**1. Introduction**

80 Corrosion Resistance

Uggowitzer, P. (1991). Uggowitzer, P.; Ultrahochfeste austenitische Stähle, Ergebnisse der

## **1.1 Definition of tribocorrosion**

Tribocorrosion can be defined as the study of the influence of environmental factors (chemical and/or electrochemical) on the tribological behavior of surfaces. In other words, the process leading to the degradation of a metallic and/or non-metallic material resulting from a mechanical contact (sliding, friction, impact, ...) combined to a corrosive action of the surrounding environment.

The origin of tribocorrosion is closely related to the presence of a passive film on material surfaces subject to wear and the modifications of these surfaces by friction or any other form of mechanical loading. In very general terms, the passive film (mainly oxide) is considered to be snatched in the contact area.

Oxide particles, referred to as 'debris", are released from the contacting materials. Then, the debris can be removed from the contact zone or on the contrary trapped in it. In the case of removal, the debris dissolve chemically or are dragged out by a hydraulic flow along the material surface. In this case, the tribocorrosion mechanism is based on a repeated tearing off of the oxide after each contact and eventually a removal of some of the underlying material depending on the intensity of mechanical stress acting on the contacting materials. The major concern is then to quantify and eventually to model the kinetics of repassivation as accurately as possible. This type of tribocorrosion process can be classified as an oxidative wear mechanism as, for example, the 'mild oxidative wear model' (Quinn, 1992). In the case of debris trapping, one has to consider that under appropriate hydrodynamical, chemical, and thermal contact conditions and relative speed of the two contacting bodies, the debris will remain temporarily in the contact zone mainly as colloids with a diameter usually in the range of a few hundred nanometers. Two cases may then be distinguished: (a) the debris accelerates the wear in comparison to the case of debris-free contacts is accelerated by an abrasive effect, or (b) the debris slows down the wear compared to the case where the contact zone is free of any debris, resulting in a protective effect.

Tribocorrosion: Material Behavior Under

compete on these bare surfaces, namely:

to increase with the latter.

Soon Lee et al., 1999):

n has a value between 0.3 and 1.


**1.3 Complexity of the tribocorrosion process** 

Combined Conditions of Corrosion and Mechanical Loading 83

aqueous solution is a few nanometers thick but gives them a high resistance to corrosion. The sliding of a hard counterbody material on such a surface is likely to damage that passive film what is known as a "depassivation" process by which the bare material is exposed to the corrosive environment. Various but essentially electrochemical processes can then


The following examples taken from literature illustrate quite well the numerous parameters and interactions that govern the tribocorrosion process. Lemaire & Le Calvar, 2000, described the wear of a cobalt-based alloy coating generally referred to as "stellite 6" applied on the gripper latch arms of the control rods command mechanisms in pressurized water reactors (PWR). The downwards movement of the control rods is controlled by gripper latch arms of which the protruding teeth are coated with Stellite 6. The teeth block the movement once they come in contact with the control bar at the circular grooves lining their surface. At each blocking step, there is a contact between teeth and inner part of the grooves at a moderate pressure estimated at 150 MPa. Subsequently a sliding takes place over a distance of approximately 0.1 mm before the control rods come to rest. In the middle of the primary cooling circuit stellite 6 does not undergo any significant corrosion in the absence of any mechanical stress, thanks to the protective action of the passive film on stellite 6 consisting of chromium oxides. However under field operating conditions where impact and sliding of the teeth on the control bar take place, corrosion is evident. The wear observed on the teeth was found not to depend only on the number of blocking steps as would be the case in absence of corrosion. But the wear was found to depend also on the time interval between two successive blocking steps. The wear rate for a given number of blocking steps appeared

A plausible hypothesis to interpret this behavior is to consider that between two successive blocking steps corrosion takes place on parts of the surface where the passive film was mechanically damaged in the preceding step. The wear progress is correlated with the time interval between successive blocking steps by the following simple empirical equation (Bom

in which I(t) is the evolution of the dissolution current of a metal with time starting at the time the metal becomes depassivated due to a mechanical action and extending during the film restoration where dissolution and repassivation are competitive surface processes. The parameters I0 and t0 are constants, while Ip is the passivation current under steady state, and

n 0 p 0 <sup>t</sup> I (t) I I <sup>t</sup> 

(2)


Tribocorrosion may take place in practice in a large number of very different tribological systems consisting of mechanical devices containing metallic parts that are in contact with counterparts and exhibiting a relative movement placed in an environment revealing itself to be corrosive to at least one of the contacting materials. A non-limitative list of examples might contain machinery pumps, bearings, gears, ropes, electrical connectors, hinges, microelectromechanical systems (MEMs), and orthopedic implants like hip and knee implants.

#### **1.2 Synergism between mechanical and chemical loading**

To understand the importance and complexity of the phenomena taking place under tribocorrosion, one has to consider that the corrosiveness of a medium (liquid or gas) towards a material is highly dependent on the mechanical stresses that act onto a material, particularly at its surface exposed to that environment.

In tribocorrosion, five mechanisms may explain the synergism noticed between mechanical and chemical factors acting on contacting materials, namely:


A synergistic effect occurs in tribocorrosion when the mechanical process affects the corrosion process acting in a tribological system or vice versa. In these cases, the wear, W, found on a given component in a tribological system subjected to a mechanical loading in a given corrosive environment, will be very different and often much greater than the sum of the mechanical wear, Wmo, measured as a material loss under a given mechanical load in the absence of a corrosive environment, and the material loss induced by corrosion, Wco, in the absence of any mechanical contact (see Equation 1):

$$\mathbf{W} \neq \mathbf{W}\_{\text{mo}} + \mathbf{W}\_{\text{co}} \tag{1}$$

This result is partly explained by the fact that the corrosion resistance in the case of a metal depends on the presence at its surface of reaction layers, sometimes only a few atom layers thick, resulting from an interaction between the material and the surrounding environment. Such layers can be classified as oxides, solid precipitates, adsorbed layers, or passive surface films. Some of them like dense oxide layers, precipitates, or passive films play a protective role by isolating the underlying metal from a direct contact with a surrounding corrosive environment. This is particularly true in the case of stainless steels and other alloys containing chromium. Their passive surface film formed in ambient air or in contact with an

Tribocorrosion may take place in practice in a large number of very different tribological systems consisting of mechanical devices containing metallic parts that are in contact with counterparts and exhibiting a relative movement placed in an environment revealing itself to be corrosive to at least one of the contacting materials. A non-limitative list of examples might contain machinery pumps, bearings, gears, ropes, electrical connectors, hinges, microelectromechanical systems (MEMs), and orthopedic implants like hip and knee

To understand the importance and complexity of the phenomena taking place under tribocorrosion, one has to consider that the corrosiveness of a medium (liquid or gas) towards a material is highly dependent on the mechanical stresses that act onto a material,

In tribocorrosion, five mechanisms may explain the synergism noticed between mechanical

1. the debris can speed up or reduce wear compared to what happens in the same environment where debris does not exist like e.g. in sliding contacts polarized at a large

2. a galvanic coupling is established between the worn and unworn areas. It accelerates

A synergistic effect occurs in tribocorrosion when the mechanical process affects the corrosion process acting in a tribological system or vice versa. In these cases, the wear, W, found on a given component in a tribological system subjected to a mechanical loading in a given corrosive environment, will be very different and often much greater than the sum of the mechanical wear, Wmo, measured as a material loss under a given mechanical load in the absence of a corrosive environment, and the material loss induced by corrosion, Wco, in the

 W ≠ Wmo + Wco (1) This result is partly explained by the fact that the corrosion resistance in the case of a metal depends on the presence at its surface of reaction layers, sometimes only a few atom layers thick, resulting from an interaction between the material and the surrounding environment. Such layers can be classified as oxides, solid precipitates, adsorbed layers, or passive surface films. Some of them like dense oxide layers, precipitates, or passive films play a protective role by isolating the underlying metal from a direct contact with a surrounding corrosive environment. This is particularly true in the case of stainless steels and other alloys containing chromium. Their passive surface film formed in ambient air or in contact with an

3. a galvanic coupling may be established between the two contacting counterparts, 4. an accumulation of dissolved species may take place in the liquid surrounding the contact. This may render the medium chemically or electrochemically more aggressive, 5. the mechanical loading in the contact area and its nearby zone may causes a work hardening of the materials. This work hardening can alter the kinetics of corrosion

the anodic dissolution in the area where the metal is depassivated,

**1.2 Synergism between mechanical and chemical loading** 

particularly at its surface exposed to that environment.

and chemical factors acting on contacting materials, namely:

implants.

cathodic potential,

and/or repassivation processes.

absence of any mechanical contact (see Equation 1):

aqueous solution is a few nanometers thick but gives them a high resistance to corrosion. The sliding of a hard counterbody material on such a surface is likely to damage that passive film what is known as a "depassivation" process by which the bare material is exposed to the corrosive environment. Various but essentially electrochemical processes can then compete on these bare surfaces, namely:


#### **1.3 Complexity of the tribocorrosion process**

The following examples taken from literature illustrate quite well the numerous parameters and interactions that govern the tribocorrosion process. Lemaire & Le Calvar, 2000, described the wear of a cobalt-based alloy coating generally referred to as "stellite 6" applied on the gripper latch arms of the control rods command mechanisms in pressurized water reactors (PWR). The downwards movement of the control rods is controlled by gripper latch arms of which the protruding teeth are coated with Stellite 6. The teeth block the movement once they come in contact with the control bar at the circular grooves lining their surface. At each blocking step, there is a contact between teeth and inner part of the grooves at a moderate pressure estimated at 150 MPa. Subsequently a sliding takes place over a distance of approximately 0.1 mm before the control rods come to rest. In the middle of the primary cooling circuit stellite 6 does not undergo any significant corrosion in the absence of any mechanical stress, thanks to the protective action of the passive film on stellite 6 consisting of chromium oxides. However under field operating conditions where impact and sliding of the teeth on the control bar take place, corrosion is evident. The wear observed on the teeth was found not to depend only on the number of blocking steps as would be the case in absence of corrosion. But the wear was found to depend also on the time interval between two successive blocking steps. The wear rate for a given number of blocking steps appeared to increase with the latter.

A plausible hypothesis to interpret this behavior is to consider that between two successive blocking steps corrosion takes place on parts of the surface where the passive film was mechanically damaged in the preceding step. The wear progress is correlated with the time interval between successive blocking steps by the following simple empirical equation (Bom Soon Lee et al., 1999):

$$\mathbf{I}\ \mathbf{\dot{r}}\ \mathbf{\dot{t}} = \mathbf{I}\_0 \left(\frac{\mathbf{t}}{\mathbf{t}\_0}\right)^{-\mathbf{n}} + \mathbf{I}\_\mathbf{p} \tag{2}$$

in which I(t) is the evolution of the dissolution current of a metal with time starting at the time the metal becomes depassivated due to a mechanical action and extending during the film restoration where dissolution and repassivation are competitive surface processes. The parameters I0 and t0 are constants, while Ip is the passivation current under steady state, and n has a value between 0.3 and 1.

Tribocorrosion: Material Behavior Under

**techniques** 

on-flat,...),

parameters, like:

stirred,..).

residual stress ...),

Combined Conditions of Corrosion and Mechanical Loading 85

Fig. 1. Example of a third body resulting from a tribological process during reciprocating sliding. Some abrasive grooves are also visible in the central area of the sliding track.

When performing a tribocorrosion test, one has to implement not only the following


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




These parameters determine the electrochemical reactivity of the surfaces and in consequence

influence the contact conditions (wear regime, existence of a third body, friction ...).

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


microstructure, surface film composition and structure ...), and

traditional concepts of tribological testing, namely:

However, the effect of sliding on the electrochemical reactivity of the surface of a metal is not always confined to the partial destruction of surface layers. Other phenomena resulting directly or indirectly from contacts between parts can influence the corrosion behavior of their surfaces. For example, under reciprocating sliding conditions at small displacement amplitude, known as fretting (Carton et al., 1995; Godet et al, 1991) cracks may appear at the rim of the contact zone even after only a small number of contact events. Another parameter to be considered in tribocorrosion is the stirring of the corrosive environment along the surfaces of contacting parts caused by their relative movement. It affects tribocorrosion since such a stirring modifies the transport kinetics of chemical species that are generated in the vicinity of the surfaces due to the corrosionrelated reactions.

The effect of the environment (mechanical or physico-chemical) on the crack propagation is evident. This returns us, somehow, to the notion of stress corrosion with the nuance that the cracks are induced at a mechanical loading which is not constant in the fretting test under consideration.

In addition one has to consider the possible role of strain hardening and/or structural transformations induced by the sliding action on the electrochemical reactivity of the surface, speeding up or slowing down some reactions. The resulting material transformation may end up in the most stable phase of the material considered being a supersaturated solid solution obtained by the gradual dissolution of pre-existing precipitates. It may also become a nano-crystalline network of a few tens of nanometers in average size that contains a high density of dislocations with no preferred orientation. This structure is very hard but also very fragile. Its intrinsic reactivity with the environment differs necessarily from the one of the original surface. Moreover, starting with the formation of a network of micro-cracks on it, wear particles (known as 'debris') are generated.

In the case of tribocorrosion, it is important to consider the galvanic coupling that might result from the heterogeneity of the electrochemical state of non-rubbed surfaces, and rubbed surfaces undergoing a strain hardening and on which the surface layers are altered. This galvanic coupling causes the polarization of non-rubbed and rubbed areas, and modifies the kinetics of reactions in these areas.

One has also to consider the existence of a third body that consists of wear particles as visible in Figure 1. At first, this third body may interact with the environment to form oxides or hydroxides. If they are ejected out of the contact zone, they become strictly speaking 'debris' and contribute to a material loss. If they remain in the contact zone, they can modify the mechanical response of the system by favoring a sliding action between counterparts, by acting as an abrasive agent promoting the so-called "abrasive wear", or by affecting the reactivity of material surfaces.

In return, a corrosion process can modify the surface states of materials and in consequence the contact conditions. In that way corrosion can affect the sliding conditions (coefficient of friction, wear regime, ...). The interaction between friction and corrosion therefore induces a complex phenomenon of synergy.

However, the effect of sliding on the electrochemical reactivity of the surface of a metal is not always confined to the partial destruction of surface layers. Other phenomena resulting directly or indirectly from contacts between parts can influence the corrosion behavior of their surfaces. For example, under reciprocating sliding conditions at small displacement amplitude, known as fretting (Carton et al., 1995; Godet et al, 1991) cracks may appear at the rim of the contact zone even after only a small number of contact events. Another parameter to be considered in tribocorrosion is the stirring of the corrosive environment along the surfaces of contacting parts caused by their relative movement. It affects tribocorrosion since such a stirring modifies the transport kinetics of chemical species that are generated in the vicinity of the surfaces due to the corrosion-

The effect of the environment (mechanical or physico-chemical) on the crack propagation is evident. This returns us, somehow, to the notion of stress corrosion with the nuance that the cracks are induced at a mechanical loading which is not constant in the fretting test under

In addition one has to consider the possible role of strain hardening and/or structural transformations induced by the sliding action on the electrochemical reactivity of the surface, speeding up or slowing down some reactions. The resulting material transformation may end up in the most stable phase of the material considered being a supersaturated solid solution obtained by the gradual dissolution of pre-existing precipitates. It may also become a nano-crystalline network of a few tens of nanometers in average size that contains a high density of dislocations with no preferred orientation. This structure is very hard but also very fragile. Its intrinsic reactivity with the environment differs necessarily from the one of the original surface. Moreover, starting with the formation of a network of micro-cracks on it, wear particles (known as 'debris')

In the case of tribocorrosion, it is important to consider the galvanic coupling that might result from the heterogeneity of the electrochemical state of non-rubbed surfaces, and rubbed surfaces undergoing a strain hardening and on which the surface layers are altered. This galvanic coupling causes the polarization of non-rubbed and rubbed areas, and

One has also to consider the existence of a third body that consists of wear particles as visible in Figure 1. At first, this third body may interact with the environment to form oxides or hydroxides. If they are ejected out of the contact zone, they become strictly speaking 'debris' and contribute to a material loss. If they remain in the contact zone, they can modify the mechanical response of the system by favoring a sliding action between counterparts, by acting as an abrasive agent promoting the so-called "abrasive wear", or by affecting the

In return, a corrosion process can modify the surface states of materials and in consequence the contact conditions. In that way corrosion can affect the sliding conditions (coefficient of friction, wear regime, ...). The interaction between friction and corrosion therefore induces a

related reactions.

consideration.

are generated.

modifies the kinetics of reactions in these areas.

reactivity of material surfaces.

complex phenomenon of synergy.

Fig. 1. Example of a third body resulting from a tribological process during reciprocating sliding. Some abrasive grooves are also visible in the central area of the sliding track.
