In Situ Scanning Tunneling Microscopy Study of Grain-Dependent

Oct 8, 2014 - Reza Parvizi , Anthony E. Hughes , Mike Y. Tan , Ross K.W. Marceau ... Hu Chen , Mohamed Bettayeb , Vincent Maurice , Lorena H. Klein ...
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In Situ Scanning Tunneling Microscopy Study of Grain-Dependent Corrosion on Microcrystalline Copper Esther Martinez-Lombardia,† Vincent Maurice,‡ Linsey Lapeire,§ Iris De Graeve,† Kim Verbeken,§ Leo Kestens,§ Philippe Marcus,‡ and Herman Terryn*,† †

Research Group Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium PSL Research University, Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France § Department of Materials Science and Engineering, Ghent University (UGent), Technologiepark 903, B-9052 Zwijnaarde (Ghent), Belgium ‡

ABSTRACT: In situ electrochemical scanning tunneling microscopy (ECSTM) was applied to analyze the local susceptibility to corrosion at different grains of cryogenically rolled microcrystalline copper in a HCl solution, and combined with electron backscatter diffraction (EBSD) and field emission scanning electron miscroscopy (FE-SEM) to discuss the relationship between nanometer scale corrosion resistance and crystallographic orientation. The results show that the thickness of the air-formed oxide layer is grain-dependent with the largest values exceeding locally by a factor of 2 the macroscopic value (2.8 nm) measured electrochemically. Anodic dissolution is also grain-dependent with dissolving grains observed to neighbor corrosion-resistant grains. A nearly random texture prevented an EBSD-based assignment of the crystallographic orientation of the grains observed by ECSTM. However, comparison of the etched surface morphology measured in situ by ECSTM and ex situ by FESEM suggested that the faster dissolving grains were oriented closer to ⟨111⟩//ND or in between ⟨111⟩//ND and ⟨110⟩//ND while the neighboring corrosion-resistant grains were oriented closer to ⟨001⟩//ND. The higher step density measured by ECSTM on the grains corroding faster despite possibly ⟨111⟩//ND oriented terraces confirms the role of surface defects related to misorientation on the corrosion susceptibility.

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INTRODUCTION Most metallic materials are polycrystalline and consist of many grains with different crystallographic orientations. Local studies on individual grains have demonstrated that the crystallographic orientation of a specific grain at the metal surface affects its corrosion properties.1−5 These differences between single grains on a microscopic level cause the anisotropic corrosion properties of metals. Understanding the role of the crystallographic orientation on the initial stages of corrosion of a polycrystal is a challenging topic and requires electrochemical techniques with high lateral resolution. The grain-dependent corrosion behavior of polycrystals has been studied with different local electrochemical techniques. The microcapillary cell technique has been used to study the critical current density for the transition from active to passive behavior of a polycrystalline FeAlCr ferritic (bcc) steel, and it was reported that the critical current density on the (111) plane is 53% higher than for the (001) plane.6 The technique was also used to investigate the electrochemical behavior of single Zn grains of different crystallographic orientations, and it was suggested that the oxide formation is faster on close-packed planes.7 The growth of an oxide film on titanium was also studied using this technique, and it was confirmed that the electrochemical reactivity of the grown oxide was depending on the crystallographic orientation of the titanium grains.8 Scanning electrochemical microscopy9 has been widely used © 2014 American Chemical Society

to study the electrochemical activity and the local dissolution of different metals, demonstrating that the crystallographic orientations at the metal surface have an effect on the corrosion properties.10−15 All of these published works confirm that there is an increasing interest to quantify the local electrochemical behavior of polycrystalline materials, and to link this behavior to the different microstructural variables. One of the most powerful techniques for obtaining topographic information on metal surfaces with high lateral resolution is scanning tunneling microscopy (STM). Most studies nevertheless focused on single crystals because this enables one to get new insight on the properties of wellcontrolled surfaces at the atomic level. On copper, several groups have characterized the in situ electrochemical behavior of single crystals immersed in various electrolytes by using the electrochemical STM (ECSTM). 16−22 In our recently published work, ECSTM was also proven to be a powerful tool for providing highly accurate topographic information to study the susceptibility to intergranular corrosion of different types of grain boundaries.23 In the present study, ECSTM was used to investigate the submicrometer scale morphological changes at different grains Received: July 16, 2014 Revised: October 4, 2014 Published: October 8, 2014 25421

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After cathodic reduction, anodic dissolution was forced by scanning the potential in the positive direction until an anodic current density not exceeding 50 μA cm−2 was measured. The potential was then scanned back to the starting value of −0.05 V/SHE, and new images of the same area were taken at this potential. The STM tip was withdrawn from the surface while polarizing the sample and re-engaged afterward, allowing analyzing the very same area before and after several anodic cycles. The evolution of the topography could thus be followed after several anodic cycles.

produced by reduction of the air-formed oxide and by anodic dissolution. It is shown that it is possible to study in situ the local corrosion on a polycrystalline material by means of ECSTM. The relation of the observed corrosion properties with crystallographic orientation is discussed based on electron backscatter diffraction (EBSD) and field emission scanning electron miscroscopy (FE-SEM) analysis. The advantages of using ECSTM are the higher spatial resolution, both perpendicular and parallel to the surface, that can be attained compared with other local electrochemical techniques and that it allows addressing nanometer scale surface alterations occurring in the initial stages of corrosion that are not detectable by other techniques.

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RESULTS AND DISCUSSION Grain-Dependent Air-Formed Oxide Film. As a seminoble metal, copper develops an air-formed oxide layer that can be easily reduced by scanning the potential to an appropriate negative electrode potential. Figure 1 shows the cyclic

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EXPERIMENTAL METHODS The material used in this work was a specially designed microcrystalline copper. The starting material was cast Electrolytic Tough Pitch (ETP-) Cu, obtained from Aurubis (Belgium). Chemical analysis confirmed that it contained nearly no sulfur (less than 5 ppm) and a minimal amount (27 ppm) of oxygen. Grain size and crystallographic texture could be controlled to a certain extent by varying the deformation and annealing parameters. For this study, a suitable grain size for the STM field of view was of great importance. Cryogenic deformation, previously proposed to produce microcrystalline or nanocrystalline materials,24,25 was applied. The samples were prepared by cryogenic rolling in liquid nitrogen. The final reduction of the sample thickness was 91%. After this mechanical treatment, the samples were annealed for 1 min at 200 °C to ensure full recrystallization of the grains while maintaining a suitable grain size for the field of view of the ECSTM scanner (10 μm × 10 μm). The surface was prepared by mechanical polishing with diamond paste down to 0.25 μm grade, followed by electrochemical polishing in 60% orthophosphoric acid during 4 min at 1.4 V versus a copper electrode. The texture of the microcrystalline copper samples was characterized by EBSD. The EBSD system is attached to an FEI FE-SEM (Quanta) operated at 20 kV. Measurements were carried out with a step size of 0.1 μm. The inverse pole figure maps (IPF), grain size, and the orientation data were calculated and analyzed by the commercial orientation imaging software package OIM-TSL. The texture is described by the orientation distribution function (ODF). All ECSTM experiments were performed at room temperature with an Agilent Technologies PicoSPM system. The selected electrolyte was a 10 mM HCl aqueous solution. The ECSTM cell was mounted on top of the microcrystalline copper sample with a working area of 0.16 cm2 exposed to the electrolyte. Two Pt wires served as pseudo reference electrode and counter electrode. The tungsten STM tips were prepared from 0.25 mm diameter wire electrochemically etched in 3 M NaOH and covered by Apiezon wax. All images were obtained in the constant current mode. All potentials reported are relative to the standard hydrogen electrode (V/SHE). The samples were exposed to the electrolyte at −0.05 V/ SHE to avoid any copper dissolution before starting the measurements. This value was selected based on the measured open-circuit potential value of 0.1 V/SHE. The air-formed native oxide was first reduced by scanning the potential down to −0.4 V/SHE and backward to the starting value of −0.05 V/ SHE. Images of the microcrystalline copper surface were taken before and after the cathodic reduction of the native oxide film.

Figure 1. Cyclic voltammograms for the cathodic pretreatment of microcrystalline copper in 10 mM HCl as recorded in the ECSTM cell; scan rate = 10 mV/s.

voltammogram for the cathodic pretreatment in the ECSTM cell. The reduction peak is observed at −0.20 V/SHE in the first CV. A second CV produced nearly no peak, showing that the first CV was effective in reducing almost totally the airformed oxide. The thickness (δ) of the cathodically reduced oxide layer was calculated from the total charge density (q) of the reduction peaks for the first and second CVs according to eq 1 δ = qV /zF

(1)

where V is the molar volume of the reduced oxide, F is the Faraday constant, and z is the number of exchanged electrons. The measured reduction charge density, calculated as the sum of the charge density of the reduction peak from the two CVs, is 1143 μA·cm−2. Assuming that the oxide layer consists of Cu2O (V = 23.9 cm3·mol−1, z = 1), the calculated thickness of the Cu2O oxide layer is 2.8 nm. Recent IR-RAS and XPS data demonstrate that pure copper is covered only with Cu2O cuprous oxide in a short term after sample preparation,26 as prepared in the present work. The surface oxide layer is covered with adsorbed H2O and OH, without any evidence of CuO present.27 Figure 2 shows the sample topography prior to and after two cathodic reduction cycles, i.e., after full reduction of the air25422

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Figure 2. Topographic ECSTM images of the surface of polycrystalline copper covered by the air-formed oxide (a) and metallic surface of the same area after cathodic reduction of the oxide film (b) (X = Y = 7000 nm, ΔZ = 5 nm, ECu = −0.05 V, Etip = −0.30 V, Itip = 3 nA). Height profiles across grains 1-2-3 (c) and grains 4-5 (d). Green arrows mark grain boundaries.

formed oxide layer. When the surface morphology is examined prior to cathodic reduction (Figure 2a), the grains can be easily distinguished and their different reduction behavior is revealed after the cathodic pretreatment (Figure 2b). Two line scans are displayed for analysis of the local reduction of the native oxide layer (Figure 2c,d). In the case of grains labeled 1, 3, and 4, the height profiles obtained before and after pretreatment are similar, and these grains are thus taken as height reference for comparing the profiles. It does not mean, however, that no reduction of the air-formed oxide layer took place on these grains; it only indicates that the lowering of the surface level associated with the reduction of the air-formed oxide film cannot be appreciated. In contrast, grains labeled 2 and 5 present significantly lower surface levels after reduction, showing that the air-formed oxide layer was thicker on these grains. Thus, it is clearly observed that the air-formed oxide film thickness is grain-dependent. Grains 2 and 5 show a rougher surface after cathodic reduction of the oxide film than the surrounding grains, possibly owing to a less homogeneous redistribution of the reduced copper atoms. For grain 2, the minimum height level is measured around 3 nm below the surface level of the surrounding grains 1 and 3 and the mean value is around 1.5 nm. For grain 5, the mean value of the surface level is around

2.5 nm below that of grain 4. Comparison of these ECSTM data with the average thickness of the oxide calculated from the CV data must take into account that the air-formed oxide layer is reduced to metallic copper. The Pilling−Bedworth ratio is 1.71 as calculated from the molar volume of Cu2O (23.9 cm3· mol−1) and Cu (7 cm3·mol−1). Thus, a 2.8 nm thick Cu2O oxide layer reduces to a 1.6 nm thick Cu metallic layer, yielding a lowering of the surface level of about 1.2 nm if one assumes a homogeneous distribution of the reduced copper atoms. The mean values measured for grains 2 and 5 are both higher than 1.2 nm, indicating that the air-formed oxide layer was thicker than the average on these grains. Thickness values of 3.6 and 6 nm can be calculated for grains 2 and 5, respectively, from the measured mean values of the surface level lowering, showing that the highest local value of the thickness can exceed by a factor of 2 the average value (2.8 nm) measured at a macroscopic scale by electrochemistry. Counter balance imposes then that the thickness of the oxide layer was thinner than average on the grains where the effect of the cathodic reduction was not appreciated, i.e., on grains 1, 3, and 4 in Figure 2, thus providing a possible explanation for the absence of a marked difference after reduction. These results thus show that the native oxide layer thickness is grain-dependent and most likely influenced by the grain 25423

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Figure 3. Topographic ECSTM images of metallic surface of polycrystalline copper after cathodic reduction of air-formed oxide film (a), after 3 anodic dissolution cycles (b), and after 5 anodic dissolution cycles (c) (X = Y = 1500 nm, ΔZ = 8 nm, ECu = −0.05 V, Etip = −0.40 V, Itip = 2 nA). Cyclic voltammogram for the anodic dissolution of microcrystalline copper in 10 mM HCl as recorded in the non-deaerated ECSTM cell; scan rate = 10 mV/s (d). Height profile across grains A-B (e). Green arrows mark grain boundaries.

orientation. Differences of reduction behavior related to crystallinity or defect density of the oxide are not retained in the present case since the cathodic pretreatment is effective in totally reducing the oxide layer (Figure 1). Potentiodynamic polarization curves of copper single crystals obtained in NaOH by Kunze et al.17 revealed clear differences in the cathodic peaks of reduction of the passive oxide films grown on the Cu(001) and Cu(111) orientations and showed that the thickness of the anodic oxide films was larger on Cu(111). Gao et al. also studied the influence of crystallographic orientation on copper oxidation and observed that the oxide films grown on Cu(001) and Cu(110) were thinner than those on Cu(311) and Cu(111).28 The present data suggest that the crystallographic orientations of grains 2 and 5 in Figure 2, for which the airformed oxide film is thicker, are possibly closer to the ⟨111⟩// ND than to ⟨001⟩//ND or to ⟨110⟩//ND. Grain-Dependent Anodic Dissolution. Figure 3 shows a first example of the grain-dependent anodic dissolution of copper. On the metallic surface (Figure 3a), i.e., after cathodic reduction of the air-formed oxide layer and prior to anodic treatment, two grains (labeled A and B) can be easily distinguished by their different surface morphology. Both grains display a stepped surface at the nanometer scale, but the step orientation is different, which is indicative of a different crystallographic orientation. Besides, the orientation of the atomic terraces may also be different, as discussed further on.

The step linear density was measured for both grains as the inverse of the distance between two consecutive steps. The obtained values are (5 ± 0.3)·10−2 nm−1 for grain A and (8 ± 0.5)·10−2 nm−1 for grain B, showing a higher misorientation for grain B with respect to the orientation of the terraces. Anodic potentiodynamic polarization, as illustrated by the CV presented in Figure 3d, was performed in order to induce copper dissolution. Images in Figure 3b,c were taken after 3 and 5 anodic cycles, respectively. After 3 anodic cycles, a considerable difference of attack of the two grains is revealed at the nanometer scale. Grain B dissolved, as shown by the appearance of an etched surface morphology, whereas grain A was resistant to attack, as shown by its essentially unchanged stepped surface. After 5 anodic cycles, grain A appears still resistant against dissolution while changes on the surface of grain B are further enhanced. The evolution of the surface morphology of the two grains is further illustrated by crosssectional analysis. Figure 3e compares the height profile across grains A and B before and after 5 anodic cycles. The positions of the two grain boundaries (GB) delimiting grain B are marked, and it can be seen that grain boundaries are preferential sites of corrosive attack, confirming our results for random grain boundaries, as previously discussed.23 The height profile on grain B is much more corrugated after anodic dissolution, which confirms preferential localized attack of the surface on this grain. Interestingly, one notices that, in certain 25424

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Figure 4. Topographic ECSTM images of metallic surface of polycrystalline copper after cathodic reduction of air-formed oxide film (a), after 2 anodic dissolution cycles (b), and after 3 anodic dissolution cycles (c) (X = Y = 1500 nm, ΔZ = 4 nm, ECu = −0.05 V, Etip = −0.35 V, Itip = 2 nA). Height profile across grains C−D (d). Green arrow marks the grain boundary.

areas on grain B, the measured surface level is higher after anodic dissolution, which is surprising since the material is being dissolved, and the contrary is expected after the anodic treatment. However, in the CV performed to induce dissolution (Figure 3d), the potential was scanned starting from −0.05 V/ SHE into the positive direction and backward to the starting value. As a consequence, a cathodic peak is observed around 0.1 V/SHE after dissolution that, most likely, corresponds to the redeposition of dissolved copper. This redeposition of copper could explain the higher surface level measured on certain sites of grain B after anodic dissolution. Figure 4 displays another example of the grain-dependent dissolution of copper. The displayed area was selected on a different microcrystalline copper sample. Figure 4a was taken after cathodic reduction of the air-formed oxide and before any anodic treatment. Images (b) and (c) in Figure 4 were taken after 2 and 3 anodic cycles, respectively. The anodic dissolution cycles were the same as the one presented in Figure 3d. In this example, several grains were present in the field of view and we focused the analysis on the evolution of grains labeled C and D after several anodic cycles. In this case, the linear step density is measured to be (9 ± 1)·10−2 nm−1 for grain C and (6 ± 0.3)· 10−2 nm−1 for grain D before the anodic treatment, showing a higher misorientation for grain C with respect to the orientation of the terraces. Dissolution on grain C can already be observed after 2 anodic cycles, as shown by the development of an etched surface morphology, and it becomes more marked

after 3 anodic cycles. In contrast, the changes of the surface topography on grain D indicate very little dissolution. When comparing the topography of these two grains with height profiles (Figure 4d), it is confirmed that surface roughening caused by dissolution and redeposition of copper mostly occurred on grain C, while grain D appears barely affected. Thus, with this example, it is confirmed that microcrystalline copper shows significantly heterogeneous anodic dissolution, observed as an unequal nanometer scale corrosive attack of the grains. Here again, this grain-dependent heterogeneous corrosion behavior is most likely affected by the crystallographic orientations of the grains. Link with Crystallographic Orientation. EBSD analysis revealed an interesting feature of our cryogenically prepared copper. Figure 5a shows the inverse pole figure (IPF) map in which the crystal orientation of the grains is distinguished by the color code. The average grain size is 1.1 μm, which is in the order of the grain sizes observed by ECSTM. In order to evaluate the texture, a larger area was analyzed and the orientation distribution function (ODF) was derived from the EBSD scans containing about 10 000 grains. The three-dimensional Euler space for a cubic system can be seen in Figure 5b. Any orientation (or texture component) expressed in terms of Euler angles can be represented in this Euler space. The ODF sections presented in Figure 5c show two-dimensional sections of Euler space in which one of the Euler angles is kept constant. An ODF yields isointensity lines, 25425

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Figure 5. Inverse pole figure (IPF) map (a), schematic representation of fcc rolling textures in Euler space (b), and experimental ODF sections of microcrystalline copper sample (c).

Figure 6. FE SEM micrographs of copper grains after anodic dissolution in 0.1 M HCl. Orientation close to ⟨001⟩//ND (a, c), ⟨111⟩//ND oriented grain (b), and ⟨110⟩//ND oriented grain (d). 25426

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resistant grains (grain A in Figure 3 and grain D in Figure 4) maintain a smoother surface more typical of that expected for slow corroding grains having an orientation on the left side of the stereographic triangle and thus closer to ⟨001⟩//ND. In our previous papers,31,15 we showed that the largest dissolution rate was measured for the ⟨111⟩//ND or closely oriented grains, mainly when neighbored by slowly dissolving ⟨100⟩//ND or closely oriented grains. These results suggested that not only grain orientation but also the orientation of neighboring grains plays a decisive role in the dissolution rates. The ECSTM data discussed here are along this line. The grains showing faster corrosion rates are possibly oriented closer to ⟨111⟩//ND or in between ⟨111⟩//ND and ⟨110⟩//ND, as supported by their surface morphology after dissolution, whereas the slower dissolving neighboring grains are possibly oriented closer to ⟨001⟩//ND. Faster dissolution of {111} planes compared to {001} planes is counterintuitive for an fcc system because of the higher coordination of the atoms in the {111} planes. However, a larger misorientation of the grains with respect to the orientation of the terraces will produce a higher density of atoms of lower coordination promoting the reaction of dissolution, as confirmed by selective dissolution of step edges observed on copper single crystals.18−21 In the present work, the higher step linear density measured on the faster dissolving grains may then be the cause of their lower corrosion resistance despite the possible ⟨111⟩//ND orientation of their terraces, confirming thus the decisive role of surface defects related to misorientation on the local corrosion of metal surfaces.

conventionally scaled in random units, that give the density of grains having a particular orientation. A randomly textured material will exhibit a level (intensity) close to unity. The ODF of our microcrystalline copper can be seen in Figure 5c. As the most important fcc texture components are presented in a limited set of sections, only three ODF sections (with φ2 = 0, 45, and 65°) need to be shown to describe the texture characteristics. The strength of the isolines seen in this figure are 1.0 and 1.641 with the highest intensity being 2.5 times random. This means that our sample has a nearly random texture. This is in contrast with cryogenically rolled copper that displays strong Brass-type texture ({011}⟨211⟩).25 However, in our cases, samples needed an extra annealing step to ensure full recrystallization of the sample and a limited grain size appropriate for ECSTM examination, which resulted in the observed nearly random texture. As a consequence of the quasi-random texture, it is not possible to determine from the EBSD data the most likely crystallographic orientation of the grains studied by ECSTM. Besides, the ECSTM system having no visual access to the area being measured, EBSD analysis cannot be restricted to the exact same area prior to or after the ECSTM measurements, so as to have more local information on the texture. It is known that, when a copper surface is corroded or etched, the morphologies of the attack differ depending on the crystallographic orientation, and this dependence was tentatively used here to assign the grain orientation. Kondo et al.29 studied how etched surface morphologies of single crystals of copper vary with crystal orientation, and the observations were classified into three types with (i) {100} single crystals showing a squared-etched morphology, (ii) {110} single crystals showing a triangular columnar morphology, and (iii) {111} single crystals showing a triangular pyramidal morphology. Moreno et al.30 also observed different surface morphologies after sputtering copper monocrystals with helium, and their classification is similar to that reported by Kondo et al. Figure 6 shows FE-SEM images of four different grains of our microcrystalline copper, cut from the same cryogenically rolled material, subjected to anodic dissolution in 0.1 M HCl using a classical electrochemical cell after the same surface preparation as that for ECSTM experiments. Different etched morphologies of the copper surface are revealed after corrosion, and the morphologies of the attack vary depending on the crystallographic orientation of the grains measured by EBSD. Figure 6b shows the triangular pyramidal morphology obtained on a ⟨111⟩//ND oriented grain and typical for this orientation. Figure 6d shows the as-expected triangular columnar morphology observed on a ⟨110⟩//ND oriented grain. The typical square-etched morphology was not clearly observed on the ⟨001⟩//ND oriented grains (Figure 6a,c), possibly owing to a slower corrosion rate, but the observed morphology clearly differs from those observed for ⟨111⟩//ND and ⟨110⟩//ND oriented grains. Examining the ECSTM images, the grains that corrode faster (grain B in Figure 3 and grain C in Figure 4) develop a surface etched morphology similar to that observed on single crystals oriented closer to ⟨111⟩//ND or ⟨110⟩//ND than to ⟨001⟩// ND. Even though the exact orientation of these grains cannot be determined, it is suggested that these grains are oriented closer to ⟨111⟩//ND or ⟨110⟩//ND (or in between these two poles) than to ⟨001⟩//ND. This corresponds to an orientation located on the right of the vertical dashed line in the stereographic triangle in Figure 6. In contrast, the corrosion-

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CONCLUSIONS For the first time to our knowledge, ECSTM was used to study in situ on copper the grain-dependent corrosion properties on a polycrystalline metallic material. The high resolution of the ECSTM, both perpendicular and parallel to the surface, is proven to be very useful to observe in situ at the nanometer scale the local effect of the microstructure on the air-formed oxide layer and in the initial stages of anodic dissolution. Differences in thickness of the air-formed oxide layer were observed for different grains with the largest values exceeding locally by a factor of 2 the average value (2.8 nm) measured electrochemically. On the basis of the literature, the grains with a thicker oxide layer may be ⟨111⟩//ND or closely oriented grains. The evolution of the polycrystalline surface after several anodic dissolution cycles was followed and revealed the graindependent corrosion resistance of the material. After several anodic cycles, certain grains were being dissolved preferentially while neighboring grains remained barely affected. EBSD was used to determine the texture of the cryogenically rolled microcrystalline copper sample after a short recrystallization treatment. The orientations were found to be almost randomly distributed, and the exact orientation of the grains examined by ECSTM could not be determined. However, comparison of the surface etched morphologies observed in situ by ECSTM and ex situ by FE-SEM after anodic dissolution suggested that the faster dissolving grains at the nanometer scale were oriented closer to ⟨111⟩//ND or in between ⟨111⟩//ND and ⟨110⟩//ND while the neighboring corrosionresistant grains were oriented closer to ⟨001⟩//ND. The higher step linear density measured on the grains corroding faster despite possibly ⟨111⟩//ND oriented terraces confirms the 25427

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(16) Maurice, V.; Strehblow, H.-H.; Marcus, P. In Situ STM Study of the Initial Stages of Oxidation of Cu (111) in Aqueous Solution. Surf. Sci. 2000, 458, 185−194. (17) Kunze, J.; Maurice, V.; Klein, L. H.; Strehblow, H.-H.; Marcus, P. In Situ STM Study of the Duplex Passive Films Formed on Cu (111) and Cu (001) in 0.1 M NaOH. Corros. Sci. 2004, 46, 245−264. (18) Lennartz, M.; Broekmann, P.; Arenz, M.; Stuhlmann, C.; Wandelt, K. Sulfate Adsorption on Cu (111) Studied by in-Situ IRRAS and STM: Revealing the Adsorption Site and Desorption Behavior. Surf. Sci. 1999, 442, 215−222. (19) Magnussen, O.; Zitzler, L.; Gleich, B.; Vogt, M.; Behm, R. In Situ Atomic-Scale Studies of the Mechanisms and Dynamics of Metal Dissolution by High-Speed STM. Electrochim. Acta 2001, 46, 3725− 3733. (20) Vogt, M.; Lachenwitzer, A.; Magnussen, O.; Behm, R. In Situ STM Study of the Initial Stages of Corrosion of Cu (100) Electrodes in Sulfuric and Hydrochloric Acid Solution. Surf. Sci. 1998, 399, 49− 69. (21) Kruft, M.; Wohlmann, B.; Stuhlmann, C.; Wandelt, K. Chloride Adsorption on Cu (111) Electrodes in Dilute HCl Solutions. Surf. Sci. 1997, 377, 601−604. (22) Wan, L.-J.; Itaya, K. In Situ Scanning Tunneling Microscopy of Cu (110): Atomic Structures of Halide Adlayers and Anodic Dissolution. J. Electroanal. Chem. 1999, 473, 10−18. (23) Martinez-Lombardia, E.; Lapeire, L.; Maurice, V.; Graeve, I. D.; Verbeken, K.; Klein, L.; Kestens, L.; Marcus, P.; Terryn, H. In Situ Scanning Tunneling Microscopy Study of the Intergranular Corrosion of Copper. Electrochem. Commun. 2014, 41, 1−4. (24) Huang, Y.; Prangnell, P. The Effect of Cryogenic Temperature and Change in Deformation Mode on the Limiting Grain Size in a Severely Deformed Dilute Aluminium Alloy. Acta Mater. 2008, 56, 1619−1632. (25) Konkova, T.; Mironov, S.; Korznikov, A.; Semiatin, S. Microstructural Response of Pure Copper to Cryogenic Rolling. Acta Mater. 2010, 58, 5262−5273. (26) Itoh, J.; Sasaki, T.; Ohtsuka, T. The Influence of Oxide Layers on Initial Corrosion Behavior of Copper in Air Containing Water Vapor and Sulfur Dioxide. Corros. Sci. 2000, 42, 1539−1551. (27) Chawla, S.; Rickett, B.; Sankarraman, N.; Payer, J. An X-ray Photo-Electron Spectroscopic Investigation of the Air-Formed Film on Copper. Corros. Sci. 1992, 33, 1617−1631. (28) Gao, J.; Hu, A.; Li, M.; Mao, D. Influence of Crystal Orientation on Copper Oxidation Failure. Appl. Surf. Sci. 2009, 255, 5943−5947. (29) Kondo, K.; Hiroaki, K.; Murakami, H. Morphology Evolution of Single Crystal Copper by Etching. ECS Meet. Abstr. 2006, 17, 687 , Abstr. MA2005-02 687. (30) Moreno, D.; Eliezer, D. Sputtering and Roughness of the (0 01), (01 1) and (111) Copper Single-Crystal Planes. J. Mater. Sci. Lett. 1994, 13, 1591−1593. (31) Lapeire, L.; Martinez Lombardia, E.; Verbeken, K.; De Graeve, I.; Kestens, L.; Terryn, H. Effect of Neighbouring Grains on the Microscopic Corrosion Behaviour of a Grain in Polycrystalline Copper. Corros. Sci. 2012, 67, 179−183.

effect of misorientation-induced surface defects on the corrosion susceptibility.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.M.-L and L.L. wish to thank the Agency for Innovation by Science and Technology in Flanders (IWT) and the Fund for Scientific Research in Flanders (FWO) for financial support (FWO10/PRJ/336). Aurubis is acknowledged for the ETP-Cu.

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REFERENCES

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dx.doi.org/10.1021/jp507089f | J. Phys. Chem. C 2014, 118, 25421−25428