**1.1 Separate processes in erosion-corrosion**

Erosion processes include cutting, plastic deformation, or contact fatigue caused by cavitation from fluid flow or solid particle collisions. Therefore, erosion is

#### **Figure 1.**

*Schematic representation of the interaction of erosive corrosion phenomena [2].*

essentially a process of mechanical loss of material. The process of corrosion is the decomposition of atoms of matter by an anodic current. Atoms of matter in solution are decomposed into their ionic state. Therefore, corrosion has the essence of an electrochemical phenomenon. Together, these two processes can create an additional effect that leads to a complex problem. Therefore, to study this problem, it is necessary to understand each of the separate phenomena in erosive corrosion.

#### **1.2 Pure corrosion**

Bradford [3] defined corrosion as "damage caused to a metal due to reaction with the environment." Corrosion always occurs at the surface of the metal where it is in contact with the environment, such as soil, a solution, or even moist air. From a thermodynamic point of view, most metals are unstable when in contact with the environment. They tend to lose electrons and become more stable, that is, cations, oxides, or other chemical compounds, which is the opposite of extractive metallurgy. As mentioned above, there is charge transfer in corrosion processes; hence, this process is considered electrochemical. When corrosion occurs, metal atoms are absorbed into the solution. In most cases, there are three basic steps to this process: (a) transfer of reactant to the electrode surface, (b) a surface electrochemical reaction, (c) transfer of products away from the electrode surface. Each of these steps has its complexity [4].

#### *1.2.1 Two differences in corrosion systems*

The corrosion system can be divided into two major parts based on what becomes a protective layer on the surface. One is the active or non-passivated system, and the other is the passive system. In active systems, the material surface is not protected by an oxide layer. In these systems, uniform corrosion always occurs. When the metal

#### *Erosion-corrosion DOI: http://dx.doi.org/10.5772/intechopen.109106*

atoms react with the electrolyte, some products may form on the surface. Typically, these products are neither dense nor protective. This shows that the metal atoms can dissolve in the electrolyte without any significant hindrance. Galvanic corrosion, intergranular corrosion, and crevice corrosion occur in these active systems. In these systems, the controlling step is charge transfer on the metal surface or resistance to mass transfer in the fluid environment or both.

In passive systems, the surface of the material always becomes a protective oxide layer. This protective film can be either a surface absorption film or a threedimensional oxide film. The passive film can reduce the corrosion rate by preventing the material from contacting the electrolyte or by limiting the movement of ions and atoms. Therefore, the structure and characteristics of the passive film can affect the corrosion behavior of the material. Passive systems always have a lower corrosion rate than active systems. However, in these systems, localized corrosion always occurs and causes severe damage, such as pitting corrosion. For example, in pitting corrosion, holes are created in the pitted area, but the rest of the area is still covered by the protective layer, so it is not corroded. This is a typical system with a small anode and a large cathode, so the rate of anode dissolution is significantly increased [4]. Over time, the holes become deeper and deeper and eventually penetrate the entire material.

#### *1.2.2 Corrosion in fluid conditions*

In a corrosive environment, the current affects the corrosion rate. A fluid medium increases mass transfer, thus increasing the overall corrosion rate. The presence of a fluid medium increases the corrosion rate when the material is subject to general or uniform corrosion. At high speeds, the current can delay the formation of protective films or the passivation process. On the other hand, metal materials themselves suffer from mechanical damage at appropriate high speeds [5, 6].

#### **1.3 Erosion**

Erosion is a special state of wear in which the process of tearing off pieces from the surface of the part and thinning due to interfacial tensions applied by a fluid (liquid or gas) or collision of suspended particles in the fluid occurs [7]. A type of erosion called spark erosion is caused by the impact of sparks or an electric arc on the surface of the material, which is used in the industry for machining materials. Another type of erosion occurs at very high speeds called "ultrasonic effects." In the present discussion, erosion means erosion caused by mechanical factors in common industry conditions. The pure mechanical erosion rate (ER) depends on the fluid velocity as follows [8]:

$$ER\left(\frac{mm}{year}\right) = K\_m K\_{En} \varepsilon . v \,\nu^n f(\beta) \tag{1}$$

In this formula, Km is the material factor (depending on hardness and flexibility), KEn is the environmental factor (including size, shape, hardness, and density of particles suspended in the fluid), c is the concentration of particles, n is the power of velocity, v is the velocity of particles, and β is the angle of impact of particles on the surface. Are. The value of n is about 3 in most cases. The effect of the impact angle (β) on the erosion rate depends on the type of material (brittle or flexible). This issue is shown in **Figure 2**.

**Figure 2.** *Effect of impact angle on the rate of erosion of soft and brittle materials [8].*
