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In vitro corrosion behavior of cast iron–platinum magnetic alloys
dental materials Dental Materials 17 (2001) 217±220
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In vitro corrosion behavior of cast iron±platinum magnetic alloys I. Watanabe a,*, K. Hai a, Y. Tanaka b, K. Hisatsune b, M. Atsuta a a
b
Department of Fixed Prosthodontics, Nagasaki University, School of Dentistry, Nagasaki, Japan Department of Dental Materials Sciences, Nagasaki University, School of Dentistry, Nagasaki, Japan Received 5 January 2000; revised 22 May 2000; accepted 8 June 2000
Abstract Objective: The objective of this study was to investigate the corrosion resistance of cast Fe±Pt alloys of varying compositions for use as attachment keepers and to make a comparison with the corrosion resistance of magnetic stainless steel. Methods: The corrosion behavior of cast Fe±Pt alloy keepers (Fe±40 at%Pt, Fe±38 at%Pt, Fe±37 at%Pt and Fe±36 at%Pt) was evaluated by means of an immersion test and an anodic polarization test. The solutions used were a 1.0% lactic acid aqueous solution (pH 2.3) (10 ml) and 0.9% NaCl solution (pH 7.3) (10 ml). As a control, the corrosion resistance of a magnetic stainless steel keeper (SUS 447J1: HICOREX) was also measured. Results: Chromium and platinum ions were not detected in either the 1.0% lactic acid or 0.9% NaCl solutions. The only released ions detected were the Fe ions in the 1.0% lactic acid solution. The amounts of Fe ions released from the Fe±40 at%Pt and Fe±38 at%Pt alloys were signi®cantly p , 0:05 lower than from the Fe±37 at%Pt, Fe±36 at%Pt and SUS 447J1 alloys. In the anodic polarization test, the potentials at the beginning of passivation for the four Fe±Pt alloys were higher than for the SUS 447J1 alloy in both solutions. Signi®cance: The Fe±Pt alloys, especially the alloys with higher Pt percentages (Fe±40 and 38 at%Pt), indicated a high corrosion resistance compared to the magnetic stainless steel keeper. A reduction in the Pt percentage may decrease the corrosion resistance in the oral environment. q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Corrosion resistance; Castable magnet; Fe±Pt alloy; Immersion test; Anodic polarization
1. Introduction Magnetic attachments have been used in prosthodontics to anchor removable partial or full dentures to the abutment teeth in the oral cavity [1±3]. A magnetic attachment system consists of a magnet (Fe14Nd2B or SmCo5) covered with a yoke cap and its keeper (attachment keeper). In general, the magnet is embedded in the removable prosthesis, whereas the attachment keeper is cast-bonded, brazed, or screwed into the root cap or dowel core, then bonded to the root canal of the abutment tooth. The attachment keeper and the yoke cap of the magnet in a magnetic attachment system are usually fabricated from magnetic stainless steel with soft magnetic properties. One of the problems associated with magnetic stainless steel is a lack of corrosion resistance in the oral environment [4±6], especially when it is castbonded or in contact with other types of dental alloys, which is known as ªgalvanic corrosionº [6,7]. This magnetic stainless steel has been reported to release ferric and chromium ions in 1% lactic acid or 0.9% sodium chloride aqueous solutions [8,9]. Tanaka et al. [5] reported that * Corresponding author. Tel.: 181-95-849-7688; fax: 181-95-849-7689.
the corrosion of the yoke cap destroyed the main body of the magnet in the magnetic attachment, and resulted in the loss of attractive force. It has recently been found that the Fe±Pt alloy system yields magnetic properties [10±12], leading to developments in dental applications [13] and medical treatment [14]. In the Fe±Pt alloy system, compositions of approximately Fe±39.5 at%Pt produce hard magnetic properties and can be used for castable magnets in the magnetic attachment systems for dentistry. Tanaka et al. [15] found that Fe±Pt alloys with Pt compositions lower than Fe± 39.5 at%Pt show higher saturation magnetization values and have soft magnetic properties. This means that Fe±Pt alloys produce a great attractive force to the magnet and can be used as magnetic attachment keepers [16]. Furthermore, the Fe±Pt alloys contain large amounts of platinum (around 70 wt% Pt) and are expected to yield excellent corrosion resistance. If a magnet and its keeper can be made from the same Fe±Pt alloy, the corrosion resistance may be outstanding since it would not be necessary to cast-bond, braze or contact two types of dental casting alloys in the same magnetic attachment system. The purpose of this study was to investigate the corrosion resistance of cast
0109-5641/01/$20.00 + 0.00 q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S 0109-564 1(00)00072-5
218
I. Watanabe et al. / Dental Materials 17 (2001) 217±220
Fe±Pt alloys of varying compositions for use as attachment keepers and to make a comparison to the corrosion resistance of magnetic stainless steel. 2. Materials and methods 2.1. Materials The compositions of the four Fe±Pt alloys employed were: Fe±40 at%(70.0 wt%)Pt, Fe±38 at%(68.2 wt%)Pt, Fe±37 at%(67.2 wt%)Pt and Fe±36 at%(66.3 wt%)Pt. Ingots of each alloy were prepared by melting the compositional elements under an argon atmosphere. As the control material, the magnetic stainless steel (SUS 447J1, 67.6 wt% Fe, 30.0 wt% Cr, 2.0 wt% Mo, 0.18 wt% Ni, 0.003 wt% C) used in a dental magnetic attachment system (HICOREX 4513, Hitachi Metals Ltd, Tokyo, Japan) was selected. 2.2. Casting Disc-shaped plastic patterns (4.5 mm in diameter, 0.8 mm thick) duplicated from a dental magnetic stainless steel keeper (SUS 447J1, HICOREX) were prepared. Five patterns were invested in a mold ring with a magnesia-based investment (Titavest CB, Morita, Japan). The molds were then cast with the Fe±Pt alloys in a high-frequency centrifugal casting machine (Eagle, Jelenko/Morita, Tokyo, Japan). The overall burn-out schedule of the investments before casting followed the manufacturer's instructions. After casting, the cast discs of the Fe±Pt alloy were vacuum-enclosed in quartz-glass tubes and solution-treated at 13258C for 45 min, then quenched in ice water. 2.3. Immersion test The surfaces of the cast Fe±Pt and SUS 447J1 alloy specimens were polished with No. 1000 SiC paper and ultrasonically cleaned with acetone. Five specimens of each alloy were separately immersed in 10 ml of 1.0% lactic acid aqueous solution (pH 2.3) or 10 ml of 0.9% NaCl solution (pH 7.3). Both solutions were deaerated by bubbling nitrogen gas through the solution at least 30 min before the experiment. The closed capsules containing the specimens in the solution were placed in a 378C water bath and were shaken in a reciprocating shaker for 7 days at 60 cycles/min. After 7 days' immersion, the amounts of ferric, chromium, and platinum ions released from the specimens into each solution were measured by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES, SPS 1500VR, Seiko Instruments, Tokyo, Japan) with a detection limit of 5 ppb. The amounts of released ions divided by the specimen surface area were calculated for each specimen. For each condition, the mean value and standard deviation of ®ve specimens were statistically evaluated using an ANOVA and post hoc Tukey±Kramer multiple comparisons test at a statistical signi®cance of p 0:05:
2.4. Anodic polarization The corrosion behavior was evaluated by means of voltammetry (GPIB, HA-151, HB-111, Hokuto Denko K.K., Japan) in 1.0% lactic acid and 0.9% NaCl solutions kept at 378C in a water bath. The solutions were deaerated by nitrogen gas before the anodic polarization test. Single anodic polarization curves were measured and the potential was changed at a rate of 50 mV/min from 2500 to 11700 mV against an Hg/Hg2Cl2/KCl reference electrode. For each alloy and each solution, the anodic polarization test was repeated three times (three specimens). Since all tests were reproducible, the median curve of the three measurements was selected for results.
3. Results Neither Cr nor Pt ions were detected in either the 1.0% lactic acid or the 0.9% NaCl solution. The only released ions detected were ferric ions in the 1.0% lactic acid solution. Table 1 shows the amounts of ferric ions released from the SUS 447J1 and Fe±Pt alloy specimens. The Fe±Pt alloys immersed in the1.0% lactic acid solution showed two different types of data. The values for the Fe±40 at%Pt (1.12 mg/ cm 2) and Fe±38 at%Pt alloys (1.18 mg/cm 2) were lower than for the Fe±37 at%Pt (1.66 mg/cm 2) and Fe±36 at%Pt (1.65 mg/cm 2) alloys. The latter values were similar (no statistical difference at p . 0:05 to those for SUS 447J1 (1.60 mg/cm 2). The amounts of Fe ions released from the Fe±40 at%Pt and Fe±38 at%Pt alloys were signi®cantly lower than from Fe±37 at%Pt, Fe±36 at%Pt and SUS 447J1. Fig. 1 shows the potential/current density curve of the SUS 447J1 and the four Fe±Pt alloys in the 1.0% lactic acid solution. The passive current densities for the Fe±40 at%Pt and Fe±38 at%Pt were particularly lower than for the other alloys. All the Fe±Pt alloys showed a higher distinctive passive region (approx. ,1.2 V) compared to SUS 447J1 in the transpassive region. Fig. 2 shows the potential/current density curves in the 0.9% NaCl solution. Even though the passive current densities of SUS 447J1 and the four Fe±Pt alloys were very similar, the Fe±Pt alloys exhibited a higher distinctive passive region Table 1 Amounts of ferric ions released from magnetic stainless steel and Fe±Pt alloy specimens in 1.0% lactic acid solution (identical letters in the Difference column indicate no statistical differences p . 0:05 by post hoc Tukey±Kramer multiple comparisons test n 5 Alloy
Mean (SD)
Min. value
Max. value
Difference
Fe±40 at%Pt Fe±38 at%Pt Fe±37 at%Pt Fe±36 at%Pt SUS 447J1
1.12 (0.05) 1.18 (0.07) 1.66 (0.02) 1.65 (0.03) 1.60 (0.01)
0.92 0.98 1.54 1.52 1.54
1.24 1.35 1.81 1.85 1.63
a a b b b
I. Watanabe et al. / Dental Materials 17 (2001) 217±220
Fig. 1. Potential/current density curve of magnetic stainless steel and four Fe±Pt alloys in 1.0% lactic acid solution.
(approx. ,1.18 V) compared to SUS 447J1 in the transpassive region. The potentials at the beginning of passivation for the four Fe±Pt alloys were higher than for the SUS 447J1 alloy in both solutions. 4. Discussion Corrosion behavior is generally in¯uenced by the spontaneous potential (self-potential) of the material in the solution, which mainly depends on pH, temperature, and amounts of dissolved oxygen in the solution. In this study, two types of solutions, 1.0% lactic acid and 0.9% NaCl solutions, were employed. When the alloys are immersed in these solutions, it is thought that different corrosion processes take place because of the different pH values: 1.0% lactic acid solution Ð pH 2.3, 0.9% NaCl solution Ð pH 7.3. In a neutral solution like the NaCl solution, corrosion occurs due to oxygen diffusion on the surface of the specimen. In the present study, no ions were
219
detected in the 0.9% NaCl solution because the solutions were deaerated by bubbling nitrogen gas before the immersion test, which reduced the oxidation factors. However, due to its low pH value, lactic acid has oxide action itself. This is the main reason why ferric ions were detected only in the 1.0% lactic acid solution. The amounts of Fe ion released from the Fe±40 at%Pt and Fe±38 at%Pt alloys in lactic acid solution were lower compared to the other alloys. The higher amounts of Fe ions released from the Fe±37 at%Pt and Fe±36 at%Pt alloys, which showed values similar to SUS 447J1, could be attributed to small casting defects on the surface, since there were a few small pores on these alloy surfaces after polishing with No. 1000 SiC paper. These casting defects may have also affected the anodic polarization test results. The passive current density for Fe±40 at%Pt and Fe±38 at%Pt was markedly lower than for Fe±37 at%Pt and Fe±36 at%Pt in the 1.0% lactic acid solution. The potentials at the beginning of passivation for Fe±40 at%Pt (Fig. 1) and Fe±38 at%Pt were higher than for Fe±37 at%Pt and Fe±36 at%Pt in the 0.9% NaCl solution (Fig. 2). In a previous study of Fe±Pt alloys, Watanabe et al. [16] investigated the attractive force of Fe±Pt alloys to dental Fe14Nd2B magnets using the same experimental alloys as in this study. They reported that decreasing the Pt percentage (thus increasing the Fe percentage) increased the saturation magnetization values and resulted in an increase in attractive force to the magnet. Of the Fe±Pt alloys tested, Fe±36 at%Pt yielded the greatest values of saturation magnetization and attractive force to dental Fe14Nd2B magnets. In the present study, the decrease in Pt percentage (Fe±37 at%Pt and Fe±36 at%Pt) increased the amounts of Fe ions released into the 1.0% lactic acid solution. The defects on the cast surface may also play a role in the reduction of corrosion resistance. However, the anodic polarization results indicated a higher corrosion resistance by the Fe±Pt alloys compared to the SUS 447J1 alloy. It is generally assumed that increasing the amount of Fe (and decreasing the Pt percentage to less than 36 at%) improves magnetic properties (increase of attractive force); however, at the same time, it reduces corrosion resistance. From this point of view, Fe±36 at%Pt might be the appropriate composition for magnetic attachment keepers. If Fe±36 at%Pt is selected to make castable attachment keepers, the addition of a small amount of another element such as Si would be necessary to improve castability and reduce casting defects. Acknowledgements
Fig. 2. Potential/current density curve of SUS 447J1 and four Fe±Pt alloys in 0.9% NaCl solution.
This investigation was supported in part by Grant-in-Aid for young scientists, (A)11771229 and (A) 11771232, and Grant-in-Aid for scienti®c research (B)11694293 from the Ministry of Education, Science, Sports and Culture, Tokyo, Japan.
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I. Watanabe et al. / Dental Materials 17 (2001) 217±220
References [1] Gillings BRD. Magnetic retention for complete and partial overdentures, Part I. J Prosthet Dent 1981;45:484±91. [2] Gillings BRD. Magnetic retention for complete and partial over-dentures, Part II. J Prosthet Dent 1983;49:607±18. [3] Laird WRE, Grant AA, Smith GA. The use of magnetic forces in prosthetic dentistry. J Dent 1981;9:328±35. [4] Garl J, Grago CJ. Tarnish and corrosion with the use of intraoral magnets. J Prosthet Dent 1991;66:536±40. [5] Tanaka Y, Honkura Y, Furushima Y, Nagamachi N, Imamura T, Gotoh H, Siranuma K. Basic study of a sandwich-type dental magnetic attachment. J Jpn Prosthet Soc 1991;35:167±77. [6] Kitsugi A, Okuno O, Nakano T, Hamanaka H, Kuroda T. The corrosion behavior of Nd2Fe14B and SmCo5 magnets. Dent Mater J 1992;11:119±29. [7] Fujimoto T, Ueda M. Application of magnetic attachment to BraÈnemark implant system. Quint Dent Implant 1996;3:327±30. [8] Imuro F, Yoneyama T, Okuno O. Corrosion of coupled metals in a dental magnetic attachment system. Dent Mater J 1993;12:136± 44. [9] Takada Y, Okuno O. Reversing potentials and increasing amount of
[10] [11] [12]
[13]
[14]
[15]
[16]
released ions with contact corrosion between stainless steels and Au± Ag±Pd alloys. J Jpn Soc Dent Appar Mater 1996;15:525±31. Watanabe K, Masumoto H. On the high energy product of Fe±Pt permanent magnet alloys. J Jpn Inst Metals 1983;47:699±703. Watanabe K. Permanent magnet properties in Fe±Pt±Nb system alloys. J Jpn Inst Metals 1990;54:1284±90. Watanabe K. Permanent magnet properties and their temperature dependence in the Fe±Pt±Nb alloy system. Mater Trans JIM 1991;32:292±8. Tanaka Y, Kimura N, Hono K, Yasuda T. Microstructure and magnetic properties of Fe±Pt permanent magnets. J Magn Mater 1997;170:289±97. Tohnai T, Goto Y, Hayashi Y, Ueda M, Kobayashi T, Matsui M. Preoperative thermochemotherapy of oral cancer using magnetic induction hyperthermia. Int J Hyperth 1996;12:37±47. Tanaka Y, Watanabe I, Udoh K, Atsuta M, Hisatsune K, Nakano M, Fukunaga H. Relations between micro- and magnetic-structures and magnetic properties of Fe±Pt ferromagnetic alloys. J Jpn Soc Dent Mater Device 1998;17:44. Watanabe I, Tanaka Y, Udoh K, Fukunaga T, Hisatsune K, Atsuta M. Attractive force of castable magnetic alloys to dental Fe14Nd2B magnet. J Dent Res (special issue) 1999;78:237 (Abstract no. 1055).
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In vitro corrosion behavior of cast iron±platinum magnetic alloys I. Watanabe a,*, K. Hai a, Y. Tanaka b, K. Hisatsune b, M. Atsuta a a
b
Department of Fixed Prosthodontics, Nagasaki University, School of Dentistry, Nagasaki, Japan Department of Dental Materials Sciences, Nagasaki University, School of Dentistry, Nagasaki, Japan Received 5 January 2000; revised 22 May 2000; accepted 8 June 2000
Abstract Objective: The objective of this study was to investigate the corrosion resistance of cast Fe±Pt alloys of varying compositions for use as attachment keepers and to make a comparison with the corrosion resistance of magnetic stainless steel. Methods: The corrosion behavior of cast Fe±Pt alloy keepers (Fe±40 at%Pt, Fe±38 at%Pt, Fe±37 at%Pt and Fe±36 at%Pt) was evaluated by means of an immersion test and an anodic polarization test. The solutions used were a 1.0% lactic acid aqueous solution (pH 2.3) (10 ml) and 0.9% NaCl solution (pH 7.3) (10 ml). As a control, the corrosion resistance of a magnetic stainless steel keeper (SUS 447J1: HICOREX) was also measured. Results: Chromium and platinum ions were not detected in either the 1.0% lactic acid or 0.9% NaCl solutions. The only released ions detected were the Fe ions in the 1.0% lactic acid solution. The amounts of Fe ions released from the Fe±40 at%Pt and Fe±38 at%Pt alloys were signi®cantly p , 0:05 lower than from the Fe±37 at%Pt, Fe±36 at%Pt and SUS 447J1 alloys. In the anodic polarization test, the potentials at the beginning of passivation for the four Fe±Pt alloys were higher than for the SUS 447J1 alloy in both solutions. Signi®cance: The Fe±Pt alloys, especially the alloys with higher Pt percentages (Fe±40 and 38 at%Pt), indicated a high corrosion resistance compared to the magnetic stainless steel keeper. A reduction in the Pt percentage may decrease the corrosion resistance in the oral environment. q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Corrosion resistance; Castable magnet; Fe±Pt alloy; Immersion test; Anodic polarization
1. Introduction Magnetic attachments have been used in prosthodontics to anchor removable partial or full dentures to the abutment teeth in the oral cavity [1±3]. A magnetic attachment system consists of a magnet (Fe14Nd2B or SmCo5) covered with a yoke cap and its keeper (attachment keeper). In general, the magnet is embedded in the removable prosthesis, whereas the attachment keeper is cast-bonded, brazed, or screwed into the root cap or dowel core, then bonded to the root canal of the abutment tooth. The attachment keeper and the yoke cap of the magnet in a magnetic attachment system are usually fabricated from magnetic stainless steel with soft magnetic properties. One of the problems associated with magnetic stainless steel is a lack of corrosion resistance in the oral environment [4±6], especially when it is castbonded or in contact with other types of dental alloys, which is known as ªgalvanic corrosionº [6,7]. This magnetic stainless steel has been reported to release ferric and chromium ions in 1% lactic acid or 0.9% sodium chloride aqueous solutions [8,9]. Tanaka et al. [5] reported that * Corresponding author. Tel.: 181-95-849-7688; fax: 181-95-849-7689.
the corrosion of the yoke cap destroyed the main body of the magnet in the magnetic attachment, and resulted in the loss of attractive force. It has recently been found that the Fe±Pt alloy system yields magnetic properties [10±12], leading to developments in dental applications [13] and medical treatment [14]. In the Fe±Pt alloy system, compositions of approximately Fe±39.5 at%Pt produce hard magnetic properties and can be used for castable magnets in the magnetic attachment systems for dentistry. Tanaka et al. [15] found that Fe±Pt alloys with Pt compositions lower than Fe± 39.5 at%Pt show higher saturation magnetization values and have soft magnetic properties. This means that Fe±Pt alloys produce a great attractive force to the magnet and can be used as magnetic attachment keepers [16]. Furthermore, the Fe±Pt alloys contain large amounts of platinum (around 70 wt% Pt) and are expected to yield excellent corrosion resistance. If a magnet and its keeper can be made from the same Fe±Pt alloy, the corrosion resistance may be outstanding since it would not be necessary to cast-bond, braze or contact two types of dental casting alloys in the same magnetic attachment system. The purpose of this study was to investigate the corrosion resistance of cast
0109-5641/01/$20.00 + 0.00 q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S 0109-564 1(00)00072-5
218
I. Watanabe et al. / Dental Materials 17 (2001) 217±220
Fe±Pt alloys of varying compositions for use as attachment keepers and to make a comparison to the corrosion resistance of magnetic stainless steel. 2. Materials and methods 2.1. Materials The compositions of the four Fe±Pt alloys employed were: Fe±40 at%(70.0 wt%)Pt, Fe±38 at%(68.2 wt%)Pt, Fe±37 at%(67.2 wt%)Pt and Fe±36 at%(66.3 wt%)Pt. Ingots of each alloy were prepared by melting the compositional elements under an argon atmosphere. As the control material, the magnetic stainless steel (SUS 447J1, 67.6 wt% Fe, 30.0 wt% Cr, 2.0 wt% Mo, 0.18 wt% Ni, 0.003 wt% C) used in a dental magnetic attachment system (HICOREX 4513, Hitachi Metals Ltd, Tokyo, Japan) was selected. 2.2. Casting Disc-shaped plastic patterns (4.5 mm in diameter, 0.8 mm thick) duplicated from a dental magnetic stainless steel keeper (SUS 447J1, HICOREX) were prepared. Five patterns were invested in a mold ring with a magnesia-based investment (Titavest CB, Morita, Japan). The molds were then cast with the Fe±Pt alloys in a high-frequency centrifugal casting machine (Eagle, Jelenko/Morita, Tokyo, Japan). The overall burn-out schedule of the investments before casting followed the manufacturer's instructions. After casting, the cast discs of the Fe±Pt alloy were vacuum-enclosed in quartz-glass tubes and solution-treated at 13258C for 45 min, then quenched in ice water. 2.3. Immersion test The surfaces of the cast Fe±Pt and SUS 447J1 alloy specimens were polished with No. 1000 SiC paper and ultrasonically cleaned with acetone. Five specimens of each alloy were separately immersed in 10 ml of 1.0% lactic acid aqueous solution (pH 2.3) or 10 ml of 0.9% NaCl solution (pH 7.3). Both solutions were deaerated by bubbling nitrogen gas through the solution at least 30 min before the experiment. The closed capsules containing the specimens in the solution were placed in a 378C water bath and were shaken in a reciprocating shaker for 7 days at 60 cycles/min. After 7 days' immersion, the amounts of ferric, chromium, and platinum ions released from the specimens into each solution were measured by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES, SPS 1500VR, Seiko Instruments, Tokyo, Japan) with a detection limit of 5 ppb. The amounts of released ions divided by the specimen surface area were calculated for each specimen. For each condition, the mean value and standard deviation of ®ve specimens were statistically evaluated using an ANOVA and post hoc Tukey±Kramer multiple comparisons test at a statistical signi®cance of p 0:05:
2.4. Anodic polarization The corrosion behavior was evaluated by means of voltammetry (GPIB, HA-151, HB-111, Hokuto Denko K.K., Japan) in 1.0% lactic acid and 0.9% NaCl solutions kept at 378C in a water bath. The solutions were deaerated by nitrogen gas before the anodic polarization test. Single anodic polarization curves were measured and the potential was changed at a rate of 50 mV/min from 2500 to 11700 mV against an Hg/Hg2Cl2/KCl reference electrode. For each alloy and each solution, the anodic polarization test was repeated three times (three specimens). Since all tests were reproducible, the median curve of the three measurements was selected for results.
3. Results Neither Cr nor Pt ions were detected in either the 1.0% lactic acid or the 0.9% NaCl solution. The only released ions detected were ferric ions in the 1.0% lactic acid solution. Table 1 shows the amounts of ferric ions released from the SUS 447J1 and Fe±Pt alloy specimens. The Fe±Pt alloys immersed in the1.0% lactic acid solution showed two different types of data. The values for the Fe±40 at%Pt (1.12 mg/ cm 2) and Fe±38 at%Pt alloys (1.18 mg/cm 2) were lower than for the Fe±37 at%Pt (1.66 mg/cm 2) and Fe±36 at%Pt (1.65 mg/cm 2) alloys. The latter values were similar (no statistical difference at p . 0:05 to those for SUS 447J1 (1.60 mg/cm 2). The amounts of Fe ions released from the Fe±40 at%Pt and Fe±38 at%Pt alloys were signi®cantly lower than from Fe±37 at%Pt, Fe±36 at%Pt and SUS 447J1. Fig. 1 shows the potential/current density curve of the SUS 447J1 and the four Fe±Pt alloys in the 1.0% lactic acid solution. The passive current densities for the Fe±40 at%Pt and Fe±38 at%Pt were particularly lower than for the other alloys. All the Fe±Pt alloys showed a higher distinctive passive region (approx. ,1.2 V) compared to SUS 447J1 in the transpassive region. Fig. 2 shows the potential/current density curves in the 0.9% NaCl solution. Even though the passive current densities of SUS 447J1 and the four Fe±Pt alloys were very similar, the Fe±Pt alloys exhibited a higher distinctive passive region Table 1 Amounts of ferric ions released from magnetic stainless steel and Fe±Pt alloy specimens in 1.0% lactic acid solution (identical letters in the Difference column indicate no statistical differences p . 0:05 by post hoc Tukey±Kramer multiple comparisons test n 5 Alloy
Mean (SD)
Min. value
Max. value
Difference
Fe±40 at%Pt Fe±38 at%Pt Fe±37 at%Pt Fe±36 at%Pt SUS 447J1
1.12 (0.05) 1.18 (0.07) 1.66 (0.02) 1.65 (0.03) 1.60 (0.01)
0.92 0.98 1.54 1.52 1.54
1.24 1.35 1.81 1.85 1.63
a a b b b
I. Watanabe et al. / Dental Materials 17 (2001) 217±220
Fig. 1. Potential/current density curve of magnetic stainless steel and four Fe±Pt alloys in 1.0% lactic acid solution.
(approx. ,1.18 V) compared to SUS 447J1 in the transpassive region. The potentials at the beginning of passivation for the four Fe±Pt alloys were higher than for the SUS 447J1 alloy in both solutions. 4. Discussion Corrosion behavior is generally in¯uenced by the spontaneous potential (self-potential) of the material in the solution, which mainly depends on pH, temperature, and amounts of dissolved oxygen in the solution. In this study, two types of solutions, 1.0% lactic acid and 0.9% NaCl solutions, were employed. When the alloys are immersed in these solutions, it is thought that different corrosion processes take place because of the different pH values: 1.0% lactic acid solution Ð pH 2.3, 0.9% NaCl solution Ð pH 7.3. In a neutral solution like the NaCl solution, corrosion occurs due to oxygen diffusion on the surface of the specimen. In the present study, no ions were
219
detected in the 0.9% NaCl solution because the solutions were deaerated by bubbling nitrogen gas before the immersion test, which reduced the oxidation factors. However, due to its low pH value, lactic acid has oxide action itself. This is the main reason why ferric ions were detected only in the 1.0% lactic acid solution. The amounts of Fe ion released from the Fe±40 at%Pt and Fe±38 at%Pt alloys in lactic acid solution were lower compared to the other alloys. The higher amounts of Fe ions released from the Fe±37 at%Pt and Fe±36 at%Pt alloys, which showed values similar to SUS 447J1, could be attributed to small casting defects on the surface, since there were a few small pores on these alloy surfaces after polishing with No. 1000 SiC paper. These casting defects may have also affected the anodic polarization test results. The passive current density for Fe±40 at%Pt and Fe±38 at%Pt was markedly lower than for Fe±37 at%Pt and Fe±36 at%Pt in the 1.0% lactic acid solution. The potentials at the beginning of passivation for Fe±40 at%Pt (Fig. 1) and Fe±38 at%Pt were higher than for Fe±37 at%Pt and Fe±36 at%Pt in the 0.9% NaCl solution (Fig. 2). In a previous study of Fe±Pt alloys, Watanabe et al. [16] investigated the attractive force of Fe±Pt alloys to dental Fe14Nd2B magnets using the same experimental alloys as in this study. They reported that decreasing the Pt percentage (thus increasing the Fe percentage) increased the saturation magnetization values and resulted in an increase in attractive force to the magnet. Of the Fe±Pt alloys tested, Fe±36 at%Pt yielded the greatest values of saturation magnetization and attractive force to dental Fe14Nd2B magnets. In the present study, the decrease in Pt percentage (Fe±37 at%Pt and Fe±36 at%Pt) increased the amounts of Fe ions released into the 1.0% lactic acid solution. The defects on the cast surface may also play a role in the reduction of corrosion resistance. However, the anodic polarization results indicated a higher corrosion resistance by the Fe±Pt alloys compared to the SUS 447J1 alloy. It is generally assumed that increasing the amount of Fe (and decreasing the Pt percentage to less than 36 at%) improves magnetic properties (increase of attractive force); however, at the same time, it reduces corrosion resistance. From this point of view, Fe±36 at%Pt might be the appropriate composition for magnetic attachment keepers. If Fe±36 at%Pt is selected to make castable attachment keepers, the addition of a small amount of another element such as Si would be necessary to improve castability and reduce casting defects. Acknowledgements
Fig. 2. Potential/current density curve of SUS 447J1 and four Fe±Pt alloys in 0.9% NaCl solution.
This investigation was supported in part by Grant-in-Aid for young scientists, (A)11771229 and (A) 11771232, and Grant-in-Aid for scienti®c research (B)11694293 from the Ministry of Education, Science, Sports and Culture, Tokyo, Japan.
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I. Watanabe et al. / Dental Materials 17 (2001) 217±220
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