**4. Tribocorrosion testing**

96 Corrosion Resistance

0 p 0 j (t)dt *<sup>t</sup>*

When the component jp related to passive zones can be neglected, C0 becomes zero and

*M aa*

The depassivated area during one cycle, Aa, can hardly be assessed. Some authors assume in first instance that it is equal to the apparent area of the sliding track. However, it is well known that the contact takes place only on a fraction of that area. An evaluation of the depassivated area from currents resulting from an electrochemical depassivation achieved by a potential jump was proposed (Garcia et al., 2001). This method also allowed them to evaluate the oxide thickness formed in between two successive depassivation events. They

Another approach was developed (Jemmely et al., 2000). The authors proposed to express

Rdep = f.Aa (17)

with Qrep the charge density for repassivation. The rate at which an active area is generated per unit of time depends on the morphology and hardness of the surfaces in contact. A derivation of Rdep from the scratching of a ductile material by a hard abrasive one was proposed (Adler & Walters, 1996). That approach was taken over (Jemmely et al., 2000) and

extended in more general terms (Mischler et al., 1998) in the following expression:

between 0.5 and 1 in the general case.

loss by corrosion under sliding, C, can thus finally be written as:

R K.V dep

with K an empirical constant, V the sliding velocity, FN the applied load, H the hardness of

One empirical constant K remains in this model which approximates Archard's constant and which is related to the probability that a given contact becomes depassivated. The mass

> *<sup>M</sup> <sup>F</sup> C KV Q f nF H*

*NF H* 

= 0.5 in the case of a rough and hard body against a smooth and ductile

( ) *<sup>N</sup> rep*

= 1 in the case of a hard and smooth body against a rough and ductile

= 0.5 in the case of a contact between two counterparts with a similar

1/

0

( ) *f*

*<sup>M</sup> C A*

equation (4) expressing the mass loss by corrosion under sliding becomes then:

obtained oxide layer thicknesses in the range of a few nanometers.

with f the contact frequency. The currents can then be expressed as:

the tested material,

roughness,

counterpart,

counterpart, and

the depassivated area in terms of a depassivation ratio per unit of time, Rdep :

1/f

*nF* (15)

*<sup>M</sup> C C A j t dt nF* (16)

I = Rdep.Qrep (18)

(19)

(20)
