Effect of surface finishing of a Zr-based bulk metallic glass on its corrosion behaviour

Corrosion Science 52 (2010) 1711–1720

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Effect of surface finishing of a Zr-based bulk metallic glass on its corrosion behaviour A. Gebert, P.F. Gostin *, L. Schultz Leibniz-Institute for Solid State and Materials Research IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany

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Article history: Received 29 September 2009 Accepted 16 January 2010 Available online 22 January 2010 Keywords: A. Zirconium A. Alloy B. AES C. Amorphous structures C. Passivity C. Pitting corrosion

a b s t r a c t Zr-based metallic glasses passivate spontaneously, but exhibit also a certain pitting susceptibility. On the example of the Zr59Ti3Cu20Al10Ni8 alloy studied in 0.01 M Na2SO4 + x M NaCl (x = 0–0.1) electrolytes it is demonstrated that the surface finishing state and the pre-exposure conditions can significantly influence the free corrosion and anodic polarisation behaviour. Mechanical fine-polishing procedures can lead to extremely smooth topographies but also to Cu enrichment at the surface. This yields a pronounced Cu dissolution at low anodic polarisation prior to stable passivity and increases the pitting initiation susceptibility as compared to mechanically ground surface states. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Among the bulk glass-forming alloy systems those on Zr-based are presently most investigated regarding atomic short-range order structure, casting processability, phase formation, thermal stability and mechanical behaviour [1]. Multi-component Zr-based bulk metallic glasses (BMG’s) exhibit excellent mechanical performances, i.e., high yield strength and high elastic strain limit combined with a low Young’s modulus. Therefore, they are very attractive for applications under high mechanical load, e.g., as components of sportive goods, as springs or parts of pressure sensors for automobiles or case parts of portable electronic devices [1,2]. New exploration fields for potential applications of those BMG’s are related with the development of components with very small size (e.g., plates, gears) or with fine surface patterns for use in micro-electromechanical devices, micro-medical surgery tools, as micro-moulds or in printing tools or optical mirrors [3–7]. With the lowering of the dimensions of the components the properties of the BMG surface become more important. For example, very smooth surface topographies are required to meet the tolerance limits of high precision small tool parts. The surfaces of cast BMG samples are in principle very smooth, but often comprise local structural defects [8]. Further cutting and machining of cast samples increases the surface roughness [9]. Therefore, additional surface finishing by fine-polishing procedures is indispensible for the realization of defined very smooth BMG surface states.

* Corresponding author. Tel./fax: +49 351 4659 275/541. E-mail addresses: [email protected], fl[email protected] (P.F. Gostin). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.01.027

Another important requirement for the applicability of Zr-based bulk metallic glass components is a high corrosion resistance in different environments. Meanwhile various fundamental corrosion studies have been conducted on bulk glassy alloys of compositional types (Zr,Ti,Nb)–(Cu,Ni,Co,Fe)–(Al,Be,Ag,Pd) [10–14]. At room temperature for all these bulk metallic glasses excellent passivity was detected in halide-free electrolytes within a wide pH value range. A number of surface analytical studies revealed consistently, that this is mainly due to the composition of the passive films which naturally form in air or during electrolyte exposure under open circuit or anodic conditions. These films were found to comprise the oxides of the valve-metal components, i.e., mainly Zr- and Al-oxides, whereas in particular Cu (and Ni) is enriched in a metallic state at the inner metal/film interface [8,15–19]. This corresponds well with the typical surface state of the top side of melt-spun amorphous Cu–Zr ribbon samples with Cu P50 at%, for which a surface segregation of Zr due to selective oxidation was stated [20–22]. Nevertheless, Zr-based BMG’s exhibit commonly a poor pitting resistance [10,11,23]. Chemical or physical defects in the cast bulk samples, e.g., crystalline inclusions, gas pockets or scratches, were identified as preferential initiation points for pitting [8,15,24]. There an easy breakthrough of the very thin passive films is possible. A commonly accepted pit growth mechanism comprises the selective dissolution of Zr and Al and other valve-metal components, which leads to a local enrichment of Cu in the pit zone. Cu can then locally interact with chloride ions Cl’ to form cuprous chloride CuCl, which subsequently undergoes hydrolysis to form cuprous oxide Cu2O. The local presence of Cu-rich species may give rise to galvanic coupling effects, which trigger the local dissolution of the glassy phase and therefore, may explain the observed low

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re-passivation ability [16,18,19,25–27]. Additives of other valvemetals components like Ti and Nb can improve the passive film properties and thus, lead to a certain improvement of the pitting resistance. But at the same time they deteriorate the glass-forming ability [28–31]. In various studies it was demonstrated that the kind of surface finish, i.e., the degree of mechanical grinding and polishing, can remarkably influence the passivity and localized corrosion behaviour of conventional metallic alloys, e.g., of the alloy AA 5083 [32], of stainless steel 316L [33] or of Ti [34]. However, for Zr-based metallic glasses so far those effects were only scarcely considered. For example, for melt-spun glassy Zr65Al7.5Ni10Cu17.5 ribbon samples after mechanical grinding an ennoblement of the corrosion potential and a reduction of the pitting resistance compared to the non-polished state was detected in phosphate buffered saline solution [35]. But commonly, detailed information on the experimental procedure for the glassy sample surface preparation prior to electrochemical studies are rarely given, except for example in [27–31], and is often restricted to mechanical grinding with emery paper. The present paper reports on a recent study to systematically analyze the effect of different surface finishings on the passivity and pitting behaviour of a Zr-based bulk metallic glass. It will be demonstrated that in particular selected fine-polishing procedures lead to significant changes of the surface chemistry, which has remarkable consequences for the anodic behaviour of the alloy and alters its local corrosion susceptibility. Moreover, it will be shown that the electrochemical response of the glassy alloy surface sensitively depends on the particular experimental conditions chosen, e.g., on exposure time under open circuit conditions prior to polarisation. 2. Experimental 2.1. Sample preparation and surface finishing For alloy preparation an ingot of the nominal composition Zr59Ti3Cu20Al10Ni8 was produced by arc melting the pure elements in a highly purified Ar atmosphere. The ingot was re-melted several times for homogenization. From this ingot a rod sample with 3 mm diameter and 50 mm length was prepared by injection copper mould casting. Details are described elsewhere [36]. From the lower part of the rod, test samples of 1 or 10 mm thickness were obtained by cross-sectional cuts using a laboratory cutting machine Accutom 50 (Struers) with a diamond disc at 3000 rpm and a load of 50 g under lubricant cooling. The cross-sectional areas of these samples were the subject of further studies. Different states of surface finishing were obtained by using an automatic grinding machine TegraPol-31 (Struers) with sample holder TegraForce-5 (Struers): (i) SiC grit 4000 – mechanical grinding using emery SiC paper from grit 400 down to grit 4000 (Buehler) for about 10 min for each grinding step. (ii) Diamond 1 lm – additional fine-polishing using water-based suspensions with polycrystalline diamond particles up to 1 lm (MetaDi Supreme, Buehler) for 5 min. (iii) Final SiO2 0.1 lm – additional fine-polishing using a waterbased suspension with 0.1 lm SiO2 particles (Final, Buehler) for 5 min. (iv) MasterMet SiO2 0.02 lm –additional fine-polishing using a water-based suspension with 0.02 lm SiO2 particles, (MasterMet 2, Buehler) for 5 min. After these grinding and polishing procedures the samples were carefully cleaned with bi-distilled water and ethanol and dried in air.

2.2. Microstructural and surface analytical studies The microstructural state of the cast and cut rod samples (1 mm discs) has been analysed by X-ray diffraction, scanning and transmission electron microscopy and differential scanning calorimetry. By these methods the bulk glassy nature of the samples with some randomly occurring crystalline defects with <10 lm dimension in a low volume fraction was verified, see also [36]. The surface state of the cross-sectional sample areas after the different grinding and fine-polishing procedures has been firstly characterized by scanning electron microscopy (SEM) using a JSM 6400 (JEOL). For more detailed analyses of the surface topographies of the fine-polished samples atomic force microscopy (AFM) has been employed using a Park-XE100. AFM measurements were performed in a non-contact mode with a scanning frequency of 0.5 Hz. For each disc sample the surface topography was investigated at three or more different sites in the centre and in the rim region in a window of 5  5 lm. Auger electron spectroscopy (AES) with sputter depth profiling has been applied to analyze the chemical composition of the surface regions of alloy disc samples (1 mm thickness) after different surface finishing treatments (see Section 2.1) and after anodic polarisation at selected potentials (see Section 2.3). A PHI 660 Auger microprobe with primary electrons of 10 keV at an electron current of 100 nA was used. On each sample surface the beam was scanned over probe areas of about 10 lm  10 lm at various selected sites in the centre and in the rim region of the disc. For depth profiling the sample surfaces were sputtered by a scanned beam of 1.5 keV Ar+ ions and spectra were recorded after intervals of 0.3 min. The corresponding sputter rate in silicon oxide SiO2 was determined to be 2.8 nm/min. Besides the alloy components also silicon Si and carbon C were considered in the spectra analysis to identify impurity effects that may origin from the polishing agents. The AES studies on differently surface finished samples have been repeated up to three times to ensure high reliability of the data. 2.3. Electrochemical methods For electrochemical studies alloy rod samples with 1 cm length were electrically connected and embedded in epoxy resin. The cross-sectional areas have been ground and polished as described above (see Section 2.1). For reference measurements samples of high purity Cu and Zr have been similarly prepared. Corrosion studies were carried out by means of a Solartron SI 1287 electrochemical interface connected to a cell with SCE reference electrode (E(SHE) = 0.241 V) and Pt net counter electrode. As base electrolyte a nitrogen-purged 0.01 M Na2SO4 electrolyte (pH 7) was used. For pitting tests 0.001, 0.01 or 0.1 M NaCl were added to the base electrolyte. In most of the cases the samples were immersed into the electrolyte directly after the surface finishing. Only for some tests the finished sample surfaces were aged for 20 h in air prior to immersion. Polarisation studies were started after 0 min (several seconds), 30 min, 120 min or 20 h of immersion under open circuit conditions. During this period the open circuit potential (OCP) was recorded vs. time. Cyclic potentiodynamic polarisation measurements were performed in the base electrolyte starting at the OCP firstly in anodic direction and scanning the potential up to 2.5 V vs. SCE at a rate of 10 mV/s. The cathodic limit was [OCP 50 mV]. In each experiment 2 cycles were recorded. Anodic current transient measurements were conducted by stepping the potential from the OCP (after 30 min immersion) to selected anodic potentials in the passive region and holding this value for 1 h. Linear anodic polarisation curves were measured after 30 min of establishment of the open circuit potential starting at a cathodic potential [OCP 50 mV] and applying a scan rate of 0.5 mV/s.

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All those electrochemical measurements have been repeated two to four times to ensure high reliability of the results. After electrochemical studies selected samples were examined by means of SEM in particular regarding local corrosion damage morphologies and by AES regarding passive film compositions. 3. Results and discussion 3.1. Analyses of the initial state after surface finishing of the glassy alloy After applying different mechanical grinding and fine-polishing procedures, the surfaces of cross-sectional areas of bulk glassy Zr59Ti3Cu20Al10Ni8 alloy samples have been characterized regarding topography and composition. Fig. 1 shows SEM images of typical surface topographies. The as-cast surface (Fig. 1a) is principally very smooth, but various defects like a crystalline defect (right upper corner), a scratch and air pockets (dark spherical spots) occur. After mechanical grinding with SiC emery papers up to grit 4000 the surface appears visible to the naked eye already mirrorlike, but is quite rough when observed under high magnification with SEM (Fig. 1b). Grinding grooves are clearly recognizable. After applying an additional fine-polishing with diamond particle suspensions (Fig. 1c), the topography is much smoother; only weak contrasts of narrow, fine grooves are visible. After additional fine-polishing with SiO2 particle suspensions, for both Final and MasterMet with the maximum topography contrast resolution of this SEM no contrasts are identifiable anymore (Fig. 1d). To further analyze the surface state of the fine-polished samples AFM studies have been conducted. Typical images together with a line-scan recorded at a selected site are summarized in Fig. 2. For the diamond-polished sample (Fig. 2a) a quite diffuse smooth surface pattern is revealed. The line-scan exhibits a wavy shape with periodic minima in a distance of 1 lm and amplitudes of 1–2 nm. This corresponds well with the size of 1 lm of the diamond particles, which generate the grooves in the polishing process. Despite the nearly contrast-less SEM images obtained for the Final and MasterMet fine-polished sample surfaces, which suggested very smooth surface states, with AFM a significant roughness at the nano-scale is revealed (Fig. 2b and c). Numerous round-shaped

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islands with diameters of up to several hundreds of nanometres and heights of 3–5 nm (single islands up to 25 nm) are detected. To identify the compositional nature of the differently ground and polished surface states, Auger electron spectroscopy has been applied. For this directly after the surface treatments, i.e., within about 30 min, the samples were transferred into the vacuum chamber of the Auger microprobe. Characteristic depth profiles of elemental concentrations are shown in Fig. 3. First of all it should be emphasized that in all spectra (not shown here) no significant Si peaks were detected. Also C was only identified in the first spectra of the non-sputtered surfaces and is attributed to hydrocarbons that are known to easily adsorb from the environment on the glassy alloy surface. This indicates that the cleaning processes after the grinding and fine-polishing steps were done without leaving residues of polishing agents. In the AES profiles shown in Fig. 3b–d, in the steady state of sputtering, i.e., after more than 4 min, the determined elemental concentrations do not exactly match the nominal alloy composition. This must be mainly attributed to sputter effects and to strong peak overlaps, e.g., that of Cu and Ni [17]. Nevertheless, when comparing the surface-near regions that were analysed under the same experimental conditions, significant differences can be identified. In the AES profile of the sample surface which was mechanically ground to 4000, shown in Fig. 3a, the O concentration has initially a maximum and then gradually decays with increasing sputter time. Correspondingly, the Zr concentration gradually increases. This suggests that a thin oxide film is present at the surface. But due to the high surface roughness in the small probe areas of about 10 lm  10 lm (compare also Fig. 1a), the concentration curves in Fig. 3a appear smeared due to low depth resolution [37]. A clearer elemental distribution is obtained for the much smoother diamond fine-polished surface (Fig. 3b). After three sputter steps the O concentration sharply decreases and reaches after about 1.3 min of sputtering the half value between maximum and base level, which is regarded as film/metal interface region. From the distribution of the alloy components in this profile shown in Fig. 3b and similarly from that shown in Fig. 3a, it can be derived that the oxide films natively grown on a mechanically ground (4000) and on a diamond fine-polished glassy surface are mainly composed of Zr- and Al-oxides. On the diamond-polished surface

Fig. 1. SEM images of topographies of differently finished surfaces of a bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample (cross-sectional area): a, mechanically ground SiC grit 4000; b, fine-polished 1 lm diamond suspension; c, fine-polished 0.1 lm SiO2 suspension (Final) 0.02 lm SiO2 suspension (MasterMet).

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Fig. 2. AFM images of topographies of fine-polished surfaces of a bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample: a, 1 lm diamond suspension; b, 0.1 lm SiO2 suspension (Final); c, 0.02 lm SiO2 suspension (MasterMet).

Cu appears slightly enriched at the film/metal interface. This finding corresponds very well with that what is frequently reported in the literature for as-cast or mechanically ground surfaces of Zrbased BMG’s, which were exposed to atmospheric conditions [16–18,29].

In contrast, a clearly different surface composition is detected after fine-polishing using water-based suspensions with submicron-sized SiO2 particles. The profiles in Fig. 3c and d reveal that after both – Final and MasterMet polishing – an enrichment of Cu at the outermost surface regions occurs. The Cu concentration has initially a maximum and then decreases within a few sputter steps, before it increases again in the zone close to the oxide film/metal interface. Accordingly, the initial levels of O and Zr concentrations are depleted. Considering the sputter times at which the Cu concentration reaches its first minimum, i.e., at about 1.2 min in Fig. 3c and at about 1.5 min in Fig. 3d, and applying the reference sputter rate in SiO2, the surface zones of Cu enrichment are in the order of 3–4 nm. This corresponds well with the height dimensions of the topographic islands identified on those fine-polished surfaces by AFM (Fig. 2b and c). After repeated AES analyses of Final and MasterMet-polished surfaces no clear difference in the degree of the Cu enrichment in dependence on the finepolishing agent used can be stated. In summary the topographical and chemical analyses of the differently finished surface states of the bulk glassy Zr59Ti3Cu20Al10Ni8 alloy revealed considerable effects. Surfaces that were mechanically ground with SiC paper up to grit 4000 and those that were additionally fine-polished with diamond suspensions exhibit a certain residual surface roughness. Natively grown oxide films in air are composed of valve-metal oxides, i.e., mainly Zr- and Al-oxides, and Cu is weakly enriched underneath. In contrast, alloy surfaces fine-polished with SiO2-based suspensions (0.1 and 0.02 lm particle size) are principally very smooth. But many Cu-rich islands of nanometre dimensions are detectable and seem to be embedded in the natively grown valvemetal oxide film. The question arises now: what is the reason for this Cu enrichment? This can not be clearly answered at present. It is assumed that the water-based SiO2 suspensions with pH P 9 are chemically inert to the glassy alloy surface. SiO2 particles do typically not react with the alloy constituents at room temperature. In a weakly alkaline aqueous electrolytes, a possible galvanic coupling between Cu and less noble constituents like the main constituent Zr would lead to an anodic switching of the latter. In result, spontaneous formation of stable ZrO2 in large fractions should occur on the glassy alloy surface. But the opposite is the case. Therefore, one may consider an ease of physically driven Cu segregation processes at extremely smooth surface states. In general, Zr–Cu-based (bulk) glassy alloys are known to exhibit Cu segregation in presence of driving forces like hydrogen [21,38] and temperature [39] or by micro-alloying with metallic components [40]. In an earlier study Asami et al. [20] investigated very detailed the air oxidation of melt-spun Zr40Cu60 – an alloy with low glass-forming ability and much higher Cu content than the typical bulk glass-forming compositions. They detected the gradual segregation of Cu at the nontreated surface of as-spun ribbons after long-term exposure for 34 months in air. But this occurred only on the wheel side with rough surface topography, whereas the smooth top side remained stable with a thin ZrO2 film and underlying metallic Cu. The Cu segregation on the rough wheel side was supposed to be possible due to the defective nature of the initially grown tetragonal ZrO2 film, which even cracks when it gradually transforms to the monoclinic oxide during exposure in humid air. Moreover, formation of Cu2O and CuO on an amorphous Zr–Cu alloy in air with a relative humidity of 62 % at room temperature was also reported by Kimura et al. [41]. However, on the freshly fine-polished very smooth surface of the bulk glassy Zr59Ti3Cu20Al10Ni8 alloy the valve-metal oxides are principally expected to grow very uniformly and mainly defect-free. But obviously at these surface conditions the oxide film formation is inhibited and Cu segregation is preferred, establishing

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Fig. 3. AES depth profiles showing the elemental distribution at differently finished surfaces of a bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample: a, SiC grit 4000; b, 1 lm diamond suspension; c, 0.1 lm SiO2 suspension (Final); d, 0.02 lm SiO2 suspension (MasterMet).

the energetically most favourable surface state of the metastable alloy. Surface segregation of Cu is a frequently observed phenomenon for various intermetallic compounds of Cu with d-transition metals. Lack of atomic long-range order of the material seems to support this effect [21,22]. This might be due to a certain higher mobility of the metal atoms in the short-range ordered structure which eases the adjustment of a surface state of the glassy alloy that corresponds to the lowest possible energy level. However, more surface analytical studies and considerations of the thermodynamics and electronic properties of Zr-based glassy alloy surfaces are needed to verify this hypothesis.

typical valve-metal behaviour like Zr and the Ni oxidation proceeds in a similar potential regime like that of Cu. Fig. 4 shows the evolution of the electrode potentials (OCP) of mechanically ground and fine-polished alloy surfaces during exposure under open circuit conditions in comparison to those of the pure main single components. The initial OCP of a fresh mechanically ground Zr surface adjusts close to 0.6 V vs. SCE and with progressing exposure time it gradually increases up to 0.19 V

3.2. Anodic behaviour of differently finished alloy surfaces in neutral sulphate solution Electrochemical studies focused firstly on the analysis of surface finishing effects on the corrosion under open circuit conditions and on the anodic passivation behaviour of the glassy Zr59Ti3Cu20Al10Ni8 alloy in low concentrated sodium sulphate solution, 0.01 M Na2SO4 with pH 7. For a better understanding of the surface reactions of the alloy comparative measurements on the pure main components Zr and Cu have been conducted. The other alloy components are certainly also involved in the alloys corrosion and passivation processes. But their contribution can not be particularly discussed since they are present only in lower concentrations (610 at%) and thus, in part difficult to analyze with surface-sensitive methods. Also, their oxidation reactions are mostly superimposed with those of the main components, i.e., Al, Ti exhibit the

Fig. 4. Time dependence of the open circuit potential of differently finished surfaces of a bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample and of Zr and Cu in 0.01 M Na2SO4.

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vs. SCE. This can be attributed to a spontaneous passivation under formation of a duplex-type film with an external hydrated zirconium oxide [ZrO2[H2O]n] layer and an internal anhydrous monoclinic ZrO2 layer [42,43]. The initial OCP of Cu is much more positive, i.e., at  0.070 V vs. SCE and changes slightly with exposure time reaching  0.030 V vs. SCE after 2 h. In aqueous environments with pH 7 and under open circuit conditions Cu can be principally stable and its surface transforms gradually to cuprous oxide Cu2O [20,44]. In comparison, the OCP’s of the differently finished multi-component glassy Zr59Ti3Cu20Al10Ni8 sample surfaces establish initially in between the two values of Zr and Cu. After mechanical grinding and diamond fine-polishing the OCP increases slightly with time and transfers into a stationary state already after 30 min of exposure leading to an OCP value of  0.140 V vs. SCE after 2 h. This is close to that for Zr and suggests a dominance of Zr oxide growth at the glassy alloy surfaces. However, after extreme fine-polishing with SiO2 suspensions the initial OCP’s are slightly more positive and gradually increase within 2 h up to the OCP level of pure Cu indicating the dominance of Cu reactions at the alloy surface. Thus, the OCP differences between mechanically ground and diamond-polished surfaces on one side and SiO2 finepolished surfaces on the other side are in the order of more than 100 mV. Early studies on glassy Zr–Cu alloys with Cu P48 at% and with as-spun or mechanically ground surface states regarding their anodic polarisation behaviour in acidic and neutral sulphate solutions [45–47] revealed that in the low polarisation regime the active Cu dissolution is the governing reaction before the surface strongly passivates due to oxidation of Zr and that the transpassive activity of the alloys is enhanced compared to that of Zr. This is similarly the case for the bulk glassy Zr59Ti3Cu20Al10Ni8 alloy with much lower Cu content, but the detectable superimposition of the surface reactions of the main alloying components depends strongly on the state after surface finishing and on the exposure duration prior to the polarisation test. Fig. 5 shows cyclic potentiodynamic anodization curves for the multi-component glassy alloy after mechanical grinding with emery paper SiC grit 4000. For comparison, the behaviour of the single components Zr and Cu was measured under similar conditions and is shown in Fig. 6. Zr progressively passivates during the polarisation; this is reflected in a significant plateau current density of 100 lA/cm2 in the first cycle which drops to 1 lA/cm2 in the second cycle and in the diminution of the transpassive activity. However, Cu is not stable in this neutral sulphate solution, but actively dissolves

Fig. 5. Cyclic potentiodynamic polarisation curves of a mechanically ground bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample in 0.01 M Na2SO4 recorded after different pre-exposure times at OCP (scan-rate 10 mV/s, starting anodic, 2 cycles).

Fig. 6. Cyclic potentiodynamic polarisation curves of a mechanically ground bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample and Zr, Cu in 0.01 M Na2SO4 after a few seconds of exposure at OCP (Cu shows similar behaviour after 2 h OCP).

already at very low anodic potentials [44]. This behaviour is also observed after 2 h of pre-exposure at OCP. When the glassy Zr59Ti3Cu20Al10Ni8 alloy is polarised, immediately after the mechanical grinding process (OCP 0 min) indeed at low anodic potentials of the first cycle, i.e., at 0.08 V vs. SCE a Cu oxidation peak is visible followed by a wide shoulder, which is attributed to the oxidation of valve-metal components, i.e., mainly Zr but presumably also Al and Ti. In the second polarisation cycle the surface behaves completely passive. However, with prolonged exposure time under open circuit conditions prior to the anodization, the Cu peak vanishes and the shoulder shifts to more anodic potentials. A similar trend is observed when ageing the sample surface for 20 h in air prior to electrolyte exposure. These features indicate in correspondence with the observed OCP behaviour (Fig. 4) that during both air ageing and pre-exposure in the sulphate electrolyte a valve-metal oxide film progressively grows on the mechanically ground alloy surface while Cu is more and more replaced from the outer surface-near regions. With beginning anodic polarisation residues of Cu at the surface-near regions (in particular after short exposure times) will be dissolved while the valve-metal oxidation will be enhanced. Therefore, anodized glassy alloy surfaces exhibit the typical compositional profile, i.e., oxidized Zr-, Al- (and Ti-) species in the outermost surface film and metallic Cu only at the inner film/metal interface, as it is frequently reported for as-cast and mechanically ground bulk glassy Zr-based alloy samples [8,15–19]. However, other fine-polishing procedures lead to significant changes in the anodic polarisation behaviour. Fig. 7 compares the cyclic potentiodynamic curves recorded for the glassy Zr59Ti3Cu20Al10Ni8 alloy sample at different surface states after 30 min of immersion in neutral sulphate solution. Similar as described above for the mechanically ground state (4000), for the as-cast as well as for the diamond fine-polished state a spontaneous surface passivation with weakly pronounced Cu oxidation peak (see inset) is observed. In contrast, the anodic curves for surfaces which were extremely fine-polished with SiO2 suspensions (Final, MasterMet) exhibit in the first cycle initially a pronounced Cu oxidation peak with maximum at 0.09–0.1 V vs. SCE. Then they transfer into a wide passive range – similar as the as-cast and mechanically ground states and with only slightly increased passive current density, i.e., in the order of 0.04 mA/cm2 compared to 0.02 mA/cm2. In the second polarisation cycle the Cu peak does not occur. Furthermore, in repeated measurements no clear differences were detected in the transpassive region of the differently finished samples. This characteristic polarisation behaviour suggests – also with respect to Fig. 6 – that with beginning anodic polarisation the

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Fig. 7. Cyclic potentiodynamic polarisation curves of differently finished surfaces of a bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample in 0.01 M Na2SO4 recorded after 30 min exposure at OCP (scan-rate 10 mV/s, starting anodic, 2 cycles) inset: magnification of selected curves in the potential range 0.3 to 0.8 V vs. SCE.

Cu that was enriched at the surface in result of the fine-polishing with SiO2 suspensions (Figs. 3 and 4) initially dissolves, then the alloy surface is passivated by the formation of Zr-, Al- (and Ti-) oxides. This is also supported by polarisation studies under potentiostatic control. Fig. 8 shows anodic current density transients that were recorded for mechanically ground (4000) and extremely fine-polished (MasterMet) alloy surfaces in sodium sulphate solution. The potential was stepped from the OCP value which established after 30 min of immersion to a potential above the Cu oxidation peak (Fig. 7), i.e., to 0.5 V vs. SCE, and this potential was held for 1 h. For the mechanically ground surface the transient comprises two nearly linear regions. A first flat region with m  0.6 is attributable to the initial anodic passive film formation with weak competitive dissolution processes. This transfers after 10 s into the transient region with slope m  1 which is indicative for a high-field controlled growth of barrier-type passive films as typically known for valve metals like Zr, Al or Ti and as already

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demonstrated for as-cast surfaces of Zr-based glasses [16]. However, the anodic transient of the MasterMet fine-polished alloy surface comprises initially a wide pronounced maximum, before it enters after about 10 s into the high-field controlled regime at a similar current density level like that of the mechanically ground sample surface. This indicates an initially dominating corrosion– dissolution of the Cu-rich alloy surface which transfers into stable strong passivation when the excess of Cu is mostly removed. Indeed, further AES studies of the potentiostatically passivated surfaces of the ground and fine-polished alloy samples did not reveal significant differences in the elemental distribution (in the limits of this method) but confirmed the typically detected passive film composition of the oxides of the valve-metal components [16,17]. Exemplarily, an AES profile of a MasterMet fine-polished sample surface measured after potentiostatic polarisation is shown in the inset in Fig. 8. Further cyclic polarisation studies were conducted on the extremely fine-polished glassy alloy samples after longer pre-exposure time, i.e., after 2 h under open circuit conditions. Fig. 9 shows sections of the anodic curves in the low polarisation regime in comparison to those measured on the mechanically ground state. Diamondpolished surfaces were found to behave similar like ground surfaces; their curves are not shown here. While the mechanically ground surface (4000) exhibits the spontaneous passive behaviour (see Fig. 5), the fine-polished sample surfaces (Final, MasterMet) show even more pronounced Cu oxidation reactions in terms of two characteristic peaks, i.e., a small peak at 0.21 V vs. SCE and a sharp one at 0.35 V vs. SCE. This clearly confirms the conclusions drawn from the results shown in Fig. 4, namely that after 2 h of exposure under open circuit conditions Cu species are still enriched at the fine-polished glassy alloy surface. But within this pre-exposure period a large fraction of the metallic Cu is transferred into cuprous oxide Cu2O, according to 2Cu + H2O =Cu2O + 2H+ + 2e [44]. At the first weak peak residues of Cu are also oxidized. The transformation to Cu2O is related with a local acidification of the electrolyte close to the electrode surface. Therefore, the following anodic reaction corresponding to the second sharp peak is mainly attributed to the further oxidation to Cu2+ ions according to Cu2O + 2H+ = 2Cu2+ + H2O + 2e [44] and thus, the removal of Cu species from the alloy surface.

Fig. 8. Anodic current density transients recorded for the bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample with mechanically ground and fine-polished surface states in 0.01 M Na2SO4 after a potential step from OCP (30 min) to 0.5 V vs. SCE Inset: AES depth profile taken after potentiostatic polarisation of the MasterMet-polished sample.

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Fig. 9. Cyclic potentiodynamic polarisation curves of differently finished surfaces of a bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample in 0.01 M Na2SO4 recorded after 2 h exposure at OCP (scan-rate 10 mV/s, starting anodic, 2 cycles).

3.3. Chloride induced pitting susceptibility of differently finished glassy alloy surfaces In order to study the effect of different surface finishing of the glassy Zr59Ti3Cu20Al10Ni8 alloy sample anodic polarisation studies were conducted in neutral sodium sulphate solutions containing varying concentrations of sodium chloride, 0.01 M Na2SO4 + x M NaCl (x = 0; 0.001; 0.01; 0.1) with pH 7. In part the samples were not directly transferred to the electrochemical cell but were aged in air for 20 h after the grinding or fine-polishing step to let the native oxidation proceed. Fig. 10 summarizes selected characteristic linear anodic polarisation curves recorded on mechanically ground (4000 – Fig. 10a) and extremely fine-polished (Final – Fig. 10b, MasterMet – Fig. 10c) glassy alloy surfaces after pre-exposure under open circuit conditions for 30 min. It must be emphasized that for diamond fine-polished surfaces an anodic behaviour very similar to that of the ground state was observed [48], therefore it is not shown here again. In neutral chloride-free sulphate solution, a mechanically ground alloy surface exhibits also under quasi-stationary anodic polarisation conditions a dominating passive behaviour. At 0.03 V vs. SCE an only weakly pronounced Cu peak occurs and this is followed by a wide passive current density plateau at 1 lA/cm2. Furthermore, for the sample surfaces which were fine-polished with SiO2 suspensions the active Cu oxidation–dissolution is markedly pronounced in the low anodic polarisation regime prior to stable passivity at higher anodic potentials [48]. In both cases the passive current density level is slightly increased compared to the ground surface, i.e., reaches values of 2.5 lA/cm2 for Final-polished and 3 lA/cm2 for MasterMet-polished surfaces (measured at 0.8 V vs. SCE). This indicates a slightly lower protective effect of the growing passive films. For all states after surface finishing, no significant effect of the pre-ageing in air for 20 h on the slow dynamic polarisation behaviour was noticed. With the addition of various sodium chloride concentrations to the base electrolyte for all states after surface finishing principally a negative shift of the corrosion potential is observed. This indicates a more active state of the glassy alloy surface in the presence of chloride ions. Though only a limited reproducibility was observed for the adjustment of the corrosion potential in a chloride-containing solution under particular experimental conditions, the trend of a negative corrosion potential shift was found to be less pronounced for fine-polished Cu-enriched sample surfaces. Under anodic polarisation conditions the mechanically ground alloy sample (Fig. 10a) shows a stable

Fig. 10. Linear anodic polarisation curves of differently finished surfaces of a bulk glassy Zr59Ti3Cu20Al10Ni8 alloy sample recorded in 0.01 M Na2SO4 + x M NaCl (x = 0; 0.001; 0.01; 0.1) after 30 min exposure at OCP (scan-rate 0.5 mV/s) (in part recorded after ageing the sample surface for 20 h in air): a, SiC grit 4000; b, 0.1 lm SiO2 suspension (Final); c, 0.02 lm SiO2 suspension (MasterMet).

passivation in electrolytes with up to 0.01 M NaCl, while in 0.1 M NaCl passive layer breakdown occurs at a potential Epit of 0.13 V vs. SCE followed by a steep rise of the current density which has been found to be typical for the pit growth on Zr-based glassy alloy surfaces [11– 16,24–30,48]. Additional ageing of the sample surface for 20 h in air results in a positive shift of the pitting potential by about 0.3 V. This can be attributed to the progressive native valve-metal oxide film growth prior to the electrochemical measurement. In contrast, both extremely fine-polished sample surface states – with Final and MasterMet – show a much higher susceptibility

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for the initiation of pitting processes. This is expressed in the fact, that a passive layer breakdown is observed already at much lower chloride concentrations. Stable anodic passivity is given only in the electrolyte containing 0.001 M NaCl, while already in 0.01 M NaCl pitting occurs. This critical concentration is one order of magnitude lower than that for the mechanically ground (Fig. 10a) or diamondpolished surface states [48]. The pitting initiation is typically observed in the potential regime, which directly follows the pronounced Cu oxidation peak. The pitting potentials at this critical chloride concentration for fine-polished surfaces are slightly less positive than those values measured at the higher critical concentration for mechanically ground samples. Moreover, for the finepolished surface states no clear effect of pre-ageing is detectable. The particular role of Cu species in the chloride-induced local corrosion process of Zr-based glassy alloys has been already described – see introduction [16,18,19,25–27]. Considering this, it appears to be reasonable to assume that Cu which is accumulated at the extremely fine-polished sample surfaces only gradually dissolves during the initial stages of the anodic polarisation while a surface passivation is locally hindered. Consequently, residues of Cu species may be present on the alloy surface, which can give rise to local galvanic coupling effects, where Cu species act as local cathodes. Under these conditions the reaction of the valve-metal components Zr, Al (and Ti) with chloride ions can be facilitated. This may explain the observed increased pitting initiation susceptibility of the extremely fine-polished glassy alloy sample surfaces. Finally it should be clearly stated, that no significant change in the typical morphology of the grown pits was detected when applying the different surface finishing treatments [48]. This indicates that only the pitting initiation is governed by the surface state of the alloy.

4. Summary and conclusions The study demonstrated that the state of surface finishing of a bulk glassy Zr-based alloy can remarkably influence its corrosion and passivity. Native oxide films grown in air on alloy surfaces that were mechanically ground with SiC paper up to grit 4000 and those that were additionally fine-polished with diamond suspensions are composed of valve-metal oxides and Cu is weakly enriched underneath. During exposure under open circuit conditions in neutral sulphate solution the growth of these oxides also dominates, while Cu species are gradually depleted from surface-near regions. This effect is further enhanced with progressing anodic polarisation leading to stable passivity in a wide potential range. The initiation of pitting is only observed in neutral sodium sulphate solutions containing P0.1 M sodium chloride. In contrast, on glassy alloy surfaces which were extremely finepolished with SiO2-based suspensions nanometre-sized Cu-rich islands are detectable and seem to be embedded in the natively grown oxide film. The Cu enrichment is mainly explained with an ease of Cu segregation processes on the freshly fine-polished surfaces, which lead to the energetically most favourable surface state. During exposure in neutral sodium sulphate solution under open circuit conditions the gradual transfer of Cu into Cu2O is the dominating surface reaction. With beginning anodic polarisation oxidation and dissolution of Cu species takes place before at higher potentials stable passivity is attained. In consequence of the Cu enrichment, a significant increase of the susceptibility for pitting initiation is noticed. This is reflected in the reduction of the critical chloride concentration at which pitting is firstly observed by one order of magnitude to 0.01 M sodium chloride. The findings of this study are firstly of practical importance with respect to the choice of suitable surface finishing methods.

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These should yield on one hand the required surface properties of a bulk glassy alloy for particular applications, e.g., smoothness and homogeneity. But on the other hand they should not detrimentally modify the surface chemistry. Furthermore, the knowledge gained from this study is of fundamental scientific importance. Since the effects of surface finishing and pre-exposure conditions are so far scarcely considered aspects in corrosion studies on Zr-based glassy alloys, a comparison of corrosion data reported in literature even for one particular alloy composition appears to be critical. Therefore, a more careful consideration of the discussed aspects in future corrosion studies is recommended.

Acknowledgements The authors gratefully acknowledge the support of M. Frey and S. Donath with sample preparation and of M. Johne with electrochemical measurements. They thank J. Eckert and M. Uhlemann (IFW Dresden), L. Battezatti (Univ. Torino, Italy) and M.M. Lohrengel (Univ. Duesseldorf, Germany) for fruitful discussions.

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