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Corrosion protection of Nd–Fe–B type permanent magnets by zinc phosphate surface conversion coatings
Intermetallics 9 (2001) 515–519 www.elsevier.com/locate/intermet
Corrosion protection of Nd–Fe–B type permanent magnets by zinc phosphate surface conversion coatings H. Balaa, N.M. Trepakb, S. Szymurac,d,*, A.A. Lukine, V.A. Gaudynf, L.A. Isaichevab, G. Pawowskaa, L.A. Ilinab a
Department of Chemistry, Technical University of Czcec˛stochowa, 42-200 Czcec˛stochowa, Poland b Saratov University, Saratov, 410026, Russia c Department of Physics, Technical University of Opole, 45-370 Opole, Poland d Institute of Physics, University of Opole, 45-052 Opole, Poland e Centre of Magnetic Technologies, Moscow, 117246, Russia f Insitute of Chemistry, Pedagogical University of Czcec˛stochowa, 42-200 Czcec˛stochowa, Poland Received 13 April 2001; accepted 18 April 2001
Abstract The use of protective zinc phosphate top coatings to protect Nd–Fe–B type permanent magnets against corrosion is discussed. The progress of the phosphatisation process has been tested by simultaneous measurement of pH near surface, corrosion potential, substrate mass loss and phosphate coating mass gain. The corrosion behaviour of the magnet in phosphate solution was analyzed by the anodic polarization technique and the general resistance of the magnet to corrosion was evaluated by the Akimov drop test. It is shown that immersion of the magnet in the acidified (pH=2) phosphating solution containing Zn(II), nitrate and fluoride ions resulted in the formation of well adhered, thin and tight hopeite layers, causing prolonged corrosion resistance of the magnet. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Rare-earth intermetallics; B. Corrosion; G. Magnetic applications
1. Introduction A poor corrosion resistance of Nd–Fe–B type magnets in many aggressive environments [1–3] is associated with the presence of 35 wt.% of neodymium in their composition. Neodymium, along with other rare earth (RE) elements belongs among the most electrochemically active metals (E O Nd3þ =Nd ¼ 2:431V) [4]. The Nd–Fe–B type magnets are highly susceptible to attack from both climatic and corrosive environments, with this attack resulting in corrosion of the alloy and deterioration in both its physical and magnetic properties. Numerous attempts to improve corrosion behaviour of Nd–Fe–B type magnets by alloy additions [5–7], metallic coatings [8–10], surface amorphisation [11] or stabilisation of grain boundaries [12] have not appeared to be very effective. Organic coatings seem to be the
* Corresponding author. E-mail address: [email protected] (S. Szymura).
most promissing in protection of Nd–Fe–B type magnets against atmospheric corrosion [10,13–15]. It has been established that application of organic coatings is the most effective on various types of primers, among which zinc phosphate coatings are especially advantageous [16,17]. The organic coating adhesion is improved then, because the zinc phosphate conversion coating provides a clean, uniform grease-free surface. In addition, adhesion is improved through an increase of surface area of the substrate thus leading to an increase of the possibility of bond formation. A dense uniform zinc phosphate conversion coating provides a stable insulating layer which inhibits corrosion if the paint film is damaged [16]. Attempts to protect the Nd–Fe–B magnets by surface phosphatisation are rather rare in the literature. Chin et al. [18] noticed that Nd–Fe–B type magnets undergo passivation in 10% H3PO4 solution. These magnets did not exhibit similar behaviour in 0.5 M H2SO4 or in Ringer solution (synthetic saliva). Pawlowska et al. [19] found the particular ability of neodymium to undergo passivation in 0.5 M phosphate
0966-9795/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(01)00035-8
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solutions, especially when pH52. The effectiveness of Nd passivation slightly decreased in the presence of F and NO 3 ions in the phosphating bath [19]. Kim [20] pointed out that a Nd–Fe–B type magnet treated with H3PO4 solution, or its combination with H2CrO4 and HF, exhibited a significant improvement in corrosion resistance as compared with the untreated one. Costa et al. [21] reported that the immersion of RE– Fe–B type magnets in diluted NaH2PO4 solution produced on the magnet surface a protective film consisting of phosphate/pyrophosphate and oxide which caused prolonged corrosion protection of the magnet during long storage period. The authors suggested the possibility of using this treatment for corrosion protection or as a pre-treatment step before organic coating application. Ribitch [16] described the commercial zinc phosphate coating processes (Parker/Amchem Bounderite bath) on Nd–Fe–B type magnets, followed by cathodic epoxy electrocoating. Attempts to apply a zinc phosphate coating to the surface of Nd–Fe–B magnets were not successful until the surface oxide film had been removed by a mild acid solution. Removal of surface oxides by etching in sulphuric acid solution led to substrate activation and rendered it phosphatable. The author also described the effect of fluoride ions addition to phosphating bath. As the F ions concentration increased, the coating weight on the Nd–Fe–B substrate distinctly decreased [16]. Some cations present in zinc phosphate coating baths (e.g. Al3+, Ti3+, Fe3+) clearly worsened the quality of the final phosphate coatings. Addition of F ions, forming stable complexes with many metals, effectively eliminated the poisoning effect of these cations [22]. Phosphatisation is an important method of protection against corrosion widely applied for many iron-base alloys and several other industrial metallic materials [22– 25]. The zinc phosphate conversion coatings are usually obtained by metal immersion in acidified (pH 2) Zn(H2PO4)2 solution. During the phosphatisation process certain complicated electrochemical and chemical processes take place, including the active dissolution of the metallic surface layers together with the depolarizer cathodic reduction processes, leading to an increase of pH at the surface. This favours hydrolysis of the soluble Zn(H2PO4), and causes deposition of hydrated Zn3(PO4)2 (hopeite) on the metal surface: þ ( 3Zn2þ þ 2H2 PO 4 + Zn3 ðPO4 Þ2 þ4H :
ð1Þ
The equilibrium of the reaction (1) depends strongly on temperature and on pH (the deposition is possible when pH>2). The equilibrium constant of the reaction (1) is 0.013 at 25 C and 0.74 at 98 C [23]. Simplifying the matter somewhat, the surface layer consists of insoluble hopeite, Zn3(PO4)2.4H2O, for which a solubility
product is Lhopeite=1033. There exists also conception that the zinc phosphate conversion layer crystallises on the thin, inner iron oxide layer [26]. Although phosphates of Fe(II) and Fe(III) also show very low solubilities, the ferric ions form a stable complex with phosphate ions [the stability constant of Fe(HPO4)+ complex is on the order of 108]. As a result, iron does not passivate in phosphoric acid solutions [26]. Introduction of NO 3 ions into a phosphating bath on the one hand oxidizes Fe2+ ions to Fe3+ and on the other, leads to a change in the character of cathodic þ processes, including reduction of NO 3 to NH4 [23]. Observations show that the phosphatisation of the iron alloys can proceed at not very high temperatures in the presence of NO 3 ions, which is an important advantage of this addition [22,23]. The main purpose of the present paper is to evaluate the usefulness of phosphatisation in effective protection of high-coercivity Nd–Fe–B type magnets against corrosion. The mechanism of the corrosion process of Nd– Fe–B type magnets consists in preferential oxidation of the most electrochemically active phase (the so-called Nd-rich phase — Nd4Fe) situated between grains of ferromagnetic phase (Nd2Fe14B) [2,3,27]. As was found by Pawlowska et al. [19] pure neodymium easily passivates in weak acid (pH 2) phosphate solutions. Taking into account that Nd does not undergo passivation in acidified solutions containing other anions, e.g. in sulphate solutions (pH=1–4) [28], one can conclude that the protective layer on Nd in phosphate solution may contain insoluble NdPO4 (the solubility product of this compound is LNdPO4=1023). 2. Experimental details Tests were carried out on the high-coercivity sintered (Nd,Dy)–(Fe,Ti)-B magnet (wt.%: Nd 32.0, Dy 1.0, Fe 64.7, Ti 1.0 and B 1.3) in the demagnetized state. The rectangular samples (10108 mm) were prepared by the conventional powder metallurgy route as described in Refs. [5,29,30]. Their magnetic properties were as follows: remanence Br=1.28 T, intrinsic coercivity iHc=1100 kA/ m and the energy product (BH)max=310 kJ/m3. The anodic potentiokinetic polarisation curves for the tested magnets were measured in air-saturated 0.5 M phosphate solution with pH=2 and at a temperature of 45 C. The polarisation tests were performed both for phosphate solutions containing 0.1 M NO 3 ions (introduced as NaNO3) and for phosphate solutions containing 0.1 M NO ions [introduced as 3 ions and 0.1 M F (NH4)2F2]. For comparative purposes the polarisation tests were made also on the spectroscopically pure metals, iron and neodymium. An electrochemical cell with a working volume of 50 cm3 was equipped with a Luggin’s capillary and a platinum rod
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auxiliary electrode. The working electrodes had an operating surface area of 1 cm2. The anodic polarisation curves were carried out starting with a stationary corrosion potential of the sample and going up to a potential of +0.3 V with a potential sweep rate of 2 mV/s. All potentials in the present paper were measured versus a standard Ag/AgCl electrode but their values are expressed in a standard hydrogen scale. The phosphatisation process of the magnet samples was carried out by immersion of the samples in one of two alternative baths: Bath A was a basic solution containing: 1.5 M H3PO4, 0.75 M Zn2+ and 0.1 M NO 3 with pH=2, achieved by suitable addition of NaOH. Bath B had a similar chemical composition but it contained an addition of 0.1 M NO 3 ions [introduced in the form of (NH4)2F2]. Prior to immersion in the phosphating bath the specimen surface was polished on waterproof emery paper (No. 800), rinsed with water, then with ethanol and finally dried. After drying, the sample was weighed to an accuracy of 0.1 mg. The process of sample phosphating lasted 30 min and after this time neither further increase of the thickness of the zinc phosphate layer, nor mass changes of the sample were observed. The phosphatisation process was carried out in non-stirred solutions in contact with air. During the phosphatisation process both corrosion potential (Ecorr) and near-electrode pH value (pHs) were continuously measured. The pHs was detected using a glass microelectrode (0.6–0.8 mm2) situated in 40–60 mm distance from the electrode surface (the sides of the glass electrode were isolated from the solution with paraffin wax and the shift of the electrode was realized by micrometer screw). The mass of the dissolved material substrate per surface unit ( m/S) and the surface mass of the zinc conversion phosphate layer ( mf /S) were determined after 1, 3, 5, 10, 15, 20 and 30 min of sample exposure in the phosphating bath on the basis of gravimetric measurements: mass gain after phosphating and mass loss after removal of the phosphate layer (etching in 5% aqueous solution of CrO3). In analogous intervals of time, the corrosion resistance of the zinc phosphate conversion coating was determined using the long-established Akimov drop test [31]: a drop of the test solution containing 0.26 M CuSO4, 0.56 M NaCl and 0.0024 M HCl (20 ) was placed on the coating (rinsed with water, then with alcohol and dried), and the time x (the resistance time) was measured until the colour of the drop changed from blue to red (as a result of corrosion of iron-base substrate and copper deposition). The longer the time x, the better protective properties of the coating.1
1 The zinc phosphate conversion coating on carbon steel substrate shows ‘‘medium protective properties’’ when 5 120 s; and it shows ‘‘increased protective properties’’ when x 5 300 s [31].
3. Results and discussion In Fig. 1a, the changes of corrosion potential (Ecorr) and near-surface pH versus phosphatisation time are presented. Fig. 1b presents mass loss of the magnet substrate ( m/S), mass gain of the formed phosphate layer per surface unit ( mf /S) and corrosion resistance of the coating characterized by the resistance time (x ) as a function of time of immersion in the phosphating solution A. Directly after immersion of the magnet into solution A the Ecorr shows positive values, then it rapidly drops, and after 1 min of exposure, it reaches a minimum value ( 0.24 V). Further exposure of the magnet to Bath A causes slow increase of the corrosion potential and, after 15–20 min, it reaches practically stationary value of 0.15 V. As it is seen from Fig. 1a, after initial short-lasting (20 s), rapid decrease of pHs, the near-surface pH value gradually increases and stabilizes on the level of pHs 3 after 15 min of phosphatisation. The initial drop of pHs (to the value of 1) can be attributed to the reaction of the surface oxide layer or adsorption of dihydrophosphate ions, e.g.: 2M þ 3H2 O 6e ! M2 O3 þ 6Hþ M þ 3H3 PO4 ! M H2 PO 4
3;ads
þ3Hþ
ð2Þ ð3Þ
where M symbolizes the metallic magnetic material. Further increase of pHs results from an active anodic substrate dissolution balanced by hydrogen ions and
Fig. 1. Dependence of near-surface pH and corrosion potential of the (Nd, Dy)–(Fe, Ti)–B magnet (a) and substrate mass loss as a result of etching ( m/S), mass gain as a result of zinc phosphate layer formation ( mf /S) and time of substrate protection against corrosion ( x) determined by Akimov test (b). Phosphating bath (A): 1.5 M H3PO4, 0.75 M Zn2+, 0.1 M NO 3 ; (pH=2), 45 C.
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dissolved oxygen depolarisation processes — both accompanied with consumption of H+ ions: Anodic reaction: M 3e ! M3þ
ð4Þ
Cathodic reactions: 2Hþ þ 2e ! H2
ð5Þ
O2 þ 4Hþ þ 4e ! 3H2 O
ð6Þ
Increase of pHs to values of 3 facilitates deposition of hopeite according to the reaction (1) and, possibly, NdPO4. After 15 min of exposure, the pHs slightly decreases with time, although this dependence is not generally reproducible, and therefore it is not presented in Fig. 1a. After 15 min of phosphating, the zinc phosphate layer reaches 92% of its final mass; simultaneously, the mass of etched substrate material after this time consists of 89% of its final mass. Thus, despite the relatively significant thickness of the zinc phosphate layer (its surface mass is 24 g/m2), it does not sufficiently inhibit the substrate dissolution process. The corrosion resistance of the layer, represented by time x, continuously increases with the progress of the phosphatisation process; however, the final resistance of the coating is not very high: after 30 min of exposure x ffi 100 s. The main reason of the poor protective properties of the zinc phosphate conversion coating obtained in solution A seems to be its considerable porosity. In Fig. 2, the anodic potentiokinetic polarisation curves of the magnet in tested aerated phosphate solution (pH=2) in the absence or in presence of 0.1 M F
Fig. 2. Anodic potentiokinetic polarisation curves of the tested (Nd, Dy)–(Fe, Ti)–B magnet and pure elements: Fe and Nd in 0.5 M PO3 4 +0.1 M NO3 (pH=2) solution without (solid line, A) and with addition of 0.1 M F ions (B, dashed line). Experimental conditions: 2 mV/s, no stirring, 45 C.
ions are shown. For comparative purposes, corresponding curves for pure metals, Fe and Nd, are also presented in the figure. It can be seen from Fig. 2 that the addition of F ions scarcely worsens the passivity of neodymium and slightly inhibits the active dissolution of iron. However, the presence of fluoride ions in the phosphate solution clearly influences the anodic behaviour of the (Nd, Dy)–(Fe, Ti)–B magnet. In the range of potentials close to 0 V, the F ions enhance the anodic oxidation process of the magnet 10 times. At potentials less than 0.3 V, the situation is reversed and the active dissolution process rate in the presence of F ions is noticeably slower than in a pure phosphate solution. One can notice by comparing polarisation curves of Fe, Nd and (Nd, Dy)–(Fe, Ti)–B magnet, that, in the presence of fluoride ions, the anodic behaviour (active dissolution) of the magnet becomes rather similar to that of pure iron whereas, in the absence of F ions, the behaviour of the magnet (tendency to passivation) is similar to that of neodymium. It can be assumed that, in the presence of F ions, the tested magnet corrodes with preferential dissolution of iron, owing to the formation of stable FeF3n complexes (Nd does not form any stable comn plexes with F ions). Consequently, the surface layer of the magnet becomes enriched with Nd when the potential increases. At potentials greater than 0 V, the surface is enriched with Nd to such an extent that further anodic polarisation decreases the anodic current density towards the values typical for neodymium. One can expect that, during phosphatization of the Nd–Fe–B type magnets in solutions containing fluoride ions, the hopeite layer is formed on the surface enriched with Nd. It is possible that neodymium reacts with phosphoric acid or phosphate ions to produce a thin insoluble NdPO4 layer, directly adhering to the metal substrate (LNdPO4 ¼ 1023 ). The presence of NdPO4 in the hopeite phase has been confirmed by XRD analysis of the samples phosphated in solution containing F ions. The existence of a thin interphase containing Nd, situated between magnet substrate and hopeite layer, should affect the properties of the final coating. The protective properties of phosphate coatings produced on the surface of the tested magnets by immersion in baths without F ions (Bath A) and with the addition of 0.1 M F ions (Bath B) are compared in Fig. 3. As can be seen from Fig. 3a, the presence of fluoride ions in the phosphating solution results in a decrease of the surface mass of the layer by 30%, and it leads to a distinct increase of corrosion resistance of the layer when compared to the solution without F ions ( x,B 4 x,A, compare to Fig. 3b). Addition of F ions additionally shortens the time necessary for the coating formation: after 15 min of magnet exposure in Bath B, the zinc phosphate conversion coating is practically completed, very tight and well adhered to the magnet substrate.
H. Bala et al. / Intermetallics 9 (2001) 515–519
Fig. 3. Mass gain per surface unit as a result of zinc phosphate layer deposition on the surface of the tested magnet (a) and the coat protection time ( x) determined by Akimov test (b) versus exposure time 2+ +0.1 M NO in 1.5 M PO3 4 +0.75 M Zn 3 (pH=2) solution without (bath A, solid line) and with addition of 0.1 M F ions (bath B, dashed line).
4. Conclusions The investigation of the usefulness of phosphatisation in view of effective protection of Nd–Fe–B type magnets against corrosion has revealed the following results: 1. zinc phosphate conversion coating satisfactorily protects Nd–Fe–B type permanent magnets against corrosion; 2. addition of 0.1 M of fluoride ions into the phosphating bath is very advantageous in terms of the coating corrosion resistance, its thickness and tightness; 3. the positive role of F ions in the effectiveness of Nd–Fe–B type magnet phosphating process presumably consists in preferential dissolution of iron in the initial stages of the process, enrichment of the metal surface with Nd and formation of well adhered layer (most likely consisting of neodymium phosphate) covered by a basic hopeite coat.
References [1] Jacobson J, Kim A. J Appl Phys 1987;61:3763. [2] Mitchell P. IEEE Trans Magn 1990;26:1933.
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[3] Warren GW, Gao G, Li Q. J Appl Phys 1991;70:6609. [4] Pourbaix M. In: Atlas of electrochemical equilibria in aqueous solutions. Bruxelles: NACE, Houston et Cebelcor, 1975. [5] Bala H, Pawlowska G, Szymura S, Sergeev VV, Rabinovich YuM. J Magn Magn Materials 1990;87:L255. [6] Nozieres JP, Taylor DW, Bala H, Szymura S, Malik M, Stoklosa H. J Alloys Compounds 1992;186:201. [7] Bala H, Malik M, Szymura S. Metallurgy Foundry Eng 1992;18:221. [8] Ahmed A, McGuiness PJ, Harris IR. IEEE Trans Magn 1990;26:2625. [9] Hamada T, Hayakawa T, Matsuura Y. Process for producing magnets having improved corrosion resistance. US Patent No. 4837114 (1989). [10] Bala H, Geˆga J, Szymura S, Sergeev VV, Vetrov VV. Powoki Ochronne 1993;32:121–2 (in Polish). [11] Armostrong JV, Coey JMD, Lunney JG, Lyons MEG. Hyperfine Interactions 1989;46:467. [12] Sagawa F, Hirosawa Y. European Patent Appl. 87111257.9 (1987). [13] Cheng CW, Man HC, Cheng FT. IEEE Trans Magn 1997;33:3910. [14] Tokuhara K, Hirosawa S. Appl Phys 1991;69:5521. [15] Hartmann M, Witter K, Reck J, Tolle HJ. IEEE Trans Magn 1986;22:943. [16] Ribitch RW. In: Proc. XI International Workshop on Rare Earth Magnets and their Applications. Carnegie Mellon University Pittsburgh, 21–24 October 1990, paper No. W.3.1. [17] Malakhov AI, Tyutina KM, Korroziya i osnovy galvanostegii. Khimiya, Moskva, 1977. [18] Chin TS, Chang RT, Tsai TW, Hung MP. IEEE Trans Magn 1988;24:1927. [19] Pawlowska G, Bala H, Kucharska B. In Mater. VI Ogolnopol. . Symposium N-T ‘‘Nowe Osicca˛ gnicce˛cia w Badaniach i Inzynierii Korozyjnej’’. Poraj, 15–17 November 2000, p. 37 (in Polish). [20] Kim AS. In: ASM World Materials Congress. McCormick Place, Chicago, 24–30 September 1988, J Mater Eng 1989;11:95. [21] Costa I, Sayeg IJ, Faria RN. IEEE Trans Magn 1997;33:3907. [22] Kubicki J. In: Pracownia Techniki Przeciwkorozyjnej. WSiP, Warszawa, 1977 (in Polish). [23] Lipkin YA, Shtanko VM.In: Kchimicheskaya i elktrokchimicheskaya obrabotka stalnykh trub. Metallurgiya, Moskva, 1974. [24] Machu W. Werkstoffe Korros 1963;14:237. [25] Ghali EL, Potivin RJA. Corros Sci 1972;12:582. [26] Kwiatkowski L. Ochrona przed Korozja (Special Issue), 1999;42:413 (in Polish). [27] Bala H, Pawlowska G, Szymura S, Rabinovich YuM. British Corros J 1998;33:37. . [28] Bala H, Szymura S. Inzynieria Materiaowa 1995;16:119 (in Polish). [29] Szymura S, Bala H, Rabinovich YuM, Sergeev VV, Pawlowska G. J Magn Magn Mater 1991;94:113. [30] Szymura S, Rabinovich YuM, Bala H, Sergeyeva TK, Lukin AA. Kovove Mater 2001;39:55. [31] Tomashov ND, Zhuk NP, Titov VA, Vedeneeva MA. In: Laboratornye raboty po korrozii i zashchite metallov, 2 izd. Metallurgiya, Moskva, 1971.
Corrosion protection of Nd–Fe–B type permanent magnets by zinc phosphate surface conversion coatings H. Balaa, N.M. Trepakb, S. Szymurac,d,*, A.A. Lukine, V.A. Gaudynf, L.A. Isaichevab, G. Pawowskaa, L.A. Ilinab a
Department of Chemistry, Technical University of Czcec˛stochowa, 42-200 Czcec˛stochowa, Poland b Saratov University, Saratov, 410026, Russia c Department of Physics, Technical University of Opole, 45-370 Opole, Poland d Institute of Physics, University of Opole, 45-052 Opole, Poland e Centre of Magnetic Technologies, Moscow, 117246, Russia f Insitute of Chemistry, Pedagogical University of Czcec˛stochowa, 42-200 Czcec˛stochowa, Poland Received 13 April 2001; accepted 18 April 2001
Abstract The use of protective zinc phosphate top coatings to protect Nd–Fe–B type permanent magnets against corrosion is discussed. The progress of the phosphatisation process has been tested by simultaneous measurement of pH near surface, corrosion potential, substrate mass loss and phosphate coating mass gain. The corrosion behaviour of the magnet in phosphate solution was analyzed by the anodic polarization technique and the general resistance of the magnet to corrosion was evaluated by the Akimov drop test. It is shown that immersion of the magnet in the acidified (pH=2) phosphating solution containing Zn(II), nitrate and fluoride ions resulted in the formation of well adhered, thin and tight hopeite layers, causing prolonged corrosion resistance of the magnet. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Rare-earth intermetallics; B. Corrosion; G. Magnetic applications
1. Introduction A poor corrosion resistance of Nd–Fe–B type magnets in many aggressive environments [1–3] is associated with the presence of 35 wt.% of neodymium in their composition. Neodymium, along with other rare earth (RE) elements belongs among the most electrochemically active metals (E O Nd3þ =Nd ¼ 2:431V) [4]. The Nd–Fe–B type magnets are highly susceptible to attack from both climatic and corrosive environments, with this attack resulting in corrosion of the alloy and deterioration in both its physical and magnetic properties. Numerous attempts to improve corrosion behaviour of Nd–Fe–B type magnets by alloy additions [5–7], metallic coatings [8–10], surface amorphisation [11] or stabilisation of grain boundaries [12] have not appeared to be very effective. Organic coatings seem to be the
* Corresponding author. E-mail address: [email protected] (S. Szymura).
most promissing in protection of Nd–Fe–B type magnets against atmospheric corrosion [10,13–15]. It has been established that application of organic coatings is the most effective on various types of primers, among which zinc phosphate coatings are especially advantageous [16,17]. The organic coating adhesion is improved then, because the zinc phosphate conversion coating provides a clean, uniform grease-free surface. In addition, adhesion is improved through an increase of surface area of the substrate thus leading to an increase of the possibility of bond formation. A dense uniform zinc phosphate conversion coating provides a stable insulating layer which inhibits corrosion if the paint film is damaged [16]. Attempts to protect the Nd–Fe–B magnets by surface phosphatisation are rather rare in the literature. Chin et al. [18] noticed that Nd–Fe–B type magnets undergo passivation in 10% H3PO4 solution. These magnets did not exhibit similar behaviour in 0.5 M H2SO4 or in Ringer solution (synthetic saliva). Pawlowska et al. [19] found the particular ability of neodymium to undergo passivation in 0.5 M phosphate
0966-9795/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(01)00035-8
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solutions, especially when pH52. The effectiveness of Nd passivation slightly decreased in the presence of F and NO 3 ions in the phosphating bath [19]. Kim [20] pointed out that a Nd–Fe–B type magnet treated with H3PO4 solution, or its combination with H2CrO4 and HF, exhibited a significant improvement in corrosion resistance as compared with the untreated one. Costa et al. [21] reported that the immersion of RE– Fe–B type magnets in diluted NaH2PO4 solution produced on the magnet surface a protective film consisting of phosphate/pyrophosphate and oxide which caused prolonged corrosion protection of the magnet during long storage period. The authors suggested the possibility of using this treatment for corrosion protection or as a pre-treatment step before organic coating application. Ribitch [16] described the commercial zinc phosphate coating processes (Parker/Amchem Bounderite bath) on Nd–Fe–B type magnets, followed by cathodic epoxy electrocoating. Attempts to apply a zinc phosphate coating to the surface of Nd–Fe–B magnets were not successful until the surface oxide film had been removed by a mild acid solution. Removal of surface oxides by etching in sulphuric acid solution led to substrate activation and rendered it phosphatable. The author also described the effect of fluoride ions addition to phosphating bath. As the F ions concentration increased, the coating weight on the Nd–Fe–B substrate distinctly decreased [16]. Some cations present in zinc phosphate coating baths (e.g. Al3+, Ti3+, Fe3+) clearly worsened the quality of the final phosphate coatings. Addition of F ions, forming stable complexes with many metals, effectively eliminated the poisoning effect of these cations [22]. Phosphatisation is an important method of protection against corrosion widely applied for many iron-base alloys and several other industrial metallic materials [22– 25]. The zinc phosphate conversion coatings are usually obtained by metal immersion in acidified (pH 2) Zn(H2PO4)2 solution. During the phosphatisation process certain complicated electrochemical and chemical processes take place, including the active dissolution of the metallic surface layers together with the depolarizer cathodic reduction processes, leading to an increase of pH at the surface. This favours hydrolysis of the soluble Zn(H2PO4), and causes deposition of hydrated Zn3(PO4)2 (hopeite) on the metal surface: þ ( 3Zn2þ þ 2H2 PO 4 + Zn3 ðPO4 Þ2 þ4H :
ð1Þ
The equilibrium of the reaction (1) depends strongly on temperature and on pH (the deposition is possible when pH>2). The equilibrium constant of the reaction (1) is 0.013 at 25 C and 0.74 at 98 C [23]. Simplifying the matter somewhat, the surface layer consists of insoluble hopeite, Zn3(PO4)2.4H2O, for which a solubility
product is Lhopeite=1033. There exists also conception that the zinc phosphate conversion layer crystallises on the thin, inner iron oxide layer [26]. Although phosphates of Fe(II) and Fe(III) also show very low solubilities, the ferric ions form a stable complex with phosphate ions [the stability constant of Fe(HPO4)+ complex is on the order of 108]. As a result, iron does not passivate in phosphoric acid solutions [26]. Introduction of NO 3 ions into a phosphating bath on the one hand oxidizes Fe2+ ions to Fe3+ and on the other, leads to a change in the character of cathodic þ processes, including reduction of NO 3 to NH4 [23]. Observations show that the phosphatisation of the iron alloys can proceed at not very high temperatures in the presence of NO 3 ions, which is an important advantage of this addition [22,23]. The main purpose of the present paper is to evaluate the usefulness of phosphatisation in effective protection of high-coercivity Nd–Fe–B type magnets against corrosion. The mechanism of the corrosion process of Nd– Fe–B type magnets consists in preferential oxidation of the most electrochemically active phase (the so-called Nd-rich phase — Nd4Fe) situated between grains of ferromagnetic phase (Nd2Fe14B) [2,3,27]. As was found by Pawlowska et al. [19] pure neodymium easily passivates in weak acid (pH 2) phosphate solutions. Taking into account that Nd does not undergo passivation in acidified solutions containing other anions, e.g. in sulphate solutions (pH=1–4) [28], one can conclude that the protective layer on Nd in phosphate solution may contain insoluble NdPO4 (the solubility product of this compound is LNdPO4=1023). 2. Experimental details Tests were carried out on the high-coercivity sintered (Nd,Dy)–(Fe,Ti)-B magnet (wt.%: Nd 32.0, Dy 1.0, Fe 64.7, Ti 1.0 and B 1.3) in the demagnetized state. The rectangular samples (10108 mm) were prepared by the conventional powder metallurgy route as described in Refs. [5,29,30]. Their magnetic properties were as follows: remanence Br=1.28 T, intrinsic coercivity iHc=1100 kA/ m and the energy product (BH)max=310 kJ/m3. The anodic potentiokinetic polarisation curves for the tested magnets were measured in air-saturated 0.5 M phosphate solution with pH=2 and at a temperature of 45 C. The polarisation tests were performed both for phosphate solutions containing 0.1 M NO 3 ions (introduced as NaNO3) and for phosphate solutions containing 0.1 M NO ions [introduced as 3 ions and 0.1 M F (NH4)2F2]. For comparative purposes the polarisation tests were made also on the spectroscopically pure metals, iron and neodymium. An electrochemical cell with a working volume of 50 cm3 was equipped with a Luggin’s capillary and a platinum rod
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auxiliary electrode. The working electrodes had an operating surface area of 1 cm2. The anodic polarisation curves were carried out starting with a stationary corrosion potential of the sample and going up to a potential of +0.3 V with a potential sweep rate of 2 mV/s. All potentials in the present paper were measured versus a standard Ag/AgCl electrode but their values are expressed in a standard hydrogen scale. The phosphatisation process of the magnet samples was carried out by immersion of the samples in one of two alternative baths: Bath A was a basic solution containing: 1.5 M H3PO4, 0.75 M Zn2+ and 0.1 M NO 3 with pH=2, achieved by suitable addition of NaOH. Bath B had a similar chemical composition but it contained an addition of 0.1 M NO 3 ions [introduced in the form of (NH4)2F2]. Prior to immersion in the phosphating bath the specimen surface was polished on waterproof emery paper (No. 800), rinsed with water, then with ethanol and finally dried. After drying, the sample was weighed to an accuracy of 0.1 mg. The process of sample phosphating lasted 30 min and after this time neither further increase of the thickness of the zinc phosphate layer, nor mass changes of the sample were observed. The phosphatisation process was carried out in non-stirred solutions in contact with air. During the phosphatisation process both corrosion potential (Ecorr) and near-electrode pH value (pHs) were continuously measured. The pHs was detected using a glass microelectrode (0.6–0.8 mm2) situated in 40–60 mm distance from the electrode surface (the sides of the glass electrode were isolated from the solution with paraffin wax and the shift of the electrode was realized by micrometer screw). The mass of the dissolved material substrate per surface unit ( m/S) and the surface mass of the zinc conversion phosphate layer ( mf /S) were determined after 1, 3, 5, 10, 15, 20 and 30 min of sample exposure in the phosphating bath on the basis of gravimetric measurements: mass gain after phosphating and mass loss after removal of the phosphate layer (etching in 5% aqueous solution of CrO3). In analogous intervals of time, the corrosion resistance of the zinc phosphate conversion coating was determined using the long-established Akimov drop test [31]: a drop of the test solution containing 0.26 M CuSO4, 0.56 M NaCl and 0.0024 M HCl (20 ) was placed on the coating (rinsed with water, then with alcohol and dried), and the time x (the resistance time) was measured until the colour of the drop changed from blue to red (as a result of corrosion of iron-base substrate and copper deposition). The longer the time x, the better protective properties of the coating.1
1 The zinc phosphate conversion coating on carbon steel substrate shows ‘‘medium protective properties’’ when 5 120 s; and it shows ‘‘increased protective properties’’ when x 5 300 s [31].
3. Results and discussion In Fig. 1a, the changes of corrosion potential (Ecorr) and near-surface pH versus phosphatisation time are presented. Fig. 1b presents mass loss of the magnet substrate ( m/S), mass gain of the formed phosphate layer per surface unit ( mf /S) and corrosion resistance of the coating characterized by the resistance time (x ) as a function of time of immersion in the phosphating solution A. Directly after immersion of the magnet into solution A the Ecorr shows positive values, then it rapidly drops, and after 1 min of exposure, it reaches a minimum value ( 0.24 V). Further exposure of the magnet to Bath A causes slow increase of the corrosion potential and, after 15–20 min, it reaches practically stationary value of 0.15 V. As it is seen from Fig. 1a, after initial short-lasting (20 s), rapid decrease of pHs, the near-surface pH value gradually increases and stabilizes on the level of pHs 3 after 15 min of phosphatisation. The initial drop of pHs (to the value of 1) can be attributed to the reaction of the surface oxide layer or adsorption of dihydrophosphate ions, e.g.: 2M þ 3H2 O 6e ! M2 O3 þ 6Hþ M þ 3H3 PO4 ! M H2 PO 4
3;ads
þ3Hþ
ð2Þ ð3Þ
where M symbolizes the metallic magnetic material. Further increase of pHs results from an active anodic substrate dissolution balanced by hydrogen ions and
Fig. 1. Dependence of near-surface pH and corrosion potential of the (Nd, Dy)–(Fe, Ti)–B magnet (a) and substrate mass loss as a result of etching ( m/S), mass gain as a result of zinc phosphate layer formation ( mf /S) and time of substrate protection against corrosion ( x) determined by Akimov test (b). Phosphating bath (A): 1.5 M H3PO4, 0.75 M Zn2+, 0.1 M NO 3 ; (pH=2), 45 C.
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dissolved oxygen depolarisation processes — both accompanied with consumption of H+ ions: Anodic reaction: M 3e ! M3þ
ð4Þ
Cathodic reactions: 2Hþ þ 2e ! H2
ð5Þ
O2 þ 4Hþ þ 4e ! 3H2 O
ð6Þ
Increase of pHs to values of 3 facilitates deposition of hopeite according to the reaction (1) and, possibly, NdPO4. After 15 min of exposure, the pHs slightly decreases with time, although this dependence is not generally reproducible, and therefore it is not presented in Fig. 1a. After 15 min of phosphating, the zinc phosphate layer reaches 92% of its final mass; simultaneously, the mass of etched substrate material after this time consists of 89% of its final mass. Thus, despite the relatively significant thickness of the zinc phosphate layer (its surface mass is 24 g/m2), it does not sufficiently inhibit the substrate dissolution process. The corrosion resistance of the layer, represented by time x, continuously increases with the progress of the phosphatisation process; however, the final resistance of the coating is not very high: after 30 min of exposure x ffi 100 s. The main reason of the poor protective properties of the zinc phosphate conversion coating obtained in solution A seems to be its considerable porosity. In Fig. 2, the anodic potentiokinetic polarisation curves of the magnet in tested aerated phosphate solution (pH=2) in the absence or in presence of 0.1 M F
Fig. 2. Anodic potentiokinetic polarisation curves of the tested (Nd, Dy)–(Fe, Ti)–B magnet and pure elements: Fe and Nd in 0.5 M PO3 4 +0.1 M NO3 (pH=2) solution without (solid line, A) and with addition of 0.1 M F ions (B, dashed line). Experimental conditions: 2 mV/s, no stirring, 45 C.
ions are shown. For comparative purposes, corresponding curves for pure metals, Fe and Nd, are also presented in the figure. It can be seen from Fig. 2 that the addition of F ions scarcely worsens the passivity of neodymium and slightly inhibits the active dissolution of iron. However, the presence of fluoride ions in the phosphate solution clearly influences the anodic behaviour of the (Nd, Dy)–(Fe, Ti)–B magnet. In the range of potentials close to 0 V, the F ions enhance the anodic oxidation process of the magnet 10 times. At potentials less than 0.3 V, the situation is reversed and the active dissolution process rate in the presence of F ions is noticeably slower than in a pure phosphate solution. One can notice by comparing polarisation curves of Fe, Nd and (Nd, Dy)–(Fe, Ti)–B magnet, that, in the presence of fluoride ions, the anodic behaviour (active dissolution) of the magnet becomes rather similar to that of pure iron whereas, in the absence of F ions, the behaviour of the magnet (tendency to passivation) is similar to that of neodymium. It can be assumed that, in the presence of F ions, the tested magnet corrodes with preferential dissolution of iron, owing to the formation of stable FeF3n complexes (Nd does not form any stable comn plexes with F ions). Consequently, the surface layer of the magnet becomes enriched with Nd when the potential increases. At potentials greater than 0 V, the surface is enriched with Nd to such an extent that further anodic polarisation decreases the anodic current density towards the values typical for neodymium. One can expect that, during phosphatization of the Nd–Fe–B type magnets in solutions containing fluoride ions, the hopeite layer is formed on the surface enriched with Nd. It is possible that neodymium reacts with phosphoric acid or phosphate ions to produce a thin insoluble NdPO4 layer, directly adhering to the metal substrate (LNdPO4 ¼ 1023 ). The presence of NdPO4 in the hopeite phase has been confirmed by XRD analysis of the samples phosphated in solution containing F ions. The existence of a thin interphase containing Nd, situated between magnet substrate and hopeite layer, should affect the properties of the final coating. The protective properties of phosphate coatings produced on the surface of the tested magnets by immersion in baths without F ions (Bath A) and with the addition of 0.1 M F ions (Bath B) are compared in Fig. 3. As can be seen from Fig. 3a, the presence of fluoride ions in the phosphating solution results in a decrease of the surface mass of the layer by 30%, and it leads to a distinct increase of corrosion resistance of the layer when compared to the solution without F ions ( x,B 4 x,A, compare to Fig. 3b). Addition of F ions additionally shortens the time necessary for the coating formation: after 15 min of magnet exposure in Bath B, the zinc phosphate conversion coating is practically completed, very tight and well adhered to the magnet substrate.
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Fig. 3. Mass gain per surface unit as a result of zinc phosphate layer deposition on the surface of the tested magnet (a) and the coat protection time ( x) determined by Akimov test (b) versus exposure time 2+ +0.1 M NO in 1.5 M PO3 4 +0.75 M Zn 3 (pH=2) solution without (bath A, solid line) and with addition of 0.1 M F ions (bath B, dashed line).
4. Conclusions The investigation of the usefulness of phosphatisation in view of effective protection of Nd–Fe–B type magnets against corrosion has revealed the following results: 1. zinc phosphate conversion coating satisfactorily protects Nd–Fe–B type permanent magnets against corrosion; 2. addition of 0.1 M of fluoride ions into the phosphating bath is very advantageous in terms of the coating corrosion resistance, its thickness and tightness; 3. the positive role of F ions in the effectiveness of Nd–Fe–B type magnet phosphating process presumably consists in preferential dissolution of iron in the initial stages of the process, enrichment of the metal surface with Nd and formation of well adhered layer (most likely consisting of neodymium phosphate) covered by a basic hopeite coat.
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