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Corrosion of amorphous and nanocrystalline Zr-based alloys
Materials Science and Engineering A 375–377 (2004) 53–59
Corrosion of amorphous and nanocrystalline Zr-based alloys Daniela Zander, Uwe Köster∗ Department of Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany
Abstract Due to the high potential of Zirconium and Zr-based alloys regarding corrosion it is of interest to develop even better Zr-based alloys as well as to improve the understanding of their corrosion mechanisms. Production techniques such as rapid quenching and mechanical alloying are used for the design of amorphous or nanocrystalline materials and give the possibility to improve Zr-based alloys, not only by alloying, but also by changing the microstructure. Corrosion of metallic alloys is strongly influenced by the alloying elements as well as by their microstructure. The aim of this paper is to review the current knowledge on the design and corrosion of nanocrystalline Zr-based alloys in comparison to amorphous and crystalline materials. Based on these results, trends for further research and expectation of an improved corrosion behavior of nanocrystalline alloys are outlined. © 2003 Elsevier B.V. All rights reserved. Keywords: Corrosion; Nanocrystalline; Metallic glass; Zr-alloys; Zr–Cu–Ni–Al
1. Introduction Zirconium and its alloys are very interesting materials for applications in the nuclear industry due to their low neutron absorption coefficient. Current trends towards extended burn-up of the nuclear fuel in pressurized water reactors have accentuated the demand for Zr-based alloys with higher uniform corrosion resistance under irradiation and lower hydrogen absorption. Zirconium is used as a getter in vacuum tubes and as a component in surgical appliances. In addition Zr-based alloys find also major use in chemical plants, especially if, e.g. Ni-based alloys or stainless steels are not appropriate materials. Sectors for applications in the chemical industry are the synthesis of acetic acid, reactions with hydrochloric acid or other aggressive organic solutions. The corrosion of metallic alloys is strongly influenced by alloying elements as well as by the microstructure. Zirconium and crystalline Zr-based alloys are known to exhibit some unique chemical properties, such as chemical as well as electrochemical corrosion resistance, oxidation resistance, and toxicity. Among their chemical properties, corrosion resistance was well studied and many alloy systems were found to show extremely high corrosion resistance in a
∗
Corresponding author. E-mail address: [email protected] (U. Köster).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.230
broad range of aqueous environments due to the easy formation of a protective oxide layer at the surface [1]. However, during the last few decades there was considerable interest on the effect of microstructure, e.g. the variation of grain size, on the corrosion of materials. In recent years a variety of non-equilibrium methods were developed to design new materials with amorphous, nano- or quasicrystalline structure that exhibit very promising physical, mechanical and chemical properties, e.g. high corrosion resistance. 1.1. Corrosion of amorphous alloys Since the extremely high corrosion resistance of amorphous Fe–Cr–P–C alloys was reported already in 1974 [2] a variety of corrosion resistant amorphous Fe-based alloys were developed mostly by melt spinning and sputter deposition methods [3–7]. It was also shown that alloying elements have a great influence on the corrosion resistance besides the aspect of microstructure; glasses exhibit far less restriction on alloying than crystalline materials. Cr is the most effective alloying element for increasing the corrosion resistance as a result of enrichment of Cr ions in the passive film formed on amorphous Fe–Cr–P–C [3,8,9]. The high corrosion resistance of amorphous alloys was also attributed to their chemical homogeneity and lack of structural defects [10]. This excellent corrosion behavior is not limited on melt–spun metallic glasses, but was currently also observed in bulk amorphous alloys prepared by conventional casting
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techniques. Studies on bulk amorphous Fe–Cr–Mo–C–B alloys exhibiting a large supercooled liquid region showed lowered corrosion rates in the range of 10−4 to 10−2 mm per year in 1–12 mol/l HCl solutions at room temperature as well as a wide passive region and low passive current density in the range of 10−2 to 100 A/m2 [11]. Other bulk amorphous alloys e.g. Ni–Cr–Ta–Mo–P–B alloys were also observed to form spontaneously a passive layer in 6 and 12 M HCl, similar to their melt–spun amorphous counterparts [12]. Further, studies of a number of binary TM–TM metallic glasses (TM: transition metal), e.g. Ni–Zr [13,14], Cu–Zr [15,16] or Al–Zr [17] revealed an increased corrosion resistance compared to their crystalline counterparts. During the last 20 years studies on corrosion of magnetic amorphous Fe-based alloys revealed its tremendous effect on the magnetic properties [18]. Amorphous alloys, e.g. Fe–M–B (M: Zr, Nb) based alloys, attracted attention due to their magnetic properties such as effective permeability and saturation flux density [18–21]. These properties indicate that these materials may have application in several magnetic parts and devices such as power transformers, electromagnetic interference prevention components, magnetic heads, sensors, magnetic shielding and reactors [22]. Corrosion studies of polycrystalline Fe84 Nb7 B9 Fe8 showed that corrosion causes a substantial decrease in the saturation magnetic flux density [23]. Since atmospheric corrosion is unavoidable in many applications of soft-magnetic alloys there is still need for investigations regarding the influence of microstructure and composition on corrosion of these materials. Only recently the effect of composition on corrosion resistance of amorphous Fe–M–B (M: Zr, Nb) in a 0.1–0.5 M H2 SO4 solution was reported [24]. A greater corrosion resistance in Nb containing alloys was observed in comparison to Zr containing alloys, which was attributed to the formation of a protective niobium oxide layer. Besides studying the influence of metallic glasses on the corrosion resistance investigations of its influence on the formation of amorphous passive layers on the corrosion behavior became also of great interest. The improved corrosion resistance associated with the addition of 18% Cr to e.g. Fe is attributed, in part, to a change in the protective oxide structure from a well oriented spinel structure at 0–12% Cr to a non-crystalline structure at 18% Cr [25,26]. A retardation in ionic transport may occur because noncrystalline films have fewer defects or grain boundaries to enhance ionic movement. In addition these films might exhibit a higher homogeneity than the crystalline substrate. 1.2. Corrosion of nanocrystalline alloys Nanocrystalline materials with grain size of less than 100 nm are a relatively new and novel class of advanced materials receiving considerable attention in the scientific and industrial communities during the last decade. Since the early 1980s [27] scientific activities in the area of synthesis,
microstructural characterization and property determination of these materials resulted in the development of a number of manufacturing techniques for the production of various nanocrystalline materials with improved physical, chemical and mechanical properties. Current efforts toward large-scale production of nanostructured solids are concerned with consolidating nanocrystalline precursor powders produced by techniques such as gas condensation, ball milling or spray conversion. Film deposition techniques, such as physical and chemical vapor deposition, and sol–gel techniques are also under intensive investigations as well as processing nanocrystalline alloys by partial crystallization (e.g. [28]). Electroplating was identified as a technologically feasible technique for production of nanocrystalline pure metals and alloys as well as nanocomposites [29]. Developments in the synthesis of these materials, particularly by conventional electroplating techniques [30], stimulated interest in protective and functional coating applications. However, the variability of e.g. protective nanocrystalline coatings will depend on its corrosion resistance. The corrosion behavior of nanocrystalline materials received very little attention until now. Nanocrystalline stainless steel films with grain size of 25 nm, prepared by sputter-deposition, exhibited a more uniform corrosion and superior localized corrosion resistance as compared to the conventional counterparts in 0.3 wt.% NaCl solution at room temperature [31]. The breakdown potential of the sputtered 304 type stainless steel films is approximately 850 mV higher as the potential of conventional material and close to those of amorphous Fe–Cr–P–C alloys. The improved corrosion resistance of the sputtered films is attributed to the fine grain size and the resulting homogeneity of nanocrystalline materials. The ultrafine microstructure seems to permit a uniform distribution of impurities and to provide a homogenous substrate for the formation of a stable passive layer. Up to now sintered Nd–Fe–B alloys are the most favorable alloys for application as high performance permanent magnets; although they are sensitive to some climatic and corrosive environmental conditions. The poor corrosion resistance of Nd–Fe–B magnets is attributed to the formation of typically three phases in the microstructure: The ferromagnetic Nd2 Fe14 B matrix and the Nd- and B-rich intergranular phases which are very sensitive to corrosion [32,33]. The corrosion mechanisms of such magnets can be summarized as a preferential dissolution of intergranular phases followed by separation of ferromagnetic grains from the surface before their dissolution. The rapid surface degradation and pulverization can be attributed to the lack of corrosion resistance of the intergranular phases as well as the high content of Nd which is one of the most reactive elements [34,35]. Partial substitution of Fe with Al, Co, Cu and Ga was found to improve the corrosion resistance of Nd–Fe–B alloys in many corrosive environments as a result of changes in microstructure, phase composition as well as segregation of the mentioned additions into intergranular phase regions.
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Thus, the substitution of Fe may reduce the electrochemical potential difference between ferromagnetic and intergranular phases, leading to a lower driving force for galvanic corrosion [36,37]. Nevertheless, the influence of Al, Co, Cu and Ga additions on, e.g. grain size, phase distribution, and sensitivity to surface hydrogen which can affect corrosion has only be considered. Only recent studies revealed in few cases relations between the corrosion property and the microstructure of Nd–Fe–B alloys with additional Co and Ga in a 0.1 M H2 SO4 solution [38]. First results on the corrosion of crystallized melt–spun materials were obtained in Fe–Ni–Cr–P–B alloys in chloride-sulfate and sulfuric acid solutions [39,40]. It was determined that the corrosion resistance of nanocrystalline Fe32 Ni36 Cr14 P12 B6 was significantly greater than that of the amorphous counterpart. This improved corrosion resistance was attributed to the observed enrichment of Cr in the passive layer due to rapid interface boundary diffusion. Nanocrystalline Zr-based alloys can be produced by, e.g. annealing of the amorphous precursor [41–43] and are interesting due to their better mechanical properties for industrial applications. Zr–Cu–Ni–Al belongs to the best glass forming systems known [44], furthermore the formation of icosahedral quasicrystals of different size (10–200 nm) in this system is possible [45,46]. As reviewed in the following sections only few investigations were conducted on the corrosion of nanocrystalline materials, especially based on Zr. This lack of data highlights the urgent need for further investigations. The current knowledge reviewed is expected to provide significant conclusions for the corrosion properties of these new metastable Zr-based materials, in particular for their use as structural materials as well as on the influence of a particular microstructure on corrosion.
2. Design of nanocrystalline Zr-based alloys New routes of materials preparation leading to metastable materials such as metallic glasses, nanocrystalline or quasicrystalline alloys are seen as a promising approach to improve a number of properties. During the last decade special attention was directed to the preparation of nanocrystalline alloys because of their improved mechanical and chemical properties. In nanocrystalline materials, i.e. crystalline solids with crystal diameters in the range of a few nanometer, the grain boundary volume represents up to about 50% of the total volume. Therefore, these solids can be assumed to consist of two different types of atomic structure: a crystalline structure with long range order for all the atoms far from the grain boundaries and a disordered structure with some short-range order at the interfacial region, more comparable to a gas-phase structure. It is the presence of two structural components (crystallites and the interfacial component) of comparable volume fractions that characterize nanocrystalline materials and was claimed to be the origin of novel
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Fig. 1. Effect grain size on the calculated volume fractions for intercrystalline regions, grain boundaries and triple junctions [49].
and improved properties [47,48]. Fig. 1 shows the effect of grain size on the calculated volume fractions for intercrystalline regions, grain boundaries and triple junction, assuming a grain boundary thickness of 1 nm [49]. Zr-based nanocrystalline alloys can be prepared directly by rapid quenching techniques, e.g. melt-spinning, by nanocrystallization of amorphous precursor alloys, vacuum condensation or by mechanical alloying followed by compaction. In the case of mechanical alloying followed by compaction one has to consider the porosity while investigating the corrosion behavior. In Zr–Cu–Ni–Al for example depending on the alloy composition and the thermal treatment a variety of nanostructured materials can be designed containing different crystalline phases with a fcc (so called big-cube phase) or tetragonal (Al2 Cu- or MoSi2 -type) structure, as well as icosahedral quasicrystals [50]. Within our work nanocrystalline Zr–Cu–Ni–Al was prepared mainly by controlled crystallization of a metallic glass precursor into a nanocrystalline state.
3. Corrosion of metastable Zr-based alloys Crystalline Zr possesses good corrosion resistance in many environments ranging from highly acidic to highly alkaline aqueous solutions. This is usually ascribed to the presence of a protective oxide film present on the surface, formed either during air exposure or during the initial period of exposure to the aqueous solution. Recently, a new class of amorphous and partially nanocrystalline alloys with various alloy compositions based on Zr has been developed. Such nanocrystalline alloys exhibit for example greater mechanical strength than conventional polycrystalline alloys while simultaneously retaining the macroscopic corrosion resistance of the fully amorphous state. Thus, the often observed problem between high strength and good corrosion resistance can be avoided in this new class of materials but depends critically on the nanocrystal size.
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Thus, the combination of the superior chemical properties of Zr-based alloys with an optimized, e.g. nanocrystalline microstructure should lead to improved corrosion resistance. 3.1. Amorphous materials It is well known that glassy alloys offer superior resistance to corrosion and vastly improved mechanical properties over their conventional counterparts. Amorphous alloys have often a corrosion resistance one to two orders of magnitude higher than polycrystalline alloys with the same alloy composition [51,52]. One theory states that the improved resistance of amorphous alloys is not only based on its structure itself but also on their ability to promote amorphous oxide formation. In high temperature gas, vitreous or amorphous oxides offer improved oxidation resistance due to the absence of oxide grain boundaries, which provide a rapid diffusion path for concentration gradient driven ion movement [53,54]. In the case of metal passivity in aqueous solution, ion transport is mostly driven by the electric field across the oxide film. The lack of oxide grain boundaries may lower ion migration rates, rendering the passive film more protective [53–55]. The rapid transport and accumulation of cation vacancies at the metal/oxide interface is used in one theory accounting for oxide breakdown [56]. Moreover, vitreous oxides on amorphous alloys perform well due to enhanced bond flexibility since the vitreous or amorphous material can rearrange to accommodate lattice mismatch and strain between the oxide and the metal. As a result of this flexibility, almost all surface atoms can bond with oxygen or OH− without requiring an optimal epitaxial relationship between an ordered metal substrate and the oxide [57,58]. In summary, a desirable amorphous oxide formed on metallic glasses includes defect minimization, film ductility, bond flexibility and efficient, rapid film repassivation. As already mentioned above, new bulk amorphous Zr-based alloys such as Zr–Al–TM, La–Al–TM and Ti–Zr–TM (TM: transition metals) reveal good mechanical
Fig. 2. SEM image of the corrosive layer on amorphous Zr69.5 Cu12 Ni11 Al7.5 after salt spray test (cross-sectional fracture surface; SEM image) [61].
properties, e.g. high static as well as dynamic strength, and high toughness. For this reason exemplary the corrosion behavior of amorphous Zr–Cu–Ni–Al alloys will be discussed. In order to enable the use of these amorphous Zr-based alloys as structural materials it is essential to have good corrosion resistance in a variety of different corrosive atmospheres. First investigations on the corrosion behavior of the bulk amorphous Zr55 Cu30 Ni5 Al10 alloy in 1997 showed in a wide pH and potential range the rapidly formation of passivating surface layers in 0.1 M Na2 SO4 (pH = 2–8) and 0.1 M NaOH (pH = 13) solutions [59]. Later investigations on the passivation behavior of bulk amorphous Zr55 Cu30 Ni5 Al10 compared to the crystalline counterpart and zirconium in weakly alkaline sulfate solutions showed that the barrier effect of anodic films grown on the amorphous alloy were slightly lower than on zirconium. Anodic layers on the surface of Zr–Cu–Ni–Al consist apparently of a mixture of simple oxides or complex oxidic compounds of all alloying elements [60]. Our own microstructural investigations by SEM (Fig. 2) as well as TEM (Fig. 3) of amorphous Zr69.5 Cu12 Ni11 Al7.5 after corrosion in a salt spray test in 5% NaCl solution
Fig. 3. Passive film observed on an amorphous Zr69.5 Cu12 Ni11 Al7.5 ribbon after salt spray test [61]: (a) TEM image and (b) electron diffraction pattern.
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resistance can be obtained for Nb [63] followed by Ta, Ti and Cr. Table 1 summarizes the corrosion rates of some amorphous Zr-based alloys in 6 M HCl solution at 295 K. Alloying with Nb in a range of 5–20 at.% Nb, which still allows good glass formability, led to very low corrosion rates of 0.06–0.07 mm per year that was independent of the Nb content. With increasing Ta content above 5 at.% a significant increase in the corrosion rate was observed due to pitting. The reduction of corrosion resistance by the generation of pits was also noticed with the addition of Ti. The corrosion resistance of Zr60 Ni10 Cu20 Al10 as well as Zr55 Cu20 Ni10 Al12.5 Ti2.5 and Zr55 Cu20 Ni10 Al10 Nb5 was investigated in 3% NaCl solution at 298 K [62]. From these investigations it can be concluded that the corrosion resistance under these parameters is also the highest for the addition of Nb, even in comparison to pure crystalline Zr. Additional alloying elements such as Ti, Cr, Nb or Ta lead to the conclusion that the addition of Nb revealed the best improvement of corrosion resistance in Zr–Cu–Ni–Al alloys. Since Zr55 Cu20 Ni10 Al10 Nb5 not only exhibits good corrosion resistance but also the ability to form bulk metallic glass, the use of amorphous Zr–Cu–Ni–Al–Nb alloys as machinery structural parts in a corrosive environment is of great interest for further investigations. Recently, the use of Zr–Cu–Ni–Al alloys in biomedical applications was discussed [64]. Amorphous Zr-based alloys should be promising biomaterials due to their high strength, good corrosion resistance and low Young’s modulus in comparison to crystalline alloys. In particular, the high corrosion resistance is the most important property to avoid biological damages, such as allergy and carcinogenicity due to metal ions. First evaluation of the use of amorphous Zr65 Cu17.5 Ni10 Al7.5 as biomedical materials in phosphate buffered solutions with varying chloride concentration, pH and dissolved oxygen content indicated that this alloy is a promising biomaterial but will require further investigation.
Fig. 4. Corrosion rates of amorphous Zr60−x TMx Al10 Ni10 Cu20 (TM: Ti, Cr, Nb or Ta) alloys after immersion in 6M HCl solution [62].
(pH = 6.5) revealed an amorphous passive layer with a small amount of crystals. TEM diffraction revealed mainly an amorphous structure with small amounts tetragonal ZrO2 (a = 0.512 nm, c = 0.525 nm) and an unknown fcc phase (a = 0.37 nm), probably a Cu(Ni, Al) solid solution phase [61]. The influence of additional alloying elements such as Ti, Cr, Nb or Ta were studied to improve the corrosion resistance of amorphous Zr–Al–Ni–Cu alloys in HCl as well as NaCl solutions [62]. Fig. 4 shows the corrosion rates of amorphous Zr60−x TMx Al10 Ni10 Cu20 (TM: Ti, Cr, Nb or Ta) alloys after immersion for 16 h and 64 h in 6M HCl solution at 295 K. It was observed that the corrosion rates of amorphous Zr–Cu–Ni–Al and Zr–Cu–Ni–Al–Cr were too high to be measured after immersion for 64 h, even though the corrosion rates of their amorphous alloys in 1 M HCl show very low values below 0.01 mm per year. In comparison the addition of Nb and Ta exhibited rather low corrosion rates below 0.1 mm per year after immersion for 64 h; the largest effect of additional elements on the corrosion
Table 1 Corrosion rates of Zr-based amorphous alloys in 6 M HCl solution at 295 K [62] Alloy
Crystallinity
Density (mg/m3 )
Corrosion rate (mm per year) 16 h
64 h
Zr55 Cu20 Ni10 Al10 Cr5 Zr50 Cu20 Ni10 Al10 Cr10 Zr55 Cu20 Ni10 Al10 Nb5 Zr50 Cu20 Ni10 Al10 Nb10 Zr45 Cu20 Ni10 Al10 Nb15 Zr40 Cu20 Ni10 Al10 Nb20 Zr55 Cu20 Ni10 Al10 Ta5 Zr50 Cu20 Ni10 Al10 Ta10 Zr55 Cu20 Ni10 Al10 Ti5 Zr50 Cu20 Ni10 Al10 Ti10
Amorphous Amorphous + crystallinity Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous
6.59 6.61 6.66 6.75 6.86 6.96 7.05 7.55 6.43 6.47
0.312 – 0.166 0.152 0.421 0.113 0.081 0.044 0.098 0.105
Dissolved – 0.075 0.068 0.062 0.062 0.039 0.129 pitting 0.909 pitting 0.969 pitting
Zr60 Cu20 Ni10 Al10 Zr60 Cu30 Al10
Amorphous Amorphous
6.56 7.05
0.131 Dissolved immediately
Dissolved –
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Fig. 5. Corrosive layer on nanocrystalline Zr69.5 Cu12 Ni11 Al7.5 after salt spray test (cross-sectional fracture surface; SEM image) [61].
3.2. Nanocrystalline materials In general nanocrystallinity is assumed to improve the corrosion resistance of an alloy as compared to its conventional counterpart. A reason for such a behavior might be a higher diffusivity due to the high ratio of surface to volume due to the presence of large number of grain boundaries, thus improving the formation of a protective layer. Furthermore, the higher homogeneity allows a better distribution of contamination because of the increased area of grain boundaries, thus reducing local segregation. Unfortunately, there is still a lack of fundamental understanding of the corrosion mechanisms in such materials. In particular, understanding is required why nanocrystals of a critical size, spacing, composition and structure control both, the resistance to form of micrometer scale local corrosion pits as well as the dissolution properties in strong reducing acids. Corrosion of nanocrystalline Zr69.5 Cu12 Ni11 Al7.5 ribbons was investigated similar to their amorphous precursor by a salt spray test in 5% NaCl solution (pH = 6.5) [61]. SEM (Fig. 5) as well as TEM (Fig. 6) investigations showed a nanocrystalline passive layer with grain sizes of about 10 nm
and only some amorphous areas in comparison to amorphous Zr69.5 Cu12 Ni11 Al7.5 , which formed mainly an amorphous passive layer. Electron diffraction of the nanocrystalline passive layer showed also the pattern of the unknown fcc phase with a lattice parameter of a = 0.37 nm (probably Cu(Ni,Al) solid solution) similar as formed on the amorphous counterpart. EDS analysis revealed twice as much oxygen in the passive layer on the amorphous Zr69.5 Cu12 Ni11 Al7.5 than on the nanocrystalline ribbon. All the metals were homogeneously distributed in both layers. However, amorphous and nanocrystalline Zr–Cu–Ni–Al alloys were observed to be very sensitive against corrosion as compared to pure crystalline Zr. Furthermore, it was observed that the corrosion resistance of nanocrystalline Zr–Cu–Ni–Al did not improve compared to the amorphous state of the alloy or crystalline Zr. Therefore, it would be of great interest to find out whether Nb additions have a similar beneficial effect for a nanocrystalline microstructure as in the amorphous state. 4. Trends for further investigations Design of improved Zr alloys can proceed by, e.g. rapid quenching or ball milling leading to amorphous and nanocrystalline alloys as well as nanocomposites. It was shown that additives as well as a special microstructure can affect the corrosion resistance of both, the amorphous as well as the nanocrystalline Zr-based alloys. However, there are still a lack of understanding regarding the influence of microstructure on the corrosion resistance, especially in the case of nanocrystalline materials. Further investigations are necessary in particular on partially nanocrystalline materials as recently mentioned by Scully [65] regarding: (a) The effect of the nanocomposite structure, i.e. nanocrystals embedded in an amorphous matrix, (including lattice parameter changes and surface curvature effects,
Fig. 6. Passive film observed on a nanocrystalline Zr69.5 Cu12 Ni11 Al7.5 ribbon after salt spray test [61]: (a) TEM image and (b) electron diffraction pattern.
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Corrosion of amorphous and nanocrystalline Zr-based alloys Daniela Zander, Uwe Köster∗ Department of Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany
Abstract Due to the high potential of Zirconium and Zr-based alloys regarding corrosion it is of interest to develop even better Zr-based alloys as well as to improve the understanding of their corrosion mechanisms. Production techniques such as rapid quenching and mechanical alloying are used for the design of amorphous or nanocrystalline materials and give the possibility to improve Zr-based alloys, not only by alloying, but also by changing the microstructure. Corrosion of metallic alloys is strongly influenced by the alloying elements as well as by their microstructure. The aim of this paper is to review the current knowledge on the design and corrosion of nanocrystalline Zr-based alloys in comparison to amorphous and crystalline materials. Based on these results, trends for further research and expectation of an improved corrosion behavior of nanocrystalline alloys are outlined. © 2003 Elsevier B.V. All rights reserved. Keywords: Corrosion; Nanocrystalline; Metallic glass; Zr-alloys; Zr–Cu–Ni–Al
1. Introduction Zirconium and its alloys are very interesting materials for applications in the nuclear industry due to their low neutron absorption coefficient. Current trends towards extended burn-up of the nuclear fuel in pressurized water reactors have accentuated the demand for Zr-based alloys with higher uniform corrosion resistance under irradiation and lower hydrogen absorption. Zirconium is used as a getter in vacuum tubes and as a component in surgical appliances. In addition Zr-based alloys find also major use in chemical plants, especially if, e.g. Ni-based alloys or stainless steels are not appropriate materials. Sectors for applications in the chemical industry are the synthesis of acetic acid, reactions with hydrochloric acid or other aggressive organic solutions. The corrosion of metallic alloys is strongly influenced by alloying elements as well as by the microstructure. Zirconium and crystalline Zr-based alloys are known to exhibit some unique chemical properties, such as chemical as well as electrochemical corrosion resistance, oxidation resistance, and toxicity. Among their chemical properties, corrosion resistance was well studied and many alloy systems were found to show extremely high corrosion resistance in a
∗
Corresponding author. E-mail address: [email protected] (U. Köster).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.230
broad range of aqueous environments due to the easy formation of a protective oxide layer at the surface [1]. However, during the last few decades there was considerable interest on the effect of microstructure, e.g. the variation of grain size, on the corrosion of materials. In recent years a variety of non-equilibrium methods were developed to design new materials with amorphous, nano- or quasicrystalline structure that exhibit very promising physical, mechanical and chemical properties, e.g. high corrosion resistance. 1.1. Corrosion of amorphous alloys Since the extremely high corrosion resistance of amorphous Fe–Cr–P–C alloys was reported already in 1974 [2] a variety of corrosion resistant amorphous Fe-based alloys were developed mostly by melt spinning and sputter deposition methods [3–7]. It was also shown that alloying elements have a great influence on the corrosion resistance besides the aspect of microstructure; glasses exhibit far less restriction on alloying than crystalline materials. Cr is the most effective alloying element for increasing the corrosion resistance as a result of enrichment of Cr ions in the passive film formed on amorphous Fe–Cr–P–C [3,8,9]. The high corrosion resistance of amorphous alloys was also attributed to their chemical homogeneity and lack of structural defects [10]. This excellent corrosion behavior is not limited on melt–spun metallic glasses, but was currently also observed in bulk amorphous alloys prepared by conventional casting
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techniques. Studies on bulk amorphous Fe–Cr–Mo–C–B alloys exhibiting a large supercooled liquid region showed lowered corrosion rates in the range of 10−4 to 10−2 mm per year in 1–12 mol/l HCl solutions at room temperature as well as a wide passive region and low passive current density in the range of 10−2 to 100 A/m2 [11]. Other bulk amorphous alloys e.g. Ni–Cr–Ta–Mo–P–B alloys were also observed to form spontaneously a passive layer in 6 and 12 M HCl, similar to their melt–spun amorphous counterparts [12]. Further, studies of a number of binary TM–TM metallic glasses (TM: transition metal), e.g. Ni–Zr [13,14], Cu–Zr [15,16] or Al–Zr [17] revealed an increased corrosion resistance compared to their crystalline counterparts. During the last 20 years studies on corrosion of magnetic amorphous Fe-based alloys revealed its tremendous effect on the magnetic properties [18]. Amorphous alloys, e.g. Fe–M–B (M: Zr, Nb) based alloys, attracted attention due to their magnetic properties such as effective permeability and saturation flux density [18–21]. These properties indicate that these materials may have application in several magnetic parts and devices such as power transformers, electromagnetic interference prevention components, magnetic heads, sensors, magnetic shielding and reactors [22]. Corrosion studies of polycrystalline Fe84 Nb7 B9 Fe8 showed that corrosion causes a substantial decrease in the saturation magnetic flux density [23]. Since atmospheric corrosion is unavoidable in many applications of soft-magnetic alloys there is still need for investigations regarding the influence of microstructure and composition on corrosion of these materials. Only recently the effect of composition on corrosion resistance of amorphous Fe–M–B (M: Zr, Nb) in a 0.1–0.5 M H2 SO4 solution was reported [24]. A greater corrosion resistance in Nb containing alloys was observed in comparison to Zr containing alloys, which was attributed to the formation of a protective niobium oxide layer. Besides studying the influence of metallic glasses on the corrosion resistance investigations of its influence on the formation of amorphous passive layers on the corrosion behavior became also of great interest. The improved corrosion resistance associated with the addition of 18% Cr to e.g. Fe is attributed, in part, to a change in the protective oxide structure from a well oriented spinel structure at 0–12% Cr to a non-crystalline structure at 18% Cr [25,26]. A retardation in ionic transport may occur because noncrystalline films have fewer defects or grain boundaries to enhance ionic movement. In addition these films might exhibit a higher homogeneity than the crystalline substrate. 1.2. Corrosion of nanocrystalline alloys Nanocrystalline materials with grain size of less than 100 nm are a relatively new and novel class of advanced materials receiving considerable attention in the scientific and industrial communities during the last decade. Since the early 1980s [27] scientific activities in the area of synthesis,
microstructural characterization and property determination of these materials resulted in the development of a number of manufacturing techniques for the production of various nanocrystalline materials with improved physical, chemical and mechanical properties. Current efforts toward large-scale production of nanostructured solids are concerned with consolidating nanocrystalline precursor powders produced by techniques such as gas condensation, ball milling or spray conversion. Film deposition techniques, such as physical and chemical vapor deposition, and sol–gel techniques are also under intensive investigations as well as processing nanocrystalline alloys by partial crystallization (e.g. [28]). Electroplating was identified as a technologically feasible technique for production of nanocrystalline pure metals and alloys as well as nanocomposites [29]. Developments in the synthesis of these materials, particularly by conventional electroplating techniques [30], stimulated interest in protective and functional coating applications. However, the variability of e.g. protective nanocrystalline coatings will depend on its corrosion resistance. The corrosion behavior of nanocrystalline materials received very little attention until now. Nanocrystalline stainless steel films with grain size of 25 nm, prepared by sputter-deposition, exhibited a more uniform corrosion and superior localized corrosion resistance as compared to the conventional counterparts in 0.3 wt.% NaCl solution at room temperature [31]. The breakdown potential of the sputtered 304 type stainless steel films is approximately 850 mV higher as the potential of conventional material and close to those of amorphous Fe–Cr–P–C alloys. The improved corrosion resistance of the sputtered films is attributed to the fine grain size and the resulting homogeneity of nanocrystalline materials. The ultrafine microstructure seems to permit a uniform distribution of impurities and to provide a homogenous substrate for the formation of a stable passive layer. Up to now sintered Nd–Fe–B alloys are the most favorable alloys for application as high performance permanent magnets; although they are sensitive to some climatic and corrosive environmental conditions. The poor corrosion resistance of Nd–Fe–B magnets is attributed to the formation of typically three phases in the microstructure: The ferromagnetic Nd2 Fe14 B matrix and the Nd- and B-rich intergranular phases which are very sensitive to corrosion [32,33]. The corrosion mechanisms of such magnets can be summarized as a preferential dissolution of intergranular phases followed by separation of ferromagnetic grains from the surface before their dissolution. The rapid surface degradation and pulverization can be attributed to the lack of corrosion resistance of the intergranular phases as well as the high content of Nd which is one of the most reactive elements [34,35]. Partial substitution of Fe with Al, Co, Cu and Ga was found to improve the corrosion resistance of Nd–Fe–B alloys in many corrosive environments as a result of changes in microstructure, phase composition as well as segregation of the mentioned additions into intergranular phase regions.
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Thus, the substitution of Fe may reduce the electrochemical potential difference between ferromagnetic and intergranular phases, leading to a lower driving force for galvanic corrosion [36,37]. Nevertheless, the influence of Al, Co, Cu and Ga additions on, e.g. grain size, phase distribution, and sensitivity to surface hydrogen which can affect corrosion has only be considered. Only recent studies revealed in few cases relations between the corrosion property and the microstructure of Nd–Fe–B alloys with additional Co and Ga in a 0.1 M H2 SO4 solution [38]. First results on the corrosion of crystallized melt–spun materials were obtained in Fe–Ni–Cr–P–B alloys in chloride-sulfate and sulfuric acid solutions [39,40]. It was determined that the corrosion resistance of nanocrystalline Fe32 Ni36 Cr14 P12 B6 was significantly greater than that of the amorphous counterpart. This improved corrosion resistance was attributed to the observed enrichment of Cr in the passive layer due to rapid interface boundary diffusion. Nanocrystalline Zr-based alloys can be produced by, e.g. annealing of the amorphous precursor [41–43] and are interesting due to their better mechanical properties for industrial applications. Zr–Cu–Ni–Al belongs to the best glass forming systems known [44], furthermore the formation of icosahedral quasicrystals of different size (10–200 nm) in this system is possible [45,46]. As reviewed in the following sections only few investigations were conducted on the corrosion of nanocrystalline materials, especially based on Zr. This lack of data highlights the urgent need for further investigations. The current knowledge reviewed is expected to provide significant conclusions for the corrosion properties of these new metastable Zr-based materials, in particular for their use as structural materials as well as on the influence of a particular microstructure on corrosion.
2. Design of nanocrystalline Zr-based alloys New routes of materials preparation leading to metastable materials such as metallic glasses, nanocrystalline or quasicrystalline alloys are seen as a promising approach to improve a number of properties. During the last decade special attention was directed to the preparation of nanocrystalline alloys because of their improved mechanical and chemical properties. In nanocrystalline materials, i.e. crystalline solids with crystal diameters in the range of a few nanometer, the grain boundary volume represents up to about 50% of the total volume. Therefore, these solids can be assumed to consist of two different types of atomic structure: a crystalline structure with long range order for all the atoms far from the grain boundaries and a disordered structure with some short-range order at the interfacial region, more comparable to a gas-phase structure. It is the presence of two structural components (crystallites and the interfacial component) of comparable volume fractions that characterize nanocrystalline materials and was claimed to be the origin of novel
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Fig. 1. Effect grain size on the calculated volume fractions for intercrystalline regions, grain boundaries and triple junctions [49].
and improved properties [47,48]. Fig. 1 shows the effect of grain size on the calculated volume fractions for intercrystalline regions, grain boundaries and triple junction, assuming a grain boundary thickness of 1 nm [49]. Zr-based nanocrystalline alloys can be prepared directly by rapid quenching techniques, e.g. melt-spinning, by nanocrystallization of amorphous precursor alloys, vacuum condensation or by mechanical alloying followed by compaction. In the case of mechanical alloying followed by compaction one has to consider the porosity while investigating the corrosion behavior. In Zr–Cu–Ni–Al for example depending on the alloy composition and the thermal treatment a variety of nanostructured materials can be designed containing different crystalline phases with a fcc (so called big-cube phase) or tetragonal (Al2 Cu- or MoSi2 -type) structure, as well as icosahedral quasicrystals [50]. Within our work nanocrystalline Zr–Cu–Ni–Al was prepared mainly by controlled crystallization of a metallic glass precursor into a nanocrystalline state.
3. Corrosion of metastable Zr-based alloys Crystalline Zr possesses good corrosion resistance in many environments ranging from highly acidic to highly alkaline aqueous solutions. This is usually ascribed to the presence of a protective oxide film present on the surface, formed either during air exposure or during the initial period of exposure to the aqueous solution. Recently, a new class of amorphous and partially nanocrystalline alloys with various alloy compositions based on Zr has been developed. Such nanocrystalline alloys exhibit for example greater mechanical strength than conventional polycrystalline alloys while simultaneously retaining the macroscopic corrosion resistance of the fully amorphous state. Thus, the often observed problem between high strength and good corrosion resistance can be avoided in this new class of materials but depends critically on the nanocrystal size.
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Thus, the combination of the superior chemical properties of Zr-based alloys with an optimized, e.g. nanocrystalline microstructure should lead to improved corrosion resistance. 3.1. Amorphous materials It is well known that glassy alloys offer superior resistance to corrosion and vastly improved mechanical properties over their conventional counterparts. Amorphous alloys have often a corrosion resistance one to two orders of magnitude higher than polycrystalline alloys with the same alloy composition [51,52]. One theory states that the improved resistance of amorphous alloys is not only based on its structure itself but also on their ability to promote amorphous oxide formation. In high temperature gas, vitreous or amorphous oxides offer improved oxidation resistance due to the absence of oxide grain boundaries, which provide a rapid diffusion path for concentration gradient driven ion movement [53,54]. In the case of metal passivity in aqueous solution, ion transport is mostly driven by the electric field across the oxide film. The lack of oxide grain boundaries may lower ion migration rates, rendering the passive film more protective [53–55]. The rapid transport and accumulation of cation vacancies at the metal/oxide interface is used in one theory accounting for oxide breakdown [56]. Moreover, vitreous oxides on amorphous alloys perform well due to enhanced bond flexibility since the vitreous or amorphous material can rearrange to accommodate lattice mismatch and strain between the oxide and the metal. As a result of this flexibility, almost all surface atoms can bond with oxygen or OH− without requiring an optimal epitaxial relationship between an ordered metal substrate and the oxide [57,58]. In summary, a desirable amorphous oxide formed on metallic glasses includes defect minimization, film ductility, bond flexibility and efficient, rapid film repassivation. As already mentioned above, new bulk amorphous Zr-based alloys such as Zr–Al–TM, La–Al–TM and Ti–Zr–TM (TM: transition metals) reveal good mechanical
Fig. 2. SEM image of the corrosive layer on amorphous Zr69.5 Cu12 Ni11 Al7.5 after salt spray test (cross-sectional fracture surface; SEM image) [61].
properties, e.g. high static as well as dynamic strength, and high toughness. For this reason exemplary the corrosion behavior of amorphous Zr–Cu–Ni–Al alloys will be discussed. In order to enable the use of these amorphous Zr-based alloys as structural materials it is essential to have good corrosion resistance in a variety of different corrosive atmospheres. First investigations on the corrosion behavior of the bulk amorphous Zr55 Cu30 Ni5 Al10 alloy in 1997 showed in a wide pH and potential range the rapidly formation of passivating surface layers in 0.1 M Na2 SO4 (pH = 2–8) and 0.1 M NaOH (pH = 13) solutions [59]. Later investigations on the passivation behavior of bulk amorphous Zr55 Cu30 Ni5 Al10 compared to the crystalline counterpart and zirconium in weakly alkaline sulfate solutions showed that the barrier effect of anodic films grown on the amorphous alloy were slightly lower than on zirconium. Anodic layers on the surface of Zr–Cu–Ni–Al consist apparently of a mixture of simple oxides or complex oxidic compounds of all alloying elements [60]. Our own microstructural investigations by SEM (Fig. 2) as well as TEM (Fig. 3) of amorphous Zr69.5 Cu12 Ni11 Al7.5 after corrosion in a salt spray test in 5% NaCl solution
Fig. 3. Passive film observed on an amorphous Zr69.5 Cu12 Ni11 Al7.5 ribbon after salt spray test [61]: (a) TEM image and (b) electron diffraction pattern.
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resistance can be obtained for Nb [63] followed by Ta, Ti and Cr. Table 1 summarizes the corrosion rates of some amorphous Zr-based alloys in 6 M HCl solution at 295 K. Alloying with Nb in a range of 5–20 at.% Nb, which still allows good glass formability, led to very low corrosion rates of 0.06–0.07 mm per year that was independent of the Nb content. With increasing Ta content above 5 at.% a significant increase in the corrosion rate was observed due to pitting. The reduction of corrosion resistance by the generation of pits was also noticed with the addition of Ti. The corrosion resistance of Zr60 Ni10 Cu20 Al10 as well as Zr55 Cu20 Ni10 Al12.5 Ti2.5 and Zr55 Cu20 Ni10 Al10 Nb5 was investigated in 3% NaCl solution at 298 K [62]. From these investigations it can be concluded that the corrosion resistance under these parameters is also the highest for the addition of Nb, even in comparison to pure crystalline Zr. Additional alloying elements such as Ti, Cr, Nb or Ta lead to the conclusion that the addition of Nb revealed the best improvement of corrosion resistance in Zr–Cu–Ni–Al alloys. Since Zr55 Cu20 Ni10 Al10 Nb5 not only exhibits good corrosion resistance but also the ability to form bulk metallic glass, the use of amorphous Zr–Cu–Ni–Al–Nb alloys as machinery structural parts in a corrosive environment is of great interest for further investigations. Recently, the use of Zr–Cu–Ni–Al alloys in biomedical applications was discussed [64]. Amorphous Zr-based alloys should be promising biomaterials due to their high strength, good corrosion resistance and low Young’s modulus in comparison to crystalline alloys. In particular, the high corrosion resistance is the most important property to avoid biological damages, such as allergy and carcinogenicity due to metal ions. First evaluation of the use of amorphous Zr65 Cu17.5 Ni10 Al7.5 as biomedical materials in phosphate buffered solutions with varying chloride concentration, pH and dissolved oxygen content indicated that this alloy is a promising biomaterial but will require further investigation.
Fig. 4. Corrosion rates of amorphous Zr60−x TMx Al10 Ni10 Cu20 (TM: Ti, Cr, Nb or Ta) alloys after immersion in 6M HCl solution [62].
(pH = 6.5) revealed an amorphous passive layer with a small amount of crystals. TEM diffraction revealed mainly an amorphous structure with small amounts tetragonal ZrO2 (a = 0.512 nm, c = 0.525 nm) and an unknown fcc phase (a = 0.37 nm), probably a Cu(Ni, Al) solid solution phase [61]. The influence of additional alloying elements such as Ti, Cr, Nb or Ta were studied to improve the corrosion resistance of amorphous Zr–Al–Ni–Cu alloys in HCl as well as NaCl solutions [62]. Fig. 4 shows the corrosion rates of amorphous Zr60−x TMx Al10 Ni10 Cu20 (TM: Ti, Cr, Nb or Ta) alloys after immersion for 16 h and 64 h in 6M HCl solution at 295 K. It was observed that the corrosion rates of amorphous Zr–Cu–Ni–Al and Zr–Cu–Ni–Al–Cr were too high to be measured after immersion for 64 h, even though the corrosion rates of their amorphous alloys in 1 M HCl show very low values below 0.01 mm per year. In comparison the addition of Nb and Ta exhibited rather low corrosion rates below 0.1 mm per year after immersion for 64 h; the largest effect of additional elements on the corrosion
Table 1 Corrosion rates of Zr-based amorphous alloys in 6 M HCl solution at 295 K [62] Alloy
Crystallinity
Density (mg/m3 )
Corrosion rate (mm per year) 16 h
64 h
Zr55 Cu20 Ni10 Al10 Cr5 Zr50 Cu20 Ni10 Al10 Cr10 Zr55 Cu20 Ni10 Al10 Nb5 Zr50 Cu20 Ni10 Al10 Nb10 Zr45 Cu20 Ni10 Al10 Nb15 Zr40 Cu20 Ni10 Al10 Nb20 Zr55 Cu20 Ni10 Al10 Ta5 Zr50 Cu20 Ni10 Al10 Ta10 Zr55 Cu20 Ni10 Al10 Ti5 Zr50 Cu20 Ni10 Al10 Ti10
Amorphous Amorphous + crystallinity Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous
6.59 6.61 6.66 6.75 6.86 6.96 7.05 7.55 6.43 6.47
0.312 – 0.166 0.152 0.421 0.113 0.081 0.044 0.098 0.105
Dissolved – 0.075 0.068 0.062 0.062 0.039 0.129 pitting 0.909 pitting 0.969 pitting
Zr60 Cu20 Ni10 Al10 Zr60 Cu30 Al10
Amorphous Amorphous
6.56 7.05
0.131 Dissolved immediately
Dissolved –
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Fig. 5. Corrosive layer on nanocrystalline Zr69.5 Cu12 Ni11 Al7.5 after salt spray test (cross-sectional fracture surface; SEM image) [61].
3.2. Nanocrystalline materials In general nanocrystallinity is assumed to improve the corrosion resistance of an alloy as compared to its conventional counterpart. A reason for such a behavior might be a higher diffusivity due to the high ratio of surface to volume due to the presence of large number of grain boundaries, thus improving the formation of a protective layer. Furthermore, the higher homogeneity allows a better distribution of contamination because of the increased area of grain boundaries, thus reducing local segregation. Unfortunately, there is still a lack of fundamental understanding of the corrosion mechanisms in such materials. In particular, understanding is required why nanocrystals of a critical size, spacing, composition and structure control both, the resistance to form of micrometer scale local corrosion pits as well as the dissolution properties in strong reducing acids. Corrosion of nanocrystalline Zr69.5 Cu12 Ni11 Al7.5 ribbons was investigated similar to their amorphous precursor by a salt spray test in 5% NaCl solution (pH = 6.5) [61]. SEM (Fig. 5) as well as TEM (Fig. 6) investigations showed a nanocrystalline passive layer with grain sizes of about 10 nm
and only some amorphous areas in comparison to amorphous Zr69.5 Cu12 Ni11 Al7.5 , which formed mainly an amorphous passive layer. Electron diffraction of the nanocrystalline passive layer showed also the pattern of the unknown fcc phase with a lattice parameter of a = 0.37 nm (probably Cu(Ni,Al) solid solution) similar as formed on the amorphous counterpart. EDS analysis revealed twice as much oxygen in the passive layer on the amorphous Zr69.5 Cu12 Ni11 Al7.5 than on the nanocrystalline ribbon. All the metals were homogeneously distributed in both layers. However, amorphous and nanocrystalline Zr–Cu–Ni–Al alloys were observed to be very sensitive against corrosion as compared to pure crystalline Zr. Furthermore, it was observed that the corrosion resistance of nanocrystalline Zr–Cu–Ni–Al did not improve compared to the amorphous state of the alloy or crystalline Zr. Therefore, it would be of great interest to find out whether Nb additions have a similar beneficial effect for a nanocrystalline microstructure as in the amorphous state. 4. Trends for further investigations Design of improved Zr alloys can proceed by, e.g. rapid quenching or ball milling leading to amorphous and nanocrystalline alloys as well as nanocomposites. It was shown that additives as well as a special microstructure can affect the corrosion resistance of both, the amorphous as well as the nanocrystalline Zr-based alloys. However, there are still a lack of understanding regarding the influence of microstructure on the corrosion resistance, especially in the case of nanocrystalline materials. Further investigations are necessary in particular on partially nanocrystalline materials as recently mentioned by Scully [65] regarding: (a) The effect of the nanocomposite structure, i.e. nanocrystals embedded in an amorphous matrix, (including lattice parameter changes and surface curvature effects,
Fig. 6. Passive film observed on a nanocrystalline Zr69.5 Cu12 Ni11 Al7.5 ribbon after salt spray test [61]: (a) TEM image and (b) electron diffraction pattern.
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