A Comparative Electrochemical and Morphological Investigation on the Behavior of NiCr and CoCr Dental Alloys at Various Temperatures

: The purpose of our study is to compare the behavior of two reprocessed dental alloys (NiCr and CoCr) at different temperatures considering the idea that food and drinks in the oral cavity create various compositions at different pH levels; the novelty is the investigation of temperature effect on corrosion parameters and ion release of dental alloys. Electrochemical stability was studied together with morphology, elemental composition and ions release determination. The results obtained are in good concordance: electrochemistry studies reveal that the corrosion rate is increasing by increasing the temperature. From SEM coupled with EDS, the oxide ﬁlm formed on the surface of the alloys is stable at low temperatures and a trend to break after 310K. ICP-MS results evidence that in accordance with increasing temperature, the quantities of ions released from the alloys immersed in artiﬁcial saliva also increase, though they still remain small, less than 20 ppm.


Introduction
In the long history of dental biomaterials, precious and nonprecious metals and alloys with Au, Ag, Ti, Fe, Cr, Co, Al, have been frequently selected due to their numerous favorable, chemical, biological physical features as well as mechanical properties [1][2][3][4]. These materials have significant mechanical properties, such as stiffness strength and corrosion resistance and are more easily used and cheaper to elaborate; processed CoCr and NiCr alloys are still frequently investigated and used in dental laboratories as convenient choices for restorative dental works such as crow, bridge, and denture bases [5][6][7][8][9]. Extended use of metals as regenerative biomaterials starting the end of the last century has led to a strong development of titanium and titanium alloys materials introducing intensive binary and ternary alloys with Al, Nb, V, and Zr as selected alloys [10][11][12]. Due to an easier processing method in common dental restorative works, NiCr and CoCr alloys remain widely used.
The main aim of the above cited papers was to determine the corrosion behavior of dental alloys in various media, starting from the idea that food and drinks in the oral cavity introduce various compositions establishing different pH levels ranging from acid to basic values [6]. Corrosion of dental alloys in the intra-oral cavity is leading not only to the deterioration of restorative works, but also involves a release of ions that is related to their biocompatibility [13]. In the literature, there are few data [14] about corrosion resistance properties of Co-Cr alloys which have been enhanced at various temperatures compared to those relating to Ni-Cr alloys; however, the results are incomplete and need extensive investigations especially nowadays when discussion about their toxicological risks are more aggressive. Considering that commercial dental alloys are the subject of a reprocessing Before electrochemical measurement, both types of studied alloy were subjected to thermal reprocessing. Samples of cylindrical shape purchased from the two companies, with a diameter of 0.8 cm and height of 2 cm, were casted according to manufacturer's recommendations using a laboratory lost wax technique. A centrifugal casting induction foundry machine and ceramic crucibles were used to melt and cast the samples. [36,37]. The samples obtained had a disc shape with a diameter of 2 cm. The weight of the samples was 3.42 ± 0.15 g. The number of each sample used for all experiments is 5. Then, all the samples were wet grinded with SiC paper of 350, 800 and 1200 grits. After polishing, the NiCr and CoCr alloys were washed with water and ultrasonicated for 10 min in acetone to remove impurities on the surface because of polishing and drying at room temperature. As always, the surface exposed to electrochemical testing was 1 cm 2 .
The morphology of the samples, by SEM analysis, before the electrochemical testing is shown in Figure 1. Secondary electron images were acquired at different magnifications: 30× and 1k×. The surface of the samples is smooth and homogeneous. There is no well-defined structure that is visible on the surface before the electrochemical procedure. The surface of both alloys looks "clean". with a diameter of 0.8 cm and height of 2 cm, were casted according to manufacturer's recommendations using a laboratory lost wax technique. A centrifugal casting induction foundry machine and ceramic crucibles were used to melt and cast the samples. [36,37]. The samples obtained had a disc shape with a diameter of 2 cm. The weight of the samples was 3.42 ± 0.15 g. The number of each sample used for all experiments is 5. Then, all the samples were wet grinded with SiC paper of 350, 800 and 1200 grits. After polishing, the NiCr and CoCr alloys were washed with water and ultrasonicated for 10 min in acetone to remove impurities on the surface because of polishing and drying at room temperature. As always, the surface exposed to electrochemical testing was 1 cm 2 .
The morphology of the samples, by SEM analysis, before the electrochemical testing is shown in Figure 1. Secondary electron images were acquired at different magnifications: 30x and 1 kx. The surface of the samples is smooth and homogeneous. There is no welldefined structure that is visible on the surface before the electrochemical procedure. The surface of both alloys looks "clean".

The Electrochemical Procedures
The electrochemical tests have been open circuit potential determination, Tafel procedure and electrochemical impedance spectroscopy (EIS). The electrochemical measurements were performed on AutoLab PGSTAT 12 EcoChemie potentiostat/galvanostat (Radiometer Analytical SAS, Lyon, France), at various temperatures ranging from 290 K until 330 K. The potentiodynamic polarization tests have been performed starting from cathodic region with very low scan rate of 2 mV/s [22]. EIS characterization was carried out within 10 5 Hz and 10 −2 Hz frequency range, with an ac voltage amplitude of 10 mV at open circuit potential. The impedances were represented as Nyquist and Bode spectra.
All electrochemical experiments were performed with a classical electrochemical three-electrodes cell (Metrohm, Bucharest, Romania) as follows: a Pt plate for counter electrode Ag/AgCl, KCl system as reference electrode and working electrode.

Surface Analysis and Ion Release
The surface morphology of NiCr and CoCr alloy after potentiodynamic polarization in artificial saliva at different temperatures was studied by scanning electron microscopy (SEM, model Quanta 650 FEG from FEI company) (Hillsboro, OR, USA) at a pressure of 0.7 torrs at 10 and/or 15 kV. EDS analyses were performed to identify the elemental composition of the samples.
To get information about the hydrophilic/hydrophobic properties of the studied alloy, the static contact angles were measured. These measurements were performed using a contact angle CAM 100 equipment (KSV Instruments Ltd, Helsinki, Finland) and the same solution as for electrochemical measurements. Five measurements on five different areas of the same specimen were made [39].

The Electrochemical Procedures
The electrochemical tests have been open circuit potential determination, Tafel procedure and electrochemical impedance spectroscopy (EIS). The electrochemical measurements were performed on AutoLab PGSTAT 12 EcoChemie potentiostat/galvanostat (Radiometer Analytical SAS, Lyon, France), at various temperatures ranging from 290 K until 330 K. The potentiodynamic polarization tests have been performed starting from cathodic region with very low scan rate of 2 mV/s [22]. EIS characterization was carried out within 10 5 Hz and 10 −2 Hz frequency range, with an ac voltage amplitude of 10 mV at open circuit potential. The impedances were represented as Nyquist and Bode spectra.
All electrochemical experiments were performed with a classical electrochemical threeelectrodes cell (Metrohm, Bucharest, Romania) as follows: a Pt plate for counter electrode Ag/AgCl, KCl system as reference electrode and working electrode.

Surface Analysis and Ion Release
The surface morphology of NiCr and CoCr alloy after potentiodynamic polarization in artificial saliva at different temperatures was studied by scanning electron microscopy (SEM, model Quanta 650 FEG from FEI company) (Hillsboro, OR, USA) at a pressure of 0.7 torrs at 10 and/or 15 kV. EDS analyses were performed to identify the elemental composition of the samples.
To get information about the hydrophilic/hydrophobic properties of the studied alloy, the static contact angles were measured. These measurements were performed using a contact angle CAM 100 equipment (KSV Instruments Ltd, Helsinki, Finland) and the same solution as for electrochemical measurements. Five measurements on five different areas of the same specimen were made [39].
Ions release was quantified using inductively coupled plasma mass spectrometry (ICP-MS), work that has been carried out under constant temperature and humidity conditions of an ELAN DRC-e ICP-MS equipment from Perkin Elmer Company (Waltham, MA, USA). The conditions were ensured by working in an atmosphere free from contaminants of clean class ISO class 7 enclosures.
For electrochemical tests, but also for EDS and ICP-MS measurements, statistical analyzes were performed.

Open Circuit Potential Tests
Open circuit potential (OCP) measurements were made for 10 min for NiCr as well as for CoCr alloy. As we can see in Figure 2, in the first seconds of immersion in artificial saliva the potential values shift rapidly towards electronegative values. This first stage is followed by a much slower movement towards electronegative values, ending with a plateau. As can be observed from Figure 2, for both studied alloys, the stable value of the reached potential has increasingly electropositive values as the temperature of the artificial saliva increases.
Ions release was quantified using inductively coupled plasma mass spectrometry (ICP-MS), work that has been carried out under constant temperature and humidity conditions of an ELAN DRC-e ICP-MS equipment from Perkin Elmer Company (Waltham, MA, USA). The conditions were ensured by working in an atmosphere free from contaminants of clean class ISO class 7 enclosures.
For electrochemical tests, but also for EDS and ICP-MS measurements, statistical analyzes were performed.

Open Circuit Potential Tests
Open circuit potential (OCP) measurements were made for 10 min for NiCr as well as for CoCr alloy. As we can see in Figure 2, in the first seconds of immersion in artificial saliva the potential values shift rapidly towards electronegative values. This first stage is followed by a much slower movement towards electronegative values, ending with a plateau. As can be observed from Figure 2, for both studied alloys, the stable value of the reached potential has increasingly electropositive values as the temperature of the artificial saliva increases. It is important to mention that such aspects of potential variation in time for both dental alloys represent a trend to stable state in short terms with the formation of specific oxide films [40]. In the case of CoCr, the mixture of oxides is of all present elements meaning Co, Cr, Mo, Mn, W and Ga. In the case of NiCr alloy, the film contains oxides of Ni, Cr, Mo, Si and Fe.

Electrochemical Impedance Spectroscopy Measurements
The NiCr and CoCr alloys behavior was also studied by electrochemical impedance measurements at open circuit potential after 10 min of immersion in Fusayama-Meyer artificial saliva. Electrochemical impedance spectroscopy (EIS) tests for NiCr alloy in artificial saliva have led to the recording of the spectra presented in the Nyquist and Bode diagrams as we can see from Figure 3. As we can see from the Nyquist diagrams ( Figure  3a), a single open capacitive appears. The diameters of semicircles decrease with temperature increase, indicating a decrease in polarization resistance value, which leads to an increase in the corrosion rate. The Bode diagram (Figure 3b) reveals the presence of one single time constant corresponding to the capacitive semicircles from the Nyquist diagrams. As we can observe, the maximum phase angle moves to higher frequencies and decreases slightly once the temperature of artificial saliva increases. The decrease in the maximum phase angle values from −75° to −70°, indicate the corrosion of NiCr alloy. It is important to mention that such aspects of potential variation in time for both dental alloys represent a trend to stable state in short terms with the formation of specific oxide films [40]. In the case of CoCr, the mixture of oxides is of all present elements meaning Co, Cr, Mo, Mn, W and Ga. In the case of NiCr alloy, the film contains oxides of Ni, Cr, Mo, Si and Fe.

Electrochemical Impedance Spectroscopy Measurements
The NiCr and CoCr alloys behavior was also studied by electrochemical impedance measurements at open circuit potential after 10 min of immersion in Fusayama-Meyer artificial saliva. Electrochemical impedance spectroscopy (EIS) tests for NiCr alloy in artificial saliva have led to the recording of the spectra presented in the Nyquist and Bode diagrams as we can see from Figure 3. As we can see from the Nyquist diagrams ( Figure 3a), a single open capacitive appears. The diameters of semicircles decrease with temperature increase, indicating a decrease in polarization resistance value, which leads to an increase in the corrosion rate. The Bode diagram (Figure 3b) reveals the presence of one single time constant corresponding to the capacitive semicircles from the Nyquist diagrams. As we can observe, the maximum phase angle moves to higher frequencies and decreases slightly once the temperature of artificial saliva increases. The decrease in the maximum phase angle values from −75 • to −70 • , indicate the corrosion of NiCr alloy.  Electrochemical impedance spectroscopy (EIS) tests were also made for CoCr studied alloy in artificial saliva. In Figure 4, Nyquist and Bode diagrams are presented and in this case, a single open capacitive semicircle can be seen in the Nyquist diagram (Figure 4a), the diameter of which decreases as the temperature of the saliva increases. This indicates a decrease in polarization resistance value, which leads to an increase in the corrosion rate. Correspondingly, on the Bode diagram (Figure 4b), the presence of one single time constants can be observed. The maximum phase angle moves to higher frequencies and decreases slightly once the temperature of artificial saliva increases. The decrease in the maximum phase angle values from −81° to −62° is more pronounced than in the case of NiCr alloy.

Potentiodynamic Tests
The next step was to record the polarization curves for both studied alloys. First, the NiCr alloy behavior in Fusayama-Meyer artificial saliva at different temperatures using the potentiodynamic polarization method was studied. The increase in the corrosion current density values, once the temperature of the electrolyte increase, can be observed. In addition, a shift of corrosion potential towards electropositive values with increase in the temperature can be seen from Figure 5. Electrochemical impedance spectroscopy (EIS) tests were also made for CoCr studied alloy in artificial saliva. In Figure 4, Nyquist and Bode diagrams are presented and in this case, a single open capacitive semicircle can be seen in the Nyquist diagram (Figure 4a), the diameter of which decreases as the temperature of the saliva increases. This indicates a decrease in polarization resistance value, which leads to an increase in the corrosion rate. Correspondingly, on the Bode diagram (Figure 4b), the presence of one single time constants can be observed. The maximum phase angle moves to higher frequencies and decreases slightly once the temperature of artificial saliva increases. The decrease in the maximum phase angle values from −81 • to −62 • is more pronounced than in the case of NiCr alloy.  Electrochemical impedance spectroscopy (EIS) tests were also made for CoCr studied alloy in artificial saliva. In Figure 4, Nyquist and Bode diagrams are presented and in this case, a single open capacitive semicircle can be seen in the Nyquist diagram (Figure 4a), the diameter of which decreases as the temperature of the saliva increases. This indicates a decrease in polarization resistance value, which leads to an increase in the corrosion rate. Correspondingly, on the Bode diagram (Figure 4b), the presence of one single time constants can be observed. The maximum phase angle moves to higher frequencies and decreases slightly once the temperature of artificial saliva increases. The decrease in the maximum phase angle values from −81° to −62° is more pronounced than in the case of NiCr alloy.

Potentiodynamic Tests
The next step was to record the polarization curves for both studied alloys. First, the NiCr alloy behavior in Fusayama-Meyer artificial saliva at different temperatures using the potentiodynamic polarization method was studied. The increase in the corrosion current density values, once the temperature of the electrolyte increase, can be observed. In addition, a shift of corrosion potential towards electropositive values with increase in the temperature can be seen from Figure 5.

Potentiodynamic Tests
The next step was to record the polarization curves for both studied alloys. First, the NiCr alloy behavior in Fusayama-Meyer artificial saliva at different temperatures using the potentiodynamic polarization method was studied. The increase in the corrosion current density values, once the temperature of the electrolyte increase, can be observed. In addition, a shift of corrosion potential towards electropositive values with increase in the temperature can be seen from Figure 5.  The same type of behavior can be seen in Figure 6 for the CoCr studied alloy. The increase in temperature leads to an increase in the corrosion current density, indicating a more accelerated corrosion of this artificial saliva alloy. For these alloys, a passive region was observed. The decrease in the passive range occurs with the increase in the electrolyte temperature.  Table 2. The results showed similar values for the corrosion current density, regarding the method used for the analysis. Statistical analyses were performed for all parameters and they are presented as mean ±1 standard deviation.  The same type of behavior can be seen in Figure 6 for the CoCr studied alloy. The increase in temperature leads to an increase in the corrosion current density, indicating a more accelerated corrosion of this artificial saliva alloy. For these alloys, a passive region was observed. The decrease in the passive range occurs with the increase in the electrolyte temperature.  The same type of behavior can be seen in Figure 6 for the CoCr studied alloy. The increase in temperature leads to an increase in the corrosion current density, indicating a more accelerated corrosion of this artificial saliva alloy. For these alloys, a passive region was observed. The decrease in the passive range occurs with the increase in the electrolyte temperature.  Table 2. The results showed similar values for the corrosion current density, regarding the method used for the analysis. Statistical analyses were performed for all parameters and they are presented as mean ±1 standard deviation.  Corrosion kinetic parameters obtained from these polarization curves shown in Figures 4 and 5 were calculated using two methods: Tafel slopes extrapolation and polarization resistance. The values obtained for the corrosion potential (E corr ), corrosion current density (i corr ), the anodic (B a ) and cathodic (B c ) slope, polarization resistance (R P ) are shown in Table 2. The results showed similar values for the corrosion current density, regarding the method used for the analysis. Statistical analyses were performed for all parameters and they are presented as mean ±1 standard deviation.
The two methods yielded similar values of corrosion current density. The polarization resistance decreases for the studied alloys with the increase in the temperature of the electrolyte, indicating an increase in the corrosion rate of each of the biomaterials in Fusayama-Meyer artificial saliva.
The obtained results are in accordance with the data from the literature, according to which the CoCr-type alloys have a better corrosion behavior than the NiCr-type alloys [16,41]. The presence of chromium, tungsten, and molybdenum leads to passivation, which leads to improved corrosion performance of alloys containing these elements [42]. It is well known that Cr improves the corrosion resistance of alloys in a corrosive environment due to the formation of a Cr-rich, passive oxide film Cr 2 O 3 formed on the alloy surface. The Cr content determines passive film stability. A higher content of Cr 2 O 3 and MoO 3 in the passive film leads to a significant resistance to metal ion transfer through the passive film. The uniform distribution of Cr is important for better corrosion resistance compared with Cr 2 O 3 . Regarding the oxide of nickel which is more porous and has less protective ability against corrosion, it is important to mention that the film zones, rich in NiO, will act as weak regions for localized corrosion, causing dissolution of Ni-rich phases [43].

Calculation of the Kinetic Corrosion Parameters from Activation Energies
Usually, the rate of most chemical reactions increases with increasing temperature. In the case of electrochemical reactions, the increase in temperature favors the kinetics of the corrosion reactions, more precisely the speed of the anodic dissolution reaction of the metal increases. The value of the activation energy of the corrosion processes can be obtained from the Arrhenius-type graphs [44], which follow the following equation: where Ea is the process activation energy expressed in J/mol, R is the universal gas constant (8.314 J/(mol K)), T is the temperature in K, and A is a constant also called pre-exponential factor. If the Arrhenius equation is logarithmic, the following expression is obtained: Thus, the value of the activation energy can be determined from Figure 7 and the obtained values are shown in Table 3. The activation energy of an electrochemical process refers to the energy level that an electron must reach to pass through the electrode/electrolyte interface. Thus, a low value of the activation energy indicates a high corrosion rate. Moreover, the Arrhenius equation shows that the higher the value of the activation energy, the greater the dependence of corrosion rate as a function of temperature. As can be seen from the slopes of the lines shown in Figure 6, but also from the activation energy values (Table 3), the highest activation energy value is recorded in the case of CoCr alloy.
Obtaining the highest values in the case of CoCr alloy was expected because the values for the corrosion current density are the lowest at the different temperatures studied. Thus, the data previously obtained from the polarization curves are reconfirmed.
Arrhenius' theory bears some resemblance to the theory of collision activation [45]. The reaction takes place when the molecules collide with each other. The increasing rate of reaction at higher temperatures represent, in collision theory, the chance of more frequent active collisions. The mathematical form of the transition state theory can be written: where ΔH is the enthalpy of the activation process, ΔS is the entropy of the activation process, N is Avogadro's number (6.022 × 10 23 mol −1 molecules) and h is Plank's constant (6.626 × 10 −34 J s mol −1 ). If the equation is rearranged in the form: ln ( ) = ln ( ℎ ) + ∆ − ∆ (4) Figure 7. Arrhenius plots in artificial saliva for the two studied alloys. The activation energy of an electrochemical process refers to the energy level that an electron must reach to pass through the electrode/electrolyte interface. Thus, a low value of the activation energy indicates a high corrosion rate. Moreover, the Arrhenius equation shows that the higher the value of the activation energy, the greater the dependence of corrosion rate as a function of temperature.
As can be seen from the slopes of the lines shown in Figure 6, but also from the activation energy values (Table 3), the highest activation energy value is recorded in the case of CoCr alloy.
Obtaining the highest values in the case of CoCr alloy was expected because the values for the corrosion current density are the lowest at the different temperatures studied. Thus, the data previously obtained from the polarization curves are reconfirmed.
Arrhenius' theory bears some resemblance to the theory of collision activation [45]. The reaction takes place when the molecules collide with each other. The increasing rate of reaction at higher temperatures represent, in collision theory, the chance of more frequent active collisions. The mathematical form of the transition state theory can be written: where ∆H is the enthalpy of the activation process, ∆S is the entropy of the activation process, N is Avogadro's number (6.022 × 10 23 mol −1 molecules) and h is Plank's constant (6.626 × 10 −34 J s mol −1 ). If the equation is rearranged in the form: A linear dependence can be obtained from which the values of enthalpies and entropies of the activation process are calculated (Figure 8).
A linear dependence can be obtained from which the values of enthalpies and entropies of the activation process are calculated (Figure 8).
The values obtained for the three parameters (activation energy, enthalpy, and entropy) for the two biomaterials studied in artificial saliva are presented in Table 3. The enthalpy and entropy values of the activation process for both studied alloys are presented in Table 3. The lowest enthalpy value (ΔH) is obtained for NiCr alloy. This observation indicates that the corrosion reaction in Fusayama-Meyer artificial saliva of NiCr alloy requires a lower energy value to occur compared to the corrosion processes of the other alloy. It can be said that the transition state is obtained faster in case of NiCr alloy corrosion. The positive values obtained for the enthalpies of the studied corrosion processes indicate that the dissolutions of the studied biomaterials are endothermic processes.

Surface Characterization
SEM analysis highlighted the morphology of the studied samples. The central part of the sample analyzed at SEM for a small magnification (35 x) was further enlarged to record the morphology of that sample at large magnification (10 kx).
According to representative SEM images presented as examples in Figures 9 and 10 at different temperatures for NiCr and CoCr alloys in connection with the surface elemental analysis, it is possible to see that at low temperatures, oxide films are more uniform but at higher temperatures, the films become thinner and more susceptible to breakdown.  The values obtained for the three parameters (activation energy, enthalpy, and entropy) for the two biomaterials studied in artificial saliva are presented in Table 3.
The enthalpy and entropy values of the activation process for both studied alloys are presented in Table 3. The lowest enthalpy value (∆H) is obtained for NiCr alloy. This observation indicates that the corrosion reaction in Fusayama-Meyer artificial saliva of NiCr alloy requires a lower energy value to occur compared to the corrosion processes of the other alloy. It can be said that the transition state is obtained faster in case of NiCr alloy corrosion. The positive values obtained for the enthalpies of the studied corrosion processes indicate that the dissolutions of the studied biomaterials are endothermic processes.

Surface Characterization
SEM analysis highlighted the morphology of the studied samples. The central part of the sample analyzed at SEM for a small magnification (35 x) was further enlarged to record the morphology of that sample at large magnification (10 kx).
According to representative SEM images presented as examples in Figures 9 and 10 at different temperatures for NiCr and CoCr alloys in connection with the surface elemental analysis, it is possible to see that at low temperatures, oxide films are more uniform but at higher temperatures, the films become thinner and more susceptible to breakdown.
A linear dependence can be obtained from which the values of enthalpies and entropies of the activation process are calculated (Figure 8).
The values obtained for the three parameters (activation energy, enthalpy, and entropy) for the two biomaterials studied in artificial saliva are presented in Table 3. The enthalpy and entropy values of the activation process for both studied alloys are presented in Table 3. The lowest enthalpy value (ΔH) is obtained for NiCr alloy. This observation indicates that the corrosion reaction in Fusayama-Meyer artificial saliva of NiCr alloy requires a lower energy value to occur compared to the corrosion processes of the other alloy. It can be said that the transition state is obtained faster in case of NiCr alloy corrosion. The positive values obtained for the enthalpies of the studied corrosion processes indicate that the dissolutions of the studied biomaterials are endothermic processes.

Surface Characterization
SEM analysis highlighted the morphology of the studied samples. The central part of the sample analyzed at SEM for a small magnification (35 x) was further enlarged to record the morphology of that sample at large magnification (10 kx).
According to representative SEM images presented as examples in Figures 9 and 10 at different temperatures for NiCr and CoCr alloys in connection with the surface elemental analysis, it is possible to see that at low temperatures, oxide films are more uniform but at higher temperatures, the films become thinner and more susceptible to breakdown.  From SEM analysis, we can observe the morphology of the samples. On the surface of both alloys, at higher magnification some layers are visible. Before the electrochemical studies, the surface of the samples was smooth, without significative structures on the surface. After the electrochemical procedure, on the surface of both alloys, different porous structures appear, a fact that is more visible at higher magnification (10 kx). This is because oxide layers appear, a fact that is highlighted by EDS. From SEM analysis, we can observe the morphology of the samples. On the surface of both alloys, at higher magnification some layers are visible. Before the electrochemical studies, the surface of the samples was smooth, without significative structures on the surface. After the electrochemical procedure, on the surface of both alloys, different porous structures appear, a fact that is more visible at higher magnification (10 kx). This is because oxide layers appear, a fact that is highlighted by EDS.
At higher temperature, it is possible to observe some pits. It is well known that the appearance of pits is the first step of a localized corrosion named pitting corrosion which is based also on anionic penetration, usually chloride. As the chloride ions are present in the electrolyte, it is reasonable to believe that the next steps of such type of corrosion are going to take place with pits propagation and oxide breakdown.
By EDS, the spectra for the alloys immersed at different temperatures in artificial saliva were obtained. These spectra look almost similar at different temperatures; some oxygen peaks appear beside the elements from the alloy composition. These are a result of an oxide layer formed on the surface of both alloys. By increasing temperature, after 320 K the oxide layer starts to break, some pits appear, and corrosion is more highlighted. For all the samples, at different temperatures, peaks specific to the alloying elements appear in spectra (Ni, Cr, Fe, Mo, Si for NiCr alloy and Co, Cr, Mo, W, Ga, Mn for CoCr alloy). Elemental analysis (Atomic %), evidenced the presence of O besides Ni, Cr, Fe, Mo for NiCr alloy ( Figure 11) and for CoCr alloy ( Figure 12).  At higher temperature, it is possible to observe some pits. It is well known that the appearance of pits is the first step of a localized corrosion named pitting corrosion which is based also on anionic penetration, usually chloride. As the chloride ions are present in the electrolyte, it is reasonable to believe that the next steps of such type of corrosion are going to take place with pits propagation and oxide breakdown.
By EDS, the spectra for the alloys immersed at different temperatures in artificial saliva were obtained. These spectra look almost similar at different temperatures; some oxygen peaks appear beside the elements from the alloy composition. These are a result of an oxide layer formed on the surface of both alloys. By increasing temperature, after 320 K the oxide layer starts to break, some pits appear, and corrosion is more highlighted. For all the samples, at different temperatures, peaks specific to the alloying elements appear in spectra (Ni, Cr, Fe, Mo, Si for NiCr alloy and Co, Cr, Mo, W, Ga, Mn for CoCr alloy). Elemental analysis (Atomic %), evidenced the presence of O besides Ni, Cr, Fe, Mo for NiCr alloy ( Figure 11) and for CoCr alloy (Figure 12). From SEM analysis, we can observe the morphology of the samples. On the surface of both alloys, at higher magnification some layers are visible. Before the electrochemical studies, the surface of the samples was smooth, without significative structures on the surface. After the electrochemical procedure, on the surface of both alloys, different porous structures appear, a fact that is more visible at higher magnification (10 kx). This is because oxide layers appear, a fact that is highlighted by EDS.
At higher temperature, it is possible to observe some pits. It is well known that the appearance of pits is the first step of a localized corrosion named pitting corrosion which is based also on anionic penetration, usually chloride. As the chloride ions are present in the electrolyte, it is reasonable to believe that the next steps of such type of corrosion are going to take place with pits propagation and oxide breakdown.
By EDS, the spectra for the alloys immersed at different temperatures in artificial saliva were obtained. These spectra look almost similar at different temperatures; some oxygen peaks appear beside the elements from the alloy composition. These are a result of an oxide layer formed on the surface of both alloys. By increasing temperature, after 320 K the oxide layer starts to break, some pits appear, and corrosion is more highlighted. For all the samples, at different temperatures, peaks specific to the alloying elements appear in spectra (Ni, Cr, Fe, Mo, Si for NiCr alloy and Co, Cr, Mo, W, Ga, Mn for CoCr alloy). Elemental analysis (Atomic %), evidenced the presence of O besides Ni, Cr, Fe, Mo for NiCr alloy ( Figure 11) and for CoCr alloy (Figure 12). The results of SEM/EDS analyses reveal a heterogeneous morphology. It is known from the literature [43], that the presence of chromium as an alloying element determines the appearance of a primary Cr 2 O 3 passive film which is dominantly responsible for the alloy passivation.  The results of SEM/EDS analyses reveal a heterogeneous m from the literature [43], that the presence of chromium as an allo the appearance of a primary Cr2O3 passive film which is domin alloy passivation.
It seems that the increase in temperature determines the int lution process, which initially begins with only a few active cente although active centers of dissolution are present on the surface, composition of the Mo and W makes them act in the pits (active corrosion rate and generating a heterogeneous surface [46][47][48].

Contact Angle Evaluation
The wettability tests have shown a slight increase in the val the thermally reprocessed alloy samples. Thus, in the case of Ni 0.7 were obtained for the thermally reprocessed samples, comp unprocessed alloy. For the CoCr alloy the increase was from 71.8 the non-heat-treated sample) to 77.5 ± 0.9. Contact angle meas formed using the same electrolyte as the electrochemical tests.
The above values are all in the hydrophilic domain sustain and proliferation around restorative work with such alloys [49].

Ion Release by ICP-MS Measurenments
Inductively coupled plasma mass spectrometry was perfo Cr Mo, Si, Fe for NiCr alloy and Co, Cr, W, Mo, Mn, Si and Ga i from the samples at 290, 300, 310, 320 and 330 K.
For the elemental analyses, a multi-element standard refere was added as external standard. The concentration of ion release It seems that the increase in temperature determines the intensification of the dissolution process, which initially begins with only a few active centers. At low temperatures, although active centers of dissolution are present on the surface, the existence in the alloys composition of the Mo and W makes them act in the pits (active centers), reducing alloy corrosion rate and generating a heterogeneous surface [46][47][48].

Contact Angle Evaluation
The wettability tests have shown a slight increase in the value of the contact angle of the thermally reprocessed alloy samples. Thus, in the case of NiCr alloy, values of 82.5 ± 0.7 were obtained for the thermally reprocessed samples, compared to 75.4 ± 0.5 for the unprocessed alloy. For the CoCr alloy the increase was from 71.8 ± 0.8 (value obtained for the non-heat-treated sample) to 77.5 ± 0.9. Contact angle measurement tests were performed using the same electrolyte as the electrochemical tests.
The above values are all in the hydrophilic domain sustaining a better cell adhesion and proliferation around restorative work with such alloys [49].

Ion Release by ICP-MS Measurenments
Inductively coupled plasma mass spectrometry was performed to evaluate the Ni, Cr, Mo, Si, Fe for NiCr alloy and Co, Cr, W, Mo, Mn, Si and Ga ion release for CoCr alloy from the samples at 290, 300, 310, 320 and 330 K.
For the elemental analyses, a multi-element standard reference solution of 10 µg/mL was added as external standard. The concentration of ion release was studied in artificial saliva at different temperatures.
From Table 4, for NiCr sample it is shown that the concentration (ion release quantities) is in the domain of ppm (0-15 ppm). The tendency of the graph is that with the increase in the temperature, the ion release concentration is increasing with the increase in the temperature. The higher concentration is for Mo at 330 K and the smallest is for Mo at 290 K. When the temperature increases, the quantities of the elements also increasw (Table 5) for CoCr sample. The ion release is in the domain of 0-19 ppm. The higher ion release value is for Mo at 330 K and the smallest is for W at 290, 300, 310, 320 K (being constant over this temperature range). The two tested alloys release some elements such as Ni, Co, Cr, Mo. As can be seen from this study, the amounts released are well below those taken as part of the diet. However, it should be emphasized that there is no information on long-term exposure to low doses of these elements. The ion release data are in perfect accordance with the results obtained by electrochemical studies. Considering that the risk heavy metals pose in dental health is not well understood, especially for rare disease [50], such factors could lead to a decrease in the alloys' quality by changing their composition [51] or coating their surface. Such strategies have been performed and tested but still have room for better selection in tissue as the metallic ion involves a cumulative process. In a time with very aggressive bacteria, antibacterial coatings for dental alloys could be highlighted as well [52].

Conclusions
The electrochemical results indicate an increase in the corrosion rate of each of the biomaterials in Fusayama-Meyer artificial saliva with the increase in the temperature. The SEM coupled with EDS measurements evidenced the same behavior; the temperature favors the occurrence of corrosion phenomena, because of which the oxide films on the surface of the metallic biomaterial tend to break after the temperature of 310 K. ICP-MS results reveal that with an increase in the temperature, the quantities of the ions released from both dental alloys also increase, still remaining on the order of a few tens of ppm, in perfect accordance with the results obtained by electrochemical studies.