**4.1 Specificity of laboratory and industrial tribocorrosion tests**

Similarly as in classical mechanical testing, tribocorrosion tests can be classified into two categories based on their different but complementary purposes, namely fundamental and technological tests.

*Fundamental tests* are implemented in research laboratories and their objective is to clearly identify and to understand under well defined testing conditions, the basic mechanisms and their synergy that govern the phenomena of tribocorrosion. These tests require the development of experimental methodologies for both the test themselves and the techniques to be used for analyzing and measuring data and other experimental outcomes. Concerning friction in particular, two types of tests can be considered:


These tests allow the following analyses based on *in situ* and *ex situ* measurements, like the determination of the mean and local coefficient of friction, the identification of and study of the interactions between surfaces and environment, the nature of the mechanical-chemical coupling, the electrochemical or galvanic coupling due to a heterogeneous structure, the shape and location of rubbed and non-rubbed areas, or the establishment of local wear laws and their spatial distribution on the surface in view of a modelling of wear aiming at a future predictive approach.

*Technological tests* are designed to reproduce at lab scale mechanical loading and/or environmental conditions corresponding to actual operating conditions, or to mimic particular conditions intending to accelerate material degradation processes. These tests are widely used to predict precisely the behaviour of mechanical devices in actual conditions of service and to improve their reliability and durability. In that respect, they are very useful tools.

The full investigation of the tribocorrosion tests requires generally the use of *in situ* tools like open circuit measurements, polarization measurements, current transients, impedance spectroscopy, and noise measurements, and *ex situ* tools like elemental surface analysis techniques, optical or electron microscopy, micro-topography, micro and nanohardness measurements texture and internal stress analyses.

Tribocorrosion: Material Behavior Under

**without any sliding** 

Eoc

is shown in (b).

Combined Conditions of Corrosion and Mechanical Loading 99

**4.2.2 First step in the testing protocol: Electrochemical tests on passive material** 

After selecting the set of appropriate test conditions, measurements are done to collect information on the electrochemical behaviour of a material fully covered by a passive film. This is done by electrochemical tests in absence of any sliding. After immersion in the electrolyte, the open circuit potential, Eoc, is measured versus a reference electrode. In general, a stable value of Eoc is obtained after some time of immersion. From an electrochemical point of view, a stable Eoc is obtained when the long-term fluctuations of Eoc are below 1 mV min-1 during a minimum of 1 hour. The time necessary to reach such a stationary open circuit potential in the test electrolyte is an important characteristic of a passivating process, and is called in this protocol as the reaction time characteristic, treac. The evolution of Eoc from immersion time on provides useful information on the electrochemical

Eoc

t t reac

b

the behaviour of the test material in a real life application.

reactivity of the tested material in the test electrolyte (see Figure 7).

a

t

passive material, rpass, can be calculated for a test sample with a surface area, Ao, as:

Fig. 7. Schematic representation of the evolution of Eoc with immersion time in the case of (a) corrosion, and (b) passivation. The graphical determination of treac in the case of passivation

From this figure one can derive that when Eoc decreases with time, general corrosion may be suspected, and that when Eoc increases with time, passivation or adsorption is probably taking place. In this latter case, treac can be estimated from the evolution of Eoc with immersion time by drawing the tangent to the curve at the point where the slope is maximum, together with a straight line tangential to the data in the part of the curve where Eoc is stable. After achieving a long-term stable open circuit potential indicative of passivation, the polarization resistance of the passive material, Rp, is measured by electrochemical impedance spectroscopy. Based on Rp, the specific polarization resistance of

avoid plastic deformation of the tested materials. The maximum Hertzian contact pressure on the test material before starting sliding should be smaller than the yield strength. Concerning the track radius, Rtr, it should be selected in such a way that edge effects are avoided. E.g. in the case of a disc, the track radius should be by preference about half the test sample radius. Finally the number of cycles, n, depends on the type of material tested and the test conditions. It should be selected so that the wear volume is large enough to be measured accurately, while avoiding too long test durations for practical reasons. A preliminary sliding test might be necessary to determine n. In some particular cases, the selection of the number of cycles can also be done so as to reflect

#### **4.2 Testing protocol: A multiscale analysis of tribocorrosion phenomena**

A promising approach of synergy in tribocorrosion has been proposed (Diomidis et al., 2009) based on the fact that the surface state of a wear track evolves with time in a cyclic manner. That evolution is due to the repeated removal and subsequent re-growth of a passive surface film when a mechanical loading is applied. During the latency time, tlat, defined as the time between two successive contacts at a given point in the sliding track, the passivation reaction tends to restore the passive film. The fraction of the sliding track surface covered by this re-grown passive film increases with tlat. By controlling the frequency of such depassivation-repassivation events with respect to the time necessary for film growth, it is possible to measure the properties of the surface at different stages of activity and repassivation. For performing tests at different latency times, tlat, two approaches are possible, each approach having own advantages and limitations as detailed hereafter:


$$\mathbf{t}\_{lat} = \mathbf{t}\_{rot} + \mathbf{t}\_{off} \tag{21}$$

It is clear that in continuous sliding tests, Equation (21) is still valid but toff is zero. Such an approach can thus result in a protocol that provides information on the evolution of the surface with testing time, and the identification of the resulting mechanisms of material loss and surface degradation (Pourbaix, 1974).

#### **4.2.1 Selection of test conditions**

In order to characterize the sensitivity of the one or more material systems to tribocorrosion, a careful selection of the test conditions has to be done prior to any testing, so as to obtain discriminating results. The following steps are of large importance in that approach:


A promising approach of synergy in tribocorrosion has been proposed (Diomidis et al., 2009) based on the fact that the surface state of a wear track evolves with time in a cyclic manner. That evolution is due to the repeated removal and subsequent re-growth of a passive surface film when a mechanical loading is applied. During the latency time, tlat, defined as the time between two successive contacts at a given point in the sliding track, the passivation reaction tends to restore the passive film. The fraction of the sliding track surface covered by this re-grown passive film increases with tlat. By controlling the frequency of such depassivation-repassivation events with respect to the time necessary for film growth, it is possible to measure the properties of the surface at different stages of activity and repassivation. For performing tests at different latency times, tlat, two approaches are possible, each approach having own advantages and limitations as detailed



It is clear that in continuous sliding tests, Equation (21) is still valid but toff is zero. Such an approach can thus result in a protocol that provides information on the evolution of the surface with testing time, and the identification of the resulting mechanisms of material loss

In order to characterize the sensitivity of the one or more material systems to tribocorrosion, a careful selection of the test conditions has to be done prior to any testing, so as to obtain



discriminating results. The following steps are of large importance in that approach:

*lat rot off ttt* (21)

**4.2 Testing protocol: A multiscale analysis of tribocorrosion phenomena** 

hereafter:

large factor.

and surface degradation (Pourbaix, 1974).

**4.2.1 Selection of test conditions** 

should be avoided,

avoid plastic deformation of the tested materials. The maximum Hertzian contact pressure on the test material before starting sliding should be smaller than the yield strength. Concerning the track radius, Rtr, it should be selected in such a way that edge effects are avoided. E.g. in the case of a disc, the track radius should be by preference about half the test sample radius. Finally the number of cycles, n, depends on the type of material tested and the test conditions. It should be selected so that the wear volume is large enough to be measured accurately, while avoiding too long test durations for practical reasons. A preliminary sliding test might be necessary to determine n. In some particular cases, the selection of the number of cycles can also be done so as to reflect the behaviour of the test material in a real life application.

#### **4.2.2 First step in the testing protocol: Electrochemical tests on passive material without any sliding**

After selecting the set of appropriate test conditions, measurements are done to collect information on the electrochemical behaviour of a material fully covered by a passive film. This is done by electrochemical tests in absence of any sliding. After immersion in the electrolyte, the open circuit potential, Eoc, is measured versus a reference electrode. In general, a stable value of Eoc is obtained after some time of immersion. From an electrochemical point of view, a stable Eoc is obtained when the long-term fluctuations of Eoc are below 1 mV min-1 during a minimum of 1 hour. The time necessary to reach such a stationary open circuit potential in the test electrolyte is an important characteristic of a passivating process, and is called in this protocol as the reaction time characteristic, treac. The evolution of Eoc from immersion time on provides useful information on the electrochemical reactivity of the tested material in the test electrolyte (see Figure 7).

Fig. 7. Schematic representation of the evolution of Eoc with immersion time in the case of (a) corrosion, and (b) passivation. The graphical determination of treac in the case of passivation is shown in (b).

From this figure one can derive that when Eoc decreases with time, general corrosion may be suspected, and that when Eoc increases with time, passivation or adsorption is probably taking place. In this latter case, treac can be estimated from the evolution of Eoc with immersion time by drawing the tangent to the curve at the point where the slope is maximum, together with a straight line tangential to the data in the part of the curve where Eoc is stable. After achieving a long-term stable open circuit potential indicative of passivation, the polarization resistance of the passive material, Rp, is measured by electrochemical impedance spectroscopy. Based on Rp, the specific polarization resistance of passive material, rpass, can be calculated for a test sample with a surface area, Ao, as:

Tribocorrosion: Material Behavior Under

counterbody respectively.

where

and

rpass with ract in Equation (23):

the equivalent elastic modulus given by:

Combined Conditions of Corrosion and Mechanical Loading 101

with FN the applied normal load, R the radius of the tip of the curved counter-body, and E

with 1 and 2 the Poisson's ratios, and E1 and E2 the elastic moduli of the test sample and

Sliding is initiated at the time a stable Eoc is achieved. That Eoc recorded during sliding is a mixed potential resulting from the galvanic coupling of material inside (Atr) and outside (Ao – Atr) the sliding track. It is assumed that the kinetics of the redox reactions taking place on each of these areas, do not vary with the real potential of the sliding track. In other words, the ohmic drop effect is considered to be negligible in the galvanic coupling between the sliding track and the surrounding area. Electrochemical impedance spectroscopy measurements are performed during sliding to obtain the polarization resistance, Rps, of the sample surface. Similarly to Eoc, Rps may be considered as the combination of two polarization resistances, namely Ract related to the active area Aact which is equal in this case to the wear track, and Rpass which corresponds to the surrounding unworn area, (Ao – Atr):

> 11 1 *RR R ps act pass*

> > *act act tr*

> > > 0 *pass*

*tr ps pass*

*pass ps tr AR r*

> *act <sup>B</sup> <sup>i</sup>*

It can be noted that, the specific resistance of the active bare material in the sliding track, ract, is generally several orders of magnitude lower than the specific resistance of the material covered with a passive film, rpass , outside of the sliding track. Therefore, if Atr is not too small a fraction of the total area A0 of the sample, the resistance Ract related to the sliding

The corrosion current density of the active material, iact, can now be obtained by substituting

*act*

It is then possible to calculate the specific polarization resistance of the active surface, ract, as:

*r*

*tr*

<sup>0</sup>

*pass*

*R*

*act*

*r*

*<sup>r</sup> <sup>R</sup>*

11 1 *EE E* 

2 2 1 2 1 2

(28)

(29)

*<sup>A</sup>* (30)

*A A* (31)

*<sup>r</sup>* (33)

*r RA A* (32)

$$\mathbf{r}\_{\text{puss}} = \mathbf{R}\_{\text{p}} \mathbf{A}\_{\text{o}} \tag{22}$$

Specific polarization resistance values for metallic materials of 103 Ωcm2 or lower indicate the presence of an active sample surface, while values around 100 × 103 Ωcm2 or higher indicate a passive sample surface. The corrosion current density of the material covered by a passive surface film, ipass, is then calculated as follows:

$$i\_{\text{pass}} = \frac{B}{r\_{\text{pass}}} \tag{23}$$

with B a constant. For metallic materials, B normally varies between 13 and 35 mV, depending on the nature of the material and the environment. In the protocol worked out hereafter as an example, a value of 24 mV is assumed. This passive current density, ipass, is considered to correspond to the dissolution current of the material through the passive film at stationary state.

#### **4.2.3 Second step in the testing protocol: Electrochemical tests on a fully active sliding track during sliding**

The next step is the determination of the corrosion rate of the depassivated material. In order to keep continuously a part of the immersed sample surface in an active state, the passive film has to be removed by mechanical contacts. It is thus necessary to select a rotation period, trot, which is small compared to treac so that the passive film has no time to regrow in between two successive contact events. It was proposed (Diomidis et al., 2009, 2010) to take the rotation period trot equal to:

$$t\_{rot} = \frac{t\_{\text{renc}}}{10000} \tag{24}$$

A first value of tlat1 is then taken equal to trot. It is thus assumed that during such sliding tests the whole wear track area is in an active state, so that:

$$A\_{tr} = A\_{act} \tag{25}$$

Despite the fact that the width of the sliding track increases progressively due to wear during sliding tests performed against a curved counter-body, a mean sliding track area, Atr, is taken for simplicity, and defined as:

$$A\_{tr} = \frac{1}{2} (A\_{tr\max} + A\_{tr\min})\tag{26}$$

with Atr max the maximum value measured at the end of the test, and Atr min the minimum value at the end of the first cycle. Atr min is calculated by multiplying the length of the wear track, L, by the diameter of the Hertzian static contact area, e:

$$e = 2\left(\frac{3\,^{F\_N}R}{4\,^{B}E}\right)^{\frac{1}{3}}\tag{27}$$

with FN the applied normal load, R the radius of the tip of the curved counter-body, and E the equivalent elastic modulus given by:

$$\frac{1}{E} = \frac{1 - \nu\_1^2}{E\_1} + \frac{1 - \nu\_2^2}{E\_2} \tag{28}$$

with 1 and 2 the Poisson's ratios, and E1 and E2 the elastic moduli of the test sample and counterbody respectively.

Sliding is initiated at the time a stable Eoc is achieved. That Eoc recorded during sliding is a mixed potential resulting from the galvanic coupling of material inside (Atr) and outside (Ao – Atr) the sliding track. It is assumed that the kinetics of the redox reactions taking place on each of these areas, do not vary with the real potential of the sliding track. In other words, the ohmic drop effect is considered to be negligible in the galvanic coupling between the sliding track and the surrounding area. Electrochemical impedance spectroscopy measurements are performed during sliding to obtain the polarization resistance, Rps, of the sample surface. Similarly to Eoc, Rps may be considered as the combination of two polarization resistances, namely Ract related to the active area Aact which is equal in this case to the wear track, and Rpass which corresponds to the surrounding unworn area, (Ao – Atr):

$$\frac{1}{R\_{ps}} = \frac{1}{R\_{act}} + \frac{1}{R\_{pass}}\tag{29}$$

where

100 Corrosion Resistance

 rpass = Rp.Ao (22) Specific polarization resistance values for metallic materials of 103 Ωcm2 or lower indicate the presence of an active sample surface, while values around 100 × 103 Ωcm2 or higher indicate a passive sample surface. The corrosion current density of the material covered by a

*pass*

*<sup>r</sup>* (23)

*<sup>t</sup> <sup>t</sup>* (24)

*A A tr act* (25)

*AA A tr tr tr* (26)

(27)

*pass*

**4.2.3 Second step in the testing protocol: Electrochemical tests on a fully active** 

The next step is the determination of the corrosion rate of the depassivated material. In order to keep continuously a part of the immersed sample surface in an active state, the passive film has to be removed by mechanical contacts. It is thus necessary to select a rotation period, trot, which is small compared to treac so that the passive film has no time to regrow in between two successive contact events. It was proposed (Diomidis et al., 2009,

> 10000 *reac rot*

A first value of tlat1 is then taken equal to trot. It is thus assumed that during such sliding

Despite the fact that the width of the sliding track increases progressively due to wear during sliding tests performed against a curved counter-body, a mean sliding track area, Atr,

with Atr max the maximum value measured at the end of the test, and Atr min the minimum value at the end of the first cycle. Atr min is calculated by multiplying the length of the wear

> <sup>3</sup> <sup>3</sup> <sup>2</sup> 4 *F R <sup>N</sup> <sup>e</sup>*

*E* 

1 2

max min

1

*<sup>B</sup> <sup>i</sup>*

with B a constant. For metallic materials, B normally varies between 13 and 35 mV, depending on the nature of the material and the environment. In the protocol worked out hereafter as an example, a value of 24 mV is assumed. This passive current density, ipass, is considered to correspond to the dissolution current of the material through the passive film

passive surface film, ipass, is then calculated as follows:

at stationary state.

**sliding track during sliding** 

2010) to take the rotation period trot equal to:

is taken for simplicity, and defined as:

tests the whole wear track area is in an active state, so that:

track, L, by the diameter of the Hertzian static contact area, e:

$$R\_{act} = \frac{r\_{act}}{A\_{tr}}\tag{30}$$

and

$$R\_{\text{pass}} = \frac{r\_{\text{pass}}}{A\_0 - A\_{tr}} \tag{31}$$

It is then possible to calculate the specific polarization resistance of the active surface, ract, as:

$$r\_{act} = \frac{A\_{tr} \ R\_{ps} \ r\_{pass}}{r\_{pass} - R\_{ps} \left(A\_0 - A\_{tr}\right)} \tag{32}$$

The corrosion current density of the active material, iact, can now be obtained by substituting rpass with ract in Equation (23):

$$\dot{I}\_{act} = \frac{B}{r\_{act}}\tag{33}$$

It can be noted that, the specific resistance of the active bare material in the sliding track, ract, is generally several orders of magnitude lower than the specific resistance of the material covered with a passive film, rpass , outside of the sliding track. Therefore, if Atr is not too small a fraction of the total area A0 of the sample, the resistance Ract related to the sliding

Tribocorrosion: Material Behavior Under

Wtr = Wc


Arepass in re-passivated state:

with :



performed:

**5. Conclusions** 

functionality.

**4.3 Analysis and interpretation of sliding test results** 

Combined Conditions of Corrosion and Mechanical Loading 103

The total wear Wtr in a wear track of area Atr can be expressed as a sum of components related to both types of areas present on the track, the area Aact in active state, and the area

repass + Wmrepass (37)

act + Wmact + Wc

repass material loss by corrosion of repassivated material in wear track, and - Wmrepass material loss due to mechanical wear of repassivated material in the wear track. In order to assess the values of these different components and to compare them to determine the characteristics of the wear mechanism, the following analyses have to be




Detailed information on the analysis and related interpretation of sliding tests can be found

Tribocorrosion is the degradation of material surfaces by the combined action of corrosion, electrochemical passivation, and external mechanical interactions. It is essentially a surface related process, but some events like hydrogen evolution and absorption by the material, can modify the mechanical properties of the sub-surfaces on materials. Under conditions where tribocorrosion is active, the material loss depends in a complex way on many parameters like the tribological conditions in the contact, the composition of the environment, the temperature, the flow rate, the pH and eventually the applied potential. By analogy it is possible to extend this concept when, in the process previously described, a chemical or physical adsorption of inhibiting species strengthens or replaces the electrochemical passivation process. The successive repetition of some of these processes can lead to a possible synergism between mechanical stress and the effect of the environment what results in a damage of surfaces and systems through an accelerated loss of

Electrochemical methods used for corrosion studies are of interest in tribocorrosion since they allow the *in situ* monitoring and analysis of the interactions between surfaces and their

allow the study of the periodic removal and re-growth of a passive surface film,

act material loss due to corrosion of active material in wear track, - Wmact material loss due to mechanical wear of active material in wear track,

active material during sliding tests performed at low latency times,

must differentiate between continuous and intermittent sliding tests,

in a Handbook on Tribocorrosion (Celis & Ponthiaux, 2011).

track is small compared to the resistance Rpass of the area of the sample remained in passive state, outside of the sliding track: if Ract << Rpass, according to equation (29), the measured resistance Rps gives then straight an approximate value of Ract.

#### **4.2.4 Third step in the testing protocol: Electrochemical tests on a partially active sliding track during sliding**

In the preceding steps, two extreme cases were characterized, namely on the passive material and on the active one repectively. Under tribocorrosion conditions at high latency time, the surface of the material undergoes sequential events of depassivation and repassivation in-between successive contacts. This means that a part of the surface at any given time repassivates progressively. The latency time is then selected so that the regrowth of a surface film between two successive contact events is not anymore negligible as it was the case under sliding at low latency time. To achieve partially active sliding tracks, the latency times can be selected as tlat2 = treac/1000 and tlat3 = treac/100 (Diomidis et al., 2009). As a result of the increase of the latency time in this step, the wear track can now be assumed to consist of two distinct zones (Diomidis et al., 2010), namely:


$$\mathbf{A\_{tr}} = \mathbf{A\_{act}} + \mathbf{A\_{repass}} \tag{34}$$

It must be stressed that under continuous sliding, these active and repassivated areas remain constant because of stationary electrochemical state conditions. Under intermittent sliding, these active and repassivated areas on the sliding track evolve with time between two successive contact events since a gradual increase of the coverage of the repassivated area takes place within the ''off period''. By hypothesis, in both cases, the fraction of the sliding track surface covered by the passive film, Arepass/Atr, is assumed to be constant and given by the ratio tlat/treac:

$$\frac{\mathbf{A\_{response}}}{\mathbf{A\_{tr}}} = \frac{\mathbf{t\_{lat}}}{\mathbf{t\_{reac}}} \tag{35}$$

and:

$$\frac{\mathbf{A}\_{\rm act}}{\mathbf{A}\_{\rm tr}} = \mathbf{1} - \frac{\mathbf{t}\_{\rm lat}}{\mathbf{t}\_{\rm reac}} \tag{36}$$

At the latency times tlat2 = 0.001 treac and tlat3 = 0.01 treac, the relationship between repassivated and total wear track area are respectively Arepass 2 = 0.001Atr and Arepass 3 = 0.01 Atr, and thus the active wear track areas are Aact 2 = 0.999 Atr and Aact 3 = 0.99 Atr.
