Corrosion protection of zirconium surface based on Heusler alloy

XPS measurement

A typical survey spectrum of the clean Zr(0001)-(1×1) surface with identified peaks is shown in Fig. 1.

Fig. 1:

Survey photoemission spectrum of clean Zr(0001)-(1×1) surface. The inset in the upper left corner shows Zr 3d core level.

The Zr 3d spectra taken at the clean Zr(0001)-(1×1) surface are shown Fig. 2a. To the best of our knowledge, this is for the first time that two contributions have been distinguished. One of them (peak P at 179.1 eV BE) corresponds to the metallic state of Zr. The second one (peak Q) has a higher BE by about 0.2 eV. The Q peak reflects any minor structural or chemical changes of the surface. It is common for XPS measurements that the photoemission peak of a bulk origin preserves its parameters, unlike the surface one [21]. Thus, we consider the Q peak to the surface contribution either as bonds of Zr atoms with hydrogen or as a compensation of the relaxed surface. According to the theoretical prediction [6], the relaxation of the (1×1) Zr surface is insignificant, involving only top two atomic layers. It seems that the high number of steps and terraces observed in our AFM images might give rise to the Q peak. Earlier experiments demonstrated the presence of H at the surface or in the subsurface region at RT [12]. It is believed that hydrogen can be partially expelled from the sample bulk by cleaning in ultra high vacuum (UHV) [12], [27], [28]. Our impression following from the measurements is that the Q peak originates from the complex surface morphology and bonds with hydrogen are less probable.

Fig. 2:

Core level photoemission spectra of Zr(0001) surfaces with or without Fe and Si. (a) clean Zr(0001)-(1 ×1) at room temperature, (b) Zr with deposited Fe and annealed at 400°C, (c) Zr with deposited Fe and annealed at 630°C, (d) Zr with deposited Fe and Si at 45°C, (e) Zr with deposited Fe and Si and annealed at 400°C. Doniach-Šunjić line profiles were used for fitting [26].

Zr surface covered by Fe possesses amorphous structure with no LEED spots. Its chemical state is obvious from the photoelectron spectra of Zr 3d and Fe 2p in Fig. 2b and c. Zr 3d spectrum as a whole is shifted about 0.1 eV to lower BE. After annealing at 400°C, Q peak is split in two parts. The first part dominates at lower BE 179.2 eV (Q′) and the second part forms a new peak R at 179.8 eV BE. R peak intensity increases moderately with the annealing temperature up to 650°C, whereas it starts to decrease slowly above 700°C, signaling Fe dissolution. The surface becomes saturated with Fe close to the annealing temperature 630°C. Fe deposition is accompanied with reduction of the Q′ peak intensity to zero value (see Fig. 2c). The nature of the surface remains amorphous irrespective of the annealing temperature as no LEED diffraction spots were detected. The Fe 2p spectrum (Fig. 2b) displays a single peak at 707.4 eV BE for the segregated surface and the evaporated layer, as well. The value moderately differs from the one measured at a bulk sample.

Si was deposited on the Zr–Fe surface at 45°C (see Fig. 2d). The structure of the new surface retains the amorphous character. The Zr 3d spectrum is composed of three peaks (P′, Q′ and R′). The BE values of the peak P′ and Q′ are identical with pure Zr–Fe surface, except the R peak having been shifted to higher BE. The full width half maximum (FWHM) value of the Fe 2p slightly increases, signaling an increased disorder at the surface. The Si 2p spectrum exhibits an envelope corresponding to two chemical states. The first state at 99.3 eV BE represents Si atoms and the second one at 98.75 eV BE Si signalizes a bonding with one of the elements or a different adsorption site at the surface [29], [30]. Annealing at T≥400°C yields a single peak in the Si 2p spectrum at 98.8 eV BE, which implies just one type of bonding of Si Fig. 2e. After treatment at temperatures ≥400°C, the R′ (R″) peak of the Zr 3d spectrum shifts approximately by 0.5 eV to higher BE (180.3 eV). We ascribe it to the formation of Zr silicides at these temperatures [29], [30], [31]. The N structure in the Fe 2p spectrum evidences a more complex chemical nature of the bonding, like for example Zr–Fe–Si.

Topography measurement

The clean Zr(0001) surface shows characteristic (1×1) diffraction spots (Fig. 3a) in the full range of the electron beam voltage between 15 and 150 V. The images display relatively low background and not very sharp diffraction. The Zr–Fe surface with deposited Si shows a diffraction pattern which we identified to be (2√3×2√3)R30° reconstruction using the software for LEED images analysis [32]. The image consists of external six spots rotated 30° towards the (1×1) patterns of the clean Zr(0001) and 12 internal spots that are seen at the beam energy between 19 and 39 V. At the beam energy around 30 eV, faint Zr(1×1) spots are observable (see arrows marking spots as a guide to the eye in the image Fig. 3b). Annealing above ~600°C smears the (2√3×2√3)R30° patterns. Although most of Si is bonded in the new structure, there is still relatively high degree of disorder at the surface.

Fig. 3:

LEED patterns. (a) (1×1) pattern of Zr(0001) crystal, (b) and (c) diffraction spots with trigonal symmetry inside basic (1×1) pattern of Zr–Fe–Si. Arrows in panel (b) point to residual very weak (1×1) pattern of Zr(0001) substrate. Panel (c) is structure measured with beam energy 19 V and in panel (b) there is the same structure but with higher beam energy 30 V. For more details about the patterns, see [18].

AFM images taken on the single crystal surface display a hilly surface that is split in a number of small terraces, see Fig. 4. The images were acquired in the ambient atmosphere that causes high degree of the oxidation represented with white spots. The corrugation, in terms of Root Mean Square Roughness (R_RMS), is 6.9 nm for the area of 20 μm×20 μm and reaches 11.7 nm for the area of 90 μm×90 μm. It can be reduced with increasing the annealing temperature. Nevertheless, this procedure is restricted due to irreversible phase transition between hcp and bcc structures at 862°C [3] and a contaminant segregation or dissolution in the bulk. Similarly to AFM images, also STM images (Fig. 4) show the surface with many morphological features such as terraces, facets and steps. The size of terraces and facets observed at the surface are relatively small (~100 nm) in the comparison with other metal surfaces. This type of the disorder is characterized with high density of steps, vacancies, edges, etc. Atoms forming these features have certainly different BE reflected in the peak Q of the Zr 3d spectrum (Fig. 2). In Fig. 5 we can see Zr(0001) surface with a higher degree of oxidation as white islands decorating steps or borders of terraces. It is an evidence of their higher reactivity.

Fig. 4:

AFM and STM images of clean Zr(0001) surface. AFM: Left image –1μm×1μm area. Right image –20μm×20μm area. The images represent different parts of the surface more (left) or less ordered (right). Both show very high density of terraces, steps or other types of defects. STM: Left image demonstrates a typical size of the terraces at the clean surface arranged in valleys and blocks. Right image displays a strong tendency towards faceting of the surface. Similarly to AFM, high density of defects is recognized on this surface. The defects cause the presence of P and Q peaks in the Zr 3d photoelectron spectrum (Fig. 2).

Fig. 5:

STM image of oxidized Zr(0001)-(1×1) surface. The surface is covered with different structures that represent altering content of oxygen. The highest degree of oxidation is represented with small bright protrusions originating from the formation of ZrO₂ islands. They are attached mostly to surface defects.

In Fig. 6 is shown the AFM image of the Zr polycrystalline surface. The images were taken on a single crystallite. The left panel exhibits a high degree of the oxidation during the transport from the UHV conditions. The facets are covered with different number of oxide layers that copy the Zr substrate. The right panel shows the surface covered with Si. It protects the surface from the reaction with O. The white chains demonstrate an initial degree of oxidation during the Si deposition [33]. In Fig. 7, the 60 nm Si layer on the poly Zr displays worm like structure that is presumably not ideal for a surface protection.

Fig. 6:

AFM images of polycrystalline Zr surfaces. The selected crystallite shows a strong tendency of faceting on the submicron scale. Left panel – oxidized surface covered with irregular islands arranged in multilayer film. Right panel – facets of polycrystalline Zr surface covered with homogenous film of Si (2–3 monolayers). Short white chains seen in the image (right panel) originated during the Si deposition due to the evaporator degassing and they reflect surface interaction with oxygen.

Fig. 7:

AFM image of polycrystalline Zr surface covered with ~60 nm Si film as protective layer. Black spots seen on the surface are depressions in the layer. Such samples were submitted to autoclave tests (see Fig. 9).

k-space mapping

The Zr DOS of the valence band is formed of hybridized spd electrons [34], [35]. The valence band spectrum of previous experimental work [12] was done with angle resolved measurement at SR source. The Fermi edge is dominated with Zr d electrons and at –1.2 eV BE peak was prescribed to the H induced feature. The identification of this structure was done on the base of the temperature behavior: The peak is gradually erased with increasing temperature and a subsequent cooling annuls it.

We mapped the dispersion function E vs k with equi-energy cuts through the band structure providing images with acceptable momentum and energy resolutions [36]. The left panel of Fig. 8 displays selected images of local densities of electronic states (LDOS) at E_F and at –1.2 eV BE in the first BZ. At the clean Zr, the central part around the Γ point dominates the k-space E_F cut. The central hexagonal part is surrounded with six LDOS of trigonal symmetry as the entire cut corresponding to the (0001) face is. The k-space images match well the theoretical calculations of the electron structure presented in [34]. The mapping was done for the energy span between E_F and the bottom of the valence bands. The cuts show a high LDOS well localized in the center of BZ for the energies between –0.7 eV and –1.3 eV in the agreement with the previous interpretation [12] that the observed peak in LDOS results from the H–Zr interaction. Regarding the Zr–Fe–Si (2√3×2√3)R30° surface, it shows significant changes in electronic structure when compared with the clean Zr (see left panel of the Fig. 8). The cut close to E_F possesses 30° rotated symmetry with substantially lower LDOS at Γ point and flower like patterns. The Si bonding at the Zr–Fe–Si surface is fully reflected in LDOS structure forming the –1.2 eV cut. The flower like features of this cut are preserved. However, the central H induced peak is replaced with six well-separated local maxima. Thus, we believe that the (2√3×2√3)R30° structure at the Zr–Fe–Si surface is a consequence of H being depleted in the sample topmost surface layers or H possibly substituted with Si forming Zr–Si bond.

Fig. 8:

PEEM images (k-space). Top left: Zr(0001) crystal at the Fermi edge. Top right: Zr–Fe–Si at the Fermi edge. Bottom left: slice of Zr(0001) crystal representative of the energy interval spanning from –0.7 to –1.3 eV under the Fermi edge. Bottom right: slice of Zr–Fe–Si representative of the energy interval spanning from –0.7 to –1.3 eV under the Fermi edge. For more details about the images, see [18].

Electrochemical impedance spectroscopy

Impedance spectra of Si-coated and uncoated samples are given in Fig. 9. The experimental data were approximated by transfer function of the following form:

Fig. 9:

Bode plots of impedance spectra of bare (clean) Zr and Si coated Zr. Measured in autoclave after exposure for 700 h in diluted LiOH at 270°C and 8 MPa.

Here, R_e represents the electrolyte resistance. The second term represents complex capacitive (C_ox , Q_ox ) and resistive (R_ox ) response of oxide or oxide and remnants of the Si film, j stands for imaginary unit and ω for angular frequency. The third term represents mass transfer (parameters Y_o and B) and charge transfer (Y_o , B, R_ct and Q_dl ) across the oxide or oxide-Si layer. Exponents α and β express a degree of deviation of the parameters Q_ox and Q_dl , respectively from ideal capacitive behavior.

Both bare Zr/oxide and Zr/oxide/Si film impedance dispersion are characteristic with two time constants in frequency range of 10⁻¹–10⁴ Hz. Main difference between the two cases appears in the frequencies below 10⁻¹ Hz. In case of the un-coated sample, the shape of low frequency end of impedance dispersion is purely resistive, while in case of Si-coated sample new time constant appears. This part of spectra enables estimation of mass and charge transfer parameters (B and Y_o parameters in the above given function) connected with diffusion of charged species (in this case oxygen anions) across the surface layers. In case of un-coated sample, the diffusion time constant is moved to higher frequencies (approximately between 0.1 Hz and 20 Hz) and is effectively merged with a time constant formed by charge transfer resistance (R_ct ) and Helmholtz layer dispersive capacitance at the oxide-electrolyte interface (Q_dl ). The B/Y_o ratio, which has the meaning of diffusion resistance gives in the case of coated sample values around 5000–6000 Ω·cm², while for un-coated samples values around 1000–2000 Ω·cm² were found (Table 1). It means that under conditions of the experiment 2–3 times lower diffusion rate is typical for coated sample compared to the un-coated one. It corresponds to more than 2 times lower value of capacitance C_ox and, consequently, a thicker oxide, formed due to corrosion in hot LiOH solution on the un-coated sample. The Si layer and its remnants form thus an effective diffusion barrier and suppress the corrosion rate of Zr in high temperature Li+ containing water.