J Solid State Electrochem DOI 10.1007/s10008-015-2787-x
ORIGINAL PAPER
Grain boundary passivation studied by in situ scanning tunneling microscopy on microcrystalline copper Hu Chen & Vincent Maurice & Lorena H. Klein & Linsey Lapeire & Kim Verbeken & Herman Terryn & Philippe Marcus
Received: 6 January 2015 / Revised: 8 February 2015 / Accepted: 10 February 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Electrochemical scanning tunneling microscopy (ECSTM) was applied to analyze in situ the passivation of grain boundaries on microcrystalline copper in 0.1 M NaOH aqueous solution. Two types of boundaries, assigned to coherent twin and random grain boundaries, were studied in three different states of the copper surface: metallic after cathodic reduction of the air-formed native oxide film, passive after anodic polarization at 0.7 V vs. SHE to form the duplex Cu(I)/Cu(II) oxide film, and metallic after cathodic reduction of the passive film. The depth measured at several sites along the grain boundaries was extracted from statistical line profile analysis and discussed using an original model allowing differentiating metal consumption by dissolution or by passive film formation at grains and grain boundaries. The results highlight different local passivation properties between randomly oriented grains and grain boundaries and also between the two different types of grain boundaries. Both at coherent twin and random grain boundaries, it is found that more copper irreversibly dissolves during passivation and less copper is consumed to grow the passive film, suggesting a more hydroxylated/hydrated composition less dense in copper than on grains. At random grain boundaries, irreversible dissolution is found to be enhanced and the passive film to be thicker. H. Chen : V. Maurice (*) : L. H. Klein : P. Marcus (*) Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, PSL Research University, 11 rue Pierre et Marie Curie, 75005 Paris, France e-mail:
[email protected] e-mail:
[email protected] L. Lapeire : K. Verbeken Department of Materials Science and Engineering, Ghent University (UGent), Technologiepark 903, Zwijnaarde, 9052 Ghent, Belgium H. Terryn Research Group Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
Keywords Corrosion . Passive film . Grain boundary . Copper . In situ . STM
Introduction Controlling the corrosion resistance of polycrystalline materials is a challenging issue that requires understanding the effect of grain orientation and grain boundary characteristics on the corrosion properties since both are known to influence the local behavior [1–11]. Besides microcrystalline and nanocrystalline metals attracting extensive attention because of their specific properties [12, 13], the prediction of their lifetime is also conditioned by a knowledge-based control of texture and grain boundary effects on the corrosion resistance. Intergranular corrosion is a major degradation process for polycrystalline materials, and grain boundary engineering is a potential mean for increasing the corrosion resistance. Among the different grain boundary types, so-called special grain boundaries, forming a coincidence site lattice (CSL), are labeled Σn with 1/n defining the frequency of coinciding atomic positions at either side of the grain boundary. Twin boundaries, which are Σ3 boundaries, are common in facecentered cubic materials such as copper and can be subdivided in two types depending on the orientation of the grain boundary plane. Coherent twins have a {111} grain boundary plane, which is not the case for incoherent twins. Intergranular corrosion has been shown to be intimately related to the type of grain boundary [6] and to the structure and the energy of the grain boundary [7]. Coherent twin or Σ3 grain boundaries were concluded to have better corrosion resistance than random grain boundaries [9]. However, according to some authors and apart from Σ3 boundaries, the properties of other coherent grain boundaries are not different from that of
J Solid State Electrochem
random grain boundaries [9]. Moreover, it was argued that, as opposed to incoherent twins, only coherent twin boundaries display a good corrosion resistance and are resistant to intergranular corrosion [10], like also concluded for intergranular stress corrosion cracking [4]. Quite recently, it was confirmed by direct local in situ observation of active dissolution that coherent twin boundaries have indeed higher corrosion resistance than random and incoherent twin grain boundaries [11]. For passivable metals and alloys, the formation and stability of the passive film is a key aspect of the corrosion resistance that one can presume to be also strongly influenced by structural heterogeneities such as the grain boundaries and triple junctions of the substrate. On copper, an improvement of the resistance to aqueous corrosion has been reported for pure Cu and Cu-based alloyed nanocrystalline structures compared to coarse-grained counterparts with a lower length fraction of grain boundaries [14–17]. Electrochemical scanning tunneling microscopy (ECSTM) is known as a very powerful technique for obtaining in situ topographic information for surfaces with high lateral resolution. In situ corrosion-related studies were mostly performed on single crystals and brought insight into the properties of wellcontrolled surfaces at the atomic level. On copper, several studies were devoted to characterize in situ the corrosion behavior of single crystals immersed in various electrolytes including alkaline solutions in which passive films are formed [18–30]. Quite recently, ECSTM has also been proved to be a powerful tool for providing highly accurate in situ topographic information on grain and grain boundary effects on the local active dissolution of microcrystalline copper in acid solution [11, 31]. In the present study, ECSTM was applied, for the first time to our knowledge, to investigate in situ at different types of grain boundaries the nanometer scale morphological surface changes related to the passivation and subsequent electrochemical reduction of copper in an alkaline aqueous solution, and to make comparison with the grain interior. The use of high-purity copper allowed ruling out any chemical alloying or impurity effect on the intergranular properties of the material. An original model of variation of the surface and interface topographic levels at grains and grain boundaries in the studied metallic, passivated, and reduced surface states is proposed to discuss the STM data and to get insight into the local passivation properties at grain boundaries.
Experimental Sample preparation Microcrystalline copper was prepared by cryogenic rolling after immersion in liquid nitrogen and post-annealing of high-purity cast electrolytic tough pitch (ETP-) Cu, like previously detailed [11, 31–34]. For the present study, a suitable
grain size for analysis of grain boundaries by STM with a limited field of view of 10×10 μm2 was obtained with a final reduction of the sample thickness of 90 % and post-annealing performed for 2 min at 200 °C. Grain size and texture were analyzed by EBSD (electron backscatter diffraction). Average grain size was estimated to 1.4 μm with a large dispersion spanning from 0.1 to 37 μm (Fig. 1a). The texture was nearly random, i.e., with no preferential grain orientation. The grain boundary length fraction associated with ∑3 type boundaries was 66 % (Fig. 1b, c). Parallel-sided straight ∑3 boundaries could be considered as coherent twins while curved ∑3 boundaries were considered to be incoherent twin boundaries. Both types of ∑3 boundaries are colored red in Fig. 1b, c. Alternatively, random high-angle boundaries are colored in black in this figure [11]. Surface preparation is critical in order to ensure STM imaging of microcrystalline surfaces but must avoid annealing in order to not coarsen the grains. The same procedure as that adopted previously was used [11, 31]. Mechanical polishing was performed with diamond spray down to a final grade of 0.25 μm. The cold work layer was removed by electrochemical polishing in 66 % orthophosphoric acid for 5 min at 1.4 V versus a copper counter electrode. Electrochemical scanning tunneling microscopy All ECSTM experiments were performed at room temperature with an Agilent Technologies system consisting of a PicoSPM base, an STM S scanner, a PicoScan 2100 controller, a PicoStat bi-potentiostat, and the Picoscan software. A modified ECSTM cell was used. Details on the cell, its cleaning, and tip preparation can be found elsewhere [35–39]. The electrolyte was 0.1 M NaOH(aq) prepared from ultrapure NaOH and Millipore water (resistivity >18 MΩ cm). The sample was mounted in the ECSTM cell with a working area of 0.16 cm2 delimited by a Viton O-ring. Two Pt wires served as pseudo reference electrode and counter electrode. All potentials reported hereafter are relative to the standard hydrogen electrode. The tungsten tips were prepared from 0.25-mm-diameter wire electrochemically etched in 3 M NaOH(aq) and covered by Apiezon wax. The sample was exposed to the electrolyte at open-circuit potential (EOCP =−0.03 V). A first cyclic voltammogram (CV) was performed by scanning the potential down to E=−1.15 V and upward to E=−0.35 V in order to reduce the air-formed oxide layer and maintain a metallic state. It produced a cathodic peak at E = −0.62 V. Subsequent CVs (−1.15 V ≤ E ≤ −0.35 V) produced no cathodic peak, confirming full reduction of the native oxide film during the first CV. Images of the microcrystalline copper surface in the metallic state were taken after this cathodic reduction pretreatment in order to ensure that the microcrystalline structure was revealed and to localize grain boundaries of interest. Then, anodic polarization and
J Solid State Electrochem Fig. 1 EBSD inverse pole figure (IPF) map (a) and image quality (IQ) map (b, c) of the microcrystalline copper used in this study. c Enlarged view of the zone marked in (b). b, c Black lines denote random high angle (>15°) boundaries and red lines Σ3 boundaries. Coherent twins appear as parallel-sided straight boundaries and incoherent twins and random boundaries as curved boundaries
subsequent cathodic treatments were applied as detailed further on. The STM scans were stopped during these electrochemical treatments, however with the tip remaining engaged
in order to not lose the previously selected surface region of interest. Images of the passivated and cathodically reduced microcrystalline copper surfaces were recorded after each
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electrochemical treatment. Images were recorded in the constant current mode. No filtering was used. The recorded topographic images were processed with the Gwyddion software (http://gwyddion.net/).
Results and discussion Macroscopic electrochemical behavior Figure 2 shows a typical CV obtained on the microcrystalline copper sample in the ECSTM cell. The potential was first scanned anodically starting from E=−0.35 V, after reduction of the air-formed native oxide film. The characteristic anodic (AI and AII) and cathodic (CII and CI) peaks are related to the formation and decomposition of the copper anodic oxides, respectively. The passive film consists of a single layer of Cu(I) anodic oxide at EAI δG Pass). The formation of a thicker oxide film on more reactive grain boundaries is as expected for random boundaries which are known to have higher grain boundary energy. At the coherent twin boundary, the passive film is systematically found thinner than on grains − GB (δCT
