# (Bio)Tribocorrosion in Dental Implants: Principles and Techniques of Investigation
## 2 Techniques of Investigation in Tribocorrosion  
The most common experimental setup used for experimental activity on tribocorrosion consists of a classical tribometer (for mechanical wear estimation) equipped with a potentiostat (for the chemical component) and a specific chemical cell, which are all connected, operating simultaneously to guarantee the synergistic action of tribocorrosion. The cell is schematically represented in Figure 2 as follows:  
The cell is composed of an ammeter (A) and to a voltmeter (V) connected with three electrodes [23] indicated as blocks 1,2,3 follows: the first is referred to as the working electrode (WE), i.e., the sample that will undergo the test; the second is the counter (CE) that provides the current control, essentially made of gold, platinum or carbon; lastly there is the reference electrode (RE) made of copper, hydrogen, calomel, etc., necessary to calculate the potential, since it must be referred to a standard potential [24]. Each one is immersed in a solution of variable composition such as calf bovine serum (similar to the synovial fluid) [25], sulfuric acid [26], artificial seawater [27], NaCl [28], Ringer solution [29], etc. Hence, nowadays, two different methodologies exist for the corrosion approach by considering whether or not the presence of acid can complicate the experimental setup (the way of inserting it in tribological devices) and the aspect of safety. As represented in Figure 1, the three factors should be evaluated. Hence, for mechanical wear, the classical investigation is achieved by a classical tribometer, whereas for chemical wear, several techniques should be adopted according to the aim of the investigation [6], involving two or three electrodes. The first and the simplest is the OCP (open-circuit potential), as follows: it is a passive measurement since the counter electrode is bypassed with the passage of current. Hence, OCP is the free potential calculated between the working and reference electrodes [30]. Besides, it is linked with the concentration ratio of the species present in the reaction (Equations (4) and (5)) in the sense that by measuring OCP it is possible to evaluate the following concentrations involved:  
\[E_{OCP}=E_{WORKING}-E_{REFERCE} \tag{4}\]  
Measured by voltmeter, but for Nernst formula, as follows:  
\[E_{OCP}=E^{0}+\left(\frac{RT}{nF}\right)\ln\left(\frac{C_{o}^{n}}{C_{r}^{m}}\right) \tag{5}\]  
where:  
* \(E^{0}\) is the standard potential of reduction;
* \(R\) and \(F\) respectively the Universal Gas Constant and Faraday Constant;
* \(T\) the temperature expressed in Kelvin;
* \(n\) the number of electrons exchanged;
* \(C_{o}\) and \(C_{r}\) raised to the stoichiometric coefficients the molar concentration (or pressure in gas cases) of the species oxidated and reduced. The main disadvantage of this method is the scarcity of information since there is no mention of the current trend, passivation phenomena, local and total wear, etc. Consequently, it is often replaced by potentiostatic and potentiodynamic techniques. In both, a current flow occurs controlled by a potentiostat but with the following difference [31]: for the former, the potentiostat maintains a fixed potential by varying the current, whereas in the second case, the potential changes at a constant rate (potential sweep rate). This current is formed of the following two components:  
\[I_{tot}=\sum I_{Anodic}+I_{Catchodic} \tag{6}\]  
The sum is referred to the number of reactions on WE, \(I_{anodic}\) and \(I_{catchodic}\) the currents of the two semi-reactions whose values respect the potential imposed. Relation 6 yields the value of current, which appears in classical Faraday Law as follows [32]:  
\[M=\rho V=\left(\frac{C}{F}\right)Q \tag{7}\]  
Figure 2: Schematical representation of an electrochemical cell equipped with three electrodes. where \(\rho\) is the density, \(M\) and \(V\) mass and volume of material lost during the tribocorrosion, \(Q\) is the charge and C is the ratio between the molecular mass (\(MM\)) and electrons exchanged (\(n\)).