Use of Simple Binary Co-xCr Alloys (0  x  30wt.%) for Exploring the Influence of the Chromium Content in Dental Cobalt-Based Alloys on Their Passivation Behavior in a Fusayama

Patrice Berthod, Use of Simple Binary Co-xCr Alloys (0  x  30wt.%) for Exploring the Influence of the Content in Dental ---------------------------------------------------------------------------------------------------------------------------------ISSN: 2642-1747 Research Article Abstract More and more Predominantly Base alloys are used instead the expensive dental alloys containing high concentration in noble metals. Among these PB alloys the ones based on cobalt and chromium are rather interesting since they bring high mechanical resistance, and do not risk induce any nickel release in mouth. Their corrosion resistance is due to the presence of chromium the content of which may be optimized. In this work contribution to a better knowledge of the critical Cr content to necessarily exceed for a corrosion resistance high enough is done by exploring the direct relationship between the corrosion behavior and the Cr content, by choosing a particularly aggressive artificial saliva and a series of binary alloys with an increasing Cr content, in order to exclude all other possible parameters susceptible to interfere. The different electrochemical results free potential follow-up, Stern-Geary results and the cyclic polarization curves - demonstrate that 20wt.%Cr should be enough to allow cobalt-based dental alloys rapidly developing a protective passivation layer and showing a good resistance against corrosion in the buccal milieu. This suggests that the current chromium contents of commercial cobalt-based dental alloys may be lowered down to this threshold value without loss of


Introduction
Some dental prostheses involve the use of metallic alloys. This is for example the case of fixed partial dentures in which metallic alloys of different natures present under the cosmetic ceramic part and covered to be out from view, brought the major part of the mechanical resistance to the compressive and flexural solicitations induced by mastication. Alloys based on cobalt and chromium belong to a the Predominantly Base (PB) dental alloys category, which co-exists beside (Au, Pt, Pd…)-rich alloys of the Noble (N) and High Noble (HN) families, as alternative cheaper materials for dental restoration. As the other PB alloys which are based on nickel and chromium, these cobalt alloys may bring high mechanical properties and good corrosion behavior but with less risk of allergy (in case of nickel release in mouth). The chemical compositions of the cobalt-chromium PB alloys are generally complex since they may contain also tungsten, gallium, rhenium, aluminum… in addit ion to chromium the content of which may furthermore vary over a rather broad range (e.g. from 20 to 35wt.%Cr [1] in commercial alloys. Prosthetic pieces in Co-based PB alloys are generally prepared by casting (e.g. alloy melted by torch and driven in molds by centrifugal casting), but they may be also synthesized following various other elaboration routes (milling, selective laser melting… [2]). Cobalt-based alloys are known for their good mechanical behavior under many kinds of stresses [3], static [4] or dynamic studies concerning the influence of the chromium content are seemingly lacking. Binary and ternary alloys with Co and Cr as major elements were considered to investigate microstructures and mechanical/chemical properties [8], but not the corrosion properties in complex electrolytes imitating human saliva. Knowing how the corrosion behavior may vary with the chromium content can be considered as fundamental, even if many other parameters characterizing an alloy (other major elements present, minor elements, grain size, hardening state…) may also influence and more or less hide the sole effect of Cr.
This work aims to isolate the influence of the chromium content from the ones of all the chemical composition or the fabrication process factors, in order to favor the direct observation of how the corrosion behavior depends on the chromium content as single varying parameter. This was done by elaborating in a controlled manner a series of binary alloys, by preparing both electrode and electrolyte following a procedure strictly applied, and by carrying out the electrochemical tests repeatably by fixing all conditions. So the single input data was the chromium content (with six alloys with a Cr content of 0 to 30wt.% by slices of 5wt.%) and the output results were the initial behavior shortly after immersion and the response to severe anodic polarization followed by its progressive suppression.
The synthesis of each alloy started by putting the metallic charges in the metallic crucible of an induction furnace (CELES, France). This crucible, made of copper, was segmented to hinder the induced current circulation, and internally cooled by a continuous circulation of water maintained at ambient temperature. This permanent cooling aimed to avoid heating due to the Foucault currents induced by the electromagnetic field and by the proximity of the molten alloy. The Co and Cr charges being placed in the copper crucible, a silica tube was positioned around the crucible, between it and the copper coil (itself cooled by an internal continuous water flow) in which the alternative current will circulate during melting. The fusion chamber being closed, pumping was carried out until obtaining an internal pressure of about 5×10 -2 millibars.
Pure argon was then injected to rise the internal pressure up to 0,8 atm. Pumping followed by Ar injection were applied two times more and the preparation of the final atmosphere was terminated by supplementary addition of pure Ar up to 300 millibars. The gaseous environment of pure argon with a purity close to the one of the origin Ar bottle with a pressure high enough allowed avoiding both oxidation of Co and notably Cr during the heating, melting, isothermal stage and cooling, and the evaporation of Co and Cr in the chamber. The injected voltage progressively increased from 0 to 4 000 Volts (total duration: about 2 minutes) for a frequency of the resulting current (several Amperes) of about 110 000 Hertz. This provoked the heating of both Co and Cr, the melting of Co and the dissolution of Cr in the molten Co. An isovoltage stage at 4 kV-10 kHz was maintained for 5-minute s to favor the chemical homogenization of the alloy. A progressive decrease in injected voltage allowed the cooling of the levitating liquid alloy, its solidification and its solidstate cooling. After less an hour, the obtained ingot was cold enough to be removed from the furnace and handled. Three photographs illustrate the elaboration process in Figure 1.

Preparation of samples for all characterizations
Each obtained ingot was sawed using a metallographic saw

Electrochemical experiments
The experimental apparatus exploited for specifying the corrosion behavior of the alloys was composed of: Three electrodes: the working electrode the preparation of which is described above, a platinum auxilliary electrode and a potential-reference Saturated Calomel electrode.
The working electrode was placed in the bottom part of the cell, with its planar metallic part oriented upward and parallel to the platinum disk of the auxiliary electrode (distance between them: about 5mm). The electrolyte of study which was chosen was the Fusayama artificial saliva, the composition of which is presented in Table 1. This solution is commonly used for the corrosion characterization of all types of dental alloys, from the High Noble and Noble categories [9][10][11] to different kinds of Predominantly Base alloys [12][13][14]. Its pH being a major factor involved in the aggressiveness of an artificialsaliva (more corrosive in case of acidic pH), special care is to be given to it. Dental plaque pH may vary depending on the dental oral care and food habit [15,16]. In the present case, after its preparation, our Fusayama saliva was acidified to decrease its pH down to 2.3, pH recommended for such corrosion studies [17]. iii. Cyclic polarization consisting of an increase of applied potential at 1mV/s from E ocp -150mV up to Eocp +1,23V, followed by its decrease at -1mV/s down to its initial value. These results will be compared with the Pourbaix diagrams of cobalt and of chromium. These later ones are reminded in Figure 2.    Table 3. The addition of more and more chromium obviously induces a rather regular increase in hardness, from about 140 to 250 Hv 10kg .

Electrochemical Results / Corrosion Behavior: Qualitative and Semi-Quantitative Results
The first data obtained concerning the behavior in corrosion of    These assumptions found confirmation in the potential-

Electrochemical Results / Corrosion Behavior: Quantitative Results
The cyclic polarization curves were exploited to specify some data characterizing the easiness of passivation (J critical and E passiv ) and the corrosion resistance in passive state (J passiv ). J critical is the critical current that is compulsory to exceed by the alloy in order to get passive and E passiv is the corresponding passivation potential to exceed. A better easiness to become passive is demonstrated by a lower critical current (or a smaller/less extended anodic peak) and by a lower passivation potential easier to obtain in case of low presence of oxidants. J passiv is the anodic current at high potential when the alloy is covered by a passivation layer: this one is more protective if the passivation current is lower. One also made use of the Tafel-type of the E-increasing and E-decreasing parts in the neighborhood of the cathodic→anodic and anodic→cathodic transitions respectively, to apply the Tafel method in order to assess the values of corrosion density of current (J corr ) and of corrosion potential (E corr ) before and after the polarization at high potential. The determination of all these data is explained in Figure 8 and the obtained values listed in Table 5.  With corrosion currents of more than 10 µA/cm² before polarization at high potential one confirms that the alloys containing 15wt.%Cr or less are in an active state. The initial current densities of corrosion of the alloys containing more chromium are much lower: less than 0.1 µA/cm², as is to say more than two orders of magnitude below. The 30wt.%Cr-containing alloy presents particularly slow corrosion with less than 30nA/cm². This confirms that the alloys with 15wt.%Cr and more became passivated prior to the beginning of the cyclic polarization run, during the 2 hours -E ocp follow-up.
Near the end of the cyclic polarization curves the Tafel features characterizing the last parts of the E-decreasing curves were also exploited ( Table 6). For the Cr-lowest alloys (which did not passivate) as well as for the Cr-richest ones (which were already passivated before start of cyclic passivation), this led to corrosion current densities which are more or less higher than the initial J corr values (e.g. 23µA/cm² against only 11 initially for the Cr-free alloy and 0.49µ/cm² against only around 0.1 for the 25wt.%Crcontaining alloy). This is due to the oxidation of the solvent (water) in gaseous oxygen (additional O2, also dissolved in the solution) which led locally to a higher concentration in strong oxidant species.
For the alloys with intermediate Cr contents (10 and 15 wt.%Cr), such comparison cannot be done since the states before and after polarization at high potential are not the same: active state before and passive state after. The current densities of corrosion are, in both cases, much lower after polarization at high potential than before (e.g. 0.3µA/cm² against one hundred times more before, for the 15wt.%Cr-containing alloy). One can additionally mention that, with both a critical current of passivation lower than the 10wt.%Crcontaining alloy's one (1.3 against 1.7mA/cm²) and a lower passivation potential (+87 against +265mV/NHE), the 15wt.%Crcontaining alloy is easier to passivate.

Discussion
Using high purity cobalt and chromium allowed obtaining a series of binary cobalt-chromium alloys for this study devoted to the specific role of the chromium content. Using high frequency induction melting in cold crucible allowed obtaining equiaxed fine- significantly higher (neutral and basic) as this may be seen in Figure   2 (left side  Figure 3 to the right chromium oxide/hydroxide precipitation (pH = 4 to 5, as some other Fusayama saliva [19,20]) may occur for lower concentrations of Cr 3+ cations, this authorizing lower Cr content in the alloy. Because of the differential costs between cobalt and chromium (the more expensive of the two), difficulty of melting (Cr is the most refractory out of the two) and the threat of possible precipitation of the brittle Co-Cr sigma phase in some solidification/solid state cooling conditions, less Cr presents some potential advantages. New elements can be also added in small proportions to pre-passive the alloys before chromium oxide/ hydroxide precipitation occurs, as the addition of 4wt.%Mo to the 316 austenitic stainless steel does by pre-precipitation of iron molybdate allowing the 316L version to behave as an auto-passive alloy.

Conclusion
Metallic alloys based on unalterable metals such as gold or platinum are used since a long time in dental prostheses. Their undeniable interest was the total absence of corrosion in the buccal milieu, in normal conditions. Since several decades ago one knows that much cheaper alloys may be used instead, but with some