**2. Synergistic effect**

The rate of erosion-corrosion is higher than the sum of the rates of pure erosion and pure corrosion separately. The additional rate of material loss is caused by the interaction between corrosion and erosion. Many researchers have conducted extensive studies based on this interaction. Matsumura [9] reported that in a NaOH slurry, the loss of pure iron in the presence of corrosion flow was higher than that without corrosion flow, although the corrosion flow could be reduced to a very small extent. It was a measurement. Li and his colleagues [10] investigated the loss of aluminum in an acidic slurry containing 0.5 M NaCl and found that 40–50% of the total weight loss is related to the synergistic effect. Yui and his colleagues [11] reported that up to 86% of the total weight loss of white iron and chromium (chromate) in a slurry with low pH can indicate the synergistic effect. Zhang et al. [12] found that the weight loss due to the synergistic effect can be more than 92.1% for X60 tube steel, 94.6% for 321 stainless steel, and even 99.8% for 316 L stainless steel. Therefore, the important effect of the "synergistic effect" should be considered according to the above studies.

Although the erosion-corrosion map was made by Lim and Ashby [13], it can only quantitatively determine the erosion rate, corrosion rate, and ratio between them. It cannot explain how erosion and corrosion interact with each other or the mechanism of their effect. Many efforts have been made to investigate the mechanism of erosioncorrosion, but due to its specific complexity, it has not yet been fully determined. Researchers mainly divide the "synergistic effect" into two groups: erosion affecting corrosion and corrosion affecting erosion or corrosion increased by erosion and vice versa.

#### **2.1 The effect of erosion on corrosion**

As mentioned above, the effects of the erosion process include fluidity, corrosive environment, and particle impact. In a passive system, these effects lead to surface roughness, increased mass transfer acceleration, passive film failure, and increased localized corrosion. In an active system, these can lead to all of the above results except the failure of the passive layer, because there is no passive layer in the active system.

#### *2.1.1 Surface roughness*

The impact of particles makes the surface of the electrode rough and uneven. Normally, the rougher the surface, the higher the corrosion rate [14, 15]. When the electrode surface is hit by the particles, an impact cavity can form a crack or become sharp. Li and his colleagues [14, 15] used copper as a working electrode and researched the copper electrode. They found that it is easier for the electron to escape from the surface in the state of a sharp scratch than in the state of a gap. This shows that corrosion is easier in the pointed scratch mode than in the crevice and valley mode. For a completely rough surface, there is higher electron mobility and more freedom to react with the environment, so the surface is more electrochemically active. In addition, in the form of a sharp scratch, there is a tendency to lose more electrons and a more active surface. In contrast, the gap is nobler. This states that a galvanic couple can be formed, meaning that the pointed scratch acts as the anode, and the gaps act as the anode. Therefore, the localized cell formed can accelerate the corrosion process.

Burstein and his colleagues [16] investigated the relationship between surface roughness and pitting potential for 304 L stainless steel and found that surface condition is a critical parameter for determining pitting potential. They found that surface roughness caused by erosion has less pitting potential. He also noticed that during the erosion process, the potential of pitting decreases more than after the process. This shows that in addition to surface roughness, other parameters affect the pitting potential.

#### *2.1.2 Failure of the passive layer*

As mentioned, for a passive system, the surface of the material is covered by an oxide film that prevents the collision between the surface of the material and the environment. As a result of the erosion process, the surface of the passive layer may be particle collision or pitting will be damaged. Li et al. [17] and Zhao et al. [18] used the scratch test to simulate the impact of solid particles and obtained similar results to those obtained from the impact of solid particles. Some researchers [19, 20] reported that if the slurry concentration or velocity is high enough, it can replace the corrosion behavior; it means removing the passive area.

Failure of the passive film can increase pitting corrosion. When a particle hits the passive surface, its abrasion removes the passive surface and produces small holes. Burstein et al. [21] investigated the same effect on stainless steel and found that semistable pits can form after a short time below the rapid pitting potential. Also, it can increase the recurrence of semi-stable cavities compared with free erosion conditions. These small holes can shorten the period of pitting corrosion. The continued collision of particles ends the expansion of the small holes that created the large holes.

#### *2.1.3 Acceleration of mass transfer*

For a corrosion process, there are always two reactions: a cathodic reaction and an anodic reaction. When the electrolyte is fluid, it increases the transport of oxygen, the speed of the cathodic reaction increases, and as a result, the entire corrosion process improves. Also, the fluidity of solid particles in a solution leads to a disturbance in the fluid. Therefore, it can increase the transport processes of both reactants and corrosion products [12].

#### *2.1.4 Strain-hardening effect*

When particles collide with the material surface with a high and sufficient kinetic energy, they can cause both elastic and plastic deformations on the surface. This process includes a deforming effect, such as a misplaced lattice, deforming bands, or the protrusion of slip plates on the surface. Li and his colleagues [22] investigated the relationship between the effect of strain rate and electron work performance and found that a higher strain rate makes it easier for electrons to escape from the surface of the material and increases the tendency to corrosion. Li and his colleagues [23] also investigated the change of current density in stainless steel, caused by (changing) the strain rate in the solution %10*H*2*SO*4, and found that the current density increases with the increase of the strain rate. Mayozumi and his colleagues [24] found that cold working can affect the corrosion behavior of 304 stainless steel.

However, there are some conflicting results on the effect of strain and cold work. Lu et al. investigated the effect of strain on 304 stainless steel and found that in the quasi-elastic deformation process, neither elastic nor plastic deformation has significantly changed the corrosion rate of the electrode when it undergoes an anodic decomposition process. Mayozumi and his colleagues [24] also found that the effect of strain on corrosion is significantly dependent on the system. In Li's research [23], it is difficult to say that the failure of stainless steel passive film was due to tensile stress or strain rate that increased the current.

#### **2.2 The effect of corrosion on erosion**

Corrosion is a chemical process, specifically the breakdown of a surface layer of material. When solid particles hit the surface, the erosion process is much easier than when there is no corrosion flow at all, and this leads to a decrease in erosion resistance in the surface layer of the material [25, 26]. Processes such as roughening of the surface, preferential decomposition of the background phase, removal of the hardened surface, generation of vacancies, and chemical mechanical effect are studied in this section.

#### *2.2.1 Surface roughness*

When the microstructure of the material is not uniform, the decomposition process on the surface of these materials will not be uniform. Some areas are more susceptible to corrosion while others are not. As a conclusion, the more active areas act as the anode and the nobler areas act as the cathode. Therefore, we see metal decomposition in the anode. While this is not the case in the cathode. In these conditions, unevenness is created. Postlethwaite [27] investigated the erosive corrosion behavior of pipes in an aqueous slurry, with and without the presence of inhibitors. Inhibitors can slow down the corrosion process. He found that the rate of erosive corrosion and surface roughness can be reduced by adding inhibitors. Some researchers [28] explained that, because the erosion process is sensitive to the angle of impact of particles on the surface, corrosion by roughening the surface can change the angle of impact.

#### *2.2.2 Preferential decomposition of the matrix phase*

For some materials, the mechanical properties are enhanced by a secondary dispersed phase in the metal matrix composite (MMC). For example, the reinforced

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

phase can increase the surface hardness of the (MMC) and can make the material more resistant to wear and friction. The friction of the surfaces in contact is more resistant. For these materials, the mechanical properties are much more dependent on the reinforcing phases. When these materials are exposed to erosive corrosion, preferential decomposition always occurs at the junction of the ground and reinforcing phase, because it is more galvanically active; hence, they are more susceptible to corrosion [29]. This preferential degradation weakens the composite bonds and the metal substrate. Therefore, the reinforced composite may be destroyed earlier by the abrasive particles. This effect can reduce the mechanical properties of the surface of the material; as a result, the erosion resistance of the material decreases [30].
