Document09

Symposium 9 Corrosion: from Nanostructure to Structural Failure

NANOPOROUS DE-ALLOYED METALS:

57th Annual Meeting of the International Society of Electrochemistry

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Roger C. Newman*1, of Chemical Senior2 and Karl Sieradzki3 1University of Toronto, Dept Nicholas A.Engineering and Applied Chemistry, 1 200 Toronto, Dept Toronto, M5S 3E5, Ontario, Canada. University ofCollege Street,of Chemical Engineering and Applied Chemistry, 200 College Street, Toronto, M5S 3E5, Ontario, Canada. 2Corrosion and Protection Centre, School of Materials, University of Manchester, 2 Corrosion and Protection Centre, School of Materials, University of Manchester, PO Box 88, Manchester, M60 1QD, United Kingdom. PO Box 88, Manchester, M60 1QD, United Kingdom. 3Arizona State University, Ira A. Fulton School of Engineering, Department of of 3 Arizona State University, Ira A. Fulton School of Engineering, Department Mechanical && AerospaceEngineering, P.O. Box 876106,Tempe, AZ 85287-6106, USA. Mechanical Aerospace Engineering, P.O. Box 876106,Tempe, AZ 85287-6106, USA. *[email protected] *[email protected] literature on electrolytic de-alloying of homogeneous alloys has expanded recently with the realization that the resulting nanoporous metals may have interesting applications. These range from the mundane - filtration membranes, perhaps catalytic or electrically switchable - to the more challenging - actuators or mechanical sensors, specialized coatings, or templates for deposition of electrocatalytic nanoparticles. Bulk metal nanoparticles can be deposited into de-alloyed layers at an underpotential (i.e. above the normal metal/metal-ion equilibrium potential) owing to a curvature effect. De-alloyed material is nanoporous with close to zero net curvature ; normally the porosity is completely interconnected. The characteristic size scale depends on the surface diffusivity of the more noble alloy component. De-alloyed materials are usually considered as brittle solids; microscopically, they display different types of plastic fracture depending on the thickness of the ligaments. The mechanism of de-alloying in Ag-Au (for example) involves dissolution of Ag and surface diffusion of Au. The critical electrode potential to form interconnected nanoporosity is set by a balance between the rates of these two processes. Anion adsorption or monolayer Au oxidation influence the size scale of the porosity through their effects on the surface diffusivity of Au. Electrochemically controlled de-alloying in perchloric acid gives the finest porosity, and this may optionally be coarsened by thermal treatment. Another way to make very fine porosity is to use an alloy based on a higher melting-point noble metal such as Pt. The porosity of de-alloyed Ag-Au will continue to coarsen in some applications, but it is relatively stable in air. Until recently, de-alloyed materials generally had poor mechanical properties and were subject to spontaneous fracture. We find that optimization of the mechanical properties, by increasing the electrolyte temperature and avoiding monolayer Au oxidation, causes strange phenomena to appear that may be related to the surface-charge or piezoelectric effects discovered by Weissmller et al. De-alloyed layers magnify nanoscale surface effects into macroscopic mechanical behavior.

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Corrosion: from Nanostructure to Structural Failure

Symposium 9 - Keynote

FROM ATOMISTIC MODELING TO APPLICATIONS Nanoporous de-alloyed metals: Roger C. Newman*1, Nicholas A. Senior2 and Karl Sieradzki3 From atomistic modeling to applications

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Nanoscale Dissolution and Passivation Processes for an Nanoscale Dissolution and Passivation Processes for anElectrochemical TailoringDirectionally Solidified Eutectics Electrochemical Tailoring of of Directionally Solidied EutecticsAchim Walter Hassel Achim Walter Hassel Max-Planck-Institut fr Eisenforschung, Max-Planck-Str. 1, Max-Planck-Institut fr Eisenforschung, Max-Planck-Str. 1, 40237 [email protected] Dsseldorf, Germany [email protected] Dsseldorf, Germany


It may sound contradictory to apply a process that was initially designed to grow large single crystals in nanotechnology. However, if a eutectic alloy is directionally processed in a Bridgman type furnace both phases are solidifying simultaneously. Bound to the eutectic composition a coupled growth of the phases results. Eutectics with an asymmetric composition form micro or nanowires embedded in a matrix of the mayor phase. Pseudo-binary NiAl-X (X = Re, Mo, W) eutectics present a structure in which the minor phase (Re, Mo W) is composed of fibres uniformly distributed in a stable NiAl matrix. A comparative study of the Pourbaix diagrams of the 3 elements involved allows selecting conditions under which the NiAl matrix is selectively dissolved to form an array of single crystalline metallic nanowires of identical orientation and diameter [1]. Passivation of the matrix with a simultaneous electrodissolution of the minor phase on the other hand forms nanopore arrays or nanofilters. Careful selection of the conditions used allows controlling the depth of the pores [2]. Subsequently the system can be further used, e.g. by depositing gold into the nanopores that produces various microelectrode arrays [3]. Various other structures can be produced by combining these processing steps, for example Re-nanowire arrays with prominent gold heads. Pros and cons of processing route as well as possible applications will be discussed.1. 2. 3. A. W. Hassel, B. Bello Rodriguez, S. Milenkovic, A. Schneider, Electrochim. Acta. 50 (2005) 3033-3039. A. W. Hassel, B. Bello Rodriguez, S. Milenkovic, A. Schneider, Electrochim. Acta. 51 (2005) 795-801. B. Bello Rodriguez, A. Schneider, A. W. Hassel. J. Electrochem. Soc. 153 (2006) C33-C36.


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Department of Materials Science and Metallurgy University of Cambridge Pembroke Street Cambridge CB2 3QZ United Kingdom

G. T. Burstein


The passive metal surface is a remarkable entity, controlling much of the stability of metal structures and components in service. Studies of these surfaces over many decades have revealed properties of passive metals which are as unique as they are fascinating: the very nanoscopic nature of the passivating lm gives it this uniqueness. Three of the issues which concern the corrosion scientist are addressed in this talk. These matters are as follows. (i) What constitutes passivity and how do we develop total control over it. (ii) Since passivity seems to dominate metals in service, why do they corrode. (iii) What are the properties of the passive surface and how can they be exploited. Discussion of corrosion centres around pitting corrosion, a form of runaway corrosion reaction with disastrous consequences on engineering structures. However, brought under control, the phenomenon of pitting corrosion can be used to sculpt the surface of metals and generate a known predetermined microscopic morphology. Discussion is also centred on the properties of the passive metal surface which could be developed for reactive surfaces. Since heterogeneous catalysis involves catalytically reactive interfaces, the passive surface should be exploitable in this fashion. In modern development of the fuel cell, the use of platinum is a prohibitive cost barrier to a wide range of applications. Although several attempts have been made to replace this invaluable electrocatalyst, no major breakthrough has been made. This however, does not mean that there is no such possibility, but rather, that a greater research effort is required. There are several properties of such surfaces that can be usefully developed, and this forms part of the discussion of the present paper.


Developing the characteristics and properties of passive metal surfaces

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Modeling of the Barrier, Electrochemical Protection and Active Corrosion Inhibition Properties of an Al-Co-Ce Amorphous Alloy Coating Protecting AA 2024-T3F.J. Presuel-Moreno, H. Wang, M.A. Jakab, R.G. Kelly, and J.R. ScullyCenter for Electrochemical Science and Engineering University of Virginia

An Al-Co-Ce alloy coating has been developed to protect a 2024-T3 substrate against localized corrosion. The Al-Co-Ce coating can function as a corrosion barrier, a sacricial anode and a reservoir to supply soluble inhibitor ions to protect defects site exposing AA 2024-T3 to an aqueous environment. In this paper, both experiments and continuum scale modeling investigations are described that identify some of the coating attributes that optimize the ability to perform each function. Additionally, the environmental factors that challenge the ability to perform each function are assessed. Concerning barrier properties, the localized corrosion resistance of the alloy coating was assessed as a function of solid solution alloying content. Quantication of the relationships between solid solution compositions and selected electrochemical corrosion metrics was established by utilizing specialized statistical methods designed for compositional data sets. Electrochemical properties such as pitting and open circuit potential are shown to be tunable, based primarily on cobalt (Co) concentration in solid solution, and secondly on cerium (Ce) content. Therefore, barrier coatings can be tailored for a specic need via choice of alloy composition. Continuum modeling and experiments were performed to assess the sacricial anodic protection capabilities of these alloy coatings. The geometry is that of an Al-Co-Ce coated surface with the presence of a scratch simulating exposed AA2024-T3. Finite element modeling of current and potential distributions was performed to assess the scratch size that could be cathodically protection via such a sacricial coating. The criterion used was to achieve a position specic galvanic couple potential that was more negative than the pitting potential associated with 2024-T3. Lastly, the chemical throwing power of such an Al-Co-Ce metallic coating under thin electrolyte lms representative of atmospheric conditions was also modeled. The chemical throwing power was the distance over which an adequate inhibitor supply could be delivered in a reasonable time period after triggered release of inhibiting ions. Separate experiments identied critical inhibitor concentrations for suppression of corrosion of 2024-T3. The model considered the pH-dependent passive dissolution rate of an Al-Co-Ce alloy to dene the inhibitor release ux. Transport by both electro-migration and diffusion were considered. The model calculates the time necessary to accumulate Ce+3 and Co+2 inhibitors over the scratch when released from the Al-Co-Ce coating under various conditions. The effects of scratch size, initial pH, chloride concentration, and the electrochemical kinetics of the material involved were studied. Sufcient accumulation of the released inhibitor (i.e., when the Ce+3 concentration surpassed the critical inhibitor concentration over AA2024-T3 scratches) was achieved within a few hours (e.g., ~ 4 hrs for scratches of S=1500 m) when the initial solution pH was 6 or less. Coating attributes that optimize both electrochemical and chemical throwing power for protection are discussed.



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1

Department of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 Japan * [email protected] 2 Department of Materials Science and Engineering, Kansai University, 3-3-35 Yamate-Cho, Suita, Osaka 564-8680 Japan, 3 Japan Power Engineering and Inspection Corporation,

Shinji Fujimoto*,1 Masahiko Kubo1, Fahmi A. Mukhlis1, Takumi Haruna2 and Sigeo Nakahigashi3

Mach attention has been paid for high strength metals and alloys with ultra ne grains that are produced by severe plastic deformation. However, the corrosion property of such materials has not been necessarily well discussed. More recently, on the other hand, stress corrosion cracking has been reported for Type316L stainless steel of not welded but of deformed parts in light water nuclear power generating plants. In the present work, stress corrosion cracking behavior of Type 316L stainless steel cold rolled up to 80 % was examined by slow strain rate test (SSRT) in high temperature and high pressure dilute aqueous solution at 288 C. Solutions employed were 10-5 mol/l sodium sulphate with 8 ppm dissolved oxygen and conductivity of 1-2 S/cm and pure water with dissolved oxygen less than 1 ppb and conductivity of 0.3-1 S/ cm. The strain rate for SSRT was 5x10-7 s-1. Trans granular stress corrosion cracking (TGSCC) was observed for Type316L stainless steels that were deformed more than 20 % (cold rolling). The ruptured surface shows typical brittle fracture of TGSCC. The number of crack initiation increased with increasing deformation ratio for tests in both solutions that are ones with DO 8 ppm and DO 1 ppb. However, the crack development rate that is roughly evaluated by the depth of brittle fracture increased for test in DO 8 ppm but decreased for the test in 1 ppb solutions with increasing deformation. Therefore, the strain at the rapture of specimen decreased with increasing deformation ratio for DO 8 ppm, but showed a minimum at 40 % of deformation for DO 1 ppb. The variation of deformation ratio provides difference in the mechanical strength and the chemical reactivity. In the DO 8 ppm solution in which corrosion potential is more noble than that in the DO 1 ppb solution, the SCC may extends as the active path corrosion mechanism. The larger active dissolution rate for the more severely deformed steel results in the larger crack propagation rate. In DO 1 ppb solution, on the other hand, although the number of crack is larger than that in DO 8 ppm because of unstable passivity at less noble potential, the crack extension is not accelerated electrochemically but suppressed by the larger mechanical strength of more deformed steel.


14-1 Benten-cho, Tsurumi-ku, Yokohama 230-0044 Japan


Stress Corrosion Cracking of Severely Deformed Type316L Stainless Steels in High Temperature and High Pressure Aqueous Solution

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Application of Surface Analytical Methods to the Development of Environmentally Friendly CoatingsEnvironmental Sciences Department, Brookhaven National Laboratory, Upton, NY 11973,USA. *[email protected]

Dev Chidambaram*


A variety of modern, state-of the-art surface analytical techniques such as variable-angle X-ray photoelectron spectroscopy (VAXPS), secondary ion mass spectroscopy (SIMS), Fourier-transform infrared spectroscopy and mapping (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM), laser scanning confocal microscopy; synchrotron based techniques such as X-ray absorption near edge spectroscopy (XANES), synchrotron infrared micro spectroscopy (SIRMS), synchrotron radiation based grazing angle infrared spectroscopy (SR-GAIRS) have been used in conjunction with traditional electrochemical techniques such as open-circuit potential measurements (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) to study systems that range from simple air formed oxide lms, oxide lms containing various adsorbed species to complex conversion coatings formed on several substrates that include pure metals such as aluminum to complex alloys such as AA2024-T3 and depleted uranium-0.75wt% titanium. Initial work focused on understanding the effect of surface pretreatment and alloy cleaning procedures on the characteristics of subsequently formed CCC. Mechanism of protection offered by chromate conversion coatings (CCC) to aluminum-copper systems was then studied (1-4). The knowledge gained from these studies was utilized to develop an environmentally-friendly coating for corrosion protection of depleted uranium-0.75wt% Titanium alloy (5-7). The studies starting from alloy cleaning methods to the formation of complex chromate conversion coatings and its application in the development of a new environmentally benign molybdate-based protective coating will be reviewed. The corrosion properties and aging-characteristics of the molybdate coating will also be discussed. References 1. D. Chidambaram and G. P. Halada, Surf. Interface Anal., 31, 1056 (2001). 2. D. Chidambaram, C. R. Clayton, and G. P. Halada, J. Electrochem. Soc., 150, B224 (2003). 3. D. Chidambaram, C. R. Clayton, and G. P. Halada, J. Electrochem. Soc., 151, B151 (2004). 4. D. Chidambaram, C. R. Clayton, and G. P. Halada, Electrochim. Acta, Vol. in Press (2006). 5. D. F. Roeper, D. Chidambaram, C. R. Clayton, and G. P. Halada, Electrochim. Acta, 50, 3622 (2005). 6. D. F. Roeper, D. Chidambaram, C. R. Clayton, and G. P. Halada, Electrochim. Acta, 51, 545 (2005). 7. D. F. Roeper, D. Chidambaram, C. R. Clayton, G. P. Halada, and J. Derek Demaree, Electrochim. Acta, Vol in Press (2006).


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A. Afseth1, G. M. Scamans2, X. Zhou3 and G. E. Thompson31

Novelis Technology AG, Neuhausen, Switzerland 2 Innoval Technology, Banbury, UK 3 University of Manchester, Manchester, UK * [email protected]


The inuence of thermo-mechanical processing conditions on the generation of ultrane grain sized near surface microstructures has been investigated for a number of aluminium alloys of the Al-Mn, Al-Mg and Al-Mg-Si alloy families. High shear deformation processes, such as hot rolling, grinding or machining, invariably result in the formation of near-surface layers with dramatically rened microstructures compared to the underlying bulk material. These surface layers typically have grain sizes in the order of 50 to 200 nm and, depending on processing conditions, may also contain signicant amounts of second phase inclusions in the form of oxides and carbon containing compounds. The presence and distribution of second phase inclusions have been shown to have a strong effect on the thermal stability of the ne grain structure, but to be of relatively minor importance for controlling corrosion or electrochemical properties. However, during subsequent heat-treatments the precipitation of intermetallic particles is enhanced in the surface layers and this has a dominant effect in controlling corrosion behaviour. Both electrochemical characterisation and accelerated corrosion testing have been employed to understand the mechanistic relationship between near-surface microstructure and the in-service corrosion performance. Different means of controlling and exploiting the corrosion performance and surface reactivity in terms of choice of alloy, thermo-mechanical processing conditions and optimised cleaning and pre-treatment processes are discussed.

Symposium 9 - Oral

The Role of Near-surface Microstructure on Corrosion Behaviour of Wrought Aluminium Alloys

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Symposium 9 - Oral

Corrosion protection behaviour of new hybrid sol-gel thin lmsV. Barranco1, N. Carmona1, A. Jimnez-Morales2, M.A. Villegas1, J.C. Galvan1*Centro Nacional de Investigaciones Metalrgicas, CSIC Avda. Gregorio del Amo 8, E-28040 Madrid, Spain 2 Universidad Carlos III de Madrid, Avda. Universidad 30, E-28911 Legans, Spain *[email protected]

The main objectives of this work have been: (i) to design new electrochemically active sol-gel thin lms doped with Ce3+ salts (environmentally friendly anticorrosive pigments as an alternative to the use of chromates); (ii) to produce doped sol-gel thin lms as rst chemically active layer for pre-treatments of painted surfaces of mild steel and zinc, and (iii) to study the corrosion protection behaviour of the resulting paint systems. Sol-gel thin lms based on siloxane bonded units were prepared starting from an organic-inorganic hybrid system. The precursors were an alkylalkoxide (-methacry loxypropyltrimethoxysilane, MAPTMS) and a silicon alkoxide (tetramethoxysilane, TMOS). Cerium nitrate hexahydrate in three different concentrations was added. Diluted hydrochloric acid and methanol were also added for the sols preparation. These sol-gel thin lms were deposited on metal surfaces (pure zinc and mild steel) by applying dip-coating and spin-coating techniques. A model poly vinyl butyral (PVB) was applied on a series of these sol-gel lms to simulate the performance of technologically organic coatings. For this matter poly(vinyl butyral- co-vinyl alcoholco-vinyl acetate) solutions (17% w/w) were prepared in ethanol. Likewise, the remaining sols were kept to gelify at 600C and powdered to obtain suitable samples for analysing them by other characterisation techniques (Si-29 and C-13 NMR, FTIR and DTA). Electrochemical impedance spectroscopy and cathodic delamination tests were carried out. These techniques allowed to study the role of cerium ions incorporated into the hybrid sol-gel network. The effect of cerium concentration on the impedance spectra was analysed, as well as the system behaviour against the corrosive medium (aqueous NaCl 0.6 M), as a function of exposition time. In parallel studies, cathodic delamination tests were applied on the PVB/sol-gel thin lm/metal system. Just before the experiment was initiated, the centre of the sample was damaged with an intentionally scribed circular defect. The metal/paint system was immersed in 0.6 M NaCl aqueous solutions and submitted to a constant cathodic polarization (-0.8 or -1.2V vs. Ag/AgCI) at different immersion time. From the electrochemical point of view, the sol-gel thin lm behaved as a passivating layer of the metallic surface. The anticorrosive performance of the hybrid sol-gel thin lm is due to an inhibitor effect and a self-repairing mechanism awarded by the Ce3+ ions entrapped in the sol-gel network which is probably associated to the precipitation of insoluble cerium hydroxide at the cathodic sites below the coating. Acknowledges: This work has been supported by the National Program for Materials of Spain (Project MAT2003-03231)



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Kinetic Study of Model CO2 Corrosion InhibitorsInstitute for Energy Technology, Materials and Corrosion Technology Department P.O.Box 40, NO-2027 Kjeller, Norway *[email protected]

Internal corrosion of pipelines caused by presence of water and acids like CO2, acetic acid and H2S in the uid from oil and gas wells is a common problem in petroleum production. The use of carbon steel with corrosion inhibitors often constitutes an economically favourable alternative compared to the corrosion resistant materials. Commercial inhibitor products usually consist of mixtures of compounds. Quaternary ammonium salts and imidazoline salts are two major groups of inhibitor components used in such products. The corrosion inhibition performance and the kinetics of inhibition of two model corrosion inhibitors, cetyl trimethyl ammonium bromide (CTAB), and oleic imidazoline salt were investigated on high purity iron at 25C, CO2 partial pressure of 1 bar, pH 4, in 3% NaCl brine. The inhibitor performance was studied at various concentrations and it was discussed in the relation to their critical micelle concentrations (CMC). CMC is dened as the concentration at which a surfactant forms micelles in the bulk solution. Further increase in the surfactant concentration above the CMC does not increase the concentration of the monomer but it increases the number of the micelles. The focus of the presented work was on the kinetics of the inhibition; especially the processes during the initial stage of inhibition occur at a rapid rate. Thus a high frequency impedance measurement technique was developed to determine the rapid changes in polarization resistance and electrode capacitance in approximately 20 seconds interval during the initial stage of inhibition. Furthermore linear polarization resistance measurements (LPR) were used to monitor the corrosion rate during the periods of slow changes. Potentiodynamic polarization curves were measured at the end of the experiment to elucidate the effect of processes involved in CO2 inhibition on electrode part reactions. The surface of iron specimens at various time of inhibition was analyzed by x-ray photoelectron spectroscopy (XPS). The inhibition followed the same general trends for both studied compounds. Minimum effective concentration was close to the CMC. The corrosion rate was nearly constant for concentrations above the CMC. The inhibited corrosion rate of the studied compounds was independent on ow. The CO2 corrosion inhibition was found to be a combination of two processes. First a rapid process (minutes) connected to adsorption of the surfactant. Secondly there is a slower process (hours) leading to a reduction in corrosion rate. The rst process was characterized by an increase in the corrosion potential immediately after the inhibitor was added; the corrosion current density remained nearly constant. The inhibitor adsorption was conrmed by XPS; there was an increase in C and N signals for the specimens taken out immediately after inhibitor addition. There was a new N peak detected by XPS for the specimen after long time of inhibition, and there were also changes in the iron peak. This can be interpreted as a bond formation between Fe and N atom of the inhibitor.


Symposium 9 - Oral

K. Bilkova*, E. Gulbrandsen

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Symposium 9 - Oral

Corrosion studies of nanostructured composite coatings based on zinc and zinc-cobalt alloys1 Institute of Physical Chemistry, Bulgarian Academy of Sciences, 2 Institute of Polymers, Bulgarian Academy of Sciences 3 Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences * N. Boshkov - Soa 1113, Akad. G. Bonchev, bl. No. 11, Bulgaria; phone 359 2 979 39 20; fax 359 2 971 26 88; e-mail: [email protected]

N. Boshkov1*, P. Petrov2, N. Tsvetkova1, G. Raichevski1, R. Raicheff3, Ch. Tsvetanov2

Zinc and zinc-based electrogalvanized coatings have attracted remarkable interest and development because of the increasing demand for coatings with better protective ability. The traditional method to improve the corrosion resistance of these layers is their treatment in chromate or phosphate solutions. A new approach for an increase of corrosion resistance is the formation of nanostructured zinc and zinc alloy coatings containing incorporated polymeric nanoparticles. Their presence in the metal matrix spread the corrosion processes on a greater surface and this holds up the destruction in the depth of the coating. Such layers are relatively new and therefore are object of various investigations. In this study different types of polymeric nanoparticles obtained from poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide), poly(2-hydroxy-ethyl methacrylate)-b-poly(propylene oxide)-b-poly(2-hydroxy-ethyl methacrylate) or poly(ethylene oxide)-b-poly(2-hydroxyethyl methacrylate) block copolymers are used to prepare the nanostuctured composite coatings of Zn and Zn-Co. The corrosion resistance of these objects is evaluated using potentiodynamic polarization technique, polarization resistance measurements, scanning and transmission electron microscopy as well as X-ray diffraction analysis. As a model corrosion medium 5 % NaCl solution is used. It is established that the nanocomposite layers have a higher corrosion resistance compared to the resistance of the matrix coatings, the effect being stronger for pure Zn coating. The incorporation of polymeric nanoparticles in the electrodeposits affects their anodic behaviour and results in increasing of the susceptibility of the coatings to passivation.



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University Birmingham, Metallurgy and Materials, Edgbaston, Birmingham B15 2TT, UK University ofof Birmingham,Metallurgy and Materials,Edgbaston, Birmingham B15 2TT, UK 1 1University of Manchester, School of Materials, Manchester, M60 1QD, UK University of Manchester, School of Materials, Manchester, M60 1QD, UK 2 2 Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland *[email protected] *[email protected]

Alison Davenport*, Cristiano Padovani, Nick Stevens1, Tom Beale1 Alison Davenport*, Cristiano Padovani, Nick Stevens1, Tom Beale 1,, 2 Marco Stampanoni2, Amela Groso2, AmelaConnolly, TonyConnolly, Marco Stampanoni , Brian Groso2, Brian Horner, Sukanta Ghosh, and Michael PreussMichael Preuss1 Tony Horner, Sukanta Ghosh, and 1

Mg-Y-rare earth (Nd) alloys such as WE54 and WE43 are widely used, particularly in aerospace applications, owing to their combination of strength, particularly at elevated temperatures, and corrosion resistance. Yttrium is particularly effective in improving the corrosion resistance of magnesium alloys. In the present work, synchrotron microtomography is used to determine the three-dimensional distribution of Y in as cast WE43, to highlight how Y-rich regions in the matrix influence corrosion, and monitor the three-dimensional evolution of corrosion sites in situ in a corrosive environment. Figure 1(a) shows a Y distribution map for as cast WE43 and Figure 1(b) shows the evolution of a pit measured in situ over a period of 18 hours.a b 1 hour 5 hours 18 hours

0.1 mm

Figure 1 (a) 50 m thick reconstructed section of as cast WE43 showing the 3D distribution of Y-rich regions determined from tomographic measurements made above and below the Y K edge. (b) Time sequence showing evolution of corrosion for heat-treated (T6) WE43 during exposure to 0.1 M NaCl (pH 10) for 18 hours (in situ measurement).


Synchrotron X-ray microtomography is a method for imaging microstructures in three dimensions with resolution on the micron scale. It can do so non-destructively for samples immersed in water, making it an ideal method for monitoring the evolution of corrosion sites. A further benefit of the use of monochromatic synchrotron radiation for tomographic imaging is that it is possible to obtain tomographic data just above and just below the characteristic X-ray absorption edge, and from the difference in the data sets, obtain a chemical map for the element of interest. This is a particular interest for corrosion of alloys, because chemical inhomogeneity has a major influence on susceptibility to localised corrosion.

Symposium 9 - Oral

In Situ Synchrotron Microtomography Studies of In Situ Synchrotron Microtomography Studies of Localised Corrosion of an Mg-Y Alloy Localised Corrosion of an Mg-Y Alloy

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Symposium 9 - Oral

Mic monitoring in environments containing manganese oxidising bacteriaN, Arlt a, N. De Cristofarob, P. Gumpelc, M. Mengaronib, G. Mortalib and D. Schillercb

CSM, Via di Castel Romano 100- 00129, Rome; Italy. [email protected] b KTN,c Konstanz University

Stainless steels show a typical increase of corrosion potential under microbiological induced corrosion (MIC) conditions by manganese-oxidizing bacteria. This potential ennoblement makes corrosive conditions more aggressive. In the present work MIC of several stainless steels was monitored, in laboratory and in eld. The experiments were performed on a core of conventional stainless steels supplemented by various steel grades with enhanced corrosion resistance. Laboratory experiments were carried out in an aqueous salt solution matching the chemical composition of the Rhine river water contaminated with manganese oxidizing bacteria. Potentiostatic polarisation was applied to simulate the conditions emerged by manganese oxidizing bacteria. MIC was monitored in a water treatment plant by means of open circuit potential, redox potential and electrochemical impedance measurements. Electrochemical investigations, performed to simulate the electrochemical conditions emerged by the presence manganese oxidizing bacteria, showed that at potential levels known from MIC-conditions in the synthetic Rhine water and in absence of microbiological activity localised corrosion occurs only under crevice areas. In waters contaminated with manganese bacteria, an ennoblement of the open-circuit potential was always reached. The potential increases occurred, independent of the materials composition. The surface state plays an important role concerning the onset of the ennoblement. MIC eld experiments carried out in the water treatment plant showed that after 4 weeks of immersion in the water, the highest impedance modulus was showed for the EN 1.4565 and EN 1.4539 stainless steels. The behaviour is in accordance with the higher chromium, molybdenum and nitrogen content of these stainless steels. The behaviour of these steels was followed by the behaviour EN 1.4462 and EN 1.4401 stainless steels. Lower impedance values were obtained for the modied EN 1.4301 and EN 1.4301 steel. The corrosion process in the water plant was the result of the synergistic action of manganese oxidising bacteria and sulphate reducing bacteria (SRB). Manganese oxidising bacteria produce the biomineralization of MnO2 through the hydrolysis of Mn2+ present in the water. The produced MnO2 acts as a new oxidising product that can be reduced at the cathodic areas. Anaerobic SRB develop in the inner part of the aerobic lm. Sulphide products were formed in the corrosion process by SRB. Sulphide ions produced the acceleration of the anodic dissolution process and decrease the pitting potential of passive alloys.



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Centre Inter Universitaire de Recherche et dIngnierie des Matriaux, UMR CNRS 5085, ENSIACET, 118, Route de Narbonne, 31077 Toulouse Cedex 04 France b UPR 15 du CNRS, LISE, Univ. P. et M. Curie, 4, place Jussieu, 75252 Paris Cedex 05 France [email protected]

G. Barila, G. Galiciab, N. Pbrea, B. Tribolletb, V. Vivierb , C. Deslouisb *


The corrosion behaviour of pure magnesium in aerated sodium sulphate solutions was rst investigated using steady-state voltammetry and electrochemical impedance spectroscopy (EIS) with a rotating disc electrode. The analysis of impedance data obtained at the corrosion potential was consistent with the hypothesis that Mg corrosion is controlled by the presence of a very thin MgO oxide lm, and that the dissolution occurs only at free-lm spots. This hypothesis was substantiated both by the merging of the EIS diagrams obtained for different immersion times and different Na2SO4 concentrations into a single one and by scanning electrochemical microscopy (SECM) using the electrolyte resistance as electrochemical sensor. On the basis of the electrochemical results, a model was proposed to describe magnesium dissolution. (i) It was assumed that Mg+ is the species that reacts chemically with water to form an oxide/hydroxide layer. These reactions explain the negative difference effect (NDE) and the formation of a partially protective hydroxide layer. (ii) Mg dissolution occurs only at bare parts of the oxide lm whose area depends on potential and pH. The invariance of the EIS diagrams, once normalized by the maximum value of the real part, lead to conclude that the dissolution mechanism is unique in the whole anodic range investigated: it can be described as the exchange of two electrons in two successive electrochemical steps where the adsorption of the intermediate Mg+ is followed by two parallel paths, one chemical gives rise to the NDE effect and the electrochemical other one to Mg++ Simulation of the EIS diagrams was in good agreement with the experimental results.

Symposium 9 - Oral

Mechanism of pure magnesium corrosion in sodium sulphate solutions

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Symposium 9 - Oral

Characterization of Initial Films and Passivating Layers Grown on Magnesium Metal in Alkaline SolutionM. Santamaria1, L. Anicai1, F. Di Quarto1, S. Zanna, P. Marcus2Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universit di Palermo, Viale delle Scienze, 90128 Palermo, Italy 2 Ecole Natl Super Chim Paris, Lab Physicochim Surcafes, CNRS, ENSCP,UMR 7045, 11 Rue Pierre & Marie Curie, F-75231 Paris, France1

Among light metals magnesium presents very interesting engineering properties owing to: its light weight, high specic stiffness, high thermal conductivity, biocompatibility and easy recycling. These potential advantages are counteracted by the poor corrosion and wear resistance of Mg and its alloys due to the high chemical reactivity of MgO surface layer in presence of humidity or water with formation of a poorly protective hydroxide layer having also detrimental effects on the coating procedure. From a fundamental knowledge point of view the debate is still open on the mechanisms of growth and the nature of passivating layers formed on magnesium surfaces during anodizing in aqueous solution. In a complex industrial electrochemical bath containing, among other elements, uoride ions electric eld strengths across the barrier lm on the order of 7107 1108 V cm-1 have been reported during the growth under constant current density of 20 mA cm-2. The presence of such high electric eld during the anodizing process has been attributed to the formation of an inner layer of crystalline uorite covered by an amorphous and porous oxy-hydroxide layer. As for the kinetics of growth of the passive lm in aqueous solution not containing uoride ions there are very few indications if a barrier layer is formed or not and if the growth of passivating layers occurs in agreement with the Werwey or Cabrera-Mott high eld theory. According to the previously mentioned aspects we have undertaken a detailed investigation on both these aspects. We have focussed our initial study on the characterisation by different techniques (XPS, PCS) of initial lms formed on Mg in different conditions of surface preparation. Afterwards we have investigated the anodizing behaviour of Mg in aqueous solutions at high pH values ( 13) where a stable passive lm is expected to grow. The results of this preliminary study on the composition of initial lms based on the analysis of XPS and PCS data together with the experimental ndings on the kinetics of the early stage of lm growth and breakdown process will be reported. The possible nature of passivating lms formed in 1M NaOH solution will be inferred on the basis of kinetic data and tting of differential admittance plots whilst the information on the solid state properties and nature of corrosion layers gathered by PCS and XPS data analysis will be used to suggest possible mechanisms of breakdown of passive lm grown on Mg in aqueous alkaline solutions.



S9O-9

Max-Planck-Institut fuer Eisenforschung, Max-Planck-Str. 1, 40237 Duesseldorf, Germany # Christian Doppler Laboratory for Polymer/Metal Interfaces *[email protected]

N. Fink#, G. Klimow#, Ch. Stromberg, G. Grundmeier#,*

Conversion layers play a dominant role in providing excellent adhesion and corrosion resistance of polymer/metal interfaces. In the past, detailed electrochemical studies of the classical phophating process on zinc and steel substrates were done by Schultze and co-workers [1]. The authors examined the growth of Zn-phosphate crystals and localised Ni-cementation at the phosphate/metal interface. Today, the Zn-phosphating process including an activation step and the formation of crystalline tri-cation phosphate layers with few micrometer thickness is a well established process in the steel and the automotive industry. However, currently the development of new advanced conversion lms is focused on the development of thin amorphous hybrid conversion lms with a thickness of only a few tens of nanometers. In this contribution fundamental investigations on interface reactions of advanced phosphate-containing conversion solutions on ultra-smooth ZnAl-alloy coatings on steel are presented [2-3]. Surface gradient layers on these substrates were realised in order to screen the composition, electrochemical properties and corrosion resistance of the conversion coating as a function of layer thickness and processing time. For this purpose a dip-coating procedure was established yielding well-dened thin amorphous conversion gradient layers from the phosphate-containing conversion bath chemistry. These gradient samples were locally analysed by means of the Scanning Kelvin Probe and localised impedance measurements using an electrochemical capillary cell. Microscopic Infrared Reection Absorption Spectroscopy (FT-IRRAS) measurements, small-spot XPS and Tof-SIMS were used as complementary surface analytical techniques. All techniques enable a spatial resolution of better than 200 micrometers. The nucleation of thin lms was analysed by means of a high resolution AFM. Thereby, a complete variation of thin lm properties such as thickness, barrier properties, corrosion resistance and chemical composition could be measured as function of the time of lm growth on a sample with some millimetres in length. It could be shown that this surface gradient lm analysis signicantly increased the number of parameters that can be analysed per time [4]. [1] D. Zimmermann, A.G. Munoz, J.W. Schultze; Electrochim. Acta 48 (20-22) (2003) 3267-3277 [2] N. Fink, B. Wilson, G. Grundmeier ; Electrochim. Acta, in press (available online 3 October 2005) [3] B. Wilson, N. Fink, G. Grundmeier ; Electrochim, Acta, in press (available online 17 October 2005) [4] C. Stromberg, P. Thien, I. Klueppel, N. Fink, G. Grundmeier, submitted to Electrochimica Acta


Symposium 9 - Oral

Combination of Surface Gradient Chemistry and Kelvin Probe Studies of Thin Amorphous Conversion Films on Metals

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Symposium 9 - Oral

Corrosion resistance of Cr-free anodised Al 2024-T3 after long term atmospheric exposures11

CECUL, Departamento de Qumica e Bioqumica, Faculdade de Cincias da Univ. of Lisboa, Ed. C8, Rua Ernesto Vasconcelos, 1749-016 Lisboa, Portugal. 2 Instituto Superior Tcnico, Dept. of Chemical and Biological Engineering, 1049-001 Lisboa, Portugal ;Av. Rovisco Pais, 1049-001 Lisboa, Portugal 3 Univ. of Aveiro, Dept. of Ceramics and Glass Engineering, 3810-193 Aveiro, Portugal. tel.: 35121 75 00 904; [email protected]

I.T.E. Fonseca*, A.M.R. Pereira1, R. Picciochi2, M. I. Lopes1, L. F. Proena1, J.C.S. Fernandes2 and M.G.S. Ferreira2, 3.


A great number of studies for aircraft aluminium alloys have lead to some Cr-free anodising baths that might be a reasonable alternative to the traditional chromic baths. This study aims to evaluate the resistance against atmospheric corrosion of Al2024T3 alloys etched and anodised with a Cr-free bath. Samples etched and anodised in a Cr-free bath, a previously developed sulphuricboric bath, as well as samples treated by the conventional processes, chromic bath, have been submitted to eld exposures (long-term exposures), during 30 months in the urban atmosphere of the city of Lisboa (Campo Grande). The present paper deals with electrochemical data (potentiodynamic polarisation curves and open circuit potential data) of anodised Al2024-T3 material with 3, 6, 12, 24 and 30 months of atmospheric exposure. The electrochemical data is complemented by visual and optical microscopy. Pitting density was evaluated and the crystalline corrosion products formed on the anodised samples during the atmospheric exposures have been identied by Xrays powder diffraction analysis. Simultaneously to the exposures the level of pollutants in the atmosphere, namely chlorides and SO2, was monitored. It was concluded that even after 30 months of exposure both anodised processes lead to surfaces with similar resistance against atmospheric corrosion in spite of the differences found in the morphology of the anodising lms. All the data leads to the conclusion that the sulphuric boric anodising process seems to be a good alternative to the traditional anodising processes. Acknowledgements: The authors acknowledge FCT for the nancial support, under the Operational Programme of Science, Technology and Innovation (project POCTI/ CTM/39846/2001).


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Max-Planck-Insitut fuer Eisenforschung, Max-Planck-Str. 1, 40237 Duesseldorf, Germany # Christian Doppler Laboratory for Polymer/Metal Interfaces *[email protected]

G. Grundmeier#,*, G. Klimow#, K. Wapner

[1] K. Wapner, B. Schnberger, M. Stratmann, G. Grundmeier, Journal of the Electrochemical Society 152 (3) E114-E122 (2005) [2] K. Wapner, G. Grundmeier, Advanced Engineering Materials, No. 6 (2004) 163-167 [3] K. Wapner, M. Stratmann, and G. Grundmeier, Electrochimica Acta, in press (available online 24 October 2005)


A new height regulated Scanning Kelvin Probe (HR-SKP) was combined with a Blister Test (BT) to study de-adhesion processes at modied adhesive/metal interfaces under corrosive and mechanical load. The set-up was additionally equipped with a climated chamber and a three electrode set-up, allowing control of the surrounding gas phase and potentiostatic control of the defect. As an electrochemical technique the SKP measures the electrode potential at buried adhesive/oxide/metal interfaces [1-3]. Thus, it detects changes in the interfacial oxide structure and variations of the interfacial ionic conductivity with high spatial resolution of about 50 micrometer. The height regulation provides a constant distance between the sensing scanning probe and the adhesive surface, even above the spherical blister, while the interfacial electrode potential is measured simultaneous. The liquid for the Blister Test comprises of a corrosive metal halide electrolyte, whose pressure can be varied between zero and 4 bar. The electrochemical aspects of adhesion loss of adhesives from oxide covered iron and zinc, as model systems for steel and zinc coated steel, were investigated. An epoxy resin free of pigments, llers and additives was chosen as model adhesive. Selected results illustrate the measurement of pure wet de-adhesion, interfacial corrosive reactions and the acceleration of de-adhesion processes with increasing hydrostatic load of the blister. The metallic substrates were modied with conversion layers, adhesion promoting organosilanes and thin plasma polymer lms prior to the application of the adhesive to reveal the inuence of the interface chemistry on the stability of the adhesive/metal joint.

Symposium 9 - Oral

Applications of a Height-Regulated Scanning Kelvin Probe Blister Test in Corrosion and Adhesion Science

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Symposium 9 - Oral

Surface modication of passive iron by alkyl-phosphonic acid layersChemical Research Center of the Hungarian Academy of Sciences, Budapest, Hungary 1025 Budapest, Pusztaszeri t 59-67. Hungary, e-mail: [email protected] Research Group for Nuclear Methods in Structural Chemistry, Hungarian Academy of Sciences, Institute of Chemistry, Etvs Lornd University, Budapest, Hungary 1117 Budapest, Pzmny P. stny 1/a, Hungary

A. Paszternk1, S. Stichleutner2, F. Nagy2, I. Felhsi1, E. Kuzmann2, A. Vrtes2, Z. Keresztes1, E. Klmn1


Phosphonate layer formation on passive iron surface has been investigated by electrochemical, conversion electron Mssbauer spectroscopy (CEMS) and atomic force microscopy (AFM) techniques. Electrochemical methods revealed that metallic iron surface is disadvantagous for protective layer formation, while prepassivation of iron surface results in estabilization of the self-assembled layer. The rate of anodic dissolution is continually decreasing due to the time-dependent formation of protective phosphonate layer. The kinetics of phosphonate layer formation on passive iron is determined by the potential applied for preceding passive lm formation. CEMS investigations were carried out to evaluate the differences in passive layer composition as a result of phosphonate treatment. The size and shape of iron oxide grains depends slightly on he potential of passivation. Changes in morphology due to the phosphonate layer formation have been recorded by AFM.


S9O-13

GE Global Research Center 1 Research Circle, Schenectady, NY 12309, USA *(T) 518-387-6592; (F) 518-387-7007: [email protected]

Young-Jin Kim* and Peter L. Andresen


There have been well documented instances of environmentally assisted corrosion phenomena in various energy-related industries involving various subcomponents of boilers, steam turbines, piping, pressure vessels, pressurizers, steam generators, deaerators, etc. The common element in these corrosion incidents is exposure to high temperature water of structural materials such as austenitic stainless steels, nickel base alloys, turbine steels, low alloy and carbon steels, and their weld metals. It is known that the water chemistry in nuclear power plants plays an important role in several phenomena that can signicantly affect both the cost of operation and the long-term viability of reactors. The purpose of this paper is to describe advanced novel approaches to address these corrosion issues in water-cooled nuclear plants. This paper provides a fundamental description of the corrosion mechanism and describes various methods for mitigating the risk of stress corrosion cracking of reactor internals and extending the lifetime of components. These approaches are based on modifying the electrochemical nature of the metal surface, such as by the electrochemical catalysis of noble metals and/ or the insulating behavior of dielectric materials to protect reactor parts that tend to degrade in the normal reactor environment and eventually to extend plant life beyond 40 years.

Symposium 9 - Oral

Lifetime extension methods for components in watercooled nuclear reactor

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Symposium 9 - Oral

Simulation of localised corrosion: a combined probabilistic/nite element approachEngineering Mechanics, Faculty of Aerospace Engineering, Delft University of Technology, PO Box 5058, 2600 GB Delft, The Netherlands * [email protected]

L. Koene*, J.M. Lpez De La Cruz, M.A. Gutirrez

Keywords: stochastic processes, nite element methods, localised corrosion, pitting In literature localised corrosion processes like pitting, crevice corrosion and stress corrosion cracking are treated both deterministically and stochastically. In case of (metastable) pit and crack generation a stochastic approach seems most appropriate. These localised corrosion phenomena are often described as Poisson processes (e.g. [1]). On the other hand nite element simulations, i.e. deterministic approaches, of localised corrosion exist (e.g. [2]). In this paper a stochastic and a deterministic approach is combined. The electrochemical stochastic process of pit initiation is simulated using a (discretised) Poisson process, as an initial approach. A realisation of pitting is used for a 3D-nite element simulation: the deterministic part of the approach. In the corrosion of iron or steel, iron dissolution is assumed to be the anodic reaction, which takes place in the pits. Cathodic reactions are the reduction of O2 and the reduction of H+. The reduction of O2 is assumed to be negligible in the pits. For the current densities of the three electrochemical reactions Tafel equations are used. At the electrolyte boundaries the current density in the normal direction is dened to be zero. As a supporting electrolyte 0.1 M NaCl is selected. The Nernst-Planck equation describes the mass transport of species in the electrolyte domain. The electroneutrality condition was applied. The water equilibrium is accounted for using kinetic parameters of the forward and backward reaction of the protolysis reaction: H2O H+ + OH-. Distributions of current density, concentrations and the potential in solution are calculated. It will be discussed whether and when the assumption of independence of (pitting) events is justied. Both stochastic theory and nite element methods are useful instruments in the study and understanding of localised corrosion processes. Finally, possibilities for other combined stochastic and nite element treatments of localised corrosion, including stress corrosion cracking, will be discussed. References [1] J.L. Dawson, in: J.R. Kearns, J.R. Scully, P.R. Roberge and D.L. Reichert and J.L. Dawson (Eds.), Electrochemical Noise Measurements for Corrosion Applications, ASTM STP 1277, American Society for Testing and Materials, 1996, pp. 3-35. [2] J.N. Harb and R.C. Alkire, J. Electrochem. Soc., 138(9) (1991) 2594-2600.



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School of Advanced Materials Engineering, Kookmin University 861-1, Jeongneung-dong, Sungbuk-gu, Seoul 136-702, Korea Address *[email protected]

Jae-Bong Lee*, Sim-Kun Min, Se-Jung Oh


Crevice corrosion is the accelerated attack occurred in the occluded cell under a crevice on the metal surface, which is one of dangerous form of localized corrosion. Crevice corrosion shares many similar characteristics with other type of localized corrosion such as pitting corrosion, in terms of the breakdown of passive lm and its repassivation. Crevice corrosion behaviors of nickel-based alloys, Alloy 600 and Alloy 690 were investigated in acidic solution with the variation of chloride ion concentrations. Tests were carried out using the specially designed crevice cell with a very narrow Luginn capillary assembly to measure the potential inside the crevice. It is believed that crevice corrosion in active/passive system such as Alloy 600 and Alloy 690 is highly related with the properties of passive lm and its repassivation characteristics. The properties of passive lms were measured by the capacitance measurement and the reapssivation characteristics were investigated by the abrading electrode technique. The stability of repassivated lm was evaluated considering the results of EIS tests carried out before and after the abrading electrode test. An attempt was made to elucidate the relationship between crevice corrosion behavior of Alloy 600 and Alloy 690, properties of passive lm and its repassivation kinetics Keywords: Alloy 600, Alloy 690, Crevice Corrosion, Passive Film, Repassivation Kinetics,

Symposium 9 - Oral

Crevice Corrosion Behavior of Nickel-based Alloys Alloy 600 and Alloy 690 in Acidic Solution Containing Chloride Ions.

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Symposium 9 - Oral

Hydrophilic and Hydrophobic Interfaces: the Performance of Functional and Non-Functional SilaneTreated Epoxy-Coated SteelCorrosion & Protection Centre, School of Materials, University of Manchester, PO Box 88, Manchester, M60 1QD, UK *[email protected]

N.S. Ali, S.B. Lyon*

This work is concerned with developing model hydrophilic and hydrophobic interfaces on steel by surface treatment using, respectively, 3-glycidoxypropyl (i.e. epoxy-functional) trimethoxysilane and n-propyl (non-functional) trimethoxysilane. Subsequently, the performance of the organic-coated surface-treated steel was determined using electrochemical impedance spectroscopy (EIS), contact (aqueous wetting) angle measurements and pull-off adhesion. Contact angle measurements conrmed that a non-functional silane produced a signicantly more hydrophobic interface (contact angle 105) than an epoxyfunctional silane (contact angle 60), which was relatively hydrophilic. Both dry and wet adhesion measurements conrmed the expected 20% increase in adhesion for the epoxy-functional treatment but a 40% decrease in adhesion for the non-functional treatment. EIS measurements were then performed on silane-treated carbon steel substrates (i.e. without epoxy coating). After 20 minutes of immersion in 3.5% NaCl solution, the charge-transfer (polarisation) lm resistance was approximately 2000 ohm cm2 for the hydrophobic treatment compared with about 1000 ohm cm2 for the hydrophilic treatment; furthermore the lm capacitance for the hydrophobic treatment was about one-fth that of the epoxy-functional treatment indicating a considerably more diffuse double-layer. Over time (about 60 minutes) these values reverted to that of the untreated substrate. EIS results for silane-treated, epoxy-coated steel over the rst 50 hours of immersion showed that the initial coating resistance was signicantly greater (and the coating capacitance signicantly smaller) for the hydrophobic treatment. This conrms that the water uptake into the coating to the interface had been minimised by the hydrophobicity of the interface. However, after 50 hours the coating commenced de-bonding from the non-functional treated substrate due to the considerably lower adhesion strength. These results imply that if the substrate-coating interface can be made to be both hydrophobic and provide a functional bond between the substrate metal oxide and the organic coating, then the long-term performance of an organic coating is likely to be improved.



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Laboratoire de Physico-Chimie des Surfaces, CNRS-ENSCP (UMR7045), Ecole Nationale Suprieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France. E-mail: [email protected]

P. Marcus, A. Ithurbide, A. Galtayries, I. Frateur


The adsorption of proteins (BSA) was studied on passivated chromium surfaces using XPS and a switch-ow cell combined with an electrochemical quartz crystal microbalance (EQCM). The surface was initially passivated in a deaerated, proteinfree solution. Subsequently, a protein-containing solution was switched in the cell and the EQCM signals were recorded as a function of time. After protein adsorption the Cr surface was analysed by X-ray Photoelectron Spectroscopy (XPS). The kinetics of adsorption and the amount of adsorbed protein were investigated as a function of : protein concentration in the solution (2-20 mg/l) potential in the passive region pH For the interpretation of XPS data obtained after the EQCM adsorption experiments, a 3-layer model was considered : an inner Cr2O3 oxide layer and an outer Cr(OH)3 hydroxide layer covered by an adsorbed BSA layer. From the Cr 2p and N 1s core level spectra, the thicknesses of the oxide layer, of the hydroxide layer and of the adsorbed BSA layer were estimated. The thicknesses of the adsorbed protein layer measured in situ by EQCM and ex situ by XPS are in excellent agreement. The data show that the kinetics of adsorption of BSA depends strongly on the protein concentration in the solution whereas the maximum amount of adsorbed protein is similar, and corresponds to ~ 1 monolayer.

Symposium 9 - Oral

XPS and Flow-Cell EQCM Study of Protein Adsorption on Passivated Chromium Surfaces

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Symposium 9 - Oral

Fabrication of ultra thick porous oxide lms on niobium Fabrication of ultra thick porous oxide films on niobium by anodization by anodizationDepartment of Applied Chemistry, Faculty Engineering, Kogakuin University Department of Applied Chemistry, Faculty ofof Engineering, Kogakuin University 1-24-2 Nishi-shinjuku, Tokyo 163-8677, Japan 1-24-2 Nishi-shinjuku, Tokyo 163-8677, Japan *[email protected] *[email protected]

Sachiko Ono*, Takumi Nagasaka, and Hidetaka Asoh Sachiko Ono*, Takumi Nagasaka, and Hidetaka Asoh


Recently, fabrication of porous anodic films on various semiconductors such as titanium and indium phosphate has been received significant attention because of its exploitation as photocatalyst and chemical sensors. On the other hand, fabrication of mesoporous oxide of niobium has also special importance in the field of acid catalyst in addition to its potential utilization as a novel capacitor and devise applications. Therefore, an attempt has been done to fabricate ultra thick porous niobia having high specific surface area by anodizing of niobium, which was rarely reported. In the electrolytes containing HF, nano-porous amorphous films with the cell size of approximately 30 nm, pore size of 10 nm and maximum film thickness of 700 nm could be formed at relatively low current density as we reported previously (1). Dimension of the nano-porous film was independent of formation voltage except for the thickness of the barrier layer. To accelerate the oxide growth while preventing surface dissolution of the oxide, addition of some chemicals and cooling of the electrolytes were attempt. As a result, porous oxide films having thickness of 3-5 m were obtained. However, these oxide films indicate layered structure caused by fluorine ion incorporation into oxide/substrate interface which result in repeated abruption and re-growth of the oxide film as shown in Fig. 1. When a high formation voltage was applied, the thick porous layer up to 35 m was obtained after 2 hour anodizing. This thickness limitation was caused by substrate thickness of 70 m. The oxide film was identified as crystalline hexagonal Nb2O3 by XRD analysis. The cross section of the film shows granular and cellular oxide, which was grown under the layer-structured amorphous oxide (Fig. 2). Thus, we have succeeded for the first time to fabricate ultra thick porous niobium oxide film by anodization. Details of film growth mechanism and oxide properties will be discussed.(1) S. Ono et al, The ECS Meeting, Pits and Pores, Proceeding of the International Symposium, 2004, THE ELECTROCHEMICAL SOCIETY, INC. in press.

Fig.1 Cross section of anodic film formed on Nb in a fluoride solution at 20V for 2h.

Fig.2 Cross section of anodic film formed on Nb in a fluoride solution at 60V for 2h.


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Douglas P. Riemer,1 James Patrick McKinney,2 and Mark E. Orazem2*1

Hutchinson Technology, Inc., 40 West Highland Park Drive NE, Hutchinson, MN 55350 2 Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 * [email protected]

References 1. NACE International, Houston, Texas, RP0502-2002: Pipeline External Corrosion Direct Assessment Methodology, 2002. 2. E.W. Klechka, Pipeline Integrity Management and Corrosion Control, Materials Performance, 41 (2002) 24-27. 3. D.H. Kroon, External Corrosion Direct Assessment of Buried Pipelines, Materials Performance, 42 (2003) 28-32. 4. D. P. Riemer, Modeling Cathodic Protection for Pipeline Networks, Ph.D. dissertation, University of Florida, Gainesville, Florida, 2000. 5. D. P. Riemer and M. E. Orazem, Application of Boundary Element Models to Predict Effectiveness of Coupons for Assessing Cathodic Protection of Buried Structures, Corrosion, 56 (2000) 794-800. 6. D. P. Riemer and M. E. Orazem, Modeling Coating Flaws with Non-Linear Polarization Curves for Long Pipelines, in Corrosion and Cathodic Protection Modeling and Simulation, Volume 12 of Advances in Boundary Elements, R. A. Adey, editor, WIT press, Southampton, 2005, 225-259.


Buried oil and gas transmission pipelines are protected from corrosion by a combination of cathodic protection and coatings that reduce the current requirements for cathodic protection. Regular inspections, such as described in the NACE International Recommended Practice for External Corrosion Direct Assessment (ECDA),1 are required to ensure the integrity of the pipelines. The methodology behind ECDA relies heavily on close interval surveys of on-potentials (where the CP system is connected) and off-potentials (where the CP system is disconnected).2,3 The degree to which a coating holiday is detectable through a close-interval survey is not precisely known. The model CP3D, developed to model cathodic protection to pipeline networks,4-6 was extended to allow calculation of on- and off-potentials at arbitrary surfaces. When applied to the soil surface, this model can be used to ascertain the degree to which an arbitrary coating aw is detectable by such measurements and to thereby guide interpretation of ECDA indications. This paper will present the methods used to calculate potentials at arbitrary surfaces with the CP system connected and disconnected. These simulations have been used to simulate ECDA measurements and to identify general interpretation strategies. The approach presented illustrates the integration of comprehensive numerical simulation with techniques for assessment of pipe coating condition.

Symposium 9 - Oral

Models of Close-Interval Potential Surveys for Buried Pipelines

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Symposium 9 - Oral

Electrochemical and AFM investigations of dendritic growth and corrosion processes occurring in the course of the post-CMP cleaning in microelectronicsa

UPR 15 CNRS Interfaces et Systmes Electrochimiques, University P. et M. Curie, 4 Place Jussieu, 75252 Paris Cdex 05, France b Altis Semi-conductor, 224, Bd J. Kennedy 91105 Corbeil Essonnes Cedex, France [email protected]

E. Ostermanna, L. Beitoneb, C. Maceb, A. Paillereta*, C. Gabriellia

Over the last few years, copper has replaced aluminium in the fabrication process of inter-connexions in microcircuits with the help of the now well-established dualdamascene technique. This latter necessitates an intermediate step called chemicalmechanical process (or CMP) aimed at removing the excess metal deposits in order to get very smooth surfaces. The substrates are then subjected rst to a thorough cleaning using oxalic acid solutions and nally to a rinsing. Both procedures are grouped in the so-called post-CMP cleaning and they are aimed at removing abrasive particles, polishing residues, and at avoiding contamination. This nal step, during which acidic solutions are used, is actually highly critical. For example, unintentional dendritic growth phenomena were detected on favourable sites. They were actually found to result from the dissolution of a copper connexion via corrosion followed by the redeposition of copper on a neighbouring connexion. Among the various sources of copper corrosion involved in the post-CMP procedure, one can cite photo-assisted corrosion, galvanic corrosion involving the tantalum metallic barrier underlying below the copper layer as well as the drawing rules used for the various masks of the integrated circuits. In order to get a better understanding concerning the dendrite formation, we developed a comprehensive investigation using various electrochemical and imaging techniques. In this work, we relate mainly the precious contribution provided by atomic force microscopy (AFM) imaging carried out either in air or in-situ in oxalic acid containing cleaning solutions. The main results reported hereafter are focused on the morphological aspects of the dendritic growth as well as on the conrmation of a galvanic corrosion process occurring between copper and tantalum.



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Center for Electrochemical Engineering, Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA *[email protected]

Branko N. Popov*


Tremendous potential exists for the growth of metal nishing industry through the development of processes that are either environmentally friendly or are applicable at the nano scale. The chemical composition and the structure of electrodeposited metals, alloys and composites control their functional properties. This paper will discuss some of the theoretical and experimental studies done to achieve such coatings. Nanostructured alloys and innovative composite materials were developed through adaptation of existing bath chemistries. Process development was based on techniques developed in our laboratories such as: under potential deposition (UPD) of monoatomic metal layers, autocatalytic reduction and potentiostatic pulse (PP) plating of layers of amorphous and crystalline nanostructured metals and alloys. The development processes have been optimized based on superior corrosion properties of the coatings. Further renement of the coating process was accomplished through the development of rst principles based theoretical models. Electrodeposited nanostructured alloys and composites nd applications in metals and surface nishing industry. This paper will feature development of electrochemical deposition processes to synthesize secondary and ternary alloys such as Ni-Zn-X (X=P or SiO2). These materials were targeted as a replacement for Cd deposition and can inhibit corrosion and completely eliminate the hydrogen induced cracking.

Symposium 9 - Oral

Electrodeposition of Alloys with Superior Corrosion Properties

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Symposium 9 - Oral

Structure and corrosiob resistance of pulse plated nanocrystalline zinc and zinc alloy coatingsInstitute of Chemistry, A.Gotauto 9, Vilnius LT 01108, Lithuania, [email protected]

R. Ramanauskas, L. Gudaviit, R. Juknas, A. Kalinienko, O. it

The importance of texture and lattice defects on corrosion behavior of Zn and Zn alloy coatings was revealed in our previous studies [1,2], however, the role of surface morphology remains unclear. It is known that almost all mechanical properties of metals can be effectively improved by rening the grain size. However, the small grain size and the high volume of fraction of grain boundaries may result in corrosion performance different from that of polycrystalline materials. Non-stationary (pulse) electrodeposition can be used as a mean to produce unique structure, i.e. coatings with a structure and properties not obtained by direct current (dc) plating. The present study was aimed to determine the inuence of pulse electrodeposition parameters on the surface morphology, lattice imperfection and corrosion properties of Zn and Zn-Ni (12%) coatings, deposited in alkaline cyanide-free solution. AFM was applied for surface morphology examination, XRD measurements were carried out for phase composition, texture and lattice defects analysis, while electrochemical techniques were used for corrosion performance research. Pulse electrodeposition yielded a ne-grained and more homogeneous surface appearance of Zn and Zn alloy deposits, with relatively indenite grain boundaries and smaller grain size. A special attention was given for a precise determination of the latter parameter, which was obtained from AFM studies and XRD line broadening measurements. Line broadening always comprises size and strain broadening components. An attempt has been taken to evaluate separately these two factors, what resulted in attainment of very similar grain size values, obtained by two different (AFM and XRD) methods. Non-stationary electrodeposition yielded the reduction of grain size both for Zn-Ni alloy from ~ 90 nm for dc. plated, to 30-40 nm for pulse plated samples, and for Zn coatings: from ~ 65 nm to ~ 35 nm, respectively. The corrosion currents of electrodeposited coatings were determined in aerated NaCl+NaHCO3 solution and it appeared that pulse plated Zn and Zn alloy samples possessed from twice to threefold lower corrosion currents with respect to dc plated ones. Zn and Zn-Ni coatings, which possessed lower grain size, higher uniformity of grain distribution and higher number of lattice imperfections exhibited higher corrosion resistance. 1. R. Ramanauskas, Appl. Surf. Sci., 153 (1999) 53. 2. R. Ramanauskas, L. Gudaviit, A. Kalinienko, R. Juknas, J. Solid State Electrochem., 9 (2005) 900.



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Instituto Nacional de Engenharia, Tecnologia e Inovao - INETI DMTP - Electrochemistry of Materials, Pao do Lumiar, 22 1649-038 Lisboa Portugal 2 Instituto Superior de Engenharia de Lisboa, DEM Dept. Mechanical Engineering R. Conselheiro Emdio Navarro, 1 1950-062 Lisboa Portugal 3 Instituto Superior Tcnico, Dept. Chemical Eng., Av. Rovisco Pais, 1049-001 Lisboa 4 Centre National de la Recherche Scientique CECM 15 Rue Georges Urbain F94407 Vitry-sur-Seine Cedex - France *[email protected]

C.M. Rangel1*, T.M. Silva2,3 M. da Cunha Belo3,4

The role of alloying elements on the susceptibility to Stress Corrosion Cracking (SCC) of stainless steels is well documented in the literature, particularly the effect of molybdenum. However, the role of this element on lm properties is not well understood. The approach used in this work includes the study of high purity austenitic Fe-Cr-Ni model alloys, with and without Mo additions, as well as the effect of deformation. A Mott-Schottky study was conducted complimented with polarization measurements in SCC environments containing chloride ions. Results show that Mo promotes stress corrosion cracking in the otherwise nonsusceptible alloy conrming previous data. SCC is associated to the presence of a semiconducting lm, which is predominantly p-type in character (transport controlled by cationic vacancies). The doping density estimated for samples with equivalent levels of plastic deformation was found to be larger for the Mo containing alloy. Mechanisms are discussed focussing the metal/lm interface and the possible effect of dislocations on lm properties, for the SCC case. The role of Mo regarding other types of localised corrosion such as pitting will be also discussed, on the basis of phenomena in the lm/solution interface.


Symposium 9 - Oral

Susceptibility to Stress Corrosion Cracking of Stainless Steels: The Role of Molybdenum

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Symposium 9 - Oral

Mechanism of corrosion protection by conducting polymersMax-Planck-Institut fr Eisenforschung, Max-Planck-Str.1, D-40237 Dsseldorf, Germany *[email protected]

Adam Michalik, Grayna Paliwoda-Porebska, Michael Rohwerder*


Organic coatings are an efcient way to protect metal based products from corrosion. However, for reliable long-term performance pigments that release corrosion inhibiting substances need to be added to the organic coating. The most efcient pigments are those containing chromates (usually in form of strontium chromate), but because of their toxic and carcinogenic nature their use has to be progressively decreased. But nearly all powerful inhibitors may have detrimental effects on environment, when released in substantial amounts. Since in basically all pigments the release of inhibitors is based on leaching, coatings need to be highly pigmented to ensure a sufcient presence of inhibitors over years, and, of course, inhibitors are constantly released into the environment, even when they are not needed. Hence, novel approaches are desperately sought for. The potential of conducting polymer coatings for corrosion protection is a topic of current controversy. A number of possible protection mechanisms are proposed. The reason for this is that the efcacy of conducting polymers very much depends on how they are used and on the conditions of the corrosion experiment, i.e. depending on the exact conditions a conducting polymer may have excellent protection capability or may lead to a disastrously enhanced corrosive attack. This paper will focus on conducting polymer coatings with self-healing capability and on how to counteract the negative properties of the conducting polymer, so as to truly benet from the positive properties.


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M. Santamaria1, F. Di Quarto1, P. Skeldon2 and G. E. Thompson2Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universit di Palermo, Viale delle Scienze, 90128 Palermo, Italy 2 Corrosion and Protection Centre, School of Materials, The University of Manchester, PO Box 88, Manchester, M60 1QD, UK1

References 1 F. Di Quarto, M. Santamaria, C. Sunseri, Photoelectrochemical Techniques in Corrosion Studies, in Analytical Methods in Corrosion Science and Technology, Edited by P. Marcus and F. Mansfeld, Ch. 18, pp. 697-732. Taylor and Francis, Boca Raton: (Fl,USA).


The study of anodic lms grown on Al alloys is of interest for different reasons. From a practical point of view, the presence of small amounts of transition metal solute increases dramatically the pitting potential of the metallic substrate, hence improving the corrosion resistance of the alloy. On the other hand, the possibility of tailor making anodic lms with higher dielectric constant with respect to alumina lms makes such alloys relevant to the eld of electrolytic capacitors. Thus, there is need for research aimed at increased understanding of the physico-chemical properties of amorphous oxide lms on Al alloys. Of relevance, it has been proposed recently that the band gaps of crystalline binary oxides correlate with the electronegativities of their constituents, thus suggesting the possibility to predict the band gaps of ternary oxides (1), using an average electronegativity parameter for the cationic group and hence, tailoring of oxide properties. Moreover, the proposed correlation has been extended to amorphous oxides, taking into account the effect of the amorphous structure on the band gap of the corresponding crystalline oxide (1). In this work, the results of an extensive study on growth and characterisation of anodic lms on Al - valve metals alloys, containing niobium, titanium, tantalum and tungsten additions, are presented. Both in-situ Photocurrent Spectroscopy (PCS) and ex-situ Transmission Electron Microscopy (TEM) and Rutherford Backscattering Spectroscopy (RBS) are applied to the lms grown in acidic and quasi-neutral solutions in order to probe the structure, composition and electronic properties.

Symposium 9 - Oral

Effect of composition on the photoelectrochemical behaviour of anodic oxides grown on aluminium alloys

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Symposium 9 - Oral

Assessment of water transport in organic coatings using a.c. impedance in in organic coatings using Assessment of water transport ionic liquids A. a.c. impedance in2,3 and G. P. Bierwagen2 M. Simes1*, D. E. Tallman ionic liquids11

A. M. Simes1* D. E. Tallman2,3 and G. P. Bierwagen2 ICEMS / Chemical Engineering, Department, Instituto Superior Tcnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal; *[email protected] ICEMS / Chemical Engineering Department, Instituto Superior Tcnico, Av. Rovisco Pais, 1049001 Lisbon, Portugal; *[email protected]

2

2 NDSU, Department of Coatings and Polymeric Materials, 1735 NDSU Research Park Drive, NDSU, Department of Coatings and Polymeric Materials, 1735 NDSU Research Park Drive, Fargo, ND 58105-5376, USA Fargo, ND 58105-5376,USA 3

NDSU, Department of Chemistry, Fargo, ND 58105, USA

3

NDSU, Department of Chemistry, Fargo, ND 58105, USA

Water absorption is a major cause for loss of barrier properties and constitutes the first step to delamination of organic coatings. The process can be monitored by measuring the variations in the coating capacitance, by means of a.c. impedance. These measurements, however, are typically made in an aqueous solution with some salt to make it ionically conductive, which excludes the measurements of the drying processes. The advent of room temperature ionic liquids (ILs) opens new possibilities in this area, particularly in what respects the drying process. The work reported consists of monitoring an epoxy clear coat during the process of water uptake in NaCl 0.05M and of the subsequent drying in an ionic liquid. Estimation of the diffusivity D was made assuming Fickian diffusion, from the slope of a plot of the capacitance, Ct, versus the square root of time: Ct = C0 + ( C C0 ) 1 4 Dt , in which C and C0 represent L the capacitance of the saturated and of the dry coating, respectively, and L the average coating thickness. The study was carried out using a non-pigmented epoxy coating applied on aluminium panels, using as electrolytes either 0.05M NaCl, or an IL. It was observed that the coating capacitance variation was approximately reversible (Fig.1), with the capacitance increasing in the salt solution and decreasing in the IL. With a hydrophilic IL, 1-Butyl-1-methylpyrrolidinium trifluoromethanesulfonate, the apparent diffusivity was determined as 7-8x10-14 m2s-1 for the absorption step and ~19x10-14 m2s1 for the drying step, revealing that the diffusion of water in the drying process is an easier process than the absorption step. Repetition of the soaking/drying cycle revealed a nearly total reversibility, although the capacitance was slightly higher in later cycles. This could be due either to irreversible opening of the coating polymer structure caused by water ingress or to trapping of ions from the solution inside the coating matrix. The test was also repeated in a hydrophobic IL, 1-butyl-1 methyl pyrrolidinium tris(pentafluoroethyl) trifluorophosphate. The features were essentially the same, although the diffusivity was lower in the drying step. This new technique opens possibilities to the study of organic coatings and of the processes that affect degradation of coated structures by corrosion.Acknowledgements The work was done under AFOSR grant F49620-02-1-0398. A.M.S. acknowledges the sabbatical scholarship granted by the Portuguese Foundation for Science and Technology.


1


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1

ut 59/67 Hungary; 2 University of Zagreb, Faculty of Chemical Engineering and Technology, 10000 Zagreb, Mirulicev trg 19, Croatia Hungarian Academy of Sciences, Chemical Research Center, 1025 Budapest, Pusztaszeri 2 *[email protected] ut 59/67 Hungary; University of Zagreb, Faculty of Chemical Engineering and Technology, 10000 Zagreb, Mirulicev trg 19, Croatia *[email protected]

J. Telegdi1*, H. Otmacic2, T.NANOLAYERS Keszthelyi1, E. Stupnisec-Lisac2, E. Klmn1 1* 2 1 2 1 1 J. Telegdi , H. Otmacic , T. Keszthelyi , E. Stupnisec-Lisac , E. Klmn Hungarian Academy of Sciences, Chemical Research Center, 1025 Budapest, Pusztaszeri

b: 2D structure of C18 hydroxamic acid monolayer on copper visualized by AFM

a b C18 Hydroxamic acid nanolayer on copper a: SFG spectra of LB monolayer and of time-dependent SAM formation


Special hydroxamic acid amphiphiles (C10-C18) were used in self assembled molecular layers (SAM) as well as in Langmuir-Blodgett (LB) films on copper surface. The importance of the time in the SAM formation as well as of the carbon chain length in the amphiphilic molecules was in the focus of the experiments. The layer formation was characterized by sum frequency vibrational spectroscopy (Fig.1a).The efficiency of nanolayers was measured by different electrochemical techniques (EIS, polarization) and by microcalorimetry. The morphology (Fig.1b.) and the consequence of corrosion processes were visualized by atomic force microscopy. The comparative analysis of data proved that the increase in time of SAM formation up to 1 hour (under the given experimental conditions) and in the layer number in LB films enhanced the stability as well as the efficacy of nanolayers. The length of the carbon chain in the SAM layer has not significant impact on the efficiency in corrosive environment.

Symposium 9 - Oral

Copper corrosion inhibiton by amphiphiles in nanolayers COPPER CORROSION INHIBITON BY AMPHIPHILES IN

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CHARACTERIZATION OF AN OXIDE FILM Characterization of an oxide lm by eis and leis BY EIS AND LEISSymposium 9 - OralIsabelle Frateur1*, Mark E.1Orazem2E. Bernard2Tribollet3*, Tribollet3*, and 3* Isabelle Frateur *, Mark *, Orazem *, Bernard and Vincent Vivier1

Laboratoire de Physico-Chimie des Surfaces, CNRS UMR7045, ENSCP, Vincent Vivier3* 11 rue Pierre et Marie Curie, 75005 Paris,UMR7045, ENSCP, Laboratoire de Physico-Chimie des Surfaces, CNRS France 2 Department of Chemical Engineering, UniversityFrance 11 rue Pierre et Marie Curie, 75005 Paris, of Florida, Gainesville, FL, 32611 2 Department of Chemical Engineering, University of Florida, 3 Laboratoire Interfaces et Systmes Electrochimiques, CNRS UPR 15, Gainesville, FL, 32611 Universit Pierre et Marie Curie, Case 133, 4 place Jussieu 3 Laboratoire Interfaces et Systmes Electrochimiques, CNRS UPR 15, 75252 Paris Cedex 05, France Universit Pierre et Marie Curie, Case 133, 4 place Jussieu Fax: +33 1 44274074, E-mail : [email protected] 75252 Paris Cedex 05, France *ISE member1

Fax: +33 1 44274074, E-mail : [email protected] *ISE member


A metal or metallic alloy coated by a passive film can be characterized in the high frequency range by an impedance corresponding not to a pure capacitance but to a CPE. The origin of this CPE behaviour is not unique. [1] It was shown that the high frequency dispersion originates from non-uniform current and potential distributions on the disk surface that further lead to a distributed and frequency-dependent ohmic resistance [2,3], whereas, for lower frequencies, global CPE behaviour is always observed. This CPE behaviour in the medium frequency range can be explained by a gradient of the oxide layer conductivity perpendicularly to the surface. Thus, according to Youngs theory [4], the conductivity (x) within the oxide film decreases exponentially due to non-stoichiometry of the oxide layer: (x)=(0)exp(-x/), where corresponds to a characteristic length. The Young impedance for such a gradient can be expressed as: 1 + j exp( p 1 ) p ZY = ln 1 + j jCY

where p = / d is the relative penetration depth, CY = 0 S / d the oxide film capacity, and = RCY = 0 / (0) the time constant. [5,6] LEIS and EIS measurements were performed on a Fe-17 Cr surface immersed in deaerated, pH 4, diluted Na2SO4+NaCl solution. The Young impedance was used to regress the experimental LEIS and EIS data. The regression procedure yields for CY a value of a few F corresponding to an oxide film thickn

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