepartment of Materials Science and EngineeringDivision of Corrosion ScienceRoyal Institute of TechnologySE- 100 44 Stockholm, SwedenStockholm 2003ISRN KTH/M
2、SE-03/08-SE+KORR/AVHISBN 91-7283-459-5Doktorsavhandlingsom med tillstnd av Kungliga Tekniska Hgskolan i Stockholm framlgges till offentliggranskning fr avlggande av teknisk doktorsexamen, fredagen den 28 mars 2003, kl. 13:00 iKollegiesalen, Valhallavgen 79, Kungliga Tekniska Hgskolan.Fakultetsoppone
3、nt r Dr. Roland Oltra, Laboratoire de Recherches sur la Ractivit desSolides, UMR 5613, BP 47 870, 21078 Dijon Cedex, France.Marc Femenia i Nobell (2003)Corrosion Behavior of Duplex Stainless Steels in Acidic-Chloride SolutionsStudied with Micrometer ResolutionDepartment of Materials Science and Engi
4、neering, Division of Corrosion ScienceRoyal Institute of Technology, SE-100 44, Stockholm, SwedenAbstractThe local corrosion behavior of duplex stainless steel (DSS) is affected by a widevariety of factors. Localized corrosion of DSS frequently starts at micrometer scale inclusionsor precipitates, w
5、hich are often segregated in the austenite-ferrite boundary regions.Moreover, due to the partitioning of the key alloying elements of ferrite (Cr and Mo) andaustenite (N and Ni), the local interactions between the phases must also be considered.The aim of this doctoral study was to increase the know
6、ledge about the localdissolution behavior of DSS in acidic-chloride environments. The recent developments ofnew local probing techniques have opened a new frontier in corrosion science, providingvaluable local information not accessible in the past. The local techniques used includeelectrochemical s
7、canning 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 r
8、eal time. MFM was capable of imaging the phase distribution in DSS without the needof the traditional surface etching, while SKPFM revealed that the Volta potential differencebetween the two phases was measurable and significant. SAES showed that the compositiongradient at the phase boundaries is na
9、rrower 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 uniformdissolution behavior. However, the
10、 Volta potential difference between the phases proved to beof the same order for all the DSSs studied. Austenite was in general associated to regionsdisplaying a more noble Volta potential than ferrite, resulting in a higher dissolution rate ofthe ferrite next to the austenite phase.Key words: In si
11、tu, local dissolution, electrochemical, STM, SKPFM, MFM, SAES, duplexstainless steel, acidic-chloride solution.PREFACEThe 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 byElectrochemical S
12、canning Tunnelling MicroscopyM. Femenia, J. Pan, C. Leygraf, and P. LuukkonenCorrosion Science, 43, 1939 (2001)II. Corrosion Behavior of a Duplex Stainless Steel Studied by STM/AFM BasedScanning Electrochemical MicroscopyJ. Pan, M. Femenia, and C. LeygrafLocalized In Situ Methods for Investigating E
13、lectrochemical Interfaces, S. R.Taylor, A. C. Hillier, and M. Seo, Editors, PV 99-28 p. 131, The ElectrochemicalSociety Proceedings Series, Pennington, NJ (1999).III. In Situ Local Dissolution of Duplex Stainless Steels in 1M H2SO4+ 1M NaCl byElectrochemical Scanning Tunneling MicroscopyM. Femenia,
14、J. Pan, and C. LeygrafJournal of The Electrochemical Society, 149, B187 (2002)IV. Scanning Kelvin Probe Force Microscopy and Magnetic Force Microscopy forCharacterization of Duplex Stainless SteelsM. Femenia, C. Canalias, J. Pan, and C. LeygrafJournal of The Electrochemical Society, in press.V. Char
15、acterization of Ferrite-Austenite Boundary Region of Duplex Stainless Steelsby Scanning Auger Electron SpectroscopyM. Femenia, J. Pan, and C. LeygrafManuscript to be submittedIn addition, there is a conference contribution that has not been included in thethesisIn Situ Electrochemical STM Study of D
16、issolution Behavior of Duplex StainlessSteels in Aggressive SolutionsM. Femenia, P. Luukkonen, J. Pan, and C. LeygrafProceedings of Duplex Stainless 2000, Venice, Italy, pp. 261-270, AssociazioneItaliana di Metallurgia (2000).Table of contents1. Introduction 12. Experimental 52.1. Materials and Solu
17、tions 52.2. Conventional Electrochemical Techniques 62.3. Electrochemical Scanning Tunneling Microscopy andScanning Electrochemical Microsocopy 72.4. Ex situ Microstructure Characterization 102.5. Magnetic Force Microscopy andScanning Kelvin Probe Force Microscopy 112.6. Scanning Auger Electron Spec
18、troscopy 143. Summary of Results 163.1. Electrochemical Scanning Tunneling Microscopy 163.2. Scanning Kelvin Probe Force Microscopy 243.3. Scanning Auger Electron Spectroscopy 303.4. Scanning Electrochemical Microsocopy 334. Discussion 344.1. Local vs. Global Experiments 344.2. Combination of Local
19、Probing Techniques 364.3. Influence of the Alloying Elements 394.4. Influence of the Phase Boundary Regions 415. Concluding Remarks 446. Considerations for the Future 467. Acknowledgements 478. References 5011. INTRODUCTIONIn the early 20thcentury, simultaneous research in USA, England and Germany l
20、eadto the development of stainless steel. Monnard and Strauss in Germany, and Brearley inEngland are often regarded as three of the most important pioneers.1, 2, 3 The first two-phasemicrostructure was reported by Bain and Griffiths in 1927,4, 5and shortly afterwards the firstduplex stainless steels
21、 (DSSs) were already commercially available.6However, it was notuntil the 1980s, when the advent of AOD (Argon-oxygen decarburization) allowed thefabrication of low-carbon stainless steels, that DSS found widespread industrial application.7A steel is considered to be stainless when it contains more
22、than 12% Cr,*whichmakes possible the formation of a protective Cr-based passive film on the surface. DSS iscomposed of two phases that are stainless, and present in relatively large separate volumesand in approximately equal volume fractions.4, 5The term DSS has become a synonym for theferritic-aust
23、enitic steels due to their extensive use in industrial applications, although ittechnically also comprises other types of steels, such as the ferritic-martensitic.DSS solidifies as ferrite, part of which transforms to austenite during subsequentcooling, yielding the prescribed mix of the two phases
24、at room temperature.8The exactvolume fraction of each phase depends on the alloying composition and the heat treatment.However, most alloys are designed to contain similar amounts of each phase at roomtemperature.Due to its small grain size, DSS possesses higher yield strength than both ferritic and
25、austenitic stainless steel. However, it is not the attractive mechanical properties that haveincreased the interest for DSS in the past two decades, but the superior corrosion resistance,especially in chloride-containing environments, compared to austenitic steel of comparablecost.4, 5Thus, DSS has
26、been increasingly used in marine environments, and in oil and gas,pulp and paper, chemical, petrochemical, and power industries.4, 5, 8This type of environments*All compositions in this thesis are in wt%, unless noted2pose a challenge in terms of localized corrosion resistance, which has become one
27、of the mostimportant issues for DSS. The resistance to localized corrosion is strongly dependent on thechemical composition of the steel,4Cr, Mo, and N being the most beneficial alloyingelements.Localized corrosion often starts at small sites such as inclusions, precipitates, orcracks that can be on
28、 a micrometer or sub-micrometer scale. Thus, the size and distribution ofprecipitates and inclusions in the microstructure may play a crucial role in the corrosionresistance of the material.9This is especially important in the case of DSS, because the duplexstructure introduces anomalies in the dist
29、ribution of impurities and precipitates, and as aresult, phase boundaries frequently become preferential sites for the segregation of impuritiesor precipitations.4, 5, 8Moreover, the partitioning of the alloying elements between the phases(Cr and Mo partition to ferrite, and N and Ni to austenite)4,
30、 5, 8, 10-18further complicates thecorrosion behavior of DSS, since local interactions between the phases must also be taken intoaccount.10, 14-16Consequently, it is easy to realize that a deeper knowledge of the localelectrochemical activity and the corrosion processes taking place on micrometer an
31、d sub-micrometer scales would help to improve the understanding of the mechanisms behindlocalized corrosion, leading to the development of better stainless steels.Corrosion studies often involve the application of conventional electrochemicaltechniques (potentiostatic, potentiodynamic, impedance, et
32、c.) followed by subsequent ex situcharacterization by means of optical microscopy, scanning electron microscopy (SEM) orsurface analysis. These electrochemical methods are usually based on exposed electrode areasof the order of one cm2, and are therefore not capable of providing direct information a
33、boutthe local corrosion processes taking place on the sample. However, during the last decade orso, numerous high-resolution experimental techniques have been developed for corrosionstudies, and many of them permit the characterization of the solid-liquid interface in situ.Techniques used for local
34、in situ studies of corrosion of stainless steels includemicroelectrodes,19microcells,18, 20, 21localized electrochemical impedance spectroscopy(LEIS),22, 23scanning vibrating electrode technique (SVET)24, scanning reference electrode3technique (SRET),25, 26scanning tunneling microscopy (STM),27, 28a
35、tomic force microscopy(AFM),29-33scanning electrochemical microscopy (SECM),34-38and scanning Kelvin probe(SKP).39-41Combinations of these techniques have also been reported.42-45The SRET andSVET techniques are capable of mapping the current density over the surface of the electrode,and of distingui
36、shing local potential differences with a lateral resolution of a few tens ofmicrometers. The AFM/STM techniques can monitor changes in topography (3D) of thesample with sub-micron resolution. The SECM technique can provide information aboutsurface species present, or map the local faradaic current o
37、ver the surface of the sample, alsowith sub-micron resolution. The SKP technique measures the Volta potential variation overthe surface of the electrode under a thin electrolyte layer with a resolution of 50-100 m. AnAFM-based variation of this technique, scanning Kelvin probe force microscopy (SKPF
38、M),has recently been applied to corrosion studies,46-50but, to our knowledge, nothing has yet beenreported on stainless steel. The main advantage of this technique over the standard SKP is itshigh lateral resolution (in the sub-micron range), but the problem is that it cannot be used insitu. In all,
39、 the information that can be obtained with local techniques is quite comprehensive,and a combination of such techniques might be of invaluable importance for gainingadditional knowledge about corrosion processes.Acidic-chloride environments are present in many commercial applications of DSS,such as
40、marine environments or chloride-bearing pressure vessels. Moreover, in localizedcorrosion processes such as pitting and crevice corrosion, local environments inside the pits orcrevices often become acidified and enriched in chloride ions. Therefore, it seemed a highlyrelevant medium in which to stud
41、y the local corrosion behavior of DSS.The primary aim of this project was to gain deeper understanding of the localprocesses -electrochemical, chemical or physical- that may affect the corrosion resistance ofDSS in acidic-chloride environments. These processes are of vital importance for thedissolut
42、ion, passivation, and localized corrosion behavior of DSS, because they influence thelevel of performance of these steels in real-life applications. Hence, there are obvioustechnical and industrial interests; this project was implemented as a collaboration with our4industrial partners, AB Sandvik St
43、eel and Avesta Polarit AB, in January 1998. An importantprerequisite was also the joint decision made by KTH and the Swedish Institute for MetalsResearch, Stockholm, to purchase an AFM and a modification package for electrochemicalSTM and SECM facilities to be shared by both parts.This thesis survey
44、s the work performed during the five years that have past sincethese doctoral studies commenced. Section 2 provides the most significant technical detailsabout the materials and techniques used in these investigations. Section 3 gives an overviewof the results obtained with each technique, highlight
45、ing the most important findings. Section4 integrates the results by discussing them from different perspectives. Finally, section 5 liststhe most important conclusions obtained from these investigations. More detailed informationcan be found in the different publications that resulted from this work
46、, which are annexed atthe end of the thesis. Papers I to III deal with in situ EC-STM measurements, Paper IV showsthe combination of MFM and SKPFM, and Paper V contains the results obtained by theSAES measurements.52. EXPERIMENTAL2.1 Materials and SolutionsIn the course of these studies, four differ
47、ent DSSs were investigated: 2205 (Paper I),UNS S32750 (Papers II, III, IV and V), UNS S32304 (Papers III, IV and V), and UNSS31803 (Papers III, IV and V). From Table I it is possible to see that these steels present abroad variation of alloying elemental concentrations, ranging from the low-alloyed
48、UNSS32304 to the super duplex UNS S32750. The 2205 DSS in Paper I was a commercial alloy,with the same nominal composition as UNS S31803, that had undergone a heat treatment inorder to obtain a coarser grain size (10-30 m). For the other three steels, no further heattreatment was needed because of t
49、he high resolution offered by the EC-STM.Table I. Chemical composition of the DSSs investigated (wt%).*Steel Cr Ni Mo N C Si Mn P SUNS S32304 22.7 4.78 0.21 0.10 0.020 0.36 1.48 0.022 0.0012205 22.1 5.59 2.99 0.13 0.025 0.41 1.51 0.020 0.001UNS S31803 22.0 5.36 3.20 0.17 0.015 0.50 0.78 0.003 0.001UNS S32750 24.8 6.95 3.84 0.27 0.014 0.27 0.44 0.003 0.001These steels are commercially available and widely used in industrial applicationswhere high mechanical and corrosion resistance are needed. Important applications includeheat exchangers, refineries, and process systems in oil
