**3.1 Choice of technique (instrumentation)**

There are many electrochemical and non-electrochemical techniques for the study of corrosion or tribocorrosion and many factors must be taken into account when choosing a technical method. The corrosion rate can be determined by extrapolation of Tafel from a potentiodynamic polarization curve [12, 13]. It can also be determined using the Stern-Geary equation from the polarization resistance derived from a linear polarization experiment or from electrochemical impedance spectroscopy [12, 13]. Furthermore, among the recently developed techniques, those using electrochemical noise analysis as a method of determining the rate of corrosion and estimating the tribocorrosion rate of some passive materials has been shown to be beneficial, although scientists are struggling through the interpretation of some conflicting results [2, 12–15]. Sensitivity to localized corrosion is often evaluated by determining a breakdown potential and a repassivation potential for passivating materials.

### **3.2 Corrosion forms**

Obviously, the most typically known mode of corrosion is the rusting of iron, or iron oxide, and ordinary steel. Contemporary corrosion research has established several forms of corrosion, all which are important to understanding, as the best methods of preventing corrosion depend upon the form of corrosion, as shown in **Figure 2**.

• *Uniform corrosion*: the damage by general or uniform corrosion is fairly predictable; however, the damage caused by localized corrosion is rather unpredictable if no proper monitoring techniques are applied. When left uncontrolled, corrosion will not only cause costly equipment maintenance and replacement

*Corrosion Inhibitors*

**2. Importance of corrosion and tribocorrosion**

One of the primary reasons for the importance of corrosion or tribocorrosion lies on global economic losses. The destruction of nearly a quarter of the world's annual steel production is caused by corrosion. An estimate of about 150 million tons of losses per year, i.e. 300 tons per minute! [8]. Corrosion is definitely not limited to steel alloys but affects all sort of materials, namely metals, polymers, and ceramics. Corrosion and wear damage to materials, both directly and indirectly, cost industrialized countries hundreds of billions of dollars annually. For example, wear failures of metals costed the U.S. economy almost \$20 billion per year (in 1978) compared to about \$80 billion annually for corrosion during the same period [6]. The economic losses, due to friction and wear, related to these costs are estimated to be 6–10% of the Gross National Product (GNP). Wear represent 30% of the causes of dysfunction of the global mechanical engineering systems [2]. A recent study commissioned by the American Federal Highway Administration reveals that the annual direct cost of corrosion was \$ 276 billion in 1998, which represents 3.1% of the GNP [9]. Similarly, in the United Kingdom, Japan, Australia and Kuwait, the total annual cost of corrosion was estimated to range between 1 and 5% of each country's GNP [10]. Owing to many different types of expenses involved, in general, estimates of the total cost of corrosion and wear evolve over time and are sometimes difficult and uncertain. There is no doubt, however, that the cost is quite elevated. The direct losses concern replacement of corroded materials and equipment. The indirect losses are related to cost of repair and loss of production, cost of corrosion protection, and prevention. The direct losses are very often lower than indirect costs. For example, it is estimated that the price of repairing or replacing a corroded heat exchanger in a nuclear power plant is insignificant compared to the cost of lost production time. Another important aspect among major influential factors that contribute to corrosion or tribocorrosion relevance is related to reliability, or safety and preservation. Corrosion and wear can compromise the reliability and security of the operating equipment, leading to failure in-service, and at worst disasters, e.g., pressure vessels, metal reactors for toxic chemicals, turbine rotors, nuclear power plants, steering mechanisms for vehicles automobiles, and so on. Further, it requires the rebuilding or replacing the corroded structures and machinery or their components and an additional investment of the following supplies and facilities: metals, energy, water and human efforts to design these metal structures, without

**146**

sion and wear protection.

mentioning any other resources.

**3. Relevance of corrosion and tribocorrosion monitoring**

The importance of corrosion and/or tribocorrosion monitoring is that it allows people of interest to study the extent of damage due chemical and/or mechanochemical attacks and to become aware of the rate at which such damage is progressing so that the necessary measures can be taken to avoid trouble. Generally, the application of continuous and adequate on-line corrosion or tribocorrosion monitoring includes the following advantages: enhanced security, ensuring operational reliability on-time, minimizing process contamination and maximizing product quality, providing a sentinel for equipment integrity, and preventing any further risk related to material or production. The most convenient way to successfully combat corrosion or tribocorrosion impact on materials and structures is (i) to understand the main causes of corrosion and/or wear, (ii) to use all available means to prevent it, (iii) and to implement a continuous improvement approach to corro-

**Figure 2.** *Forms of corrosion.*

but may be responsible for the loss of revenue from unexpected system shutdown and the possibility for hazardous leaks causing major safety issues and possible environmental contamination [16].


**149**

*Electrochemical Techniques for Corrosion and Tribocorrosion Monitoring: Fundamentals…*

premature failure of metal structures as a result of these effects [17].

**4. Aspects of corrosion prediction in aqueous media**

• The mechanical properties of metals can be severely degraded by the combined effects of the environment and an applied stress. *Stress corrosion cracking* is the

• *Corrosion fatigue* occurs when the applied stress is fluctuating rather than being

Although, air in atmosphere is the most omnipresent environment, the aqueous solutions (e.g., atmospheric moisture, acid rain, natural waters, artificial solutions, and so on) are the most common environments associated with corrosion issues. Due to the conditions under which a material, e.g., a metal or a metallic structure is exposed to the environment, corrosion processes usually take place at the metal/ environment interface through ionic conduction processes. Corrosion is due to electrochemical reactions involved in that interface and strongly affected by a number of factors, among others, such as the power oxidizing (electrode equilibrium potential), hydrogen ion activity (pH, acidity level of the solution), driving force for metal stability (Gibbs free energy change), rate-determining step reaction or reaction rate (corrosion current), and temperature (Arrhenius oxidation rate or

The prediction of corrosion of a metal or an alloy in environments such as aqueous solutions requires information on the expected state of the metallic alloy (e.g., oxide or non-oxidized metal, etc.) and the rate at which the metallic alloy moves to that state. So first, thermodynamic principles can be applied to determine what processes can occur, and under which conditions the reactions are at an electrochemical equilibrium and, in case of deviation from that equilibrium, in which directions the reactions can proceed and what is the driving force involved. The kinetic laws then describe the reaction rates. These are strongly related to the activation energies of the electrode processes, to the mass transport and to the fundamental properties of the metal/environment interface, such as the resistance of surface films. A general method, namely the mixed potential theory, is implemented for interpreting or predicting the corrosion potential and

In what follows, a very brief reminder of corrosion principles is given.

The corrosion process is electrochemical in nature. In a more general sense, a corrosion process involving a metal/aqueous solution or any other metal/ionically conducting medium exchange or interaction interface is defined as any process causing the metal, M, to loose one or more of its loosely bound electrons in the metallic state (i.e., oxidation reaction) to generate a solvated cation in the solution, and it is simultaneously balanced, in order to ensure an exact count of the electrons involved, by a reduction reaction by which one or more atoms of a molecule or an ion (anion) of species in solution gain one or more of these electrons (conservation criteria). The aqueous corrosion sometimes requires that the oxidation product is either

2M + O2 = 2MO (1)

**4.1 Electrochemical nature of metal/electrolyte exchange reaction**

an oxide of the metal itself according to (1):

*DOI: http://dx.doi.org/10.5772/intechopen.85392*

constant [17].

the Pilling-Bedworth ratio).

reaction rates.

*Electrochemical Techniques for Corrosion and Tribocorrosion Monitoring: Fundamentals… DOI: http://dx.doi.org/10.5772/intechopen.85392*

