Microstructures, Hardness and Corrosion Behaviours in a Neutral Artificial Saliva of a Series of {Cobalt-Chromium}-Based Alloys; Influence of the Presence and Contents of the Minor Elements W, Mo and Si

Xin Li¹, Patrice Berthod^1,2* & Estelle Kretz¹

¹Institut Jean Lamour (UMR 7198), Faculty of Science and Technologies, University of Lorraine, Campus Victor Grignard, France
²Institut Jean Lamour (UMR 7198), Faculty of Science and Technologies, University of Lorraine, Campus ARTEM, France

Dr. Patrice Berthod, Institut Jean Lamour (UMR 7198), Faculty of Science and Technologies, University of Lorraine, France.

Keywords: Dental Cobalt-Chromium Alloys; Hardness; Corrosion in Artificial Saliva; Stationary Electrochemical Characterization; Impedance Spectroscopy; Tungsten Content Effect; Minor Elements Effects

Abstract

Dental cobalt alloys devoted to the constitution of frameworks strengthening fixed partial dentures may be based on cobalt and chromium. Such alloys generally contain also other elements such as tungsten, molybdenum and silicon, for example. These minor elements may be added for mechanical reasons. In contrast with the alloys of the High Noble and Noble categories which are rich in gold or platinum, these cheaper alloys may be threatened by corrosion. When it happens this phenomenon may cause health problems, allergic diseases typically. The topic of this work is to characterize the electrochemical behavior of six alloys based on cobalt and containing 25 wt.%Cr, in a NaCl 9g/L aqueous solution rated at pH=7.4 and T=37^°C. Special focus was done on the effect of the presence of W (2.5, 5 and 7.5 wt.%), Mo (5 wt.%) and/or Si (1 wt.%). These contents are typical of the ones in commercial dental alloys. The six alloys were all elaborated by high frequency induction foundry under inert atmosphere. The stationary electrochemical tests were driven according to the Stern-Geary and Tafel methods, or following the cyclic polarization technique. Electrochemical Impedance Spectroscopy was also applied. The results show that the presence of W and its content, on the one hand, and of Si, on the other hand, have significant effects on the microstructure of the alloys and of their room temperature hardness. Concerning the electrochemical results the ones issued from the stationary methods are more interesting that the ones obtained by impedance spectroscopy because of the lack of reproducibility of these EIS results. The Tafel experiments demonstrated that W did not have any systematic effect while Mo and Si are beneficial for the behaviour in corrosion. Corrosion resistance is in all case of high level, anyway, thanks to the sufficiently high Cr content present in all alloys. Thus, no significant release of cobalt ions is expected in mouth on long time.

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