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Anodic dissolution behaviour and surface texture development of cobalt under electrochemical machining conditions
Accepted Manuscript Title: Anodic dissolution behaviour and surface texture development of cobalt under ElectroChemical Machining conditions Author: M. Schneider N. Schubert S. H¨ohn A. Michaelis PII: DOI: Reference:
S0013-4686(13)00977-8 http://dx.doi.org/doi:10.1016/j.electacta.2013.05.070 EA 20558
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
22-2-2013 15-5-2013 18-5-2013
Please cite this article as: M. Schneider, N. Schubert, S. H¨ohn, A. Michaelis, Anodic dissolution behaviour and surface texture development of cobalt under ElectroChemical Machining conditions, Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.05.070 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Anodic dissolution behaviour and surface texture development of cobalt under ElectroChemical Machining conditions M. Schneider*1, N. Schubert2, S. Höhn1, A. Michaelis1,2 1
Fraunhofer IKTS Dresden, Winterbergstr. 28, 01277 Dresden, Germany Institute of Material Science, TU Dresden, Helmholtzstr.7, 01067 Dresden, Germany
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*Corresponding author: e-mail: [email protected] ; Tel.: +49 (0)351 2553 7793, fax: +49 (0)351 2554 108
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Abstract:
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The anodic dissolution of hcp-cobalt in sodium nitrate solution under near-ECM conditions was investigated in-situ by using a specially designed flow channel cell. The in-situ investigation was focused on current densities of 5A/cm2 and 10A/cm2. Cobalt dissolves actively as cobalt (II) ion with a current efficiency close
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to 100%. In contrast to copper, the current-voltage relation does not show an active-passive transition. The dissolution strongly depends on the crystallographic orientation of the grains. The lowest dissolution rate was
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observed on plains vicinal to the basal plane {0001}. No gas evolution was observed. The surface roughness
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is determined by the micro roughness on the individual grains.
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Keywords: anodic cobalt dissolution, Electrochemical Machining (ECM), grain orientation, insitu microscopy, surface texture
1. Introduction
Pure cobalt metal is used as anode material in electroplating and electroforming or as a catalyst in the chemical industry [1,2] and plays an important role as binder phase in cemented carbides [3]. Cobalt-base alloys have been applied as wear-resistant (e.g. StelliteTM) or high-temperature materials or in medical engineering (e.g. dental and orthopaedic prostheses) [1,2,4-7]. Primarily, cemented carbides and wearresistant alloys are machinable but cause extensive wear to work tools. Electrochemical Machining (ECM) is a non-conventional near-net-shape manufacturing process which is based on the controlled anodic dissolution of metals. ECM is well established as a process in machine alloyed steels [8,9], superalloys [10,11] or titanium [8,12], but not for cemented carbides. Two of the main advantages of the ECM-process are:
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(i) The metal removal of the work piece is independent of the mechanical properties (hardness, brittleness, strength, ductility) of the material. (ii) The wear of the work tool is eliminated, because no physical contact between the work piece and
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work tool takes place. Due to these advantages, ECM can be a prospective powerful process to engineer high-tensile and hard materials (e.g. cemented carbides). This being the case, it is absolutely necessary to understand the anodic
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dissolution behaviour of cobalt under near-ECM conditions. ECM is carried out far from a thermodynamic
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equilibrium. Current densities of 5A/cm2 up to 300A/cm2 and consequently high over voltage are typical for this technology. Table 1 shows the typical process parameters of ECM [13-17]. However, a detailed
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knowledge of the anodic behavior is also important for catalytic reaction and corrosion of cobalt.
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Table 1
This study focuses on the in-situ observation of the anodic dissolution of cobalt under near-ECM conditions.
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The recently published design of an electrochemical flow channel cell suitable for optical in-situ
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investigation [18-20] was used to observe the processes and the surface topography development during the
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anodic dissolution. This cell concept allows the investigation of non-steady-state processes during the anodic dissolution under near-ECM conditions and the localization of, e.g.: ! ! the oxygen evolution and the gas bubble formation over time ! ! the preferred sites of oxygen evolution, ! ! the formation and development of the viscous layer in front of the surface over time, ! ! the preferred sites of dissolution and time-resolved topography development. Such investigations were successfully performed in case of the anodic dissolution of copper and recently presented by the authors [19,20]. Therefore, the investigation of cobalt provides an opportunity to prove the generalizability of the results achieved.
2. Experimental 2.1. Materials
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The investigated material used was pure cobalt (Co 99.9%) purchased by Goodfellow [21]. The chemical composition is shown in Table 2:
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Table 2
Cylindrical cobalt rods of approximately 3 cm in length and 3 mm in diameter were prepared by wet
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grinding with SiC-paper (up to P1200). Following this, the samples were mechanically polished with a
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diamond suspension (6µm, 3µm and 1µm grit) and were finally chemically polished with a SiO2-suspension. All samples were next etched for a short time in a metallographic etchant consisting of 100ml methanol +
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6ml HNO3 (65%) at room temperature. Lastly, the samples were cleaned by rinsing water and degreased with ethanol. In all electrochemical experiments the used electrolyte was 2.9 mol/l sodium nitrate solution (250g/l,
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2.2. Methods
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pH5.6, ! =160mS/cm).
2.2.1. Electrochemical flow-through microcapillary cell
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Preliminary studies to the electrochemical behaviour of cobalt in sodium nitrate solutions were carried out in an electrochemical flow-through microcapillary cell (Fig.1) using a 3-electrode design. This microcell was specially designed for material surface investigations at large current densities by A. Moehring [22] and firstly applied to the dissolution of iron under near-ECM conditions by M.M. Lohrengel and co-workers in 2003/04 [23,24]. The microcell used in this work was an in-house reproduction of the cell design described by A. Moehring [22]. Due to the capillary diameter (d! 0.05cm), the area of the working electrode is very small (A! 0.002cm2). A gold wire situated in the outlet channel of the cell works as counter electrode. The distance between the working electrode (anode) and the counter electrode (cathode) in the used cell amounts to l! 0.1cm. The cell concept allows high current densities (j! 100A/cm2) by using commercial potentiostates (Jmax=1A). Figure 1
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2.2.2. Channel flow cell and in-situ microscopy
The aforementioned channel flow cell is schematically shown in Fig.2 [18,19]. The sample (anode,
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A=0.07cm2) was implemented in a lower section of the cell. A gold disc with opposite mounted in
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the glass lid works as cathode (A=0.13cm2). In the centre of the cathode a small conical glass lens (Ø! 0.3mm) was embedded which allows the in-situ observation of the surface during the
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electrochemical experiments.
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The gap between the anode and the cathode was adjusted to approximately 0.35mm. A detailed description of the cell, the ancillary equipment and the measurement configuration has been
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recently published in [18].
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2.2.3. Uv/vis-spectroscopy
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Figure 2
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The uv/vis-spectroscopy was carried out by an EPP2000 (StellarNet) spectrometer, which operates in a wave length range between 200-800nm. The electrolyte used was caught on the outlet of the flow channel cell. The addition of nitroso-r salt C 10 H 5 NNa2O8 S 2 leads to the formation of a red cobalt complex, which is wellsuited to determine the cobalt concentration in aqueous solutions (see e.g. [25,26]). According to the literature [25,27] the absorption intensity of this complex was evaluated at 540nm, although the peak maximum is at a lower wave length. However, at a lower wave length the reagent itself tends to be of a high absorption. The method was calibrated by a measurement of a concentration series with defined cobalt concentration [28].
2.2.4. Orientation determination by Electron Back Scattering Diffraction (EBSD) EBSD was used to analyse the orientation of the individual grains of the polycrystalline cobalt samples. The method is based on the diffraction of the quasi-elastic backscattered electrons in a near-surface range (depth 4
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! 100nm). The resulting diffraction pattern depends on the crystallographic orientation of the diffraction planes. The patterns of the diffraction were detected by a phosphor screen and a low light CCD-camera. The samples were tilted 70° to get an optimal yield of the backscattered electrons. The measurements were
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carried out using a scanning electron microscope NVision 40 with a Schottky-emitter (Fa. Carl Zeiss SMT GmbH) and a HKL NordlysF-detector. The acceleration voltage was 20kV. The generated raw datasets were
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analyzed by using the software package CHANNEL 5 (Oxford Instruments GmbH). The grain orientations
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are described by Euler-angles (! 1, ! , ! 2) and Miller-indices (h k l) and are presented as inverse pole figures.
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3. Results and discussion 3.1. Preliminary study of cobalt dissolution
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In a preliminary study the electrochemical behaviour of cobalt is investigated by cyclovoltammetry (CV) and linear sweep voltammetry (LSV). Fig.3a shows a CV on cobalt in the sodium nitrate electrolyte carried out in
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the flow-through microcapillary cell. The current density linearly increases between 1 and 10V. The
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and not in the transpassive range.
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extrapolation of the current density down to the x-axis shows that the cobalt dissolution starts in the active
A calculation of the ohmic resistant from Fig.3a results to R !
!E ! 500 ! . Considering the cell geometry !J
and the specific electrolyte resistance ( ! ! 6.25 ! cm ) the expected resistance of the electrolyte should
be R !
! !l ! 312 ! . That means the calculated resistance is slightly higher than the estimated electrolyte A
resistance. This fact can be explained by the formation of a viscous and very thin film of dissolved cobalt species in front of the sample surface. However, it has to be emphasized that the estimation of the electrolyte resistance can only be a rough approximation which is caused by the changing area of the working electrode during the experiment and the non-trivial geometry inside the microcapillary cell.
Figure 3
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A more detailed impression of the early stage of dissolution is given by the LSV on cobalt shown in Fig.3b. The absolute value of the current density is logarithmically plotted versus the potential. The potential
ECo / Co 2! ! ! 0.28VSHE
O 2 ! 2 H 2 O ! 4 e ! ! ! ! 4OH !
EO
/ OH !
) between the
(at pH5.6)
! 0.90VSHE
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2
/ OH !
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following reactions:
Co ! ! ! Co 2 ! ! 2 e !
2
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Ecorr ! ! 0.26V Ag / AgCl ! ! 0,06VSHE represents the mix potential ( Ecorr ! ECo / Co 2! / O
(1) (2)
At a higher negative overvoltage the hydrogen reaction dominates the cathodic process.
2 H ! ! 2e ! ! ! ! H 2
E H ! / H ! ! 0.33VSHE
(at pH5.6)
(3)
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2
The anodic part of the semi-logarithmic plot shows two different slopes. In region I the slope amounts to
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50mV/dec. This value is close to the anodic Tafel-factor b of a charge-transfer controlled metal dissolution according to Eq.1 ( b ! 118mV 2 for Me/Me2+) [29]. At potentials E>-0.2V (region II) the slope of the
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curve has clearly changed. A treatment of this section of the graph (region II) in decadic notation results in a The estimation of the resistance
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linear increase of the current density versus the applied potential.
approximately corresponds to the order of the electrolyte resistance again. Considering the thermodynamic
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point of view neither other anodic reactions than described in Eq.1 take place nor other reaction products exists in the potential range presented in Fig.3b. At higher potentials the following reactions are thermodynamically possible according to Fig.4 [30]: 2 Co 2 ! ! 3 H 2O ! ! ! 2 Co (OH ) 3 ! 3 H ! ! e !
E 0 ! 1 .14V SHE
(at pH5.6)
(4)
2 Co (OH ) 3 ! ! ! CoO 2 ! H 2 O ! H ! ! e !
E 0 ! 1 .62V SHE
(at pH5.6)
(5)
The literature about the existence of CoO2 is contradictory [30]. It is assumed that the cobalt peroxide is easily decomposed as follows: (6)
2CoO 2 ! 3 H 2 O ! ! ! 2Co (OH ) 3 ! 1 2 O 2
Figure 4
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With respect to the high concentration of nitrate anions in the electrolyte the formation of cobalt nitrate can be assumed according to:
Co 2 ! ! 2 NO 3! ! ! ! Co ( NO 3 ) 2
(7)
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This cobalt nitrate can be several hydrated ( Co ( NO 3 ) 2 ! xH 2 O ) [31-34]. In the case of x=6, Funk [32] reported a melting point of T=57°C. Otherwise, the nitrate anions can be reduced on the cathode according
NO 3! ! 6 H 2 O ! 8e ! ! ! ! NH 3 ! 9OH !
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NO 3! ! 7 H 2 O ! 8e ! ! ! ! NH 4 OH ! 9OH !
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to the literature [35]:
(9) (10)
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NO 3! ! 10 H 2 O ! 8e ! ! ! ! NH 4! ! 3 H 2 O ! 10OH !
(8)
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3.2. Investigation with the channel flow cell
The electrochemical experiments in the channel flow cell (schematically shown in Fig.2) were carried out
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under current control with a 2-electrode design. Fig.5 shows the potential response to the applied current
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density. Under the chosen conditions the electrolyte flow rate has apparently no influence on the potential response. Additionally, the current-voltage relation is strictly linear. The slope of the interpolated line !U ! 0.27 ! cm ² . Due to the electrode gap between anode and cathode an ohmic resistance of !i
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amounts to
! ! 7.6 ! cm can be calculated which is in good agreement with the electrolyte resistance. The difference to
the specific resistance in Fig.5a is caused by the well defined hydrodynamic conditions in front of the electrode which enhances the transport from dissolution products. More interestingly, the fact that the specific resistance is only caused by the electrolyte resistance confirms the statement of the active dissolution of cobalt in the described current-voltage range. In contrast to earlier investigations on copper, the authors did not observe the characteristic active-transpassive transition which is identified by a potential offset in the potential-current curve [19,20,36,37]. Nevertheless, the topography development is strictly dependent on the applied current density.
Figure 5a and 5b
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The current controlled pulse experiments were complementary performed in the flow-through microcapillary cell. Thereby, the waste electrolyte was caught and the cobalt content was analyzed by uv/vis-spectroscopy. Based on this analysis the authors calculated the mass loss muv/vis of cobalt (Fig.5b). Additionally, the
3d-image analysis. The removed cobalt mass mgeom was calculated according Eq.11: geom
! !
Co
!V
(11)
r
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m
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dissolved volume Vr of the sample was analysed geometrically by using a stereo microscope (Keyence) and a
Whereby, the factor ! Co describes the density of cobalt.
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The results are shown in Fig.5b. Further on, Fig.5b shows the theoretical mass loss mcoul of cobalt according to Faraday´s law under assumption of various valences z. The results allow the conclusion that:
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! ! The valence of cobalt dissolution is z=2 ! ! The current efficiency ! ! 100%
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In the following section 3.2.1 and 3.2.2 the in-situ investigation of the surface texture development under
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near-ECM conditions is focused on low current densities (j=5A/cm2 and 10A/cm2).
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3.2.1. In-situ investigation at 5A/cm2 and post-experiment SEM Fig.6a shows the EBSD-mapping of the sample section which was in-situ observed during the
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electrochemical double pulse experiment schematically presented in Fig.6c. Fig.6b shows an inverse pole figure which indicates the crystallographic orientation of the numbered grains from Fig.6a (The Euler angle and the Miller indices of the numbered grains are shown as a table in the section appendix.). Additionally, the orientation of the unit cell with respect to the surface normale (z-view) is shown in Fig.6b. Fig.6c presents the current regime of the pulse experiment. An electric charge q=5C/cm- is consumed during a single pulse. This value corresponds to a metal removal rate of 17µm/s assuming a current efficiency of
! =100% and a cobalt valence of z=2. The Roman numerous shown in Fig.6c indicates the point in time of the experiment by which images are shown in Figs.6d-h. In Figs.6d-h the network of white broken lines supports the identification of the individual grains which are numbered in Fig.6a. The white arrows are hints of special sites on the surface, whose development will be specifically discussed. The right arrow marks a triple point between three grains. Fig.6d shows the initial state of the surface under the flowing electrolyte. The surface is well prepared and ready for the experiment. Fig.6e shows the situation after 0.3s during the 8
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first pulse. Generally, the field of view is darkened when the pulse starts. This darkening is caused by the severe dissolution of metal and the formation of a product film in front of the anode. However, the grains numbered with the Arabian numeral 1 appear brighter than all other grains. It looks as if the metal
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dissolution on these grains is significantly less than on all other grains. According to the EBSD investigation the authors assume that these grains differ only slightly to the basal plane as shown in Fig.6b. The complete removal of the dissolved dissolution products by the streaming electrolyte results in only a slight brightening
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of the field of view during the pulse pause (Fig.6f). The comparison between Fig.6e and Fig.6f shows
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evidence that the darkening cannot be exclusively caused by the formation of a product film. The darkening can also be caused by the development of differing surface roughness and light scattering respectively,
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depending on the crystallographic grain orientation. In this regard the development of a small disorientated area (marked by the white arrow in the mid-position) in a grain similar to the basal plane is an interesting
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detail. The small area appears darker and larger with increasing pulse time (see also later on Figs.6g and h). This chronology clarifies a part of the darkening by diffused light scattering caused by surface roughening.
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A formation of gas bubbles as a side reaction is not observable. This is completely different from the recently published results on copper [19]. The absence of the anodic oxygen evolution supports the theory of the
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active dissolution of cobalt according to equation 1. Very recently Rataj et al. reported that the oxygen
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evolution on cobalt does not occur until current densities of j>10A/cm2 [38] are achieved. The discrimination on the different grains is enhanced during the 2nd pulse (Fig.6g). It is now possible to distinguish the appearance of the grains in bright (e.g. grain 1), medium (e.g. grain 5) and dark grains (e.g. grain 3). After the pulse experiment (Fig.6h) the dark-bright differences seem to be mainly caused by the defocusing of severe anodic dissolved regions on the surface.
Figure 6
A comparison of all images (Figs.6d-h) shows that the plane vicinal to the basal plane is less attacked in comparison to other grains. Nevertheless, small parts of this grain are more strongly affected. A correlation between the microstructure and dissolution seems natural. After the pulse experiment the sample has been ex-situ investigated by SEM. Fig.7 shows the SEM-images of the area discussed above. Fig.7a gives an
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overview of the surface range which was also observable in the flow cell. The black broken line network and the Arabian numerals make the recognition easier. The results complete and confirm the in-situ observation in an outstanding manner. As already observed during the experiment (Fig.6) grains vicinal to the basal plane
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are less affected by anodic dissolution (Fig.7b). Moreover, grain 1 shown in Fig.7b appears smoother in comparison to the surrounding grains. The black arrows mark the same sites on the surface as the white arrows in Fig.6. The severely attacked small area in the upper part of grain 1 (Fig.7b) is obviously a range
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with a significantly different orientation to grain 1. It could be that the grain was not clearly separated as an
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individual grain after the EBSD investigation. However, the completely different attack is a clear sign of a significant divergence to the orientation vicinal to the basal plane of the majority of the visible grains. The
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former triple point of grains (right arrow) shows a site of severe attack, as expected. Additionally, grain 1 is especially affected on the grain boundary marked by the left arrow. That the anodic dissolution is strongly
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influenced by the crystallographic orientation of the material has been well known for a long time in corrosion research [39-44]. In the early 1970s Landolt and co-workers reported the influence of the
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crystallographic orientation on the copper dissolution under near-ECM conditions [45]. Owing to the
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thermodynamic point of view the surface energy ! hkl decreases with increasing packing density of the planes [46]. Therefore, in case of hcp-Co the basal plane {0001} is most stable again with anodic dissolution. This
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fact is confirmed by the author’s investigation (Fig.7b). Planes vicinal to the basal plane are only slightly attacked in comparison with other planes. Additionally, the morphology of the other observed grains tends to the formation of terraces during the anodic dissolution, according to the terrace-ledge-kink (TLK) model [46]. Preferred sites of anodic dissolution are crystal defects as grain boundaries, stacking faults and dislocations as shown in Fig.7c. This fact can be also expressed by the stored energy ! . In the case of the grain boundaries the ! gb depends on the grain boundary structure which can be expressed by the misorientation angle ! between the adjacent grains and the reciprocal density of the coincidence site ! [4751]. It can be generally said that atoms located at grain boundaries are less solidly bound and can be preferentially anodic dissolved [46,52]. However, it has to be stressed that the relationship between the grain boundary energy ! gb and ! and ! is very complex. In the late 1970s Goux pointed out that the grain boundary energy ! gb is a scalar quantity while corrosion attack also depends on the direction of the grain boundary plane [53]. The fact remains, that the authors observed a different dissolution kinetic of grain boundaries, 10
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which is explained by different grain boundary energy. Otherwise, grain boundaries are often segregated sites of material impurities, which can also change the dissolution kinetic. However, this fact can be abandoned in view of the pure cobalt material used.
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The visibly isolated etch patterns in grains (marked by arrows in Fig.7c) are caused by penetration points of dislocations. The shape of the pattern is depending on the orientation of the plane were the dislocations on the surface penetrates. According to Detavernier et al. [54] who reported that the dislocation energy
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! dis>! gb>! hkl, it can be assumed that the anodic dissolution kinetic follows the same rule. This is in good
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agreement with the impression that the dislocations are severely attacked (Fig.7c). The morphology of grain
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1 suggests the crystallographic misorientation from the [0001] direction.
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Figure 7
3.2.2. In-situ investigation at 10A/cm2 and post-experiment SEM
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The set of experiments described in section 3.2.1 was repeated on a freshly prepared sample using an applied
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current density of 10A/cm2. Fig.8 illustrates the results in the same manner as in section 3.2.1. The EBSDmapping of the observed sample section, the inverse pole figure and the current density versus time regime
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are presented in Figs.8a-c. The consumed electric charge was q=10C/cm2 per pulse which corresponds to a metal removal rate of 34µm/s assuming a current efficiency of ! =100% and a cobalt valence of z=2. Fig.8d shows the initial surface state observed under streaming electrolyte. The white network shows better identification of individual surface sites in comparison to the initial state and the EBSD measurement. During the first pulse at 10A/cm2 (Fig.8e) the situation is similar to the first pulse at 5A/cm2 (Fig.6e). The darkening of the observation window is again inhomogeneous but not as distinctive as in Fig.6e. Again the darkening seems not to be caused by the formation of a supersaturated film in front of the surface because the image of the pulse pause (Fig.8f) appears in almost the same manner as Fig.8e. The darkening is obviously caused by surface roughening during the pulse time. The level of the darkness and roughness respectively seems to depend on the crystallographic orientation of the grains.
Figure 8 11
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The situation during the second pulse is completely in contrast to the first pulse. The sample surface in the observation window is surprisingly bright and shiny. Large areas of the sample seem to be levelled or
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polished during the second pulse. Only a few domains are severely anodic-dissolved and appear dark (marked by white arrows in Fig.8g). After the second pulse the image (Fig.8h) is slightly darker which means that the roughness of the sample increases again in the second part of the pulse. The image sequence
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suggests an oscillation between crystallographically orientated dissolution and polishing. Gas evolution does
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not occur during the whole experiment. This fact is in good agreement with the quantitative investigation of the metal loss shown in Fig.3b as well as the recently published literature [38].
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Fig.9 shows the results of the post-experiment investigation of the surface by SEM. At first glance the surface quality seems only slightly smoother in comparison with the sample which was machined at 5A/cm2
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(Fig.7). The anodic dissolution results in typical morphologies on the differently orientated grains as discussed in section 3.2.1 (Fig.9b). The clear orientation dependent dissolution is very impressively shown in
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Fig.9c-d. The orientation dependent surface texture of grain 4 is interrupted by grain 2 and mirrorsymmetrically continued (marked by symmetric black lines in Fig.9c) in grain 3. According to table 4 it can
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be ascertained that twin symmetry exists between grain 3 and grain 4.
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The white arrows in Fig.9d mark deep trenches. The trenches are bounded by plates (black arrows). This morphology suggests a strongly crystallographic dissolution along a defined plane. The depth of the trenches remains the same. It can be assumed that the trench ground represents the surface of an underlying grain with a completely different crystallographic orientation.
Figure 9
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4. Conclusions The authors investigated the anodic dissolution of cobalt under near-ECM conditions. The dissolution starts in the active range and increases continuously with increasing potential. The electrochemical experiments
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can be summarized as follows:
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! ! In an early stage of anodic dissolution (! <0.1V) the kinetic is characterized by charge control. At higher over voltage (! >0.1V) the dissolution is limited by the electrolyte resistant.
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! ! An active-transpassive transition as observed on copper under near-ECM conditions [19,20] of the anodic dissolution cannot be established, based on the electrochemical experiments.
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! ! The current efficiency ! determined by various methods is close to 100%. A special issue of the present work is the in-situ observation surface texture development of cobalt under
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near-ECM conditions at low current densities (5A/cm2 and 10A/cm2). It can be summarized that
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! ! The anodic dissolution starts on crystallographic defects such as penetration points of dislocations or grain boundaries. This can be explained by the stored energy inside the defects (! dis>! gb>! hkl).
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! ! The anodic dissolution of the grains strongly depends on the crystallographic orientation. The preferred dissolution along a defined lattice plain leads to the development of different morphologies on the individual grains and causes different micro roughness. The material removal mechanism is in good agreement to the TLK-model [46].
! ! No oxygen evolution is detected up to 10A/cm2. This fact supports the hypothesis that oxygen evolution needs an oxide film. Owing to the active dissolution of cobalt no oxide film is formed in the observed current-voltage range. ! ! The absence of gas bubbles confirms the current efficiency of ! ! 100% which was determined by comparison between the results of uv/vis-spectroscopy and coulometry shown in Fig.5b.
Appendix A
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The crystallographic indices (hkil) and the Euler-angle (! 1, ! , ! 2) of all numbered grains investigated at 5 A/cm2 shown in Fig.6 (table A.1) and 10 A/cm2 shown in Fig.8 (table A.2).
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Table A.1
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Table A.2
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M. Schneider, S. Schroth, S.Richter, N. Schubert, S. Höhn, A. Michaelis, Electrochimica Acta 70 (2012) 76.
14
Page 14 of 29
Data sheet product specification CO007905 Goodfellow GmbH, Bad Neuheim, 2013.
22
A. Moehring, Entwicklung einer elektrochemischen Mikrodurchflusszelle zur Untersuchung des elektrochemischen Senkens (ECM, Electrochemical Machining), Dissertation thesis, Heinrich-HeineUniversität Düsseldorf, 2004.
23
M.M. Lohrengel, I. Klüppel, C. Rosenkranz, H. Bettermann, J.W. Schultze, Electrochimica Acta 48 (2003) 3203.
24
M.M. Lohrengel, C. Rosenkranz, I. Klüppel, A. Moehring, H. Bettermann, B. Van den Bossche, J. Deconinck, Electrochimica Acta 49 (2004) 2863.
25
W.H. Shipman, J.R. Lai, Analytical Chemistry 28 (1956) 1151.
26
G. Wünsch, Talanta 26 (1979) 177.
27
J. Ghasemi, N. Shahabadi, H.R. Seraji, Anal. Chim. Acta 510 (2004) 121.
28
N. Schubert, Produktanalyse beim Electrochemical Machining, Bachelor thesis, TU Dresden, 2009.
29
E. Heitz, W. Schwenk, Werkst. u. Korr. 27 (1976) 241.
30
M. Pourbaix, Atlas of electrochemical equillibria in aqueous solutions, Pergamon Press, Oxford, 1966, pp.323-329.
31
N. Wiberg, Lehrbuch für Anorganische Chemie, 102.edition, Walter de Gruyter GmbH & Co. KG, Berlin, 2007, p.1689.
32
R. Funk, Z.anorg. Chem. 20 (1899) 393.
33
E.H. Riesenfeld, C. Milchsack, Z.anorg. Chem. 85 (1914) 401.
34
B.V. Prelesnik, F. Gabela, B. Ribar, I.R. Krstanovic, Cryst. Struct. Commun. 2 (1973) 581.
35
M. Pourbaix, Atlas of electrochemical equillibria in aqueous solutions, Pergamon Press, Oxford, 1966, pp.493-503.
37 38 39 40 41
cr
us
an
M
d
te
Ac ce p
36
ip t
21
D. Landolt, R. Acosta, R.H. Müller, C.W. Tobias, J. Electrochem. Soc. 117 (1970) 839. K. Kinoshita, D. Landolt, R.H. Müller, C.W. Tobias, J. Electrochem. Soc. 117 (1970) 1246. K. Rataj, C. Hammer, B. Walther, M.M. Lohrengel, Electrochim. Acta 90 (2013) 12. W.R. Buck, H. Leidheiser, J. Electrochem. Soc. 104 (1957) 474. H.-J. Engell, Arch. Eisenhüttenwesen 7 (1955) 393.
G.P. Camorata, L. Felloni, G. Palombarini, S.S. Traverso, Corros. 26 (1970) 129.
42
H. Roßwag, G. Eichkorn, J.W. Lorenz, Werkst. u. Korr. 2 (1974) 86.
43
M. Seo , M. Chiba, Electrochim. Acta 47 (2001) 319.
44
M. Chiba, M. Seo, J. Electrochem. Soc. 150 (2003) B525.
45
D. Landolt, R.H. Müller, C.W. Tobias, J. Electrochem. Soc. 118 (1971) 36.
46
D. Landolt, Corrosion and Surface Chemistry of Metals, EPFL Press, Lausanne, 2007, pp. 59-61 and pp. 8991.
47
G. Gottstein, Physikalische Grundlagen der Materialkunde, Springer-Verlag, Berlin, 1998, pp.73-92. 15
Page 15 of 29
F. Erdmann-Jesnitzer, Werkst. u. Korr. 8 (1958) 7.
49
P. Lin, G. Palumbo, U. Erb, K.T. Aust, Scr. Metall. Mater. 33 (1995) 1387.
50
G. Palumbo, E.M. Lehockey, P. Lin, JOM 50 (1998) 40.
51
V. Randle, Mater. Charact. 47 (2001) 411.
52
H. S. O. Chan, P. K. H. Ho, L. Zhou, N. Luo, S. C. Ng, S. F. Y. Li, Langmuir 12 (1996) 2580.
53
C. Goux, Kristall u. Technik 14 (1979) 251.
54
C. Detavernier, S. Rossnagel, C. Noyan, S. Guha, C. Cabral, C. Lavoie, J. Appl. Phys. 94 (2003) 2874.
Ac ce p
te
d
M
an
us
cr
ip t
48
16
Page 16 of 29
Table 1: Typical ECM parameter [13-17]
Current density / A cm-2 10…300
Electrolyte flow rate / m s-1 0.5…70
Electrode distance / µm 100…500
Ac ce p
te
d
M
an
us
cr
ip t
Applied potential difference /V 5 … 30
17
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Tab.2: Chemical Composition of Co according to product specification [21]
Fe 180ppm
S 150ppm
C 30ppm
Ni 800ppm
Ac ce p
te
d
M
an
us
cr
ip t
Co Remain
18
Page 18 of 29
!1 131,3
!! 161,2
! !! 42,6
2
(1 7 -8 0)
175,7
96,9
17,0
3
(1 7 -8 6)
116,8
53,2
34,2
4
(-1 5 -4 0)
118
92
5
(4 7 -11 -3)
63,9
102,7
ip t
Table A1: Grains from Fig.6 Grain (hkil) 1 (1 4 -5 -15)
33,4
Ac ce p
te
d
M
an
us
cr
47,3
19
Page 19 of 29
!1 61,0
!! 76,7
! !! 33,9
2
(-4 11 -7 0)
115,8
88,0
5,6
3
(2 8 -10 -15)
68,6
146,6
39,0
4
(0 3 -3 -4)
171,9
139,5
5
(-1 2 -1 0)
46,4
94,1
ip t
Table A2: Grains from Fig.8 Grain (hkil) 1 (0 3 -3 1)
31,6
Ac ce p
te
d
M
an
us
cr
3,9
20
Page 20 of 29
us
cr
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55
Ac ce p
te
d
M
an
Fig.1: Photo image of the used microcell for electrochemical investigation under near-ECM conditions
21
Page 21 of 29
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cr
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56
Ac ce p
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d
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an
Fig.2: Schematic sketch of the flow channel cell coupled with an optical microscope
22
Page 22 of 29
-2
12
j / mA cm
10 8
3
10
2
II
10
1
10
6
I
0
10
4
-1
10
2
-2
10
0 2
4
-0,5
6 8 10 12 E vs. (Ag/AgCl) / V
-0,4
-0,3
-0,2 -0,1 0,0 0,1 E vs. (Ag/AgCl) / V
cr
0
ip t
j / A cm
-2
57
Fig.3a: Cyclovoltammogram on cobalt in 2.94mol/l NaNO3 (dE/dt=500mV/s )
Ac ce p
te
d
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an
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Fig.3b: Semi-logarithmic plot of linear sweep voltammetry on cobalt 2.94mol/l NaNO3 (dE/dt=100mV/s )
23
Page 23 of 29
CoO2
1,5 1,0
Co(OH)3 Co++
0,5 0,0 -0,5
Co(OH)2 0
ip t
E vs. SHE / V
+++ 2,0 Co
-6
Co
-1,0 -1,5 1
2
3
4
5
6
7
8
9
10
cr
pH
Ac ce p
te
d
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an
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Fig.4: Potential-pH-diagram of cobalt according to [30]
24
Page 24 of 29
m / µg
4m/s 5m/s
200
muv/vis mcoul(z=1) mcoul(z=2)
150
mcoul(z=3)
10
5
0
0
10
20
30
40 50 -2 j / A cm
5
10
15
20
25
30
-2
j / A cm
us
0
50
ip t
mgeom
100
cr
U/V
15
Fig.5b: Mass loss of the sample calculated by coulometry (mcoul) under assumption of various valences z (z=1,2,3) in comparison with the determined mass loss by uv/visspectroscopy (muv/vis) and geometric estimation (mgeom).
Ac ce p
te
d
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Fig.5a: Potential response of current controlled pulse experiments in sodium nitrate
25
Page 25 of 29
-2
j / A cm
7,5
III
II
I
IV
V
5,0
0,0 0
d: I - initial surface state
e: II – 1st pulse 300ms
3
4
t/s
c: schematic diagram of current pulse experiment
cr
b: Inverse pole figure and crystallographic orientation of the grains numbered in Fig.4a
2
f: III – surface state during pulse pause
Ac ce p
te
d
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an
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a: EBSD-mapping of the sample section
1
ip t
2,5
g: IV – 2nd pulse 560ms
h: V - surface after 2nd pulse
Fig.6: Surface topography development of Co during a current controlled double pulse experiment
26
Page 26 of 29
ip t
Ac ce p
te
d
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an
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a: b: c: Fig.7: SEM-images of the sample surface after the double pulse experiment at 5A/cm2
27
Page 27 of 29
-2
j / A cm
I
II
III
IV
V
10
0 0
1
ip t
5
2
3
4
5
t/s
d: I - initial surface state
e: II – 1st pulse 280ms
c: schematic diagram of current pulse experiment
cr
b: Inverse pole figure and crystallographic orientation of the grains numbered in Fig.4a
f: III – surface state during pulse pause
Ac ce p
te
d
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an
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a: EBSD-mapping of the sample section
g: IV – 2nd pulse 280ms
h: V - surface after 2nd pulse
Fig.8: Surface topography development of Co during a current controlled double pulse experiment
28
Page 28 of 29
ip t
b: Detail of the surface area between grain 1 and 3
c: Detail with grain 2 bounded between twin grain boundaries
an
us
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a: Overview
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d
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d: Detail of a surface section encircled e: Schematic demonstration of the in Fig.9a basal plane Fig.9: SEM-images of the sample surface after the double pulse experiment at 10A/cm2
29
Page 29 of 29
S0013-4686(13)00977-8 http://dx.doi.org/doi:10.1016/j.electacta.2013.05.070 EA 20558
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
22-2-2013 15-5-2013 18-5-2013
Please cite this article as: M. Schneider, N. Schubert, S. H¨ohn, A. Michaelis, Anodic dissolution behaviour and surface texture development of cobalt under ElectroChemical Machining conditions, Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.05.070 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Anodic dissolution behaviour and surface texture development of cobalt under ElectroChemical Machining conditions M. Schneider*1, N. Schubert2, S. Höhn1, A. Michaelis1,2 1
Fraunhofer IKTS Dresden, Winterbergstr. 28, 01277 Dresden, Germany Institute of Material Science, TU Dresden, Helmholtzstr.7, 01067 Dresden, Germany
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*Corresponding author: e-mail: [email protected] ; Tel.: +49 (0)351 2553 7793, fax: +49 (0)351 2554 108
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2
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Abstract:
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The anodic dissolution of hcp-cobalt in sodium nitrate solution under near-ECM conditions was investigated in-situ by using a specially designed flow channel cell. The in-situ investigation was focused on current densities of 5A/cm2 and 10A/cm2. Cobalt dissolves actively as cobalt (II) ion with a current efficiency close
M
to 100%. In contrast to copper, the current-voltage relation does not show an active-passive transition. The dissolution strongly depends on the crystallographic orientation of the grains. The lowest dissolution rate was
d
observed on plains vicinal to the basal plane {0001}. No gas evolution was observed. The surface roughness
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is determined by the micro roughness on the individual grains.
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Keywords: anodic cobalt dissolution, Electrochemical Machining (ECM), grain orientation, insitu microscopy, surface texture
1. Introduction
Pure cobalt metal is used as anode material in electroplating and electroforming or as a catalyst in the chemical industry [1,2] and plays an important role as binder phase in cemented carbides [3]. Cobalt-base alloys have been applied as wear-resistant (e.g. StelliteTM) or high-temperature materials or in medical engineering (e.g. dental and orthopaedic prostheses) [1,2,4-7]. Primarily, cemented carbides and wearresistant alloys are machinable but cause extensive wear to work tools. Electrochemical Machining (ECM) is a non-conventional near-net-shape manufacturing process which is based on the controlled anodic dissolution of metals. ECM is well established as a process in machine alloyed steels [8,9], superalloys [10,11] or titanium [8,12], but not for cemented carbides. Two of the main advantages of the ECM-process are:
1
Page 1 of 29
(i) The metal removal of the work piece is independent of the mechanical properties (hardness, brittleness, strength, ductility) of the material. (ii) The wear of the work tool is eliminated, because no physical contact between the work piece and
ip t
work tool takes place. Due to these advantages, ECM can be a prospective powerful process to engineer high-tensile and hard materials (e.g. cemented carbides). This being the case, it is absolutely necessary to understand the anodic
cr
dissolution behaviour of cobalt under near-ECM conditions. ECM is carried out far from a thermodynamic
us
equilibrium. Current densities of 5A/cm2 up to 300A/cm2 and consequently high over voltage are typical for this technology. Table 1 shows the typical process parameters of ECM [13-17]. However, a detailed
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knowledge of the anodic behavior is also important for catalytic reaction and corrosion of cobalt.
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Table 1
This study focuses on the in-situ observation of the anodic dissolution of cobalt under near-ECM conditions.
d
The recently published design of an electrochemical flow channel cell suitable for optical in-situ
te
investigation [18-20] was used to observe the processes and the surface topography development during the
Ac ce p
anodic dissolution. This cell concept allows the investigation of non-steady-state processes during the anodic dissolution under near-ECM conditions and the localization of, e.g.: ! ! the oxygen evolution and the gas bubble formation over time ! ! the preferred sites of oxygen evolution, ! ! the formation and development of the viscous layer in front of the surface over time, ! ! the preferred sites of dissolution and time-resolved topography development. Such investigations were successfully performed in case of the anodic dissolution of copper and recently presented by the authors [19,20]. Therefore, the investigation of cobalt provides an opportunity to prove the generalizability of the results achieved.
2. Experimental 2.1. Materials
2
Page 2 of 29
The investigated material used was pure cobalt (Co 99.9%) purchased by Goodfellow [21]. The chemical composition is shown in Table 2:
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Table 2
Cylindrical cobalt rods of approximately 3 cm in length and 3 mm in diameter were prepared by wet
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grinding with SiC-paper (up to P1200). Following this, the samples were mechanically polished with a
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diamond suspension (6µm, 3µm and 1µm grit) and were finally chemically polished with a SiO2-suspension. All samples were next etched for a short time in a metallographic etchant consisting of 100ml methanol +
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6ml HNO3 (65%) at room temperature. Lastly, the samples were cleaned by rinsing water and degreased with ethanol. In all electrochemical experiments the used electrolyte was 2.9 mol/l sodium nitrate solution (250g/l,
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2.2. Methods
d
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pH5.6, ! =160mS/cm).
2.2.1. Electrochemical flow-through microcapillary cell
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Preliminary studies to the electrochemical behaviour of cobalt in sodium nitrate solutions were carried out in an electrochemical flow-through microcapillary cell (Fig.1) using a 3-electrode design. This microcell was specially designed for material surface investigations at large current densities by A. Moehring [22] and firstly applied to the dissolution of iron under near-ECM conditions by M.M. Lohrengel and co-workers in 2003/04 [23,24]. The microcell used in this work was an in-house reproduction of the cell design described by A. Moehring [22]. Due to the capillary diameter (d! 0.05cm), the area of the working electrode is very small (A! 0.002cm2). A gold wire situated in the outlet channel of the cell works as counter electrode. The distance between the working electrode (anode) and the counter electrode (cathode) in the used cell amounts to l! 0.1cm. The cell concept allows high current densities (j! 100A/cm2) by using commercial potentiostates (Jmax=1A). Figure 1
3
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2.2.2. Channel flow cell and in-situ microscopy
The aforementioned channel flow cell is schematically shown in Fig.2 [18,19]. The sample (anode,
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A=0.07cm2) was implemented in a lower section of the cell. A gold disc with opposite mounted in
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the glass lid works as cathode (A=0.13cm2). In the centre of the cathode a small conical glass lens (Ø! 0.3mm) was embedded which allows the in-situ observation of the surface during the
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electrochemical experiments.
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The gap between the anode and the cathode was adjusted to approximately 0.35mm. A detailed description of the cell, the ancillary equipment and the measurement configuration has been
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recently published in [18].
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2.2.3. Uv/vis-spectroscopy
d
Figure 2
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The uv/vis-spectroscopy was carried out by an EPP2000 (StellarNet) spectrometer, which operates in a wave length range between 200-800nm. The electrolyte used was caught on the outlet of the flow channel cell. The addition of nitroso-r salt C 10 H 5 NNa2O8 S 2 leads to the formation of a red cobalt complex, which is wellsuited to determine the cobalt concentration in aqueous solutions (see e.g. [25,26]). According to the literature [25,27] the absorption intensity of this complex was evaluated at 540nm, although the peak maximum is at a lower wave length. However, at a lower wave length the reagent itself tends to be of a high absorption. The method was calibrated by a measurement of a concentration series with defined cobalt concentration [28].
2.2.4. Orientation determination by Electron Back Scattering Diffraction (EBSD) EBSD was used to analyse the orientation of the individual grains of the polycrystalline cobalt samples. The method is based on the diffraction of the quasi-elastic backscattered electrons in a near-surface range (depth 4
Page 4 of 29
! 100nm). The resulting diffraction pattern depends on the crystallographic orientation of the diffraction planes. The patterns of the diffraction were detected by a phosphor screen and a low light CCD-camera. The samples were tilted 70° to get an optimal yield of the backscattered electrons. The measurements were
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carried out using a scanning electron microscope NVision 40 with a Schottky-emitter (Fa. Carl Zeiss SMT GmbH) and a HKL NordlysF-detector. The acceleration voltage was 20kV. The generated raw datasets were
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analyzed by using the software package CHANNEL 5 (Oxford Instruments GmbH). The grain orientations
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are described by Euler-angles (! 1, ! , ! 2) and Miller-indices (h k l) and are presented as inverse pole figures.
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3. Results and discussion 3.1. Preliminary study of cobalt dissolution
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In a preliminary study the electrochemical behaviour of cobalt is investigated by cyclovoltammetry (CV) and linear sweep voltammetry (LSV). Fig.3a shows a CV on cobalt in the sodium nitrate electrolyte carried out in
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the flow-through microcapillary cell. The current density linearly increases between 1 and 10V. The
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and not in the transpassive range.
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extrapolation of the current density down to the x-axis shows that the cobalt dissolution starts in the active
A calculation of the ohmic resistant from Fig.3a results to R !
!E ! 500 ! . Considering the cell geometry !J
and the specific electrolyte resistance ( ! ! 6.25 ! cm ) the expected resistance of the electrolyte should
be R !
! !l ! 312 ! . That means the calculated resistance is slightly higher than the estimated electrolyte A
resistance. This fact can be explained by the formation of a viscous and very thin film of dissolved cobalt species in front of the sample surface. However, it has to be emphasized that the estimation of the electrolyte resistance can only be a rough approximation which is caused by the changing area of the working electrode during the experiment and the non-trivial geometry inside the microcapillary cell.
Figure 3
5
Page 5 of 29
A more detailed impression of the early stage of dissolution is given by the LSV on cobalt shown in Fig.3b. The absolute value of the current density is logarithmically plotted versus the potential. The potential
ECo / Co 2! ! ! 0.28VSHE
O 2 ! 2 H 2 O ! 4 e ! ! ! ! 4OH !
EO
/ OH !
) between the
(at pH5.6)
! 0.90VSHE
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2
/ OH !
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following reactions:
Co ! ! ! Co 2 ! ! 2 e !
2
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Ecorr ! ! 0.26V Ag / AgCl ! ! 0,06VSHE represents the mix potential ( Ecorr ! ECo / Co 2! / O
(1) (2)
At a higher negative overvoltage the hydrogen reaction dominates the cathodic process.
2 H ! ! 2e ! ! ! ! H 2
E H ! / H ! ! 0.33VSHE
(at pH5.6)
(3)
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2
The anodic part of the semi-logarithmic plot shows two different slopes. In region I the slope amounts to
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50mV/dec. This value is close to the anodic Tafel-factor b of a charge-transfer controlled metal dissolution according to Eq.1 ( b ! 118mV 2 for Me/Me2+) [29]. At potentials E>-0.2V (region II) the slope of the
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curve has clearly changed. A treatment of this section of the graph (region II) in decadic notation results in a The estimation of the resistance
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linear increase of the current density versus the applied potential.
approximately corresponds to the order of the electrolyte resistance again. Considering the thermodynamic
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point of view neither other anodic reactions than described in Eq.1 take place nor other reaction products exists in the potential range presented in Fig.3b. At higher potentials the following reactions are thermodynamically possible according to Fig.4 [30]: 2 Co 2 ! ! 3 H 2O ! ! ! 2 Co (OH ) 3 ! 3 H ! ! e !
E 0 ! 1 .14V SHE
(at pH5.6)
(4)
2 Co (OH ) 3 ! ! ! CoO 2 ! H 2 O ! H ! ! e !
E 0 ! 1 .62V SHE
(at pH5.6)
(5)
The literature about the existence of CoO2 is contradictory [30]. It is assumed that the cobalt peroxide is easily decomposed as follows: (6)
2CoO 2 ! 3 H 2 O ! ! ! 2Co (OH ) 3 ! 1 2 O 2
Figure 4
6
Page 6 of 29
With respect to the high concentration of nitrate anions in the electrolyte the formation of cobalt nitrate can be assumed according to:
Co 2 ! ! 2 NO 3! ! ! ! Co ( NO 3 ) 2
(7)
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This cobalt nitrate can be several hydrated ( Co ( NO 3 ) 2 ! xH 2 O ) [31-34]. In the case of x=6, Funk [32] reported a melting point of T=57°C. Otherwise, the nitrate anions can be reduced on the cathode according
NO 3! ! 6 H 2 O ! 8e ! ! ! ! NH 3 ! 9OH !
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NO 3! ! 7 H 2 O ! 8e ! ! ! ! NH 4 OH ! 9OH !
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to the literature [35]:
(9) (10)
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NO 3! ! 10 H 2 O ! 8e ! ! ! ! NH 4! ! 3 H 2 O ! 10OH !
(8)
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3.2. Investigation with the channel flow cell
The electrochemical experiments in the channel flow cell (schematically shown in Fig.2) were carried out
d
under current control with a 2-electrode design. Fig.5 shows the potential response to the applied current
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density. Under the chosen conditions the electrolyte flow rate has apparently no influence on the potential response. Additionally, the current-voltage relation is strictly linear. The slope of the interpolated line !U ! 0.27 ! cm ² . Due to the electrode gap between anode and cathode an ohmic resistance of !i
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amounts to
! ! 7.6 ! cm can be calculated which is in good agreement with the electrolyte resistance. The difference to
the specific resistance in Fig.5a is caused by the well defined hydrodynamic conditions in front of the electrode which enhances the transport from dissolution products. More interestingly, the fact that the specific resistance is only caused by the electrolyte resistance confirms the statement of the active dissolution of cobalt in the described current-voltage range. In contrast to earlier investigations on copper, the authors did not observe the characteristic active-transpassive transition which is identified by a potential offset in the potential-current curve [19,20,36,37]. Nevertheless, the topography development is strictly dependent on the applied current density.
Figure 5a and 5b
7
Page 7 of 29
The current controlled pulse experiments were complementary performed in the flow-through microcapillary cell. Thereby, the waste electrolyte was caught and the cobalt content was analyzed by uv/vis-spectroscopy. Based on this analysis the authors calculated the mass loss muv/vis of cobalt (Fig.5b). Additionally, the
3d-image analysis. The removed cobalt mass mgeom was calculated according Eq.11: geom
! !
Co
!V
(11)
r
cr
m
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dissolved volume Vr of the sample was analysed geometrically by using a stereo microscope (Keyence) and a
Whereby, the factor ! Co describes the density of cobalt.
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The results are shown in Fig.5b. Further on, Fig.5b shows the theoretical mass loss mcoul of cobalt according to Faraday´s law under assumption of various valences z. The results allow the conclusion that:
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! ! The valence of cobalt dissolution is z=2 ! ! The current efficiency ! ! 100%
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In the following section 3.2.1 and 3.2.2 the in-situ investigation of the surface texture development under
d
near-ECM conditions is focused on low current densities (j=5A/cm2 and 10A/cm2).
te
3.2.1. In-situ investigation at 5A/cm2 and post-experiment SEM Fig.6a shows the EBSD-mapping of the sample section which was in-situ observed during the
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electrochemical double pulse experiment schematically presented in Fig.6c. Fig.6b shows an inverse pole figure which indicates the crystallographic orientation of the numbered grains from Fig.6a (The Euler angle and the Miller indices of the numbered grains are shown as a table in the section appendix.). Additionally, the orientation of the unit cell with respect to the surface normale (z-view) is shown in Fig.6b. Fig.6c presents the current regime of the pulse experiment. An electric charge q=5C/cm- is consumed during a single pulse. This value corresponds to a metal removal rate of 17µm/s assuming a current efficiency of
! =100% and a cobalt valence of z=2. The Roman numerous shown in Fig.6c indicates the point in time of the experiment by which images are shown in Figs.6d-h. In Figs.6d-h the network of white broken lines supports the identification of the individual grains which are numbered in Fig.6a. The white arrows are hints of special sites on the surface, whose development will be specifically discussed. The right arrow marks a triple point between three grains. Fig.6d shows the initial state of the surface under the flowing electrolyte. The surface is well prepared and ready for the experiment. Fig.6e shows the situation after 0.3s during the 8
Page 8 of 29
first pulse. Generally, the field of view is darkened when the pulse starts. This darkening is caused by the severe dissolution of metal and the formation of a product film in front of the anode. However, the grains numbered with the Arabian numeral 1 appear brighter than all other grains. It looks as if the metal
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dissolution on these grains is significantly less than on all other grains. According to the EBSD investigation the authors assume that these grains differ only slightly to the basal plane as shown in Fig.6b. The complete removal of the dissolved dissolution products by the streaming electrolyte results in only a slight brightening
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of the field of view during the pulse pause (Fig.6f). The comparison between Fig.6e and Fig.6f shows
us
evidence that the darkening cannot be exclusively caused by the formation of a product film. The darkening can also be caused by the development of differing surface roughness and light scattering respectively,
an
depending on the crystallographic grain orientation. In this regard the development of a small disorientated area (marked by the white arrow in the mid-position) in a grain similar to the basal plane is an interesting
M
detail. The small area appears darker and larger with increasing pulse time (see also later on Figs.6g and h). This chronology clarifies a part of the darkening by diffused light scattering caused by surface roughening.
d
A formation of gas bubbles as a side reaction is not observable. This is completely different from the recently published results on copper [19]. The absence of the anodic oxygen evolution supports the theory of the
te
active dissolution of cobalt according to equation 1. Very recently Rataj et al. reported that the oxygen
Ac ce p
evolution on cobalt does not occur until current densities of j>10A/cm2 [38] are achieved. The discrimination on the different grains is enhanced during the 2nd pulse (Fig.6g). It is now possible to distinguish the appearance of the grains in bright (e.g. grain 1), medium (e.g. grain 5) and dark grains (e.g. grain 3). After the pulse experiment (Fig.6h) the dark-bright differences seem to be mainly caused by the defocusing of severe anodic dissolved regions on the surface.
Figure 6
A comparison of all images (Figs.6d-h) shows that the plane vicinal to the basal plane is less attacked in comparison to other grains. Nevertheless, small parts of this grain are more strongly affected. A correlation between the microstructure and dissolution seems natural. After the pulse experiment the sample has been ex-situ investigated by SEM. Fig.7 shows the SEM-images of the area discussed above. Fig.7a gives an
9
Page 9 of 29
overview of the surface range which was also observable in the flow cell. The black broken line network and the Arabian numerals make the recognition easier. The results complete and confirm the in-situ observation in an outstanding manner. As already observed during the experiment (Fig.6) grains vicinal to the basal plane
ip t
are less affected by anodic dissolution (Fig.7b). Moreover, grain 1 shown in Fig.7b appears smoother in comparison to the surrounding grains. The black arrows mark the same sites on the surface as the white arrows in Fig.6. The severely attacked small area in the upper part of grain 1 (Fig.7b) is obviously a range
cr
with a significantly different orientation to grain 1. It could be that the grain was not clearly separated as an
us
individual grain after the EBSD investigation. However, the completely different attack is a clear sign of a significant divergence to the orientation vicinal to the basal plane of the majority of the visible grains. The
an
former triple point of grains (right arrow) shows a site of severe attack, as expected. Additionally, grain 1 is especially affected on the grain boundary marked by the left arrow. That the anodic dissolution is strongly
M
influenced by the crystallographic orientation of the material has been well known for a long time in corrosion research [39-44]. In the early 1970s Landolt and co-workers reported the influence of the
d
crystallographic orientation on the copper dissolution under near-ECM conditions [45]. Owing to the
te
thermodynamic point of view the surface energy ! hkl decreases with increasing packing density of the planes [46]. Therefore, in case of hcp-Co the basal plane {0001} is most stable again with anodic dissolution. This
Ac ce p
fact is confirmed by the author’s investigation (Fig.7b). Planes vicinal to the basal plane are only slightly attacked in comparison with other planes. Additionally, the morphology of the other observed grains tends to the formation of terraces during the anodic dissolution, according to the terrace-ledge-kink (TLK) model [46]. Preferred sites of anodic dissolution are crystal defects as grain boundaries, stacking faults and dislocations as shown in Fig.7c. This fact can be also expressed by the stored energy ! . In the case of the grain boundaries the ! gb depends on the grain boundary structure which can be expressed by the misorientation angle ! between the adjacent grains and the reciprocal density of the coincidence site ! [4751]. It can be generally said that atoms located at grain boundaries are less solidly bound and can be preferentially anodic dissolved [46,52]. However, it has to be stressed that the relationship between the grain boundary energy ! gb and ! and ! is very complex. In the late 1970s Goux pointed out that the grain boundary energy ! gb is a scalar quantity while corrosion attack also depends on the direction of the grain boundary plane [53]. The fact remains, that the authors observed a different dissolution kinetic of grain boundaries, 10
Page 10 of 29
which is explained by different grain boundary energy. Otherwise, grain boundaries are often segregated sites of material impurities, which can also change the dissolution kinetic. However, this fact can be abandoned in view of the pure cobalt material used.
ip t
The visibly isolated etch patterns in grains (marked by arrows in Fig.7c) are caused by penetration points of dislocations. The shape of the pattern is depending on the orientation of the plane were the dislocations on the surface penetrates. According to Detavernier et al. [54] who reported that the dislocation energy
cr
! dis>! gb>! hkl, it can be assumed that the anodic dissolution kinetic follows the same rule. This is in good
us
agreement with the impression that the dislocations are severely attacked (Fig.7c). The morphology of grain
an
1 suggests the crystallographic misorientation from the [0001] direction.
M
Figure 7
3.2.2. In-situ investigation at 10A/cm2 and post-experiment SEM
d
The set of experiments described in section 3.2.1 was repeated on a freshly prepared sample using an applied
te
current density of 10A/cm2. Fig.8 illustrates the results in the same manner as in section 3.2.1. The EBSDmapping of the observed sample section, the inverse pole figure and the current density versus time regime
Ac ce p
are presented in Figs.8a-c. The consumed electric charge was q=10C/cm2 per pulse which corresponds to a metal removal rate of 34µm/s assuming a current efficiency of ! =100% and a cobalt valence of z=2. Fig.8d shows the initial surface state observed under streaming electrolyte. The white network shows better identification of individual surface sites in comparison to the initial state and the EBSD measurement. During the first pulse at 10A/cm2 (Fig.8e) the situation is similar to the first pulse at 5A/cm2 (Fig.6e). The darkening of the observation window is again inhomogeneous but not as distinctive as in Fig.6e. Again the darkening seems not to be caused by the formation of a supersaturated film in front of the surface because the image of the pulse pause (Fig.8f) appears in almost the same manner as Fig.8e. The darkening is obviously caused by surface roughening during the pulse time. The level of the darkness and roughness respectively seems to depend on the crystallographic orientation of the grains.
Figure 8 11
Page 11 of 29
The situation during the second pulse is completely in contrast to the first pulse. The sample surface in the observation window is surprisingly bright and shiny. Large areas of the sample seem to be levelled or
ip t
polished during the second pulse. Only a few domains are severely anodic-dissolved and appear dark (marked by white arrows in Fig.8g). After the second pulse the image (Fig.8h) is slightly darker which means that the roughness of the sample increases again in the second part of the pulse. The image sequence
cr
suggests an oscillation between crystallographically orientated dissolution and polishing. Gas evolution does
us
not occur during the whole experiment. This fact is in good agreement with the quantitative investigation of the metal loss shown in Fig.3b as well as the recently published literature [38].
an
Fig.9 shows the results of the post-experiment investigation of the surface by SEM. At first glance the surface quality seems only slightly smoother in comparison with the sample which was machined at 5A/cm2
M
(Fig.7). The anodic dissolution results in typical morphologies on the differently orientated grains as discussed in section 3.2.1 (Fig.9b). The clear orientation dependent dissolution is very impressively shown in
d
Fig.9c-d. The orientation dependent surface texture of grain 4 is interrupted by grain 2 and mirrorsymmetrically continued (marked by symmetric black lines in Fig.9c) in grain 3. According to table 4 it can
te
be ascertained that twin symmetry exists between grain 3 and grain 4.
Ac ce p
The white arrows in Fig.9d mark deep trenches. The trenches are bounded by plates (black arrows). This morphology suggests a strongly crystallographic dissolution along a defined plane. The depth of the trenches remains the same. It can be assumed that the trench ground represents the surface of an underlying grain with a completely different crystallographic orientation.
Figure 9
12
Page 12 of 29
ip t
4. Conclusions The authors investigated the anodic dissolution of cobalt under near-ECM conditions. The dissolution starts in the active range and increases continuously with increasing potential. The electrochemical experiments
cr
can be summarized as follows:
us
! ! In an early stage of anodic dissolution (! <0.1V) the kinetic is characterized by charge control. At higher over voltage (! >0.1V) the dissolution is limited by the electrolyte resistant.
an
! ! An active-transpassive transition as observed on copper under near-ECM conditions [19,20] of the anodic dissolution cannot be established, based on the electrochemical experiments.
M
! ! The current efficiency ! determined by various methods is close to 100%. A special issue of the present work is the in-situ observation surface texture development of cobalt under
d
near-ECM conditions at low current densities (5A/cm2 and 10A/cm2). It can be summarized that
te
! ! The anodic dissolution starts on crystallographic defects such as penetration points of dislocations or grain boundaries. This can be explained by the stored energy inside the defects (! dis>! gb>! hkl).
Ac ce p
! ! The anodic dissolution of the grains strongly depends on the crystallographic orientation. The preferred dissolution along a defined lattice plain leads to the development of different morphologies on the individual grains and causes different micro roughness. The material removal mechanism is in good agreement to the TLK-model [46].
! ! No oxygen evolution is detected up to 10A/cm2. This fact supports the hypothesis that oxygen evolution needs an oxide film. Owing to the active dissolution of cobalt no oxide film is formed in the observed current-voltage range. ! ! The absence of gas bubbles confirms the current efficiency of ! ! 100% which was determined by comparison between the results of uv/vis-spectroscopy and coulometry shown in Fig.5b.
Appendix A
13
Page 13 of 29
The crystallographic indices (hkil) and the Euler-angle (! 1, ! , ! 2) of all numbered grains investigated at 5 A/cm2 shown in Fig.6 (table A.1) and 10 A/cm2 shown in Fig.8 (table A.2).
ip t
Table A.1
cr
Table A.2
References J. Krüger, Chemie-Ing.-Techn. 37 (1965) 27.
2
E. Kunze (Ed.), Korrosion und Korrosionsschutz, vol.2, Wiley-VCH Weinheim 2001, 1078.
3
Sandvik AB, Cemented Carbide, Sandvik new developments and applications, Ref.no. H-9116 ENG., Sandviken, 2005.
4
U. Abend, Dental kompakt (2009) 514.
5
H. ElFanity, K. Rahmouni , M. Bouanani , A. Dinia , G. Shmerber, C. Meny, P. Panissod , A. Cziraki, F. Cherkaoui , A. Berrada, Thin Solid Films 318 (1998) 227.
6
H. Heide, K. Köster, H. Lukas, Chemie-Ing.-Techn. 47 (1975) 327.
7
K. Sixtus, Archiv für Elektrotechnik 39 (1948) 260.
8
D. M. Allen, P. J. Gillbanks, Precision Eng. 7 (1985) 105.
9
M. Datta, L. T. Romankiw, J. Electrochem. Soc. 136 (1989) 285C.
10
S. Sharma, V. K. Jain, R. Shekhar, Int. J. Adv. Manuf. Technol. 19 (2002) 492.
12 13
14 15
an
M
d
te
Ac ce p
11
us
1
E. Rosset, D. Landolt, Precision Eng. 11 (1989) 79. C. Madore, D. Landolt, J. Micromech. Microeng. 7 (1997) 270. A.D. Davydov, V.M. Volgin, Electrochemical Machining, in: A.J. Bard, M. Stratmann (Eds.), Encyclopedia of Electrochemistry, vol.5, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2007. D. Landolt, Chemie-Ing.-Techn. 45 (1973) 188. S. Moser, Mikrostrukturierung von Metallen durch elektrochemischen Abtrag mit gepulstem Strom (PECM), Dissertation thesis, Heinrich-Heine-Universität Düsseldorf, 2004.
16
M.M. Lohrengel, I. Klüppel, C. Rosenkranz, H. Bettermann, J.W. Schultze, Electrochim. Acta 48 (2003) 3203.
17
M. Datta, IBM J. Res. Develop. 37 (1993) 207.
18
M. Schneider, S. Schroth, N. Schubert, A. Michaelis, Materials and Corrosion 63 (2012) 96.
19
M. Schneider, S. Schroth, S.Richter, N. Schubert, S. Höhn, A. Michaelis, Electrochimica Acta 56 (2011) 7628.
20
M. Schneider, S. Schroth, S.Richter, N. Schubert, S. Höhn, A. Michaelis, Electrochimica Acta 70 (2012) 76.
14
Page 14 of 29
Data sheet product specification CO007905 Goodfellow GmbH, Bad Neuheim, 2013.
22
A. Moehring, Entwicklung einer elektrochemischen Mikrodurchflusszelle zur Untersuchung des elektrochemischen Senkens (ECM, Electrochemical Machining), Dissertation thesis, Heinrich-HeineUniversität Düsseldorf, 2004.
23
M.M. Lohrengel, I. Klüppel, C. Rosenkranz, H. Bettermann, J.W. Schultze, Electrochimica Acta 48 (2003) 3203.
24
M.M. Lohrengel, C. Rosenkranz, I. Klüppel, A. Moehring, H. Bettermann, B. Van den Bossche, J. Deconinck, Electrochimica Acta 49 (2004) 2863.
25
W.H. Shipman, J.R. Lai, Analytical Chemistry 28 (1956) 1151.
26
G. Wünsch, Talanta 26 (1979) 177.
27
J. Ghasemi, N. Shahabadi, H.R. Seraji, Anal. Chim. Acta 510 (2004) 121.
28
N. Schubert, Produktanalyse beim Electrochemical Machining, Bachelor thesis, TU Dresden, 2009.
29
E. Heitz, W. Schwenk, Werkst. u. Korr. 27 (1976) 241.
30
M. Pourbaix, Atlas of electrochemical equillibria in aqueous solutions, Pergamon Press, Oxford, 1966, pp.323-329.
31
N. Wiberg, Lehrbuch für Anorganische Chemie, 102.edition, Walter de Gruyter GmbH & Co. KG, Berlin, 2007, p.1689.
32
R. Funk, Z.anorg. Chem. 20 (1899) 393.
33
E.H. Riesenfeld, C. Milchsack, Z.anorg. Chem. 85 (1914) 401.
34
B.V. Prelesnik, F. Gabela, B. Ribar, I.R. Krstanovic, Cryst. Struct. Commun. 2 (1973) 581.
35
M. Pourbaix, Atlas of electrochemical equillibria in aqueous solutions, Pergamon Press, Oxford, 1966, pp.493-503.
37 38 39 40 41
cr
us
an
M
d
te
Ac ce p
36
ip t
21
D. Landolt, R. Acosta, R.H. Müller, C.W. Tobias, J. Electrochem. Soc. 117 (1970) 839. K. Kinoshita, D. Landolt, R.H. Müller, C.W. Tobias, J. Electrochem. Soc. 117 (1970) 1246. K. Rataj, C. Hammer, B. Walther, M.M. Lohrengel, Electrochim. Acta 90 (2013) 12. W.R. Buck, H. Leidheiser, J. Electrochem. Soc. 104 (1957) 474. H.-J. Engell, Arch. Eisenhüttenwesen 7 (1955) 393.
G.P. Camorata, L. Felloni, G. Palombarini, S.S. Traverso, Corros. 26 (1970) 129.
42
H. Roßwag, G. Eichkorn, J.W. Lorenz, Werkst. u. Korr. 2 (1974) 86.
43
M. Seo , M. Chiba, Electrochim. Acta 47 (2001) 319.
44
M. Chiba, M. Seo, J. Electrochem. Soc. 150 (2003) B525.
45
D. Landolt, R.H. Müller, C.W. Tobias, J. Electrochem. Soc. 118 (1971) 36.
46
D. Landolt, Corrosion and Surface Chemistry of Metals, EPFL Press, Lausanne, 2007, pp. 59-61 and pp. 8991.
47
G. Gottstein, Physikalische Grundlagen der Materialkunde, Springer-Verlag, Berlin, 1998, pp.73-92. 15
Page 15 of 29
F. Erdmann-Jesnitzer, Werkst. u. Korr. 8 (1958) 7.
49
P. Lin, G. Palumbo, U. Erb, K.T. Aust, Scr. Metall. Mater. 33 (1995) 1387.
50
G. Palumbo, E.M. Lehockey, P. Lin, JOM 50 (1998) 40.
51
V. Randle, Mater. Charact. 47 (2001) 411.
52
H. S. O. Chan, P. K. H. Ho, L. Zhou, N. Luo, S. C. Ng, S. F. Y. Li, Langmuir 12 (1996) 2580.
53
C. Goux, Kristall u. Technik 14 (1979) 251.
54
C. Detavernier, S. Rossnagel, C. Noyan, S. Guha, C. Cabral, C. Lavoie, J. Appl. Phys. 94 (2003) 2874.
Ac ce p
te
d
M
an
us
cr
ip t
48
16
Page 16 of 29
Table 1: Typical ECM parameter [13-17]
Current density / A cm-2 10…300
Electrolyte flow rate / m s-1 0.5…70
Electrode distance / µm 100…500
Ac ce p
te
d
M
an
us
cr
ip t
Applied potential difference /V 5 … 30
17
Page 17 of 29
Tab.2: Chemical Composition of Co according to product specification [21]
Fe 180ppm
S 150ppm
C 30ppm
Ni 800ppm
Ac ce p
te
d
M
an
us
cr
ip t
Co Remain
18
Page 18 of 29
!1 131,3
!! 161,2
! !! 42,6
2
(1 7 -8 0)
175,7
96,9
17,0
3
(1 7 -8 6)
116,8
53,2
34,2
4
(-1 5 -4 0)
118
92
5
(4 7 -11 -3)
63,9
102,7
ip t
Table A1: Grains from Fig.6 Grain (hkil) 1 (1 4 -5 -15)
33,4
Ac ce p
te
d
M
an
us
cr
47,3
19
Page 19 of 29
!1 61,0
!! 76,7
! !! 33,9
2
(-4 11 -7 0)
115,8
88,0
5,6
3
(2 8 -10 -15)
68,6
146,6
39,0
4
(0 3 -3 -4)
171,9
139,5
5
(-1 2 -1 0)
46,4
94,1
ip t
Table A2: Grains from Fig.8 Grain (hkil) 1 (0 3 -3 1)
31,6
Ac ce p
te
d
M
an
us
cr
3,9
20
Page 20 of 29
us
cr
ip t
55
Ac ce p
te
d
M
an
Fig.1: Photo image of the used microcell for electrochemical investigation under near-ECM conditions
21
Page 21 of 29
us
cr
ip t
56
Ac ce p
te
d
M
an
Fig.2: Schematic sketch of the flow channel cell coupled with an optical microscope
22
Page 22 of 29
-2
12
j / mA cm
10 8
3
10
2
II
10
1
10
6
I
0
10
4
-1
10
2
-2
10
0 2
4
-0,5
6 8 10 12 E vs. (Ag/AgCl) / V
-0,4
-0,3
-0,2 -0,1 0,0 0,1 E vs. (Ag/AgCl) / V
cr
0
ip t
j / A cm
-2
57
Fig.3a: Cyclovoltammogram on cobalt in 2.94mol/l NaNO3 (dE/dt=500mV/s )
Ac ce p
te
d
M
an
us
Fig.3b: Semi-logarithmic plot of linear sweep voltammetry on cobalt 2.94mol/l NaNO3 (dE/dt=100mV/s )
23
Page 23 of 29
CoO2
1,5 1,0
Co(OH)3 Co++
0,5 0,0 -0,5
Co(OH)2 0
ip t
E vs. SHE / V
+++ 2,0 Co
-6
Co
-1,0 -1,5 1
2
3
4
5
6
7
8
9
10
cr
pH
Ac ce p
te
d
M
an
us
Fig.4: Potential-pH-diagram of cobalt according to [30]
24
Page 24 of 29
m / µg
4m/s 5m/s
200
muv/vis mcoul(z=1) mcoul(z=2)
150
mcoul(z=3)
10
5
0
0
10
20
30
40 50 -2 j / A cm
5
10
15
20
25
30
-2
j / A cm
us
0
50
ip t
mgeom
100
cr
U/V
15
Fig.5b: Mass loss of the sample calculated by coulometry (mcoul) under assumption of various valences z (z=1,2,3) in comparison with the determined mass loss by uv/visspectroscopy (muv/vis) and geometric estimation (mgeom).
Ac ce p
te
d
M
an
Fig.5a: Potential response of current controlled pulse experiments in sodium nitrate
25
Page 25 of 29
-2
j / A cm
7,5
III
II
I
IV
V
5,0
0,0 0
d: I - initial surface state
e: II – 1st pulse 300ms
3
4
t/s
c: schematic diagram of current pulse experiment
cr
b: Inverse pole figure and crystallographic orientation of the grains numbered in Fig.4a
2
f: III – surface state during pulse pause
Ac ce p
te
d
M
an
us
a: EBSD-mapping of the sample section
1
ip t
2,5
g: IV – 2nd pulse 560ms
h: V - surface after 2nd pulse
Fig.6: Surface topography development of Co during a current controlled double pulse experiment
26
Page 26 of 29
ip t
Ac ce p
te
d
M
an
us
cr
a: b: c: Fig.7: SEM-images of the sample surface after the double pulse experiment at 5A/cm2
27
Page 27 of 29
-2
j / A cm
I
II
III
IV
V
10
0 0
1
ip t
5
2
3
4
5
t/s
d: I - initial surface state
e: II – 1st pulse 280ms
c: schematic diagram of current pulse experiment
cr
b: Inverse pole figure and crystallographic orientation of the grains numbered in Fig.4a
f: III – surface state during pulse pause
Ac ce p
te
d
M
an
us
a: EBSD-mapping of the sample section
g: IV – 2nd pulse 280ms
h: V - surface after 2nd pulse
Fig.8: Surface topography development of Co during a current controlled double pulse experiment
28
Page 28 of 29
ip t
b: Detail of the surface area between grain 1 and 3
c: Detail with grain 2 bounded between twin grain boundaries
an
us
cr
a: Overview
Ac ce p
te
d
M
d: Detail of a surface section encircled e: Schematic demonstration of the in Fig.9a basal plane Fig.9: SEM-images of the sample surface after the double pulse experiment at 10A/cm2
29
Page 29 of 29