Stainless Steel Brackets: In Vitro Corrosion Behaviour and Mechanical Debonding Test

Abstract:

The purpose of the study was to investigate the microchemical, morphological, mechanical and anticorrosive characteristics of the metallic brackets by using energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), electrochemical and shear tests. The changes resulted from the exposure of the metallic brackets to artificial saliva and debonding forces were investigated under orthodontic appliances. The results have shown that metallic brackets studied in this paper present a good design, great electrochemical behaviour in artificial saliva for different pH values (2, 5 and 7), and also great mechanical properties. In the case of shear stress the obtained values were higher than the clinical accepted value. The metallic brackets studied in this paper present a good behaviour in artificial saliva, and good mechanical characteristics. The ARI investigation revealed that most of the adhesive was left on the bracket.

• Periodical: Solid State Phenomena (Volume 216)
• Pages: 187-193
• DOI: https://doi.org/10.4028/www.scientific.net/SSP.216.187
• Online since: August 2014
• Authors: Mihai Cosmin Cotrut, Mihai Tarcolea, Maria-Diana Vrânceanu*, Ana Iulia Gherghilescu, Nicolae Ghiban, Florin Baciu, Ionut Cornel Ionescu
• Keywords: Brackets, Corrosion, Debonding Test, Stainless Steel (SS)

References

[9] ARI Description 0 no adhesive left on tooth 1 less than half the adhesive left on tooth 2 more than half the adhesive left on tooth 3 enamel bonding site covered entirely with adhesive Results and discussions Scanning electron microscopy In figure 2 are presented the morphological aspects of the brackets and also the chemical composition of the metallic brackets. The morphology of the bracket provides comfort for the patient and reliability for the orthodontists. Its bracket ID makes bracket positioning easier during bonding. Figure 2. Morphological aspects of the metallic brackets at different magnification and EDS spectrum The specific design with mesh spacing can influence the penetration of adhesives, the escape of air and the effectiveness of bonding [10]. So it can be said that the mechanical retention is directly influenced by the bracket's design. Table 2. Chemical composition of stainless steel brackets Element Wt % At % Si 0.35 0.69 Mo 2.59 1.50 Cr 19.42 20.81 Mn 1.53 1.55 Fe 66.46 66.29 Ni 9.66 9.16 Total 100.000 100.000 The elemental composition of the bracket, obtained by EDS (Table 2) revealed that the composition is similar to stainless steel alloy [11], which is commonly used in orthodontics [12]. Electrochemical tests The electrochemical evaluation of a dental alloy in artificial saliva permits the estimation of the behaviour of the material in the oral cavity. The modification of the dental alloy properties could be determined using rapid electrochemical tests as a qualitative criterion to estimate their corrosion resistance [13]. After determination of the parameters from the potentiodynamic curves, the corrosion rate in different pH values (2, 5 and 7) of artificial saliva were calculated according to the ASTM G102-89 (2004), using the following equation: CR= KiIcorρ EW ([14]) where CR is the corrosion rate, Ki - a constant (3.27 x 10-3), ρ - the materials density, Icorr - the corrosion current, and EW - the equivalent weight. Figure 3. Potentiodynamic polarization curves of the stainless steel bracket Table 3. Electrochemical parameters of the stainless steel bracket in different pH values of the Fusayama – Mayer solution: open circuit potential (EOC), corrosion potential (Ecorr), corrosion current density (Icorr), corrosion rate (CR) Artificial saliva Eoc

[10] -3 mm/y] pH=2 -228.342 -222.106 185.757 23.832 pH=5 -264.151 -265.059 17.894 5.969 pH=7 -211.934 -215.808 5.339 0.755 It is considered that one material presents a better corrosion resistance if the Ecorr potential exhibits more electropositive value. If we take into account this criterion, it is noted that the bracket immersed in the artificial saliva with pH=7 showed the most electropositive corrosion potential (215.808 mV), followed by the bracket tested in pH = 2 (222.106 mV) and pH = 5 (265.059 mV). Regarding the obtained Icorr values, it can be seen that the lowest registered value was achieved in pH=7, indicating the best corrosion behaviour (Icorr = 17.894 nA). The low corrosion rate was obtained for the samples immersed in artificial saliva with pH=7 (CR = 0.755* 10-3 mm/y) compared to the ones immersed in acidic saliva (pH=2 and pH=5). The higher corrosion rate was measured for the samples immersed in pH=2. Mechanical testing In figure 4 is presented the diagram of the mechanical debonding tests. These tests revealed that the metallic bracket tested have registered similar values, the differences between them could be explained by the clinical protocol of mounting the bracket on the tooth. Figure 4. Shear bonding stress – displacement diagram of the tested brackets A higher debonding force value was registered for the bracket coded with M1 (~ 137 N) while for the other two brackets M2 and M3 the values were 101 and 102 N (Table 4). The shear stress value of the brackets was calculated and presented in Table 4. Tabel 4. Force and shear stress values of the mechanical tests Sample Force (N) Shear stress (MPa) M1 137.03 13.703 M2 101.78 10.178 M3 102.86 10.286 The shears stress values obtained for the system bracket – tooth, are high enough (9.98 MPa) to assure a stable and normal behaviour during its use [15]. The highest value was noticed for bracket M1 (13.703 MPa), while M2 and M3 presented very close values (around 10 MPa). One may observe that the shear stress of all samples was higher than the clinical accepted value (9.98 MPa). Adhesive Remaining Index The macroscopic evaluation of the shear bond strength (SBS) of the metallic brackets is showed in Figure 5. The images have been processed using Image J software. a) b) c) Figure 5. Macrographs aspects of the brackets after debonding: a) M1, b) M2 and c) M3 It has been shown that failure occurred at the bracket-adhesive interface (the remaining adhesive on the bracket had values between 65-75% of the base area). The obtained results are in accordance to those reported in the literature, which attested that the most common failure site of debonding is at the bracket – adhesive interface [16]. Conclusions The investigated metallic bracket presented a good design, demonstrated by SEM microscopy, and also high corrosion resistance and mechanical properties. Regarding the electrochemical tests, it can be said that the stainless steel brackets have a low and high corrosion rate in artificial saliva with pH of 7 and 2, respectively. For patients with special medical conditions, the acidity of the environment (low pH) leads to shift the electrochemical equilibrium towards unfavourable conditions lowering generally the corrosion resistance and increasing the metallic ions to be released from the metallic brackets [17]. So, in those cases, it is recommended that the orthodontic treatment to be performed with ceramic or plastic brackets. In normal circumstances, that is to say otherwise healthy patients with normal saliva flow, healthy diet and regular bracket placement, corrosion is to be found in its normal range. The mechanical test has revealed debonding values higher than minimum required for the metallic brackets, making them safely to be used in the clinical orthodontics. Also ARI shows that the most part of remaining adhesive is on brackets surfaces, desirable thing in clinicians orthodontist work. Acknowledgement This research was supported under the Romanian R&D Project no. 175/2012 - Coat4Dent Refrences

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