**2.2 Potentiodynamic polarization measurements**

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 uniform.

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 electrochemical behavior of the alloy:


Tribocorrosion: Material Behavior Under

et al., 1999).

also the state of the whole surrounding sample surface.

1979; Ponthiaux et al., 1995; Tiedemann et al., 1973).

**2.3 Repassivation current transients** 

uniform film in the damaged area.

Current

Combined Conditions of Corrosion and Mechanical Loading 89

The extent of the passivation range is in general affected by the sliding. In some cases the Flade potential, EF, shifts towards more anodic values at increasing contact pressure. This means that the stability of the passive film decreases. When the potential EF is reached, the entire surface becomes active and dissolves. However, under these experimental conditions, the disruption of the passive film is only a local disturbance in the sliding track in a relatively small area that represents generally not more than 10% of the total sample area. Therefore sliding does not only affect the electrochemical state of the rubbed surface, but

Finally it should be stressed that the coefficient of friction varies with the applied potential during a potentiodynamic polarization. Changes in the value of the coefficient of friction reveal a possible change in the surface state of the materials in the sliding track (Ponthiaux

The in-depth interpretation of the polarization curves frequently faces difficulties related to the non-uniform distributions of current and potential on the sample surface. This nonuniformity originates from the intrinsic effect of the sliding that causes an heterogeneity of the electrochemical surface reactivity, combined with the ohmic drop in the electrolyte. A full exploitation of the polarization curves in terms of local behavior is possible only if one can model the current and potential distributions under sliding conditions. This brings back to the same approach as in the case of the interpretation of open circuit potential measurements. Note that the effect of non-uniform distributions on the interpretation of polarization curves was already investigated in the absence of any sliding (Law & Newman,

Methods based on analysis of potential or current transients (Ponthiaux et al., 1995) are particularly well suited to study reciprocating sliding tests (Mischler et al., 1997). These methods are used to study between successive contact events, the rebuild of damaged surface layers (oxide, passive film ...). Under imposed polarization e.g. in the passivation range, at each stop-start event, a transient variation of current is noticed with time (see Figure 5). The charge corresponding to this transition can be attributed to the re-growth of a

Time

Fig. 5. Schematic evolution of the current transients obtained under reciprocating sliding.

Displacement of the pin

unaffected. This indicates that the dissolution process is the same on rubbed and nonrubbed materials,


Fig. 3. Schematic potentiodynamic polarization curve recorded on a passivating metallic material (Fe-30%Ni alloy) in absence of any sliding: EF = Flade potential, EM = potential at maximum dissolution current IM, IP = passivation current.

The possible evolution of the passivation current, Ip, under sliding friction with the mean contact pressure, PM, and the sliding velocity is schematically shown in Figure 4. That passivation current is increasing with these two testing parameters. That increase can be related to an increase in bare surface area due to:


Average contact pressure Pm

Fig. 4. Evolution of the passive current, Ip, with the mean contact pressure, Pm, at increasing sliding speeds. Example of a Fe-31%Ni alloy disc immersed in 0.5 M sulfuric acid and polarized at a fixed passive potential, under continuous sliding against alumina.


that the dissolution of the bare material is faster than the non-rubbed material.

rubbed materials,

Potential E (V SSE)

*vs.*

EF

EP

0

related to an increase in bare surface area due to:

V1

Current I P

speed.

maximum dissolution current IM, IP = passivation current.


V2

V4 <<<

P

unaffected. This indicates that the dissolution process is the same on rubbed and non-

Current I (A)

IP <sup>I</sup>

Fig. 3. Schematic potentiodynamic polarization curve recorded on a passivating metallic material (Fe-30%Ni alloy) in absence of any sliding: EF = Flade potential, EM = potential at

The possible evolution of the passivation current, Ip, under sliding friction with the mean contact pressure, PM, and the sliding velocity is schematically shown in Figure 4. That passivation current is increasing with these two testing parameters. That increase can be


V3 V4

Average contact pressure Pm

Fig. 4. Evolution of the passive current, Ip, with the mean contact pressure, Pm, at increasing sliding speeds. Example of a Fe-31%Ni alloy disc immersed in 0.5 M sulfuric acid and polarized at a fixed passive potential, under continuous sliding against alumina.

M

M

V1

V2

V 3 The extent of the passivation range is in general affected by the sliding. In some cases the Flade potential, EF, shifts towards more anodic values at increasing contact pressure. This means that the stability of the passive film decreases. When the potential EF is reached, the entire surface becomes active and dissolves. However, under these experimental conditions, the disruption of the passive film is only a local disturbance in the sliding track in a relatively small area that represents generally not more than 10% of the total sample area. Therefore sliding does not only affect the electrochemical state of the rubbed surface, but also the state of the whole surrounding sample surface.

Finally it should be stressed that the coefficient of friction varies with the applied potential during a potentiodynamic polarization. Changes in the value of the coefficient of friction reveal a possible change in the surface state of the materials in the sliding track (Ponthiaux et al., 1999).

The in-depth interpretation of the polarization curves frequently faces difficulties related to the non-uniform distributions of current and potential on the sample surface. This nonuniformity originates from the intrinsic effect of the sliding that causes an heterogeneity of the electrochemical surface reactivity, combined with the ohmic drop in the electrolyte. A full exploitation of the polarization curves in terms of local behavior is possible only if one can model the current and potential distributions under sliding conditions. This brings back to the same approach as in the case of the interpretation of open circuit potential measurements. Note that the effect of non-uniform distributions on the interpretation of polarization curves was already investigated in the absence of any sliding (Law & Newman, 1979; Ponthiaux et al., 1995; Tiedemann et al., 1973).
