Electrochemical behavior and released ions of the stainless steels used for dental magnetic attachments

Y. Takada, N. Takahashi and O. Okuno

Division of Dental Biomaterials, Graduate School of dentistry, Tohoku University

Introduction

Ferritic and austenitic stainless steels such as SUS 444, SUS XM27, SUS 447J1 and SUS 316(L) are commonly used for dental magnetic attachments whose attractive force is reinforced by magnetic yokes. ("SUS" is a Japanese Industrial Standard for stainless steel.) While the dental magnetic attachments work in an oral cavity, in other words, while attractive force between the magnetic assemblies and the keepers fixes a denture in an oral cavity, the magnetic assemblies and the keepers contact with root caps made of dental precious alloys in the corrosive environment. Therefore, it is very important to investigate the galvanic corrosion of those types of stainless steel and the precious alloys composing the dental magnetic attachment system.

Objective

The objective in this study is to examine the corrosion behavior of those types of stainless steel composing the dental magnetic attachments, and also to investigate the galvanic corrosion of the stainless steels when in contact with the dental precious alloys for the root caps, according to the results of their specific electrochemical properties and released ions.

Materials and Methods

Three types of ferritic stainless steel of SUS 444, SUS XM27 and SUS 447J1, and austenitic stainless steel of SUS 316L were examined. Their chemical compositions are shown in Table 1. The dental precious alloys such as a Au-Ag-Pd alloy (CASTWELL MC, GC), a type 4 gold alloy (PGA-2, Ishifuku) and a gold alloy for metal-ceramics (KIK, Ishifuku), which were often used for the root caps, were selected for evaluation of the galvanic corrosion of the stainless steels. Their chemical compositions are also shown in Table 2.

2. Methods

2.1 Elution Test

Specimens with a size of 10 mm x 15 mm x 1 mm were cut off from rolled sheets of those types of stainless steel. Surfaces of the specimen were finally polished by an emery paper with #800-grid, and were ultrasonically cleaned in distil water for a minute. After each specimen was immersed in 0.9%NaCl and 1%lactic acid solutions with saturated dissolved oxygen at 37^oC for 7 days (n=5), released ions in the solutions were qualitatively and quantitatively analyzed using ICP (IRIS_AP, Jarrel Ash).

2.2 Electrochemical Evaluations

The dental precious alloys were cast in accordance with the manufacturers' instructions, and the castings with a size of 10 mm x 10mm x 1 mm were prepared. The specimens with the same size of the castings were cut off from the rolled sheets of the stainless steels, and were also prepared for electrochemical evaluation. Preparations for the specimens were the same as those of the elution test. Rest potentials on the stainless steels and the dental precious alloys were measured in 0.9% NaCl solution with saturated dissolved oxygen at 37^oC. (n=3) Potentio-dynamic anodic and cathodic polarization curves of the stainless steels and the precious alloys respectively were also measured at a scanning rate of 0.5 mV/sec. (n=3) Each condition of the solutions is shown in Tables 3 and 4.

Results

1. Ions released from the stainless steels

All types of the stainless steels primarily released Fe ions in the lactic acid and NaCl solutions, and Ni ions were slightly released from only SUS 316L. Total amounts of ions (W_ions) released from the stainless steels tended to decrease in both solutions as Cr content ([Cr]) of the stainless steels increased. (Figs. 1 and 2) That (W_ions) from SUS 447J1 was smallest of those from the other stainless steels. (p < 0.05) The W_ions from the ferritic stainless steels were largely in proportion to the [Cr] according to the equations as following below;

W_ions = -0.133 [Cr] + 5.908 (r = 0.999) (in the lactic acid solution)
W_ions = -0.111 [Cr] + 3.911 (r = 0.953) (in the NaCl solution)

The total amount of ions (Wions) from SUS 316L, which was one of austenitic stainless steel, was smaller than that from SUS 444, even as these stainless steels contained the same content of Cr in substance. (p < 0.05)

2. Electrochemical behavior

Rest potentials on each stainless steel at 24 hours after immersion were near 0.17V, and had no significant difference. (p > 0.05) On the other hand, rest potentials on the dental precious alloys were significantly higher than those on the stainless steels except those on the Au-Ag-Pd alloy. (p > 0.05) The rest potentials on the Au-Ag-Pd alloy were higher than those on the stainless steels for about 5 hours form right after immersion, and after that, those on the Au-Ag-Pd alloy and the stainless steels were reversed each other.

According to the anodic polarization curves (Fig. 3), pitting potentials (E_pit) on the stainless steels were in proportion to their Cr content ([Cr]) as following below;

E_pit = 0.689[Cr] - 0.702 (r=0.996)

Figure 4 reveals that the stainless steels will break down when their potentials move into the region above the line drawn by this equation (E_pit). The SUS 316L showed the lowest value of pitting potential of the stainless steels. However, all rest potentials existed within passive regions which were placed below the line by the equation (E_pit).

3. Galvanic corrosion

The anodic polarization curves of the stainless steels and cathodic polarization curves of the dental precious alloys are summarized in Fig. 5. Regarding the cathodic polarization curves, the cathodic current density, which was tenfold larger than those on the precious alloys, is additionally drawn in the figure. (This situation occurs when a surface area of the precious alloys is tenfold larger than that of the stainless steels.) When each type of stainless steel is in contact with the precious alloys, the corrosion potential can be electrochemically obtained from the intersection of the anodic and cathodic polarization curves.

The corrosion potentials on each type of stainless steel existed within passive regions, and were sufficiently lower than the pitting potentials even when in contact with the precious alloys. As the surface area of the precious alloys increases tenfold larger than that of the stainless steels, the corrosion potential also rises but does not go beyond the passive region.

Conclusions

The ferritic stainless steels composing dental magnetic attachments showed excellent corrosion resistance according to the elution test and electrochemical evaluations. Therefore, these types of stainless steel could be safely used as magnetic yokes or keepers under the corrosive condition with the galvanic corrosion in an oral cavity.

Acknowledgements

The authors greatly acknowledge NEOMAX Co., Ltd., for providing the ferritic stainless steels as well as the members of the Industrial Technology Institute, Miyagi Prefectural Government, for cooperating with the ion analyses. This study was fully supported by the NEDO grant (05IS051).

References

1. Takada Y, Corrosion resistance of dental alloys evaluated from released ions, Corrosion Engineering, 49: 669-685, 2000.

Discussion Board