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Coating substrate relationship after initial electrolyte contact in the electrodeposition of zinc on steel
Coating substrate relationship after initial electrolyte contact in the electrodeposition of zinc on steel Thomas Greul, Johann Gerdenitsch, Christian Commenda, Raffaela Sagl, Martin Arndt, Jiri Duchoslav, Achim Walter Hassel PII: DOI: Reference:
S0257-8972(14)00409-5 doi: 10.1016/j.surfcoat.2014.04.065 SCT 19390
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
27 August 2013 9 March 2014 29 April 2014
Please cite this article as: Thomas Greul, Johann Gerdenitsch, Christian Commenda, Raffaela Sagl, Martin Arndt, Jiri Duchoslav, Achim Walter Hassel, Coating substrate relationship after initial electrolyte contact in the electrodeposition of zinc on steel, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.04.065
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Coating substrate relationship after initial electrolyte contact in the
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electrodeposition of zinc on steel
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Thomas Greula,b, Johann Gerdenitschb, Christian Commendab, Raffaela Saglb, Martin Arndtc, Jiri Duchoslavc, Achim Walter Hassela,d *
Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University Linz,
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Altenberger Str. 69, 4040 Linz, Austria
Christian Doppler Laboratory for Microscopic and Spectroscopic Material Characterization, Center
Austria
CEST GmbH, Viktor Kaplan Str. 2, 2700 Wiener Neustadt, Austria
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for Surface- and Nanoanalytics, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz,
*Corresponding author:
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c
Voestalpine Stahl GmbH, voestalpine-Straße 3, 4020 Linz, Austria;
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b
Tel.: +43 732 2468 8700, Fax: +43 732 2468 8905, email: [email protected]
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ACCEPTED MANUSCRIPT Abstract Electron backscatter diffraction (EBSD) measurements show epitaxial electrochemical deposition
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of zinc on non- deformed and contrariwise random growth on deformed steel grains. Therefore,
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electrochemical investigations on the differences of zinc electrodeposition on electropolished
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respectively on temper rolled low carbon steel sheet of the same substrate are studied. These measurements show great differences in ECN (electrochemical noise) investigations and the 1st
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cycle of a CV (cyclic voltammetry) only. It is found, that the immersion of the substrate in the electrolyte prior to the measurements causes this behaviour. SEM, EDX, XPS (X-ray photoelectron
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spectroscopy), SAM (Scanning Auger microscopy) and IRRAS (Infrared reflection absorption spectroscopy) measurements are used for surface characterization and EBSD-measurements for
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determination of crystallographic orientation to reveal the influence on electrochemical growth. It is
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proven that zinc precipitates form on the temper rolled substrate during immersion and cause a
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nucleation process prior to electrodeposition and the subsequent change in electrocrystallisation as zinc is deposited on zinc instead of steel. The differences in the electrochemical measurements are
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well described by this theory.
Key words: electro deposition, zinc, crystal growth
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ACCEPTED MANUSCRIPT 1 Introduction Electrodeposition of zinc from acidic sulphate electrolytes is an important industrial process for
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corrosion protection of steel strips used in the automotive industry.
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Various aspects such as the influence of pH, temperature and current density on the
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electrodeposition of Zinc from acidic sulphate electrolytes have been studied so far [1-3]. Furthermore deposition parameters like pulse deposition with [4] or without [5] an anodic pulse
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have shown a remarkable influence on morphology and crystal size of electrodeposited zinc. Even an external magnetic field applied to the deposition cell shows changes in this system due to a
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micro magneto hydrodynamic convection [6].
The effects of inorganic impurities such as Ge [7], Pb [8], Cu, Ni, Co [9], Sb [10], Sn [11], Fe [12] organic
additives
to
acidic
sulphate
electrolytes
like
sodium
dodecylsulphate,
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and
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dodecyltrimethylammonium bromide, octylphenolpoly(ethyleneglycolether)n, n = 10 [13] PEG
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20000 [14], sodium lauryl sulphate, arabic gum [15] and tartaric acid [16], and to chloride containing electrolytes [17-19] on current efficiency or the structure of the zinc layer were investigated thoroughly.
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Another important aspect is the preparation of the surface and therefore the microstructure of the surface to be coated which plays a crucial role in the electrodeposition of zinc and the morphology and texture of the deposited zinc layer [20]. Studies have shown that there is a relationship between the texture of the steel substrate and the deposited zinc layer [21-23]. It was recently shown that zinc grows epitaxially on undeformed steel surfaces whereas on deformed steel grains random crystal growth was found [24]. One important influence which hasn’t been studied so far is the influence of the initial electrolyte contact of a steel sheet before electrodeposition. In the industrial process the electrochemical potential is applied simultaneously with the first electrolyte contact. On a laboratory scale on the 3
ACCEPTED MANUSCRIPT other hand galvanostatic experiments are usually performed in flow cells with constant electrolyte velocity which results in an electrolyte contact prior to electrochemical polarisation. In the used
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flow cell this contact time is in the range of some seconds. Thus the aim of this work is to study the
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influence of the contact of electrolyte with a steel sample on the electrodeposition of zinc.
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ACCEPTED MANUSCRIPT 2 Material and Methods
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2.1 Chemicals
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The zinc electrolyte in use was an analytical grade sulfate electrolyte with 90 g L-1 Zn from
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ZnSO4·7 H2O (p.A. from Roth) and 20 g L-1 Na from Na2SO4 (p.A. from Baker) as conducting salt with pH set at 1.55 at 55 °C with H2SO4 (p.A. from Merck).
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For the electropolishing of the substrate prior to use Struers A2 (Struers, Ballerup, Denmark) electrolyte (perchloric acid in a mixture of ethanol, 2-buthoxyethanol and H2O) was used. The
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substrate used for the measurements was a low carbon steel sheet produced by voestalpine Stahl
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2.2 Sample preparation
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GmbH.
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Temper rolled steel samples, further called TRS, were alkaline cleaned with 10 g L-1 Ridoline C72 (Henkel, Germany) and electrochemically etched with 50 mA cm-2 in 10 g L-1 H2SO4 before further investigations.
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The electropolished samples, further called EPS, were produced from mechanically polished TRS. Electropolishing was done using a Struers LECTROPOL 5 unit at 30 V for 30 s. The sample was rinsed with analytical grade ethanol after electropolishing.
2.3 Electrochemical Measurements Electrochemical Measurements were performed at room temperature in an EG & G PARC Flat Cell using an IM6e Potentiostat from Zahner® electric (Kronach, Germany). A NPROBE CorrElNoiseTM – technology device from Zahner Messtechnik was used to enhance ECN-measurement quality.
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ACCEPTED MANUSCRIPT Ag|AgCl|KCl (3M) with a potential of 0.21 V vs. SHE was used as reference electrode. Cyclic
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voltammetry was carried out from -1.2 to 0.3 V vs. SHE with a scan rate of 10 mV s-1.
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2.4 Electroplating
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Before electroplating the samples were immersed in zinc electrolyte for 20 s. Immersion was done in a flow cell at electrolyte temperature of 55 °C and a electrolyte velocity of 4 m s-1.
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Electroplating was performed in the same flow cell. The electrolyte was kept at the same conditions used during immersion and the current density was set at 400 mA cm-2 whit an EA-PS9036-240
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galvanostat (Elektro-Automatik GmbH & Co. KG Company, VIERSEN, Germany). In order to simulate the Gravitel® process [25] with 12 cells, the current was interrupted with 24 pulses (duty
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cycle: 1.7 s on, 1.0 s off). The temperature was kept at 55 °C using a Lauda PROLine RP855
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thermostat. A DSA (Dimensionally Stable Anode) made from Ti coated with IrO2 from De Nora
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(Milan, Italy) was used as an anode. The final thickness of the zinc layer was 7.5 µm.
2.5 Analytics
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Surface measurements were performed with a Zeiss Supra 35 FEG-SEM, an Oxford Channel 5 System for EBSD and an EDAX Smart Insight system for EDX. Calculations for determination of the misorientation angle were described in a previous work [24]. XPS (X-ray photo-electron spectroscopy)-measurements were performed with a Theta Probe from ThermoFischer. SAM (Scanning Auger Microscopy) was done using a JAMP-9500F Field Emission Auger Microprobe. For AAS-measurements a HITACHI Z-8230 polarized Zeeman was used downstream from a scanning droplet cell microscope (SDCM) [26]. 6
ACCEPTED MANUSCRIPT GDOES (glow discharge optical emission spectroscopy)-measurements were conducted on a GDA750 from SPECTRUMA Analytic GmbH.
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IRRAS (infrared reflection absorption spectroscopy) was done with a FTIR-Spectrometer Tensor 27
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from Bruker. An A513/3 grazing angle unit was used to enhance the surface sensibility of the
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measurements.
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ACCEPTED MANUSCRIPT 3 Results
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3.1 Cyclic Voltammetry
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Previous EBSD-measurements showed differences in the electrocrystallisation of zinc between EPS
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and TRS substrates made of low carbon steel. Therefore cyclic voltammetry was performed on these two substrates to determine their electrochemical behaviour (figure 1). Measurements of the
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1st cycle of both substrates show a typical behaviour with upd (underpotential deposition) of zinc from -0.4 to -0.8 V vs. SHE and slight differences of 10 mV in the starting potential of Zinc bulk
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deposition. Furthermore a crossover potential can be observed on TRS whereas on the EPS none was found.
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Cyclic voltammetry was carried out till the dissolution of the steel substrate, so that a fresh surface
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was generated in each cycle. Comparing later cycles of the different substrates most differences
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between the two substrates disappear. There is a slightly higher limiting current on the TRS in the upd regime compared to the EPS. But there is no indication why epitaxial growth occurs on EPS
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but not on TRS.
3.2 Electrochemical Noise (ECN) measurements Due to the fact that the CV’s have shown large differences between the 1st and the following cycles and the fact that the samples were in contact with the electrolyte about 10 - 30 s prior to these measurements, it was concluded that the surface reactivity changes as a result of the electrolyte contact. Therefore ECN in a Zn-electrolyte was measured from first electrolyte contact on (figure 2).
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ACCEPTED MANUSCRIPT On EPS a peak can be observed after 3 s, followed by a sharp drop to a constant current. In comparison to EPS this peak can’t be found on TRS. A slightly higher dissolution current due to the
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surface roughness was observed.
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3.3 Surface Characterization
In addition to the electrochemical measurements surface characterization with SEM, EDX, XPS,
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SAM and GDOES was carried out to investigate the changes of surface chemistry/composition during electrolyte contact.
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Comparing the EDX spectra of the TRS with EPS taken after the cleaning and etching procedure no differences can be found (figure 3). Both substrates show C, O and Fe-peaks only, a typical result
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for low carbon steel sheets.
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After immersion into the electrolyte the TRS shows a clear Peak at about 1 kV. After doublechecking with the Kα-Peak of Zinc which can be found at higher Energies this Peak can be assigned as Zn-Lα-Peak. That’s a major difference compared to EPS, where no zinc peak appears.
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To discern whether or not the measured zinc signal is just an artefact and to be sure that there is no superimposed Na-Signal in the EDX measurements, GDOES was performed on substrates immersed into Zn-electrolytes with different Zn-concentrations (figure 4). To make sure the measured Signal is not affected by the initiation of the plasma, carbon was sputtered onto the samples before measurement.
The GDOES-measurements from figure 4 indicate a precipitation of zinc on the TRS surface. Moreover the dependency of the amount of Zn found on the substrates on the zinc concentration of the electrolyte used is found. This means that the lower the zinc concentration in the electrolyte is 9
ACCEPTED MANUSCRIPT the lower is the signal found in the GDOES measurement. This clearly indicates a reaction of the
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electrolyte with the surface and a dependency of the reaction on the concentration of zinc.
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XPS-measurements on TRS were performed to discern whether or not the Zn-signal stems from
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residual ZnSO4. In figure 5 the XPS-measurement on top shows the comparison of a cleaned -, and etched surface and a surface immersed into electrolyte. In addition a 7.5 µm thick zinc layer
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produced from the same electrolyte used for immersion was added to the graph. Cleaned and etched surfaces were measured to form a reverence for the interpretation of the immersed samples. Both
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show neither impurities nor zinc contamination on the surface. Fe, C and O were found on the cleaned surface whereas after etching small amounts of sulphur are present on the surface due to the
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fact, that etching was performed in sulphuric acid. After immersion of TRS in electrolyte, Zn could
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be found besides Fe, C, O and small amounts of S. The concentration of S found is too low to
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correspond to Zn being present on the surface solely as ZnSO4. The fact that a Fe-Signal was detected indicates that the Zn-precipitate is either thinner than a few nm or it grows in islands. Comparison between the precipitated layer and an electrodeposited zinc with XPS show that no
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metallic zinc can be found on the immersed substrate. High-resolution measurements indicate that the Zn found on the surface is either a carbonate or an oxide. This is in good accordance to the IRRAS measurements done with a grazing angle unit to achieve high surface sensitivity. The differences between the spectra of an etched and an immersed TRS surface (figure 6) indicate that the Zn found on the immersed surface is either hydrozincate (Zn5(CO3)2(OH)6), or oxides and hydroxides of Zink. Both are typical corrosion products found on electrogalvanized steel sheets [27].
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ACCEPTED MANUSCRIPT Quantification of the zinc precipitations with a flat flow cell with downstream analytics to an AASsystem gives an amount of 5 mg m-2 of zinc, which refers to a zinc layer thickness of ~ 1 nm
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assuming a homogeneous film thickness.
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It is assumed that although the transfer time of the freshly immersed sample was kept at a minimum
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metallic zinc can’t be found on the immersed samples.
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such a small layer would be readily oxidized by the contact with air. This fact might explain why
Currentless deposition of zinc on pure iron is thermodynamically unfavourable; therefore
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immersion experiments with different steel substrates and electrodeposited iron – representing the case of pure iron – should reveal the influence of alloying elements on the precipitation of zinc.
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XPS-measurements of these samples (figure 5 bottom) showed a dependence of the amount of
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precipitated zinc on the steel grade. The highest concentration of zinc was found on pure iron
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represented by the electrodeposited iron surface. The concentration of zinc decreased with increasing alloying additions of Cr and Mn so that the least zinc was found on dual-phase steel surfaces. This indicates that the precipitation is not due to alloying additions but in the course of
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iron dissolution.
The fact, that there is no Zn precipitate on EPS but on TRS leads to the assumption that deformation on the steel grains favour the precipitation of Zn on steel. To proof this assumption a local deformation was produced on an EPS. The sample was thereafter immersed in the electrolyte and used for an Auger-mapping.
The mapping in figure 7 shows, that more zinc is found in the deformed area than on the electropolished area. The sulphur signal in the Auger-mapping is preferentially located on the 11
ACCEPTED MANUSCRIPT electropolished area. Therefore the zinc found in this area is most likely ZnSO4. It was concluded that the large amount of S and O found in this area is probably a result of insufficient cleaning. The
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droplet. In the deformed area S can’t be detected besides Zn.
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line with high sulphur concentration at the edge of the deformation is explained by drying of a
3.4 Effect of immersion on Electrodeposition
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Figure 8 shows the EBSD mapping of the cross-section of a zinc layer deposited on a TRS after immersion into electrolyte prior to electrodeposition. Determination of the misorientation angles
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between electroplated zinc and iron of a steel sample after immersion into zinc electrolyte before electroplating gave an overall angle of 42° ± 4°. It can be seen that only on highly deformed TRS
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grains the relation between zinc and steel doesn’t follow Burger’s orientation relation. Comparing
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the relations of the different zinc grains on one such deformed steel grain leads to a maximum
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standard deviation of 2.5 ° thus epitaxial growth can be assumed anyway. On steel grains with slight deformation zinc grows epitaxially following Burger’s orientation relation. Disregarding the steel grains with high deformation leads to a misorientation angle of
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43° ± 2° which is the same as previously detected on EPS [24]. The same substrate without electrolyte pre-treatment shows a misorientation angle of 34° ± 10°, thus random crystal growth.
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ACCEPTED MANUSCRIPT 4 Discussion Electrochemical measurements show that there is an influence of the immersion of a temper rolled
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substrate into the zinc electrolyte prior to the measurements. During this time zinc precipitates on
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TRS whereas on an electropolished substrate no such reaction occurs. This leads to differences in
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cyclovoltammetric scans. The absence of the current peak in ECN measurements on the temper rolled substrate in combination with the fact that zinc can be found on the surface, indicates that
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zinc-cementation is the precipitation reaction. Furthermore the difference in charge between the temper rolled substrate and the electropolished substrate from the ECN measurements calculated by
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the missing peak can explain the thickness of the Zn-layer determined by the AAS investigations. Although no metallic Zinc could be found on the steel surface by XPS-Measurements it is
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postulated that there is a cementation of zinc on the deformed steel grains. It is shown by XPS
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measurements that the zinc deposit doesn’t consist of ZnSO4. The fact that there is about 5 mg m2
detected.
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of zinc on the surface explains why after exposure to air only corrosion products of zinc were
The fact that this cementation happens even on electrodeposited iron layers indicates that there is an
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anti-galvanic reduction similar to the reduction of silver on a gold nanosystem [28]. No zinc precipitation occurs on the electropolished surface but on electrodeposited and deformed surfaces as was shown with SAM-measurements. This indicates that crystal deformation is necessary for this reaction. It can be assumed that due to the deformation of the steel grains the dissolution potential of the deformed grains is shifted to more cathodic values than the potential of undeformed or electropolished grains. This shift might be sufficient to reach a potential where underpotential deposition of zinc is possible. The fact that there might be a formation of a Zn/Fe alloy, similar to the system Zn/Cu, involved can’t be excluded by the present results. 13
ACCEPTED MANUSCRIPT The resulting Zn deposit leads to a change in electro crystallisation and furthermore to a change in the electrochemical crystal growth. Zinc is, after immersion of the TRS, deposited on a thin layer of
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zinc instead of steel. This change in surface chemistry changes the electrochemical crystal growth
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of zinc from random to epitaxial growth mostly following Burger’s orientation relation, which is
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proven by the EBSD measurements.
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ACCEPTED MANUSCRIPT 5 Conclusion
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In the course of immersion of a temper rolled substrate into zinc-electrolyte, the surface properties
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change. Zinc is deposited on iron in an anti-galvanic reaction. This reaction is possible only on
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deformed steel grains. Consequently this reaction is not observed on an electropolished substrate. Such small amounts of zinc act as crystallisation seeds, and lead to a complete change in the
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electrocrystallisation mechanism from random to epitaxial since the zinc is deposited on zinc
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instead of iron.
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Acknowledgments
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The financial support by the Austrian Federal Ministry of Economy, Family and Youth and the
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National Foundation for Research, Technology and Development is gratefully acknowledged.
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ACCEPTED MANUSCRIPT 6 References
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ACCEPTED MANUSCRIPT List of Figure captions
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Fig. 1:
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Comparison between temper rolled substrates TRS and electropolished substrates EPS in the 1st
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(C1) and 4th (C4) cycle of a cyclic voltammogram in ZnSO4 Electrolyte
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Fig. 2:
Electrochemical Noise measurement of temper rolled substrates TRSand electropolished substrates
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EPS in ZnSO4 Electrolyte
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Fig. 3:
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Comparison of temper rolled substrates TRS and electropolished substrates EPS before and after
Fig. 4:
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immersion into Zn-Electrolyte; Energy dispersive X-ray EDX measurements performed in a SEM
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Glow discharge optical emission spectroscopy GDOES depth profiles of temper rolled substrates TRS after immersion into electrolytes with a Zn-concentration of 45 g L-1 (solid line) 25 g L-1 (dashed line) and 0 g L-1 (dotted lines)
Fig. 5: XPS-measurements of steel surfaces after the different steps during electrodeposition (top) and comparison of different steel grades after immersion into electrolyte (bottom)
Fig. 6: 18
ACCEPTED MANUSCRIPT Grazing angle Infrared-Reflection-Absorption difference spectrum of an immersed and etched
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temper rolled substrate, the assignment to discussed substances is given as inset
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Fig. 7:
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Scanning Auger Microscopy-mapping of a local deformation on an electropolished surface; Zn-
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Signal (upper) and S-Signal (middle), SEM-image (bottom)
Fig. 8:
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IPF (inverse pole figure) map of a cross-section of a temper rolled substrate and corresponding zinc
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layer electrodeposited after immersion into zinc electrolyte
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TRS (C1) EPS (C1) TRS (C4) EPS (C4)
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Figures
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Zinc Steel
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Highlights:
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anti-galvanic reaction of zinc is possible on deformed steel grains
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initial Zn seeds determine further crystallisation mechanism random versus epitaxial electrodeposition is found
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EBSD-measurements demonstrates epitaxial zinc crystal growth on low carbon steel
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In depth analysis involved SDCM, SEM, SAM, XPS, GDOES, IRRAS
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S0257-8972(14)00409-5 doi: 10.1016/j.surfcoat.2014.04.065 SCT 19390
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
27 August 2013 9 March 2014 29 April 2014
Please cite this article as: Thomas Greul, Johann Gerdenitsch, Christian Commenda, Raffaela Sagl, Martin Arndt, Jiri Duchoslav, Achim Walter Hassel, Coating substrate relationship after initial electrolyte contact in the electrodeposition of zinc on steel, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.04.065
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Coating substrate relationship after initial electrolyte contact in the
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electrodeposition of zinc on steel
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Thomas Greula,b, Johann Gerdenitschb, Christian Commendab, Raffaela Saglb, Martin Arndtc, Jiri Duchoslavc, Achim Walter Hassela,d *
Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University Linz,
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Altenberger Str. 69, 4040 Linz, Austria
Christian Doppler Laboratory for Microscopic and Spectroscopic Material Characterization, Center
Austria
CEST GmbH, Viktor Kaplan Str. 2, 2700 Wiener Neustadt, Austria
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for Surface- and Nanoanalytics, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz,
*Corresponding author:
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c
Voestalpine Stahl GmbH, voestalpine-Straße 3, 4020 Linz, Austria;
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b
Tel.: +43 732 2468 8700, Fax: +43 732 2468 8905, email: [email protected]
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ACCEPTED MANUSCRIPT Abstract Electron backscatter diffraction (EBSD) measurements show epitaxial electrochemical deposition
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of zinc on non- deformed and contrariwise random growth on deformed steel grains. Therefore,
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electrochemical investigations on the differences of zinc electrodeposition on electropolished
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respectively on temper rolled low carbon steel sheet of the same substrate are studied. These measurements show great differences in ECN (electrochemical noise) investigations and the 1st
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cycle of a CV (cyclic voltammetry) only. It is found, that the immersion of the substrate in the electrolyte prior to the measurements causes this behaviour. SEM, EDX, XPS (X-ray photoelectron
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spectroscopy), SAM (Scanning Auger microscopy) and IRRAS (Infrared reflection absorption spectroscopy) measurements are used for surface characterization and EBSD-measurements for
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determination of crystallographic orientation to reveal the influence on electrochemical growth. It is
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proven that zinc precipitates form on the temper rolled substrate during immersion and cause a
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nucleation process prior to electrodeposition and the subsequent change in electrocrystallisation as zinc is deposited on zinc instead of steel. The differences in the electrochemical measurements are
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well described by this theory.
Key words: electro deposition, zinc, crystal growth
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ACCEPTED MANUSCRIPT 1 Introduction Electrodeposition of zinc from acidic sulphate electrolytes is an important industrial process for
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corrosion protection of steel strips used in the automotive industry.
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Various aspects such as the influence of pH, temperature and current density on the
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electrodeposition of Zinc from acidic sulphate electrolytes have been studied so far [1-3]. Furthermore deposition parameters like pulse deposition with [4] or without [5] an anodic pulse
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have shown a remarkable influence on morphology and crystal size of electrodeposited zinc. Even an external magnetic field applied to the deposition cell shows changes in this system due to a
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micro magneto hydrodynamic convection [6].
The effects of inorganic impurities such as Ge [7], Pb [8], Cu, Ni, Co [9], Sb [10], Sn [11], Fe [12] organic
additives
to
acidic
sulphate
electrolytes
like
sodium
dodecylsulphate,
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and
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dodecyltrimethylammonium bromide, octylphenolpoly(ethyleneglycolether)n, n = 10 [13] PEG
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20000 [14], sodium lauryl sulphate, arabic gum [15] and tartaric acid [16], and to chloride containing electrolytes [17-19] on current efficiency or the structure of the zinc layer were investigated thoroughly.
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Another important aspect is the preparation of the surface and therefore the microstructure of the surface to be coated which plays a crucial role in the electrodeposition of zinc and the morphology and texture of the deposited zinc layer [20]. Studies have shown that there is a relationship between the texture of the steel substrate and the deposited zinc layer [21-23]. It was recently shown that zinc grows epitaxially on undeformed steel surfaces whereas on deformed steel grains random crystal growth was found [24]. One important influence which hasn’t been studied so far is the influence of the initial electrolyte contact of a steel sheet before electrodeposition. In the industrial process the electrochemical potential is applied simultaneously with the first electrolyte contact. On a laboratory scale on the 3
ACCEPTED MANUSCRIPT other hand galvanostatic experiments are usually performed in flow cells with constant electrolyte velocity which results in an electrolyte contact prior to electrochemical polarisation. In the used
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flow cell this contact time is in the range of some seconds. Thus the aim of this work is to study the
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influence of the contact of electrolyte with a steel sample on the electrodeposition of zinc.
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ACCEPTED MANUSCRIPT 2 Material and Methods
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2.1 Chemicals
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The zinc electrolyte in use was an analytical grade sulfate electrolyte with 90 g L-1 Zn from
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ZnSO4·7 H2O (p.A. from Roth) and 20 g L-1 Na from Na2SO4 (p.A. from Baker) as conducting salt with pH set at 1.55 at 55 °C with H2SO4 (p.A. from Merck).
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For the electropolishing of the substrate prior to use Struers A2 (Struers, Ballerup, Denmark) electrolyte (perchloric acid in a mixture of ethanol, 2-buthoxyethanol and H2O) was used. The
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substrate used for the measurements was a low carbon steel sheet produced by voestalpine Stahl
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2.2 Sample preparation
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GmbH.
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Temper rolled steel samples, further called TRS, were alkaline cleaned with 10 g L-1 Ridoline C72 (Henkel, Germany) and electrochemically etched with 50 mA cm-2 in 10 g L-1 H2SO4 before further investigations.
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The electropolished samples, further called EPS, were produced from mechanically polished TRS. Electropolishing was done using a Struers LECTROPOL 5 unit at 30 V for 30 s. The sample was rinsed with analytical grade ethanol after electropolishing.
2.3 Electrochemical Measurements Electrochemical Measurements were performed at room temperature in an EG & G PARC Flat Cell using an IM6e Potentiostat from Zahner® electric (Kronach, Germany). A NPROBE CorrElNoiseTM – technology device from Zahner Messtechnik was used to enhance ECN-measurement quality.
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ACCEPTED MANUSCRIPT Ag|AgCl|KCl (3M) with a potential of 0.21 V vs. SHE was used as reference electrode. Cyclic
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voltammetry was carried out from -1.2 to 0.3 V vs. SHE with a scan rate of 10 mV s-1.
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2.4 Electroplating
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Before electroplating the samples were immersed in zinc electrolyte for 20 s. Immersion was done in a flow cell at electrolyte temperature of 55 °C and a electrolyte velocity of 4 m s-1.
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Electroplating was performed in the same flow cell. The electrolyte was kept at the same conditions used during immersion and the current density was set at 400 mA cm-2 whit an EA-PS9036-240
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galvanostat (Elektro-Automatik GmbH & Co. KG Company, VIERSEN, Germany). In order to simulate the Gravitel® process [25] with 12 cells, the current was interrupted with 24 pulses (duty
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cycle: 1.7 s on, 1.0 s off). The temperature was kept at 55 °C using a Lauda PROLine RP855
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thermostat. A DSA (Dimensionally Stable Anode) made from Ti coated with IrO2 from De Nora
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(Milan, Italy) was used as an anode. The final thickness of the zinc layer was 7.5 µm.
2.5 Analytics
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Surface measurements were performed with a Zeiss Supra 35 FEG-SEM, an Oxford Channel 5 System for EBSD and an EDAX Smart Insight system for EDX. Calculations for determination of the misorientation angle were described in a previous work [24]. XPS (X-ray photo-electron spectroscopy)-measurements were performed with a Theta Probe from ThermoFischer. SAM (Scanning Auger Microscopy) was done using a JAMP-9500F Field Emission Auger Microprobe. For AAS-measurements a HITACHI Z-8230 polarized Zeeman was used downstream from a scanning droplet cell microscope (SDCM) [26]. 6
ACCEPTED MANUSCRIPT GDOES (glow discharge optical emission spectroscopy)-measurements were conducted on a GDA750 from SPECTRUMA Analytic GmbH.
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IRRAS (infrared reflection absorption spectroscopy) was done with a FTIR-Spectrometer Tensor 27
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from Bruker. An A513/3 grazing angle unit was used to enhance the surface sensibility of the
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measurements.
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ACCEPTED MANUSCRIPT 3 Results
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3.1 Cyclic Voltammetry
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Previous EBSD-measurements showed differences in the electrocrystallisation of zinc between EPS
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and TRS substrates made of low carbon steel. Therefore cyclic voltammetry was performed on these two substrates to determine their electrochemical behaviour (figure 1). Measurements of the
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1st cycle of both substrates show a typical behaviour with upd (underpotential deposition) of zinc from -0.4 to -0.8 V vs. SHE and slight differences of 10 mV in the starting potential of Zinc bulk
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deposition. Furthermore a crossover potential can be observed on TRS whereas on the EPS none was found.
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Cyclic voltammetry was carried out till the dissolution of the steel substrate, so that a fresh surface
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was generated in each cycle. Comparing later cycles of the different substrates most differences
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between the two substrates disappear. There is a slightly higher limiting current on the TRS in the upd regime compared to the EPS. But there is no indication why epitaxial growth occurs on EPS
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but not on TRS.
3.2 Electrochemical Noise (ECN) measurements Due to the fact that the CV’s have shown large differences between the 1st and the following cycles and the fact that the samples were in contact with the electrolyte about 10 - 30 s prior to these measurements, it was concluded that the surface reactivity changes as a result of the electrolyte contact. Therefore ECN in a Zn-electrolyte was measured from first electrolyte contact on (figure 2).
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ACCEPTED MANUSCRIPT On EPS a peak can be observed after 3 s, followed by a sharp drop to a constant current. In comparison to EPS this peak can’t be found on TRS. A slightly higher dissolution current due to the
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surface roughness was observed.
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3.3 Surface Characterization
In addition to the electrochemical measurements surface characterization with SEM, EDX, XPS,
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SAM and GDOES was carried out to investigate the changes of surface chemistry/composition during electrolyte contact.
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Comparing the EDX spectra of the TRS with EPS taken after the cleaning and etching procedure no differences can be found (figure 3). Both substrates show C, O and Fe-peaks only, a typical result
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for low carbon steel sheets.
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After immersion into the electrolyte the TRS shows a clear Peak at about 1 kV. After doublechecking with the Kα-Peak of Zinc which can be found at higher Energies this Peak can be assigned as Zn-Lα-Peak. That’s a major difference compared to EPS, where no zinc peak appears.
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To discern whether or not the measured zinc signal is just an artefact and to be sure that there is no superimposed Na-Signal in the EDX measurements, GDOES was performed on substrates immersed into Zn-electrolytes with different Zn-concentrations (figure 4). To make sure the measured Signal is not affected by the initiation of the plasma, carbon was sputtered onto the samples before measurement.
The GDOES-measurements from figure 4 indicate a precipitation of zinc on the TRS surface. Moreover the dependency of the amount of Zn found on the substrates on the zinc concentration of the electrolyte used is found. This means that the lower the zinc concentration in the electrolyte is 9
ACCEPTED MANUSCRIPT the lower is the signal found in the GDOES measurement. This clearly indicates a reaction of the
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electrolyte with the surface and a dependency of the reaction on the concentration of zinc.
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XPS-measurements on TRS were performed to discern whether or not the Zn-signal stems from
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residual ZnSO4. In figure 5 the XPS-measurement on top shows the comparison of a cleaned -, and etched surface and a surface immersed into electrolyte. In addition a 7.5 µm thick zinc layer
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produced from the same electrolyte used for immersion was added to the graph. Cleaned and etched surfaces were measured to form a reverence for the interpretation of the immersed samples. Both
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show neither impurities nor zinc contamination on the surface. Fe, C and O were found on the cleaned surface whereas after etching small amounts of sulphur are present on the surface due to the
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fact, that etching was performed in sulphuric acid. After immersion of TRS in electrolyte, Zn could
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be found besides Fe, C, O and small amounts of S. The concentration of S found is too low to
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correspond to Zn being present on the surface solely as ZnSO4. The fact that a Fe-Signal was detected indicates that the Zn-precipitate is either thinner than a few nm or it grows in islands. Comparison between the precipitated layer and an electrodeposited zinc with XPS show that no
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metallic zinc can be found on the immersed substrate. High-resolution measurements indicate that the Zn found on the surface is either a carbonate or an oxide. This is in good accordance to the IRRAS measurements done with a grazing angle unit to achieve high surface sensitivity. The differences between the spectra of an etched and an immersed TRS surface (figure 6) indicate that the Zn found on the immersed surface is either hydrozincate (Zn5(CO3)2(OH)6), or oxides and hydroxides of Zink. Both are typical corrosion products found on electrogalvanized steel sheets [27].
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ACCEPTED MANUSCRIPT Quantification of the zinc precipitations with a flat flow cell with downstream analytics to an AASsystem gives an amount of 5 mg m-2 of zinc, which refers to a zinc layer thickness of ~ 1 nm
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assuming a homogeneous film thickness.
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It is assumed that although the transfer time of the freshly immersed sample was kept at a minimum
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metallic zinc can’t be found on the immersed samples.
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such a small layer would be readily oxidized by the contact with air. This fact might explain why
Currentless deposition of zinc on pure iron is thermodynamically unfavourable; therefore
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immersion experiments with different steel substrates and electrodeposited iron – representing the case of pure iron – should reveal the influence of alloying elements on the precipitation of zinc.
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XPS-measurements of these samples (figure 5 bottom) showed a dependence of the amount of
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precipitated zinc on the steel grade. The highest concentration of zinc was found on pure iron
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represented by the electrodeposited iron surface. The concentration of zinc decreased with increasing alloying additions of Cr and Mn so that the least zinc was found on dual-phase steel surfaces. This indicates that the precipitation is not due to alloying additions but in the course of
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iron dissolution.
The fact, that there is no Zn precipitate on EPS but on TRS leads to the assumption that deformation on the steel grains favour the precipitation of Zn on steel. To proof this assumption a local deformation was produced on an EPS. The sample was thereafter immersed in the electrolyte and used for an Auger-mapping.
The mapping in figure 7 shows, that more zinc is found in the deformed area than on the electropolished area. The sulphur signal in the Auger-mapping is preferentially located on the 11
ACCEPTED MANUSCRIPT electropolished area. Therefore the zinc found in this area is most likely ZnSO4. It was concluded that the large amount of S and O found in this area is probably a result of insufficient cleaning. The
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droplet. In the deformed area S can’t be detected besides Zn.
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line with high sulphur concentration at the edge of the deformation is explained by drying of a
3.4 Effect of immersion on Electrodeposition
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Figure 8 shows the EBSD mapping of the cross-section of a zinc layer deposited on a TRS after immersion into electrolyte prior to electrodeposition. Determination of the misorientation angles
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between electroplated zinc and iron of a steel sample after immersion into zinc electrolyte before electroplating gave an overall angle of 42° ± 4°. It can be seen that only on highly deformed TRS
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grains the relation between zinc and steel doesn’t follow Burger’s orientation relation. Comparing
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the relations of the different zinc grains on one such deformed steel grain leads to a maximum
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standard deviation of 2.5 ° thus epitaxial growth can be assumed anyway. On steel grains with slight deformation zinc grows epitaxially following Burger’s orientation relation. Disregarding the steel grains with high deformation leads to a misorientation angle of
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43° ± 2° which is the same as previously detected on EPS [24]. The same substrate without electrolyte pre-treatment shows a misorientation angle of 34° ± 10°, thus random crystal growth.
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ACCEPTED MANUSCRIPT 4 Discussion Electrochemical measurements show that there is an influence of the immersion of a temper rolled
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substrate into the zinc electrolyte prior to the measurements. During this time zinc precipitates on
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TRS whereas on an electropolished substrate no such reaction occurs. This leads to differences in
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cyclovoltammetric scans. The absence of the current peak in ECN measurements on the temper rolled substrate in combination with the fact that zinc can be found on the surface, indicates that
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zinc-cementation is the precipitation reaction. Furthermore the difference in charge between the temper rolled substrate and the electropolished substrate from the ECN measurements calculated by
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the missing peak can explain the thickness of the Zn-layer determined by the AAS investigations. Although no metallic Zinc could be found on the steel surface by XPS-Measurements it is
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postulated that there is a cementation of zinc on the deformed steel grains. It is shown by XPS
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measurements that the zinc deposit doesn’t consist of ZnSO4. The fact that there is about 5 mg m2
detected.
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of zinc on the surface explains why after exposure to air only corrosion products of zinc were
The fact that this cementation happens even on electrodeposited iron layers indicates that there is an
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anti-galvanic reduction similar to the reduction of silver on a gold nanosystem [28]. No zinc precipitation occurs on the electropolished surface but on electrodeposited and deformed surfaces as was shown with SAM-measurements. This indicates that crystal deformation is necessary for this reaction. It can be assumed that due to the deformation of the steel grains the dissolution potential of the deformed grains is shifted to more cathodic values than the potential of undeformed or electropolished grains. This shift might be sufficient to reach a potential where underpotential deposition of zinc is possible. The fact that there might be a formation of a Zn/Fe alloy, similar to the system Zn/Cu, involved can’t be excluded by the present results. 13
ACCEPTED MANUSCRIPT The resulting Zn deposit leads to a change in electro crystallisation and furthermore to a change in the electrochemical crystal growth. Zinc is, after immersion of the TRS, deposited on a thin layer of
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zinc instead of steel. This change in surface chemistry changes the electrochemical crystal growth
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of zinc from random to epitaxial growth mostly following Burger’s orientation relation, which is
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proven by the EBSD measurements.
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ACCEPTED MANUSCRIPT 5 Conclusion
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In the course of immersion of a temper rolled substrate into zinc-electrolyte, the surface properties
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change. Zinc is deposited on iron in an anti-galvanic reaction. This reaction is possible only on
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deformed steel grains. Consequently this reaction is not observed on an electropolished substrate. Such small amounts of zinc act as crystallisation seeds, and lead to a complete change in the
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electrocrystallisation mechanism from random to epitaxial since the zinc is deposited on zinc
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instead of iron.
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Acknowledgments
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The financial support by the Austrian Federal Ministry of Economy, Family and Youth and the
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National Foundation for Research, Technology and Development is gratefully acknowledged.
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ACCEPTED MANUSCRIPT 6 References
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[1] I. Zouari, F. Lapicque, An electrochemical study of zinc deposition in a sulfate medium, Electrochim. Acta 37 (1992) 439-446
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[2] P. Guillaume, N. Leclerc, C. Boulanger, J. Lecuire, F. Lapicque, Investigation of optimal conditions for zinc electrowinning from aqueous sulfuric acid electrolytes, J. Appl. Electrochem. 37 (2007) 1237-1243 [3] R. C. Salles, G. C. de Oliveira, S. L. Díaz, O. E. Barcia, O. R. Mattos, Electrodeposition of Zn in acid sulphate solutions: pH effects, Electrochim. Acta 56 (2011) 7931-7939
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[6] M. Uhlemann, K. Tschulik, A. Gebert, G. Mutschke, J. Fröhlich, A. Bund, X. G. Yang, K. Eckert, Structured electrodeposition in magnetic gradient fields, Eur. Phys. J. Spec. Top. 220 (2013) 287-302
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ACCEPTED MANUSCRIPT [15] A. Recéndiz, I. González, J. Nava, Current efficiency studies of the zinc electrowinning process on aluminum rotating cylinder electrode (RCE) in sulfuric acid medium: Influence of different additives, Electrochim. Acta 52 (2007) 6880-6887
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ACCEPTED MANUSCRIPT List of Figure captions
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Fig. 1:
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Comparison between temper rolled substrates TRS and electropolished substrates EPS in the 1st
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(C1) and 4th (C4) cycle of a cyclic voltammogram in ZnSO4 Electrolyte
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Fig. 2:
Electrochemical Noise measurement of temper rolled substrates TRSand electropolished substrates
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EPS in ZnSO4 Electrolyte
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Fig. 3:
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Comparison of temper rolled substrates TRS and electropolished substrates EPS before and after
Fig. 4:
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immersion into Zn-Electrolyte; Energy dispersive X-ray EDX measurements performed in a SEM
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Glow discharge optical emission spectroscopy GDOES depth profiles of temper rolled substrates TRS after immersion into electrolytes with a Zn-concentration of 45 g L-1 (solid line) 25 g L-1 (dashed line) and 0 g L-1 (dotted lines)
Fig. 5: XPS-measurements of steel surfaces after the different steps during electrodeposition (top) and comparison of different steel grades after immersion into electrolyte (bottom)
Fig. 6: 18
ACCEPTED MANUSCRIPT Grazing angle Infrared-Reflection-Absorption difference spectrum of an immersed and etched
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temper rolled substrate, the assignment to discussed substances is given as inset
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Fig. 7:
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Scanning Auger Microscopy-mapping of a local deformation on an electropolished surface; Zn-
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Signal (upper) and S-Signal (middle), SEM-image (bottom)
Fig. 8:
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IPF (inverse pole figure) map of a cross-section of a temper rolled substrate and corresponding zinc
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layer electrodeposited after immersion into zinc electrolyte
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TRS (C1) EPS (C1) TRS (C4) EPS (C4)
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0
-0.5
0.0
0.5
E / V vs. NHE
TRS (C1) EPS (C1) TRS (C4)
D
-10
TE
EPS (C4)
-0.8
-0.6
E / V vs. SHE
AC
CE P
-20 -1.0
Figure 1
-1.0
MA
I / mA
-100 -1.5
20
-0.4
-0.2
T
ACCEPTED MANUSCRIPT
IP
8.5
SC R
8.0 7.5
NU
6.5
TRS
MA
I / mA
7.0
D
6.0
TE
5.5
Figure 2
10
20
30
40
t/s
AC
0
CE P
5.0
EPS
21
50
1000
1200
100
T
ACCEPTED MANUSCRIPT
Fe
IP
TRS immersed EPS immersed TRS cleaned EPS cleaned
SC R
80
NU C
0 0.0
Zn
1.0
1.5
E / kV
CE P
0.5
TRS immersed EPS immersed
AC
Figure 3
O
D
20
MA
40
TE
Irel / %
60
22
2.0
2.5
3.0
ACCEPTED MANUSCRIPT
SC R
IP
T
100
75
Fe Zn O
NU
50
MA
Irel / %
Fe
25
D
Zn
10
15
d / nm
AC
Figure 4
5
CE P
0
TE
0
23
O
20
25
30
ACCEPTED MANUSCRIPT
cleaned surface etched surface immersed surface Zinc layer
IP
T
Zn
O Zn
Zn O
Zn Zn
Zn
Zn+Fe
C
Zn
S
S
Fe
MA
Fe Fe
NU
I/1
O
SC R
Zn
1200
1000
800
D
1400
600
400
200
0
CE P
TE
BE / eV
electrodeposited-Fe surface low carbon steel surface dual phase steel surface
Zn
Zn
O
O
O
Zn
AC
Fe Fe Zn Zn Zn Zn
S
Zn+Fe Fe
S Zn
I/1
C
1400
1200
1000
800
600
BE / eV Figure 5 24
400
200
0
AC
Figure 6
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
25
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
26
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
MA
Figure 7
27
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
D
Figure 8
28
Zinc Steel
ACCEPTED MANUSCRIPT
T
Highlights:
IP
anti-galvanic reaction of zinc is possible on deformed steel grains
SC R
initial Zn seeds determine further crystallisation mechanism random versus epitaxial electrodeposition is found
NU
EBSD-measurements demonstrates epitaxial zinc crystal growth on low carbon steel
AC
CE P
TE
D
MA
In depth analysis involved SDCM, SEM, SAM, XPS, GDOES, IRRAS
29