Composition and corrosion resistance of cerium conversion films on the AZ31 magnesium alloy and its relation to the salt anion

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1 Applied Surface Science 254 (2008) Composition and corrosion resistance of cerium conversion films on the AZ31 magnesium alloy and its relation to the salt anion M.F. Montemor a, *, A.M. Simões a, M.G.S. Ferreira a,b, M.J. Carmezim a,c a ICEMS DEQB, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, Lisboa, Portugal b CICECO, Universidade de Aveiro, Aveiro, Portugal c ESTSetúbal, Instituto Politécnico de Setúbal, 2910 Setúbal, Portugal Received 18 June 2007; received in revised form 20 July 2007; accepted 20 July 2007 Available online 6 August 2007 Abstract Pre-treatments based on different cerium salts were applied to the AZ31 Mg alloy. The pre-treatments were performed by immersion in solutions of various Ce(III) salts: cerium chloride, cerium nitrate, cerium sulphate and cerium phosphate. The chemical composition of the treated surfaces was investigated by X-ray photoelectron spectroscopy and Auger electron spectroscopy, whereas the corrosion behaviour of the pretreated AZ31 substrates was investigated in M NaCl solutions using potentiodynamic polarisation and open circuit potential monitoring. The surface film contained a mixture of Ce(IV) and Ce(III) salts. The film thickness depends upon the cerium salt used. The electrochemical results show that all the conversion pre-treatments reduced the corrosion activity of the AZ31 Mg alloy substrates in the presence of chloride ions. The corrosion protection efficiency is related with the anion present in the cerium salt. # 2007 Elsevier B.V. All rights reserved. Keywords: Magnesium; Cerium; XPS; AES; Corrosion 1. Introduction Magnesium alloys present very interesting properties that make them an attractive material for several industrial applications. Magnesium alloys present reduced density when compared to many other metallic materials, display good castability, easy machining and good recycling efficiency. Magnesium materials also present specific electromagnetic shielding properties, which widens the range of application to different electronic industry sectors [1]. Mg alloys spontaneously form a hydroxide layer in air. This layer is unstable in acid or neutral solutions but quite stable under alkaline conditions. Therefore, in nearly neutral conditions, even in the presence of a very small concentration of aggressive ions, there is dissolution of the Mg hydroxide film and corrosion develops at a high rate [2]. Suppression or reduction of the corrosion activity is therefore critical to improve the durability of the Mg alloys. In order to reduce the negative impact of corrosion it is necessary to protect the surface of the material. This is * Corresponding author. address: mfmontemor@ist.utl.pt (M.F. Montemor). usually achieved by forming a protective surface layer. Anodising is one of the most common procedures to reinforce the native oxide films and therefore to improve the corrosion resistance of the Mg alloys [2 4]. It has been demonstrated that corrosion resistance depends on both the current applied during anodizing and the pore structure of the anodic films. Silane treatments [5,6], sol gel coatings [7,8] and plasma vapour deposition (PVD) [9] are also surface modification procedures that can improve corrosion resistance of Mg and its alloys. These procedures add an extra layer formed over the native film, which effectively protects the underlying metallic substrate from corrosion. Among the possible alternatives for surface modification and improved corrosion resistance, deposition of conversion coatings has been studied and several formulations are proposed [10]. The most successful conversion treatments are based on the use of chromates and have been widely investigated in the past [11]. However, these formulations are now being abandoned due to the high toxicity of chromates and recent environmental legislation. Therefore, new systems, showing environmental friendliness, must be studied and developed. The deposition of conversion films from solutions of rare-earth salts has been considered a promising technical alternative for the pre-treatment of metallic /$ see front matter # 2007 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 M.F. Montemor et al. / Applied Surface Science 254 (2008) substrates. These films are usually obtained by dipping the metallic substrate in solutions containing the rare-earth salt. This procedure leads to the formation of a protective surface film that is reported to provide effective corrosion protection. The corrosion resistance of Mg substrates pre-treated with rare-earth salts formulation has been little explored in literature. The corrosion behaviour of magnesium substrates pre-treated with cerium, lanthanum and praseodymium conversion films was investigated by Rudd et al. [12]. The authors report that the conversion layers reduced the dissolution of magnesium in a ph 8.5 buffer solution. Improved corrosion resistance in NaCl solutions was also observed on AZ31 and AM50 Mg alloys pretreated with CeCl 3. The protective capability of the cerium conversion layers was improved by a first pre-treatment step using HCl [13]. Repeated immersion steps in the CeCl 3 /H 2 O 2 solutions also improved the corrosion protective behaviour of the conversion films [14]. In a previous work [15], it was demonstrated that conversion pre-treatments based on cerium nitrate or lanthanum nitrate could protect the AZ31 Mg alloy, the effectiveness of the conversion layer being dependent on the treatment time. In the present work, conversion films obtained by immersion of this alloy in solutions of different salts of trivalent cerium cerium nitrate, cerium chloride, cerium sulphate and cerium phosphate were investigated. The chemical composition of the conversion layers was studied by X-ray photoelectron spectroscopy (XPS) and by Auger electron spectroscopy (AES). The corrosion behaviour was assessed via d.c. polarisation and open circuit potential (OCP) monitoring. The results show that the chemical composition and the corrosion protection ability are dependent on the cerium salt used for the deposition of the conversion layers. Conversion layers obtained by immersion in CeCl 3 are the thickest and the most protective ones. The Auger spectra were taken using an electron beam accelerated at 10 kev and a target current of 5 na. Auger depth profiling was performed using a differentially pumped Ar + ion gun. The XPS spectra were taken in CAE mode (30 ev), using an Al (non-monochromated) anode and an accelerating voltage of 15 kv. The XPS results were treated using the procedure described elsewhere [15] Electrochemical measurements The open circuit potential measurements and the potentiodynamic polarization curves were performed using an AUTOLAB PGstat-20 apparatus. Open circuit potential measurements were carried on the samples immersed in the conversion bath, i.e. during conversion film formation and also on the samples treated for 3600 s after immersion in the aggressive M NaCl solution. Potentiodynamic polarization curves were obtained on the coupons treated for 3600 s in the conversion bath and afterwards immersed in M NaCl. The scanning rate was 1 mv/s, starting at the corrosion potential, in the anodic or cathodic directions. The area of the working electrode was 3.0 cm 2. A three-electrode cell, consisting of the working electrode, the saturated calomel electrode as reference and the platinum as counter was used. 3. Results and discussion 3.1. Study of the conversion film Fig. 1 shows the potential evolution for the AZ31 Mg alloy substrates immersed in the conversion baths for 3600 s. The 2. Experimental procedure 2.1. Materials and solutions AZ31 Mg alloy with the nominal mass composition of 96% Mg, 3% Al and 1% Zn was obtained from Goodfellow Metals (UK). The coupons were polished with SiC paper up to grit 2400, ultrasonically degreased using acetone, washed with deionised water (Millipore) and dried in air. Solutions of cerium nitrate, Ce(NO 3 ) 3, cerium chloride, CeCl 3, cerium sulphate, Ce 2 (SO 4 ) 3 (all three Sigma/Aldrich products) and cerium phosphate, CePO 4 (Alfa Aesar product), with concentrations of M were prepared. The ph of these solutions was approximately 6. The metallic coupons were immersed in the pre-treatment solution at room temperature (22 8C) for 10, 600 and 3600 s. Following immersion, the coupons were oven-dried at 80 8C for 30 min Analytical tests X-Ray photoelectron spectroscopy and Auger electron spectroscopy measurements were performed using a Microlab 310 F (from Thermo Electron former VG Scientific). Fig. 1. Open circuit potential evolution for the AZ31 coupons immersed in different cerium-containing conversion baths.

3 1808 M.F. Montemor et al. / Applied Surface Science 254 (2008) potential evolution was characterised by a drop within the first s of immersion, followed by an increase towards more positive values and tending towards stabilisation. The initial drop can be associated with the activation of the native magnesium oxide/hydroxide layer and the rise afterwards is due to the build up of the conversion layer. The values of the stable potential plateau were dependent on the cerium salt, the nobler readings being registered for the coupons immersed in cerium nitrate and the most negative values being observed in cerium chloride (approximately 1.55 V). Cerium phosphate and cerium sulphate lead to nearly identical potential values. Stabilisation of the OCP was faster for the CeCl 3 conversion bath. The coupons immersed in sulphate also revealed fast potential stabilisation, but the values were more positive than those observed in CeCl 3. For the coupons immersed in the CePO 4 bath there were more fluctuations on the potential and the stable plateau was ill defined. The OCP evolution during the conversion treatment suggests that the anion present in the cerium salt plays an important role in the build up of the conversion layer. Nitrate ion is commonly considered as anodic inhibitor and inhibitor of pitting corrosion, due to its oxidizing properties [16]. As usually happens with this type of inhibitors, increasing nitrate concentrations above a critical threshold may promote pitting corrosion in certain conditions [17,18]. Studies performed on zinc electrodes revealed that the presence of nitrate stimulates the formation of a zinc oxide protective layer [17]. In the conditions of the present work the potential evolution in the cerium nitrate bath suggests that nitrate ions promote oxidation of magnesium and aluminium, stimulating the growth of a layer of magnesium and aluminium oxide/hydroxides simultaneously with the build-up of the conversion film, as proposed in the mechanism discussed elsewhere [15]. Concerning the role of the chloride ion in the formation of the conversion layer it seems that this anion has a stronger effect in the first seconds of film deposition, as seen by the more negative readings at the early stages of the conversion treatment. The chloride ion also shows oxidising properties, promoting activation of the surface. Edington et al. [19], found that small amounts of chloride ion may affect the polarization of copper by destabilizing copper oxide, favouring cerium oxide precipitation. The rapid stabilisation of the open circuit potential at more negative values suggests a faster growth of the cerium conversion layer. Rudd et al. [12] also observed that the previous activation step with HCl improved the properties of the cerium conversion layer. The evolution of the OCP in the cerium phosphate and cerium sulphate baths was similar to the evolution reported for nitrate, however, more cathodic potential values were observed at longer immersion times. Based upon these results, the samples treated for 10, 600 and 3600 s were studied by XPS in order to assess the composition of the surface film formed during the conversion treatment. The Ce3d spectra for cerium compounds exhibit complex features related to hybridization with ligand orbital and partial occupancy of the valence orbital 4f. A typical Ce3d spectrum showing contributions from Ce(III) and Ce(IV) is depicted in Fig. 2. Ce 3d spectra evidencing the main peaks present in a surface containing Ce(III) and Ce(IV) species (see also Table 1). Fig. 2. The peak positions and the peak assignments are presented in Table 1. An important feature for the identification of the cerium is the presence of satellite peaks. Ce(IV) is clearly recognised by the satellite peak at approximately ev, which arises from a transition of the 4f 0 initial state to the 4f 0 final state and is exclusive of the presence of Ce 4+ [20,21]. Quantification of the cerium forms was reached after fitting procedures and was based on the fitting results obtained for the Ce3d 5/2 ionisation (between 875 and 894 ev) as described elsewhere [15]. The Ce3d and the O1s spectra obtained for the samples treated in the different cerium baths after distinct times are depicted in Figs After 10 s of immersion (Fig. 3(a)), the order of the relative intensities for the Ce3d peak was: PO 4 3 < NO 3 3 < SO 4 2 < Cl At this stage, the O1s ionisations showed a main peak at approximately ev, due to OH. These reflect the presence of Mg(OH) 2, since the contribution of magnesium is intense, but also of cerium hydroxides. Table 1 Ce3d characteristic peaks (as depicted in Fig. 2) Peak Binding energy (ev) Species u Ce(IV) Ce3d 3/2 v Ce(IV) Ce3d 5/2 u Ce(III) Ce3d 3/2 v Ce(III) Ce3d 5/2 u Ce(IV) Ce3d 3/2 v Ce(IV) Ce3d 5/2 u Ce(IV) Ce3d 3/2 v Ce(IV) Ce3d 5/2

4 M.F. Montemor et al. / Applied Surface Science 254 (2008) Fig. 3. XPS spectra obtained on the coupons treated for 10 s in various cerium salt solutions: (a) Ce3d and (b) O1s ionisations. After 600 s of treatment (Fig. 4), an increase of the cerium intensity was observed, especially for the film formed in the phosphate bath. Simultaneously, the O1s peak revealed an increased contribution of oxides, characterised by the growth of a shoulder in the low binding energy side, at approximately ev. This increased oxide contribution is likely to be due to the presence of CeO 2 since Ce(IV) predominates over Ce(III) (Fig. 4a). This result is more evident for the films formed in CeCl 3 and Ce(NO 3 ) 3, which are also the films that present a higher Ce(IV) content in the Ce3d spectra, at 883 ev. The O1s peak for the coupon treated in the Ce 2 (SO 4 ) 3 bath reveals essentially the presence of hydroxides, suggesting that the growth of the cerium oxide is delayed in the presence of sulphate ions. For 3600 s of conversion treatment, stable potential values were observed for all the coupons. For this treatment time the XPS spectra were nearly identical and the cerium content had increased for all the samples (Fig. 5). There is an important Fig. 4. XPS spectra obtained on the coupons treated for 600 s in various cerium salt solutions: (a) Ce3d and (b) O1s ionisations. enrichment in Ce(IV), whereas the peaks assigned to Ce(III) became weaker. The oxide contributions are also intense, especially for the CeCl 3 treated coupons, for which the oxide contribution predominated over hydroxides. Fig. 6(a) shows that for all the conversion films there was an increase of the ratio cerium (IV)/Ce total with increasing immersion time and a substantial increase of the amount of cerium relatively to magnesium and aluminium, revealing an increase of the degree of surface coverage (Fig. 6(b)). This effect is more pronounced for the films treated in CeCl 3 and Ce(NO 3 ) 3. Aluminium was detected only for the very short treatment times, suggesting that this element plays some role in the growth of the first layers of the surface film, probably because it dissolves immediately after immersion in the conversion bath and precipitates as aluminium hydroxide together with the first layers of the cerium film as proposed elsewhere [22]. No traces of nitrate or chloride could be found in the surface of the conversion films. However, small contributions of

5 1810 M.F. Montemor et al. / Applied Surface Science 254 (2008) Fig. 5. XPS spectra obtained on the coupons treated for 3600 s in various cerium salt solutions: (a) Ce3d and (b) O1s ionisations. phosphate were detected on the conversion film formed for 10 s (Fig. 7(a)). A contribution from two different forms of sulphur was found (Fig. 7(b)) in the surface of the coupons treated with cerium sulphate, for all the treatment times. The P2p spectra (Fig. 7a) revealed a weak contribution at approximately 134 ev, which can be assigned to the presence of phosphates in the conversion film. The S2p ionisation reveals two peaks: one at approximately 164 ev that is due to the presence of sulphite (SO 3 2 ) and another one at higher binding energies, 169 ev, assigned to the presence of sulphates. The presence of sulphite can be explained by the basic character of the sulphate ions, which in aqueous solution may lead to the formation of the sulphide anion in an acid basic reaction. The in-depth composition and the thickness of the conversion layers formed during 3600 s of treatment was assessed by Auger depth profiling (Fig. 8). The results show that the outer layers of conversion films formed in CeCl 3 and Fig. 6. (a) Evolution of the Ce(IV)/Ce total ratio with the conversion time. (b) Evolution of the Ce total /(Ce total + Mg + Al) ratio with the conversion time. Ce(NO 3 ) 3 are composed of cerium and oxygen. Contrarily, the films formed in CePO 4 and Ce 2 (SO 4 ) 3 revealed the presence of magnesium in the outer layers, suggesting a film with more defects and/or a lower degree of coverage in agreement with the XPS results (Fig. 6(b)). The thickness of the cerium-rich layer is clearly dependent on the conversion bath. The thickest film is the one formed with CeCl 3 whereas the thinner one is that formed with CePO 4. The CeCl 3 film is about two and four times thicker than the Ce(NO 3 ) 3 and CePO 4 films, respectively. The depth profile obtained on the coupons treated in Ce 2 (SO 4 ) 3 revealed the presence of sulphur over the entire film thickness. This results shows that the sulphite and sulphate species are incorporated in the conversion layer. This result agrees with previous literature [23], where it is reported that sulphate ions can incorporate in the conversion

6 M.F. Montemor et al. / Applied Surface Science 254 (2008) chemical composition of the film. The films formed in the CeCl 3 are richer in Ce(IV) and grow faster, providing a higher degree of coverage when compared with those formed in CePO 4. For longer conversion treatments time the composition became less dependent upon the cerium salt. However, the films thickness is affected by the nature of the cerium salt. The growth of the conversion layer is intimately related with the ph changes that occur in the metallic substrate during immersion in the conversion bath and with the formation of cerium compounds with different valence states. Although, all the solutions presented identical ph values (6) at the beginning of the conversion treatment, during the build up of the conversion film different steps may occur as proposed in a previous work [15]: 1. Dissolution of the native oxide layer, accompanied by formation of hydroxyl ions and ph rise. 2. Growth of the first layers composed of hydroxides of Ce(IV) and of Ce(III), mixed with Mg and Al hydroxides during the first instants of immersion. 3. Thickening of the surface film and slowdown of the ph changes with preferential deposition of Ce(OH) 4 and its conversion into CeO 2, forming an outer layer, richer in Ce(IV) species. The growth of the first layers and film thickening is clearly favoured for the coupons immersed in CeCl 3 and delayed in CePO 4 and Ce 2 (SO 4 ) 3. This behaviour suggests that chloride ions induce faster surface attack and ph rise when compared to the action of phosphates Corrosion behaviour of the pre-treated substrates Fig. 7. XPS spectra for the P2p and S2p ionisation obtained on the coupons treated in cerium phosphate for 10 s (a) and cerium sulphate for 10, 600 and 3600 s (b), respectively. layer, giving a fined-grained conversion layer on galvanized steel. The presence of SO 4 2 in the conversion solution forms a complex between Ce 3+ and SO 4 2, which causes the incorporation of SO 4 2 in the cerium conversion coatings during the conversion process. The following reaction was proposed to explain [23] the formation of cerium sulphate hydroxide: Ce 3þ þ 1 2 SO 4 2 þ 2OH! CeðOHÞ 2 ðso 4 Þ 0:5 The Ce(III)-hydroxide-sulphate formed at the surface can be oxidized to Ce(IV) hydroxide-sulphate by dissolved oxygen in the solution [23]. The results observed in the present work revealed that not only sulphates but also some sulphite or oxygen deficient forms of sulphate anions are also formed, being incorporated in the conversion layer. The analytical results show that for short treatment times the nature of the cerium salt plays an important role in the The corrosion resistance of the pre-treated coupons was investigated only for the conversion films formed for 3600 s of treatment in the different cerium baths. Fig. 9 depicts the evolution of the OCP during immersion in aggressive M NaCl solutions. For the blank sample there is an asymptotical increase towards more anodic values due to the simultaneous precipitation of magnesium and aluminium hydroxides that ennoble the potential. For the coupons treated in CeCl 3, CePO 4 and Ce(NO 3 ) 3, the evolution of the potential is characterised by a rise to more positive values during the first seconds of contact with the NaCl solution, followed by a slower drop. All the curves reached a minimum before it increased again towards nobler and more stable values. The potential rise during the first instants of immersion can be due to activation of the metallic substrates in the presence of aggressive NaCl solution. The drop afterwards reveals cathodic protection and the minimum potential values are independent on the nature of the salt used. The rise towards more anodic and stable values was slower for the CeCl 3 treated substrates, suggesting that the formation of magnesium corrosion products is delayed comparatively to the blank substrate. This trend can be consequence of the thicker film and larger degree of coverage as reported in the previous section.

7 1812 M.F. Montemor et al. / Applied Surface Science 254 (2008) Fig. 8. Auger depth profiles obtained for the different conversion films after 3600 s of treatment in the cerium conversion bath. The coupons treated in Ce 2 (SO 4 ) 3 revealed a distinct behaviour; the maximum is not so defined and seemed to develop after longer times. This may indicate that activation of the surface is delayed in the presence of the sulphate-containing cerium oxide/hydroxide conversion film. The potentiodynamic polarisation curves obtained for the blank substrate and for the substrates pre-treated during 3600 s are depicted in Figs. 10 and 11. The anodic polarisation curves obtained with the blank coupon showed very high current densities due to fast dissolution of the magnesium substrate. Fig. 9. Open circuit potential evolution for the AZ31 coupons immersed in NaCl M. The coupons were treated for 3600 s in the different conversion baths. Fig. 10. Anodic polarization curves for the AZ31 coupons immersed in NaCl M. The coupons were treated for 3600 s in the conversion baths.

8 M.F. Montemor et al. / Applied Surface Science 254 (2008) Fig. 11. Anodic polarization curves for the AZ31 coupons immersed in NaCl M. The coupons were treated for 3600 s in the conversion baths. The current densities showed a marked decrease for the pretreated coupons, revealing inhibition of the anodic processes. Simultaneously the shape of the curves in the Tafel region was affected, revealing changes in the kinetics of such processes. The Tafel slopes increased and the current densities were similar for all the pre-treated coupons. Among the pre-treated coupons, the one showing the higher current densities was the substrate treated in cerium nitrate. The cathodic polarisation curves (Fig. 11) are also affected by the pre-treatment salt. The curves are characterised by a limiting current density due to oxygen reduction that decreases in the presence of the pretreatment. The polarisation effect is more marked for the CeCl 3 treated samples. These reductions can be consequence of the lower fraction of area available for the corrosion reactions due to the presence of the conversion film. The OCP evolution and the polarisation curves show that the conversion film shifts the corrosion potential towards more cathodic values comparatively to the blank substrate, revealing that the cerium oxides and hydroxides conversion layer can provide significant inhibition of the cathodic processes as generally reported in literature [24 27] for these conversion films. Magnesium undergoes strong corrosion activity upon immersion in neutral NaCl solution. In this solution the anodic reactions are magnesium and aluminium oxidation and the cathodic reaction can be hydrogen evolution or oxygen reduction. The reactions between the metallic cations and the hydroxyl ions lead to the formation of insoluble corrosion products, mainly Mg(OH) 2 and Al(OH) 3. Precipitation of these products leads to a displacement of the potential towards more positive values, as observed in Fig. 9. In the presence of the conversion coating there is a significant delay of these processes and there is a marked inhibition of the corrosion processes. The corrosion mechanism of the AZ31 substrates pretreated with the cerium conversion films during immersion in NaCl solution can be described by the following steps: (i) adsorption of chloride ions and localised dissolution of the conversion film. This step is likely to be associated with the potential rise observed during the first instants of immersion and that depended upon the nature of the conversion film, being delayed for the films formed in cerium nitrate and cerium sulphate; (ii) the second step is likely to reflect the protection provided by the conversion coating. Literature [24 27] generally reports that rare-earth based conversion coatings provide cathodic inhibition due to the precipitation of oxides and hydroxides at the cathodic sites. In fact, the shift of the potential towards more cathodic values is likely to reflect this type of protection; (iii) the third step can be considered the dissolution of the conversion layer and corrosion activity in a more generalised way. This process is accompanied by a potential rise towards more positive values and latter stabilisation. These values are more negative for the films formed in cerium chloride and more positives for the ones formed in cerium phosphate and sulphate. The last two steps seem to depend on the chemical composition of the conversion film, degree of coverage and thickness. The slower potential drop and the attenuated rise in the anodic direction, reflecting the build up of corrosion products, is delayed in the presence of the film formed in cerium chloride, which is the film containing more oxides and also the one with the highest thickness. This conversion film provides a more effective barrier as confirmed in the polarisation curves. 4. Conclusions The pre-treatment of the AZ31 Mg alloy in different cerium salt solutions leads to the formation of a conversion film, whose thickness depends upon on the cerium conversion bath; the thicker film was the one formed in the CeCl 3 bath and the thinner one that formed in the CePO 4 bath. During the early stages of treatment the cerium conversion films have a solution and time-dependent composition. The films formed in CeCl 3 solutions are richer in Ce(IV) when compared to those formed in CePO 4 and Ce 2 (SO 4 ) 3 solutions. At longer treatment times (3600 s) the composition becomes nearly identical, with Ce(IV) oxides predominating over hydroxides in all the conversion films, however the films formed in CePO 4 and Ce 2 (SO 4 ) 3 revealed a higher content of magnesium in the surface. The pre-treatment in the Ce 2 (SO 4 ) 3 solutions lead to the formation of cerium conversion films containing sulphate and sulphite ions. The conversion films formed for 3600 s in the cerium salt solutions provide protection of the AZ31 Mg alloy substrates. The corrosion protection mechanism involves a clear displacement of the corrosion potential towards more cathodic values and a significant decrease of both anodic and cathodic current densities. Acknowledgement POCTI/CTM/59234/2004; Thermo Electron.

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