Dental Materials Journal 27(2)：203－210, 2008
Galvanic Corrosion of Ferritic Stainless Steels Used for Dental Magnetic Attachments in Contact with an Iron-platinum Magnet Keisuke NAKAMURA1, Yukyo TAKADA2, Masanobu YODA1, Kohei KIMURA1 and Osamu OKUNO2
Division of Fixed Prosthodontics, Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai 9808575, Japan 2 Division of Dental Biomaterials, Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai 9808575, Japan Corresponding author, Yukyo TAKADA; E-mail:
[email protected] 1
Received February 5, 2007/Accepted September 14, 2007 This study was an examination of the galvanic corrosion of ferritic stainless steels, namely SUS 444, SUS XM27, and SUS 447J1, in contact with a Fe-Pt magnet. The surface area ratio of each stainless steel to the Fe-Pt magnet was set at 1/1 or 1/10. Galvanic corrosion between the stainless steels and the magnet was evaluated by the amount of released ions and the electrochemical properties in 0.9% NaCl solution. Although each stainless steel showed sufficient corrosion resistance for clinical use, the amount of ions released from each tended to increase when the stainless steel was in contact with the magnet. When the surface area ratio was reduced to 1/10, the amount of Fe ions released from the stainless steels increased significantly more than when there was no contact. Since contact with the magnet which possessed an extremely noble potential created a very corrosive environment for the stainless steels, 447J1 was thus the recommended choice against a corrosion exposure as such. Keywords: Iron-platinum magnet alloy, Galvanic corrosion, Ferritic stainless steel
INTRODUCTION Dental magnetic attachments, which retain prostheses in the oral cavity by magnetic attractive force, are widely used in dental clinics1-6). Commercial dental magnetic attachments usually consist of a pair of magnetic assembly and keeper, which is fixed in a denture and on a root cap respectively. The magnetic assembly is usually constructed with a rareearth magnetic core covered with a ferritic stainless steel, such as SUS 444, SUS XM27, or SUS 447J1 “SUS” ( is a Japanese Industrial Standard for stainless steel). The ferritic stainless steel protects the magnetic core in corrosive environments and plays the role of yokes for magnetic circuits in order to increase the attractive force2,6). When commercial magnetic attachments are used for dental appliances, the magnetic assemblies have some limits: (1) cannot be shaped into the desirable shape by casting or cutting; (2) can be applied only to appliances with sufficient space to accommodate them; and (3) cannot be soldered or cast because heating beyond the Curie temperature would adversely affect the magnetic properties2). Since these disadvantages prevent magnetic attachments to be applied to removable crown bridges and partial dentures of relatively small size, their applicability is limited to full dentures or large partial dentures. One way to overcome these problems is to use iron-platinum magnets instead of the commercial magnetic assemblies. Some researchers have reported that iron-platinum magnet with a composition of Fe-39.5at％Pt-0.75at％Nb showed excellent
magnetic properties with a maximum energy product close to that of rare-earth magnets7). Kanno et al. reported that a iron-platinum magnet cast into a crown shape showed sufficient magnetic attraction to retain prosthetic appliances8). Additional benefits include the possibility of both casting9,10) and machining11) iron-platinum magnets and an excellent corrosion resistance close to that of platinum in vitro12). Consequently, these benefits led to a removable crown-bridge system composed of the iron-platinum magnet for external crowns and the ferritic stainless steel for inner caps8-11,13,14). Ferritic stainless steels containing not less than 19 mass％ of Cr and 1－2 mass％ of Mo are generally used for dental magnetic attachments. This is because sufficient corrosion resistance is required to endure galvanic corrosion when in contact with root caps, which are usually made of precious alloys15). It is established that ferritic stainless steels, such as SUS 444, SUS XM27, and SUS 447J1, show excellent corrosion resistance15). However, the iron-platinum magnet might create a seriously corrosive environment for the stainless steels when both are in contact, because the former has a much nobler potential than typical precious alloys used in dentistry. Takada et al. investigated the galvanic corrosion between stainless steels and dental precious alloys16). It was revealed that the amount of Fe ions released from the stainless steel of SUS 447J1 increased by about 1.5 times when the stainless steel was in contact with an Au-Ag-Pd alloy in 0.9％ NaCl solution. Further, it was expressly stated that the amount of ions increased by more than 1.7－2 times when the
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Corrosion of ferritic stainless steel
stainless steel was in contact with a gold alloy for metal-ceramic restorations, which has one of the noblest potentials of any dental alloy17). Since ironplatinum magnet has a much nobler potential than the gold alloy for metal-ceramic restorations, the contact with iron-platinum magnet might then cause fatal damages to ferritic stainless steels15,18). Therefore, it is of paramount importance to examine the galvanic corrosion between iron-platinum magnet and ferritic stainless steels in order to have successful results for the new removable crown-bridge system. The objective of this study, therefore, was to evaluate galvanic corrosion between the ferritic stainless steels and the iron-platinum magnet on the basis of released ions and specific electrochemical properties when they were in contact with each other under the conditions of dissimilar surface area ratios.
using a spinel investment compound for titanium casting (Titavest CB, J. Morita Mfg. Corp., Japan) at 600℃ on an argon pressure casting machine (Argon Caster C, Shofu Inc., Japan). After these castings of the Fe-Pt alloy were sealed in silica tubes evacuated under 5×10－5 torr and heated at 1,325℃ for 30 minutes, they were quenched in water containing ice and salt. Next, the castings were aged at 650℃ for nine hours under the same vacuum level. After aging, the castings were magnetized with a static magnetic field of 20 kG for 60 seconds (hereafter, the iron-platinum magnet will be referred to as Fe-Pt magnet). After the Fe-Pt magnets and square sheets of stainless steels were polished with 180- to 800-grit emery papers, they were ultrasonically cleaned in distilled water for one minute and rinsed with ethanol. These specimens were used for elution test and electrochemical evaluation. Elution test Each square stainless steel sheet was electrically connected to the Fe-Pt magnet with the same kind of stainless steel ribbon using a spot welder (NT-100A, Nippon Avionics Co. Ltd., Japan). After this assembly specimen was ultrasonically cleaned in ethanol for one minute, welds on the surface were sealed with water-resistant paint. The surface area ratio (SS/FePt) of the stainless steel to the magnet was set at 1/1 or 1/10, because it was expected that an external crown (the magnet) was as large as or much larger than an inner cap (stainless steel). Since 0.9％ NaCl solution induces the pitting corrosion of stainless steels19), it was chosen for the corrosion test of dental alloys. The assembly specimens were immersed in 0.9％ NaCl solution with saturated dissolved oxygen (6 mg/L) at 37℃ for seven days. The stainless steels and Fe-Pt magnet were also immersed under the same conditions. Five samples of each specimen were used for every condition. After each specimen was removed from the NaCl solution, released ions were qualitatively and quantitatively analyzed using ICP (IRIS/AP, Nippon Jarrell-Ash Co. Ltd., Japan). Statistical analyses of the released ions were performed using ANOVA
MATERIALS AND METHODS Materials Sheets (1 mm in thickness) of SUS 444 (NSS 444N, Nisshin Steel Co. Ltd, Japan), SUS XM27 (Shomac R26-1, Nippon Koshuha Steel Co. Ltd., Japan), and SUS 447J1 (Shomac 30-2, Nippon Koshuha Steel Co. Ltd., Japan), which contained dissimilar contents of Cr, were used in this study. Table 1 lists the compositions of the stainless steels. These ferritic stainless steel sheets were cut to a size of 10 mm square × 1 mm thickness. The iron-platinum magnet alloy was made from pure metals of Fe (>99.995 mass％, Toho Zinc Co. Ltd., Japan), Pt (>99.9 mass％, Tanaka Kikinzoku Kogyo Co. Ltd., Japan), and Nb (>99.9 mass％, Mitsui Mining & Smelting Co. Ltd., Japan). After the metals were weighed relative to the composition of Fe-39.5at％ Pt-0.75at％ Nb, they were melted using an argon arc melting furnace (TAM-4S, Tachibana Riko Co. Ltd., Japan) at a vacuum level below 5×10－5 torr. To ensure homogeneity, the button (15 g) was inverted over four times during the melting process. Resultant buttons of the Fe-Pt alloy were cast to a plate (7mm square×3mm thickness)
Table 1
Chemical compositions of the stainless steels (mass%) used in this study C
Si
Mn
P
SUS444
0.008
0.27
0.17
0.029
SUSXM27
0.002
0.34
0.09
SUS447J1
0.003
0.15
0.04
Ni
Cr
Mo
Fe
0.2
18.56
1.97
balance
0.019
0.17
26
1
balance
0.015
0.18
30
2
balance
NAKAMURA et al.
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(Scheffé’ s test) at a significance level of α=0.05. Electrochemical evaluation Rest potentials versus the Ag/AgCl reference electrode were measured in 0.9％ NaCl solution with saturated dissolved oxygen (6 mg/L) at 37℃ over 50 hours (n=3). The rest potentials were converted to potentials versus a normal hydrogen electrode (NHE). The potentials of the stainless steels and FePt magnet at 24 hours after immersion were also statistically analyzed using ANOVA (Scheffé’ s test) at a significance level of α=0.05. The anodic polarization curves of the stainless steels and Fe-Pt magnet were measured in 0.9％ NaCl solution, which was adjusted to a dissolved oxygen level below 0.2 mg/L (n=3). Measurement started from rest to 1.8 V (vs. NHE) immediately after immersion with a scanning rate of 0.5 mV/sec. In addition, the cathodic polarization curves of the Fe-Pt magnet were also obtained in 0.9% NaCl solution with saturated dissolved oxygen (n=3). Cathodic polarization started from rest to －1.5 V (vs. NHE) immediately or at 24 hours after immersion with the same scanning rate.
Fig. 1
Ions released from SUS 444 when in and not in contact with the Fe-Pt magnet. Fe ions were released from both SUS 444 and Fe-Pt magnet.
Fig. 2
Ions released form SUS XM27 when in and not in contact with the Fe-Pt magnet. Fe ions were released from both SUS XM27 and Fe-Pt magnet.
Fig. 3
Ions released from SUS 447J1 when in and not in contact with the Fe-Pt magnet. Fe ions were released from both SUS 447J1 and Fe-Pt magnet.
RESULTS Released ions The amount of ions released from each stainless steel in contact with the Fe-Pt magnet are summarized in Figs. 1－3. Cr and Mo ions were released from the stainless steels, whereas Fe ions were released from both the stainless steel and Fe-Pt magnet. Figure 1 shows the amounts of ions released from SUS 444 when in and not in contact with the Fe-Pt magnet. Few ions released from the magnet could be detected under the no-contact condition. Under the same condition, SUS 444 primarily released Fe ions, of which the amount was 1.68 (0.09) μg/cm2 and which was the average (standard deviation). When SUS 444 was in contact with the magnet, the amount of Fe ions released tended to increase as the surface area ratio (SS/FePt) decreased. The conditions of SS/FePt at 1/1 and 1/10 resulted in releasing Fe ions of 3.92 (2.48) μg/cm2 and 9.78 (6.15) μg/ cm2 respectively. Under the no-contact condition of SS/FePt at 1/1, there were no statistical differences between both amounts of Fe ions because of large standard deviations (p>0.05). However, under SS/ FePt at 1/10, the amount of Fe ions released significantly increased (p
