**2. The** *in situ* **study of tribocorrosion processes by electrochemical techniques**

When performing a tribocorrosion test, one has to implement not only the following traditional concepts of tribological testing, namely:


but he has also to take into account and to control simultaneously a large number of testing parameters, like:


These parameters determine the electrochemical reactivity of the surfaces and in consequence influence the contact conditions (wear regime, existence of a third body, friction ...).

Tribocorrosion: Material Behavior Under

phenomena:

place, and

complementary methods.

electrochemical behavior of the alloy:

uniform.

**2.2 Potentiodynamic polarization measurements** 

Combined Conditions of Corrosion and Mechanical Loading 87

Table 1 gives the evolution of the mean value of the open circuit potential of a 316 L stainless steel in contact with alumina immersed in artificial seawater for different normal forces and velocities (Ponthiaux et al., 1997). Alumina is taken since it has a high electrical resistance and is chemically inert in the liquid used. At a zero normal force and speed, the measured open circuit potential corresponds to the passive state of the entire stainless steel surface. When the normal force or the speed increases, the open circuit potential shifts towards lower potential values. This shift can be explained by considering the following



Potentiodynamic polarization curves obtained at increasing and decreasing potential scan in absence of any sliding is schematically shown in Figure 3. In the case the current measured originates from the whole surface of the tested sample that might be considered as being

Under sliding conditions, the currents measured during potentiodynamic polarization are in a first approach the sum of two components, namely the current originating from the rubbed area, and the one linked to the non-rubbed area. Under such conditions, the maximum dissolution current, IM, varies with the mean contact pressure and sliding speed. However, these two test parameters do not necessarily affect in the same way the


Such tests therefore need to be instrumented to control and/or to record the contact conditions like the normal or tangential force, the relative displacement, velocity, acceleration and contact frequency ...). They need also to be instrumented with electrochemical techniques enabling the control and/or the recording of electrochemical parameters like the polarization of the contacting materials.

The choice of electrochemical techniques that can be implemented in a tribocorrosion test and the development of relevant models for the interpretation of the tribocorrosion mechanism are determined by the mechanical contact conditions being continuous or reciprocating. Electrochemical measurements can be performed with both types of tribometers. However, to be implemented under conditions that allow the interpretation of results, some methods require stationary electrochemical conditions, at least prior to starting up the measurements. In the case of continuous sliding, a quasi-stationary electrochemical surface state can often be reached, and all the electrochemical techniques available for corrosion studies (polarization curves, impedance spectroscopy, electrochemical noise,...), can be used. On the contrary, when reciprocating contact conditions prevail, the interpretations of experimental results are more complex due to the non-stationary electrochemical conditions. Measuring techniques suitable for the recording of current or potential transients will be used preferentially (Mischler et al., 1997; Rosset, 1999).

#### **2.1 Open circuit potential measurements**

Under sliding conditions, the measured open circuit potential is a mean value which depends on the electromotive force induced by the surface heterogeneity resulting from the coexistence of non-rubbed and rubbed areas which are in different electrochemical states, and on the areas of these zones and their spatial distribution that determines the nonuniform distribution of potential over the whole surface (Oltra et al, 1986). When applying for example a continuous sliding, this open circuit potential responds to the mechanical loading imposed, as shown in Table 1 and Figure 2.


Table 1. Variations of the open-circuit potential EOC of 316L stainless steel in artificial sea water as a function of varying tribological contact conditions. S.C.E.: saturated calomel electrode. Fn: normal force. V: sliding speed.

Fig. 2. Electrochemical noise at the open circuit potential EOC of 316L stainless steel in artificial sea water for different tribological contact conditions. S.C.E.: saturated calomel electrode. Fn: normal force. V: sliding speed.

Such tests therefore need to be instrumented to control and/or to record the contact conditions like the normal or tangential force, the relative displacement, velocity, acceleration and contact frequency ...). They need also to be instrumented with electrochemical techniques enabling the control and/or the recording of electrochemical

The choice of electrochemical techniques that can be implemented in a tribocorrosion test and the development of relevant models for the interpretation of the tribocorrosion mechanism are determined by the mechanical contact conditions being continuous or reciprocating. Electrochemical measurements can be performed with both types of tribometers. However, to be implemented under conditions that allow the interpretation of results, some methods require stationary electrochemical conditions, at least prior to starting up the measurements. In the case of continuous sliding, a quasi-stationary electrochemical surface state can often be reached, and all the electrochemical techniques available for corrosion studies (polarization curves, impedance spectroscopy, electrochemical noise,...), can be used. On the contrary, when reciprocating contact conditions prevail, the interpretations of experimental results are more complex due to the non-stationary electrochemical conditions. Measuring techniques suitable for the recording of current or

potential transients will be used preferentially (Mischler et al., 1997; Rosset, 1999).

Under sliding conditions, the measured open circuit potential is a mean value which depends on the electromotive force induced by the surface heterogeneity resulting from the coexistence of non-rubbed and rubbed areas which are in different electrochemical states, and on the areas of these zones and their spatial distribution that determines the nonuniform distribution of potential over the whole surface (Oltra et al, 1986). When applying for example a continuous sliding, this open circuit potential responds to the mechanical

Fn (N) 0 2.2 2.2 4.2 V (cm s-1) 0 0.5 0.8 0.8 Eoc (V vs SCE) -0.15 -0.29 -0.32 -0.42 Table 1. Variations of the open-circuit potential EOC of 316L stainless steel in artificial sea water as a function of varying tribological contact conditions. S.C.E.: saturated calomel

> Fn = 2.2N v = 0.8 cm s-1

> > 5 seconds time

Fn = 4.2N v = 0.8 cm s-1

5 seconds time

EOC(V vs S.C.E.)




parameters like the polarization of the contacting materials.

**2.1 Open circuit potential measurements** 

loading imposed, as shown in Table 1 and Figure 2.

electrode. Fn: normal force. V: sliding speed.

5 seconds time

electrode. Fn: normal force. V: sliding speed.

Fn = 2.2N v = 0.5 cm s-1

EOC(V vs S.C.E.)




EOC(V vs S.C.E.)


Fig. 2. Electrochemical noise at the open circuit potential EOC of 316L stainless steel in artificial sea water for different tribological contact conditions. S.C.E.: saturated calomel



Table 1 gives the evolution of the mean value of the open circuit potential of a 316 L stainless steel in contact with alumina immersed in artificial seawater for different normal forces and velocities (Ponthiaux et al., 1997). Alumina is taken since it has a high electrical resistance and is chemically inert in the liquid used. At a zero normal force and speed, the measured open circuit potential corresponds to the passive state of the entire stainless steel surface. When the normal force or the speed increases, the open circuit potential shifts towards lower potential values. This shift can be explained by considering the following phenomena:


In Figure 2, the rapid fluctuations of the open circuit potential arising during sliding are represented schematically. They constitute an 'electrochemical noise' representing the electrochemical response to the rapid and stochastic fluctuations of the new bare metal surface generated in the real contact area by sliding friction. This noise can provide information on the mechanism of friction as well as on the mechanism of electrochemical reactions involved in the depassivation – repassivation process (Déforge et al.). A more detailed analysis of open circuit potential measurements under sliding requires a more precise knowledge of the local surface state of contacting materials. Experimentally, microelectrodes can be used to determine potential values of rubbed and non-rubbed areas. Such techniques have already been used to study localized corrosion, and models have been proposed (Lillard et al., 1995). Note that the interpretation of local potential measurements or the development of theoretical models describing the potential distribution, can only be obtained with realistic assumptions on the reaction kinetics of reactions occurring at rubbed and non-rubbed surface areas. Such Information can only be obtained by using complementary methods.
