APPLICATION OF DIFFERENT ANALYTICAL TECHNIQUES

IN THE UNDERSTANDING OF THE CORROSION

PHENOMENA OF NON-CRYSTALLINE ALLOYS

K. SEDLA KOVÁa, b, F. HAIDERb, J. SITEKa, and M. SEBERÍNIa

aDepartment of Nuclear Physics and Technology

Slovak University of Technology

Ilkovi ova 3, 812 19 Bratislava, Slovakia bInstitute of Physics, Experimentalphysics I, University of Augsburg

Universitätsstrasse 2, 861 59 Augsburg, Germany

Corresponding author: J. Sitek, e-mail: sitek@elf.stuba.sk

Abstract: The aim of this paper is to discuss the applicability of different analytical techniques to study the corrosion phenomena and mechanisms of amorphous and nanocrystalline metallic alloys. We focus here on atmospheric corrosion of soft magnetic Fe-based materials prepared by a plane-flow casting method followed by annealing. The samples were exposed at rural and industrial sites for 2 to 6 months. The techniques of (conversion electron) Mössbauer spectrometry, X-ray diffraction and transmission electron microscopy are considered. These analytical techniques provide useful information regarding the nature, composition, morphology and crystallinity of the corrosion film and are, moreover, highly suitable for understanding the possible structural rearrangement of non-crystalline materials. Substantial differences in the corrosion resistance according to the alloy composition and crystallinity have been observed. The most resistant Si-containing, partly crystallized alloy proved the presence of a pure amorphous protective oxide film on the surface. Fe oxide particles in other systems showed a needle-like morphology and poor crystalline order and were identified as lepidocrocite.

1. INTRODUCTION

Corrosion is a widespread phenomenon, responsible for many functional failures in service. It may take many forms, depending on the corrosion conditions and individual features of the material. Especially corrosion caused by various atmospheric conditions, which is nearly unavoidable during a long-term operation, accounts for more failures (involving cost and tonnage) basis than any other environment [1]. Although there are

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several techniques for accelerating corrosion under laboratory conditions, none of them

has been found satisfactory to duplicate the composition of the corrosion products produced under atmospheric exposure [2].

The aim of the study was to prove the diagnostic potential of several analytical methods to understand the corrosion mechanism of amorphous and nanocrystalline metallic alloys. The nanocrystalline materials exhibit great fundamental and technological interests because of their unique microstructural characteristics, resulting in excellent soft magnetic properties. Even though the number of papers devoted to the corrosion of these alloys is unfortunately very low (and none was found to be handling the mechanism of atmospheric rusting), a partial deterioration of their unique magnetic features observed in laboratory corrosion tests has been already indicated [3].

2. MATERIALS AND EXPERIMENTAL METHODS

Fe-based metallic alloys of the nominal composition of Fe73.5Cu1Nb3Si13.5B9 and Fe87.5Zr6.5B6 were investigated in the amorphous and nanocrystalline state. The amor-phous precursor was prepared by the plane-flow casting method at the Institute of Physics, Slovak Academy of Sciences, Bratislava, in the form of ribbons several millimetres wide and about 20 to 30 micrometers thick. To obtain the nanocrystalline structure, controlled annealing treatment of the ‘as-cast’ samples in a vacuum at a tem-perature of 540 C was carried out. Accordingly, annealed Fe73.5Cu1Nb3Si13.5B9 (so called FINEMET) and Fe87.5Zr6.5B6 (so called NANOPERM) alloys were composed of ultrafine bcc-Fe and DO3-Fe(Si) grains with DO3 structure, respectively.

The shiny sides of the amorphous and nanocrystalline samples of both compositions were exposed at rural and industrial sites for 2 to 6 months.

To analyse the corrosion mechanism and products, the methods of X-ray diffraction (XRD), Mössbauer spectrometry (MS) and transmission electron microscopy (TEM) were employed.

The X-ray diffraction patterns were recorded with the Enraf-Nonius Diffractis 585 diffraction analyser using Co K radiation. The incidence angle of the direct beam with respect to the specimen plane remained constant during the entire measurement, i.e.

11 degrees. For the selected specimens, the X-ray patterns were collected at lower incident angles as well, down to approximately 3 degrees.

Mössbauer spectra were recorded in transmission geometry by a conventional con-stant-acceleration spectrometer with 57Co(Rh) source at room temperature and for the selected samples, at liquid N2 temperature. Apart from transmission measurements, the conversion electron Mössbauer spectrometry (CEMS) was engaged. CEM spectra were taken with a back-scattering type gas-flow proportional counter, designed at

our laboratory. For evaluation of all the recorded Mössbauer spectra, the NORMOS program was used [4].

The specimens for TEM observations were prepared by ion-polishing using Gatan-PIPS 691. The samples of shiny (corroded) surfaces were prepared by ion-milling of the ribbons from the wheel-contacted side only. The corrosion products and the grain-structure of the materials were investigated using a Philips CM 120 electron micro-scope. The TEM accelerating voltage was 120 kV.

Corrosion Phenomena of Non-Crystalline Alloys 231

3. RESULTS AND DISCUSSION

3.1. X-ray diffraction

The microstructure of nanocrystalline alloys was widely investigated by X-ray diffraction in the sense of crystalline fraction, the nature of the grains and grain size determination [5]. XRD is a simple and comfortable method, which is easily applicable even to quite brittle, ribbon-shaped nanocrystalline specimens.

For corrosion studies, the composition of the rust scale can be determined quite reliably using X-ray diffraction. X-ray patterns of iron oxides can be fitted using different line profiles – preferably Voight, Pseudo-Voight or Pearson VII function – to yield accurate line positions, widths and intensities, these being the three important parameters. From these, information is obtained about the nature of the oxide, its quantity (in a mixture), its unit cell parameters and its crystallinity. Deviations from the unit cell parameters obtained from an X-ray pattern may be used to quantify the extent of Fe substitution by other cations; the line broadening provides information on the average crystal size (Scherrer formula); the changes in relative intensities of the peaks point at preferential orientation etc.

In many cases, however, the oxide film is too thin for this technique to be applied and the identification of these phases requires grazing incidence measurements or other sensitive surface chemical methods, such as electron diffraction, Auger, Mössbauer and Raman spectrometry, XPS and EXAFS. Another drawback of this method can arise when corrosion products are of a low crystallinity and for this reason, they cannot be analysed accurately. Even though, the differences in the broadening of various reflec-tions due to different degrees of development of small crystals in various directions can provide information about their crystal shape [6].

Concerning actual results, the XRD patterns of the as-quenched samples showed the presence of only broad halos confirming their amorphous structure. For the nanocrystal-line samples, the presence of rather well defined Bragg peaks corresponding to the structurally ordered crystalline grains superimposed on very broad lines typical for disordered amorphous matrix was observed. The crystalline peaks are associated with either bcc-Fe in case of NANOPERM alloys or with DO3-Fe(Si) nanograins in case of FINEMET alloys. Diffraction patterns were analysed by Rayflex program. The most in-tense peak of the first diffuse maximum was decomposed using Pseudo-Voight profiles in order to estimate the crystalline/amorphous fraction from the integral intensities of respective peak areas. The results as compared with those obtained by MS are listed further in Table II. From the full width at half maximum of the strongest peak assigned to the crystalline phase, the average grain size was assessed using the Scherrer equation (Table I).

The surface appearance of all corroded samples was inspected under optical micro-scope before further investigations. The Si-containing nanocrystalline specimens showed,even after 6-months outdoor exposure, no visible continuous oxide layer on the surface. XRD analysis yielded no information on the composition of the potentially present passive film, either. On the other hand, the amorphous precursor of FeCuNbSiB was covered by a massive orange-coloured rust layer as early as after 2 months weathering.A relatively continuous oxide film was observed on FeZrB ‘as-cast’ samples as well;

232 K. Sedla ková et al.

however, their nanocrystalline counterparts, unlike in the case of FINEMET, were corroded to the largest extent.

Figure 1. The XRD pattern of nanocrystalline FeZrB specimen after a 6-month exposure recorded at an incident angle of ca. 3 degrees

Table I. Average crystal sizes of the bcc-Fe and bcc-Fe(Si) grains of NANOPERM and FINEMET samples, respectively; and mean oxide particles sizes obtained using XRD

Crystal size (nm)

unexposed after 6-months exposure Sample

nanocrystals nanocrystals -FeOOH

FeCuNbSiB 11 12 –

FeZrB 12 13 13

Table II. Comparison of the amorphous phase fraction of nanocrystalline samples obtained using XRD and MS

Amorphous phase fraction (%) unexposed after 6-months exposure Sample

XRD MS XRD MS

FeCuNbSiB 53 50 52 53

FeZrB 62 69 34 39

The XRD analysis of the amorphous samples indicated a very low crystallinity of the oxide film preventing its identification even at lower incident angles. This method offered some relevant results on the composition of the corrosion film only in the case of the most intensively corroded nanocrystalline NANOPERM. The respective dif-fraction pattern, illustrated in Fig. 1, was found to be consistent with the orthorhombic structure of -FeOOH, i.e. lepidocrocite (Rayflex card Nr. 74-1877). The presence of

-FeOOH (goethite), which displays a similar diffraction pattern as lepidocrocite, cannot be strictly excluded; even though the reflection (130) is missing in the diffraction

Corrosion Phenomena of Non-Crystalline Alloys 233

pattern. Since from the appearance of the XRD spectra it was obvious that the oxide particles were of a very small size (broad, low intensity peaks), the Scherrer equation was applied to the most intense peaks (031 reflection) in order to get a rough estimate of their average crystal size. The mean particle size is shown in Table I together with the average sizes of Fe-nanocrystals.

3.2. Mössbauer spectrometry 57Fe Mössbauer spectrometry seems an excellent tool to investigate iron-based

nanocrystalline alloys, because this local technique is able to elucidate the nature of hyperfine interactions of the different resonating iron nuclei and to probe the nature of their immediate surroundings [5]. This technique can easily be applied to the thin and brittle ribbon-shaped samples over a wide temperature range due to a high recoilless factor [7]. Identification of Fe-containing phases, which are created during the anneal-ing process as well as elucidation of their structural (e.g. volumetric fraction of individual phases) and magnetic properties (the orientation of the net magnetic moment, the average hyperfine field, etc.), can be performed very effectively. The delicate stage of this analytical method is, however, the choice of the fitting procedure of the Mössbauer spectra. They can be, in general, quite complex due to the structural disorder, embracing up to 7 sextets attributed to different iron neighbourhoods (e.g. in case of FINEMET alloys) as well as the amorphous phase describing the distribution of hyperfine field/quadrupole splitting.

From the corrosion point of view, MS is particularly useful in systems where the iron oxide may be too low in concentration or in crystallinity to be detected by XRD. MS is the only technique, which can identify uniquely all the iron oxides (hydroxides) including the measurement of the fraction of each phase present in the corrosion coating and to obtain information about the particle size and its isomorphous substitution. For example, the size and size distribution of small particles of superparamagnetic Fe oxides can be estimated from the Mössbauer spectra by recording the transition from the doublet spectrum into a magnetically split spectrum as the temperature decreases. The temperature of such a transition decreases with decreasing the particle size [6].

The phase analysis in the transmission geometry is, however, hindered, when the oxide films are very thin and the sub-spectrum of the substrate is predominant in the Mössbauer pattern. Moreover, such a layer cannot be scraped off the surface for studying in this geometry. Here, a surface-sensitive variant of MS, i.e. conversion electron Mössbauer spectrometry (CEMS), which collects electrons associated with the 7.3 keV conversion after resonance absorption of 14.4 keV -rays comes in handy providing useful information about the corrosion products, grown to the thickness of more than 100 nm, in a non-destructive way.

The specific results showed that particularly the CEMS variant is very useful in the identification of the corrosion products. For the quite intensively corroded amorphous specimens, where the identification of corrosion products by XRD was impossible, the CEM spectra manifested the presence of a well-resolved, slightly broadened paramagnetic doublet having Mössbauer parameters of Fe(III) in the form of ferric oxyhydroxide -FeOOH (lepidocrocite). However, from the room temperature

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Mössbauer spectra, lepidocrocite and small superparamagnetic particles of goethite ( FeOOH) or maghemit ( Fe2O3) cannot be identified accurately. In order to distin-guish between these three possible paramagnetic compounds, low temperature measure-ments are required.

The relative fraction of paramagnetic (hydro)oxides registered on amorphous speci-mens varied between 6 and 49%.

The diagnostic potential of this method is, however, hindered when the more com-plex spectra are to be analysed. This is the case of nanocrystalline FINEMET, where the spectra complexity obscures the fitting procedure and further oxide describing component becomes ‘invisible’ in lower concentrations even for CEMS.

Figure 2. Changes in the structural arrangement of the nanocrystalline FeZrB alloy after 2 to 6-months exposure to the atmosphere revealed by transmission MS

Figure 3. Transmission spectra of nanocrystalline NANOPERM untreated (a) and exposed in industrial area for 6 months measured at (b) room temperature and spectra recorded at 77 K (c). Corresponding distributions of hyperfine fields are plotted at the right hand side of each graph

Corrosion Phenomena of Non-Crystalline Alloys 235

Relatively simpler features of NANOPERM spectra facilitate their interpretation. The CEMS showed that the surface of the samples even after a 2-month exposure was composed exclusively of a (hydro)oxides describing doublet. In the case of such an advanced stage of corrosion, the transmission measurements can render interesting information on the corrosion damage as well. In addition to the possibility of distin-guishing the oxide component, the bulk spectra offer unique characteristics of structural rearrangement (Fig. 2). We observed a growth of a crystalline, interfacial and paramag-netic phase component at the expense of the residual amorphous matrix. The contribu-tion of the doublet up to ca. 10% was manifested. This phenomenon could be related to the corrosion-induced growth of the existing crystalline grains.

Since, as mentioned above, lepidocrocite and other potentially superparamagnetic oxides cannot be identified separately in the Mössbauer spectrum at room temperature, the 77 K transmission spectra of NANOPERM were recorded (Fig. 3). Low temperature measurements unveiled, however, an absence of magnetically ordered iron oxides. Furthermore, the significant increase in quadrupole splitting of the doublet can attribute this component to lepidocrocite, which has a Néel temperature of about 77 K [6].

Mössbauer measurements were correlated with X-ray diffraction results mentioned above and no significant discrepancies about the fraction of the crystalline phase were observed (Table II).

3.3. Transmission electron microscopy

Transmission electron microscopy is also very useful for studying the microstruc-ture of non-crystalline materials. In addition to the grain size determination, it offers complementary information on the grain shape as well as their homogeneity, density and distribution within the amorphous matrix.

TEM is applied routinely to determine crystal morphology of common corrosion products as well. Besides the imaging mode, the electron diffraction mode is also available. The electron diffraction pattern can, unlike that of X-rays or neutrons, be related to an image of a crystal, thus enabling the structure of a particular region to be investigated. An advantage is that as electrons are scattered more efficiently than X-rays, shorter exposure times are required. In addition, comparatively small samples can be examined (selected area diffraction, SAD). On the other hand, since electrons do not penetrate the matter as easily as X-rays, only relatively thin samples (50-100 nm) can provide an electron diffraction pattern [6].

To prepare samples with such properties for the electrons transparent region is, however, critical in this method due to brittleness of the material. This can be achieved relatively successfully when ion-polishing technique is used to prepare the TEM specimens. If the samples are thinned from one side only, one can inspect the properties of the very thin, initially wheel-contacted or free surface. As far as e.g. grain size is concerned, however, these can vary significantly within the whole sample cross-section, usually being the largest on the free side and the smallest in the inner part [8]. Hence, the information on the average grain size could be obtained only if cross-section specimens are prepared by the common sandwich technique.

236 K. Sedla ková et al.

The main problem arises, when corroded samples are to be investigated. The atmos-pheric rusting results in formation of a strongly inhomogeneous oxide film, which is therefore quite difficult to analyse. Moreover, as the corrosion film is usually thicker than 50 nm, analysing of the influence of the corrosion damage on structural rearrange-ment can be difficult as well.

The preliminary TEM observations confirmed absence of the crystalline phase in the ‘as-cast’ FeCuNbSiB specimens.

Figure 4 shows the TEM micrographs of the shiny surface of nanocrystalline FINEMET. The grain size of the nanocrystals determined by an image analysis was approximately 10 nm, which was in a good agreement with the X-ray diffraction results. The early stage of the corrosion process of amorphous FeCuNbSiB was investigated and it revealed a presence of an amorphous passive film, which is to be identified (Fig. 5a). Concerning the composition, one could expect the presence of SiO2 protective layer [9]. The ‘as-cast’ specimen, after a 6-month weathering, showed the presence of a similar amorphous region too; however, in addition, some fibrous structure and needle-like particles yielding no sharper diffraction patterns were observed as well (Fig. 5b).

Figure 4. Bright-field TEM image with the a corresponding electron diffraction pattern (EDP) of the shiny surface of the annealed FeCuNbSiB alloy ribbon

Figure 5. TEM pictures of corroded ‘as-cast’ FeCuNbSiB ribbon: amorphous film observed at the early stage of the corrosion process (a); fibrous structure of the corrosion film of a 6 month weathered specimen (b)

Corrosion Phenomena of Non-Crystalline Alloys 237

Figure 6. TEM images of a 6 months corroded nanocrystalline FeCuNbSiB specimen; an amor-phous region with round particles (a), the interface between nanograins containing interface region and amorphous area (b)

On the corroded surface of a nanocrystalline FINEMET, an amorphous passive film was observed, showing scattered small round particles of the size of about 20 nm (Fig. 6a). The film thickness was, however, even after 6 months, so small that the Fe(Si) grains were still discernible in the inspected region (Fig. 6b). It was found that the nanocrystal grain size remained practically unaltered.

Figure 7. Bright-field (a) and dark-field (b) TEM image of untreated nanocrystalline FeZrB sample with corresponding SAED pattern

From the free surface of a nanocrystalline NANOPERM shown in Fig. 7, large grains of -Fe of the diameter ranging from ca. 100 to 200 nm are observable. When compared with grain size obtained by XRD (ca. 12 nm), the structure of their surface is found to be very much different from that of the inner part. Figure 8a shows the surface of the 6-months corroded NANOPERM sample. One can notice the oxide particles with needle-like morphology, showing a discernible internal structure. According to the cor-responding SAED pattern shown in Fig. 8b, these can be identified as lepidocrocite.

Table III summarizes the TEM investigations aimed at grain size determination.

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Figure 8. A bright-field TEM image of needle-like oxide particles observed on a corroded surface of nanocrystalline FeZrB ribbon (a) and the corresponding SAED pattern (b)

Table III. Approximate crystal sizes of the bcc-Fe and bcc-Fe(Si) grains of NANOPERM and FINEMET samples, respectively, and average size of oxide particles obtained using TEM (shiny side)

Crystal size (nm) unexposed after 6-months exposure Sample

nanocrystals nanocrystals -FeOOH

FeCuNbSiB 10 10 –

FeZrB 200 –10 nm wide,

up to 60 nm long

The TEM observations indicate that further investigation of the corrosion products is necessary to provide relevant information on their morphology and crystallinity.

4. CONCLUDING REMARKS

The capabilities of three different analytical techniques, namely the X-ray diffrac-tion, Mössbauer spectrometry and transmission electron microscopy, of obtaining information on the corrosion behaviour of non-crystalline Fe-based alloys were dis-cussed. The engagement of complementary techniques can lead to a better understand-ing of relevant phenomena, even though each method has its own shortcomings as far as its practical application is concerned.

Although X-ray diffractometry is a typical analytical technique for the identification of the corrosion products, it becomes unusable if the corrosion layer is very thin as is the case of the initial corrosion stage and if the corrosion products are in an amorphous state. On the other hand, TEM is able to yield relevant results even if the passive film is very thin, but the drawback appears during the preparation of specimens in advanced stages of corrosion, where the oxide film can be very inhomogeneous in its composition and depth profile. The utility of Mössbauer spectrometry techniques for the corrosion study, comprising transmission measurements and CEMS variant, was demonstrated as well. While CEMS is very suitable for different Fe-phases identification, the trans-

Corrosion Phenomena of Non-Crystalline Alloys 239

mission mode can provide relevant information on the bulk rearrangement. This can also be proved by XRD.

ACKNOWLEDGEMENTS

Contribution of the grants SGA 1/1014/04, 1/0284/03 and DAAD 08/2003 is acknowledged.

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