**3. Tribocorrosion fundamentals**

An important problem in tribology concerns the interaction between friction processes and electrochemical reactions (corrosion) occurring in reactive environments such as aqueous media or hot aggressive gases. The effect of mechanical stimuli on chemical degradation of materials and, *vice-versa*, the influence of corrosion on the mechanical response of contacting materials are of great concern in many areas of tribology. A new research area, "tribocorrosion", emerged in recent years, mainly driven by the increasing demand from the biomedical implant, power generation, marine, and offshore industries. Since tribocorrosion is ubiquitous in many technical applications, it is necessary to employ material means capable of withstanding its damage effects.

Tribocorrosion can be defined as "…the process leading to a material degradation (i.e., material loss) which results from simultaneous mechanical (materials properties, surface and sub-surface transformations, cracking, etc.), chemical, electrochemical (corrosion attack) and/ or biological material removal mechanisms (i.e., bio-tribocorrosion)…" [7]. It is basically an integration of two major areas of significance and application in mechanical systems, namely Tribology and Corrosion [8, 9]:

Tribocorrosion involves the inter-play between friction, wear (tribological), and corrosion (electrochemical) phenomena in a complex way. This includes such diverse phenomena as wear-accelerated corrosion, fretting-corrosion, erosion-corrosion, oxidational wear, chemical or corrosive wear often described in the literature. Therefore, tribocorrosion (or tribological) processes are intricate and not intrinsically fundamental, in the sense that Young's modulus for example is fundamental, but rather that they depend on the triboelectrochemical system approach. In particular they are determined by a combination of a number of more fundamental properties of the contacting materials, testing parameters, and test conditions, especially the nature of the environment in which the tests take place. An important aim of research in tribocorrosion field is precisely that of determining the nature of this dependence, so that the triboelectrochemical behavior may be predicted from a knowledge of a system approach and the more fundamental properties of interacting surfaces. Although, this aim has not been achieved yet, a fair progress has been made, and still more work is required to have a good comprehension of just which are the important distinctive features determining the surface interaction behavior in a triboelectrochemical system.

## **3.1. Elements of tribocorrosion (instrumentation)**

potentially accessible to metallic glasses extends beyond traditional benchmarks towards levels formerly inaccessible to any material (e.g., Pd-based alloys in [2]). One direct result of the unique microstructure is the high toughness-to-yield strength ratio, mainly accessible to BMGCs. Their strength exceeds that of the strength limit of known crystalline pure metals or alloys and approaches that of engineering ceramics, whereas their toughness is markedly

Some recent significant developments have been made towards the design of this kind of BMGC materials. This was achieved through the successful implementation of effective composite microstructures, which typically combine a strong glassy matrix with ductile crystalline reinforcements that suppress fracture while sustaining high strength. This variety of composite materials can only be obtained through the commitment of a nanocrystallization process [5] or *via* the reinforcement with ceramic particles [6]. Current studies on BMGCs performance are still at the development stage and concern the evaluation of either their mechanical properties or their corrosion resistance, but the perspective is very promising.

In recent years, a wide variety of industries including food, medical and pharmaceutical, aircraft components, electronics, building materials, and automobile industries have been promoting the technological development of newly composite materials including the vitreous-based composites to achieve suitable strength/density, and toughness/stiffness ratios.

An important problem in tribology concerns the interaction between friction processes and electrochemical reactions (corrosion) occurring in reactive environments such as aqueous media or hot aggressive gases. The effect of mechanical stimuli on chemical degradation of materials and, *vice-versa*, the influence of corrosion on the mechanical response of contacting materials are of great concern in many areas of tribology. A new research area, "tribocorrosion", emerged in recent years, mainly driven by the increasing demand from the biomedical implant, power generation, marine, and offshore industries. Since tribocorrosion is ubiquitous in many technical applications, it is necessary to employ material means capable of

Tribocorrosion can be defined as "…the process leading to a material degradation (i.e., material loss) which results from simultaneous mechanical (materials properties, surface and sub-surface transformations, cracking, etc.), chemical, electrochemical (corrosion attack) and/ or biological material removal mechanisms (i.e., bio-tribocorrosion)…" [7]. It is basically an integration of two major areas of significance and application in mechanical systems, namely

Tribocorrosion involves the inter-play between friction, wear (tribological), and corrosion (electrochemical) phenomena in a complex way. This includes such diverse phenomena as wear-accelerated corrosion, fretting-corrosion, erosion-corrosion, oxidational wear, chemical or corrosive wear often described in the literature. Therefore, tribocorrosion (or tribological) processes are intricate and not intrinsically fundamental, in the sense that Young's modulus

high, among metallic alloys.

80 Metallic Glasses - Properties and Processing

**3. Tribocorrosion fundamentals**

withstanding its damage effects.

Tribology and Corrosion [8, 9]:

Tribocorrosion of two contacting solids in relative motion is, just as friction, a system parameter. A triboelectrochemical system consists of implementing electrochemical techniques to a tribological designed system (i.e., tribometer type, and complete material system). That mechanical designed system is of a great importance since this will enable to simulate as much as possible the entire material system used in the field, and the constraints that have associated with it (e.g., similarity of the wear mechanisms active in the laboratory test and in the field, such as abrasion, adhesion, fatigue, penetration hardness, bending, existence or not of a third body, erosion, corrosion, their combinations, etc.).

Generally, the tribological configuration involves:


The measuring instruments in tribocorrosion tests allow to monitor in real-time and on-line the foregoing system parameters (e.g., contact conditions: normal or tangential force, relative displacement, velocity, etc.). The main focus is to promptly control any variation or change associated with these parameters or settings *in-situ*. They need also to be instrumented with electrochemical techniques, which enables the management and the recording of applied electrochemical parameters and/or their responses (e.g., polarization of the contacting materials, charge density, etc.). The ultimate goal is to obtain promptly any information on the evolution of the input and output chemical-mechanical measurements during the test. These *in-situ* data outcome combined with *ex-situ* surface characterization techniques (e.g., high resolution SEM imaging, TEM, XRD, EDAX, FIB, XPS, Auger spectroscopy, FT-IR, roughness surface profilometry, micro- or nanoindentation hardness, etc.) and chemical analyses of post-test solutions (e.g., ICP-AES, ICP-MS) allow for the disclosure of the wear-corrosion mode, and thereby contributing to a better understanding of the tribocorrosion mechanisms involved (corrosion mechanism, wear regime, friction process, etc.).

applied potential resulting in an infinitesimal disturbance of the surface (potentiostatic control), measurements of current-induced single step anodic potential pulse (for the study of film repair, or repassivation), measurements of electrochemical noise for on-line tribocorrosion monitoring, measurements of electrochemical impedance, measurement of the linear

Metallic Glasses for Triboelectrochemistry Systems http://dx.doi.org/10.5772/intechopen.78233

Since most of the undertaken studies on chemical degradation of glassy materials involve the use of cyclic potentiodynamic polarization method, it would be interesting to outline the conditions and limitations of this electrochemical technique for its use in tribocorrosion.

The susceptibility of metals to localized corrosion is usually expressed by the breakdown

or termed the protection potential, *E*p. At the breakdown potential, localized corrosion starts. The *E*pit of a metal is often associated with the potential at which the current density suddenly increases and with the breakdown of its passive surface film. The higher the potential (more noble), the less likely the alloy is to cause the initiation of localized corrosion. At the repas-

A cyclic potentiodynamic polarization technique can be employed to determine both *E*<sup>b</sup>

behavior of metallic alloy systems including the BMGs in various electrolytes [17–21].

of a corrosion system under wear (i.e., tribocorrosion) and pure corrosion conditions.

Many researchers [15–21] have successfully used the potentiodynamic anodic polarization technique to study the corrosion, the wear-corrosion synergism, and the tribocorrosion

ASTM G61-86 provides a procedure for conducting cyclic potentiodynamic polarization measurements [22]. By convention a cathodic current is negative whereas an anodic current is

This technique uses a typical three-electrode system (WE working electrode or metal being investigated, RE (SHE, reference electrode), and CE (Platinum or Graphite, counter-electrode)) controlled by a potentiostat as shown in **Figure 2**. The potential, which applies to the WE, is usually swept from the active (cathodic) direction to the noble (anodic) one, while tracking the current density continuously until it reaches a selected value of current density, where after, the scan is inverted in the active direction, until the hysteresis loop closes or until the corrosion potential is reached. The potential of the WE can be considered as the "driving force" of the corrosion system, while, the anodic current density can be regarded as proportional to the

A typical plot of polarization curve generated by this method as *E*–log (*i*) is shown in **Figure 3**. If a specified material is susceptible to localized corrosion, a hysteresis loop as shown in **Figure 3** will be observed as the potential scan is reversed. Otherwise, a uniform corrosion takes place in the transpassive or oxygen evolution region. Note that the larger the area of the

hysteresis loop, the lower the ability of the metal to repassivate.

, or designated to, as the pitting potential, *E*pit, and the repassivation potential, *E*<sup>r</sup>

and

83

polarization resistance (LPR), etc.

sivation potential, pitting stops.

potential, *E*<sup>b</sup>

positive [23].

corrosion rate of the WE.

*E*r

*3.2.1. Measurements of cyclic potentiodynamic polarization curves*

The typical configuration of a triboelectrochemical cell experiment involves an inert material (e.g., corundum counter-body) sliding against the investigated material (i.e., working electrode) under mechanoelectrochemical well-controlled conditions. Although metal-onmetal contact configurations are possible, but the use of an inert counter-body simplifies the interpretation of the electrochemical results of the corrosion-wear process. In addition, it is most advising to perform the electrochemical measurements under stationary regime conditions, at least prior to starting up of the measurements. The development of relevant models for the interpretation of the tribocorrosion mechanism concurrently depends, essentially, on the choice of electrochemical techniques to be implemented in a tribocorrosion test and the mechanical contact conditions (e.g., relative motion). The state-of-the art and reviews on triboelectrochemical techniques and experiments are available elsewhere [8–13].
