Erosion-corrosion

Abstract

Generally, almost all components moving near a corrosive fluid hitting the material surface are exposed to corrosive erosions. Meanwhile, transmission pipes of gas, oil, and water, the transmission lines of corrosion fluid in the industrial reactor, and heat exchange systems are suffering significantly from the erosion-corrosion phenomenon. Erosion-corrosion can generate material loss much greater than the sum of the pure erosion and the pure corrosion individually due to the interaction between them. Erosion-corrosion in aqueous systems is dominated by two major mechanisms: electrochemical corrosion and mechanical erosion. On account of the greater material loss than the sum of their components, the interaction between electrochemical and mechanical processes has been recognized in many works, and they have been referred to as “Synergistic” and “Additive” effects. The so-called synergistic effect is normally used to describe how corrosion can enhance erosion, while the so-called additive effect refers to the mechanism by which erosion can enhance corrosion. In general, the influencing parameters in this process include: the solid sand particles (mass, hardness, density, size, shape, velocity, and impact angle), target material (hardness, metallographic structure, strength, ductility, and toughness), and the environment (slurry composition, flow velocity, and temperature).

1. Introduction

Every year, erosion-corrosion causes severe damage in slurry transportation and related industries [1]. Many groups from the industry and universities have spent a lot of effort to understand this phenomenon and ways to prevent or reduce its effects. In erosion-corrosion, there are three separate phenomena: impact of solid particles (on the surface of the material), electrochemical reactions, and fluidity of the medium. These processes interact with each other and create erosive corrosion, forming a completely complex phenomenon. Figure 1 shows the interaction between erosion, corrosion, and the fluid environment. This shows that the erosive corrosion process can only exist if erosion, corrosion, and the fluid environment exist at the same time.

Figure 1.

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

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 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 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 three-dimensional 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]:

$$\mathit{ER}\left(\frac{\mathit{mm}}{\mathit{\text{year}}}\right)=K_m{K}_{\mathit{En}}.c.v^n.f\left(\beta \right)$$E1

In this formula, K_m is the material factor (depending on hardness and flexibility), K_En 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].

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 erosion-corrosion, 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 semi-stable 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 $\%10H_2{\mathit{SO}}_4$, 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 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].

2.2.2.1 Removal of hardened surface

Some materials have the ability to increase hardening. When the erosion process occurs, the abrasive particles continuously hit the surface, and this causes a cold working to be induced on the surface of the material. This cold working induces a hardening effect on the surface and protects the material from further loss by erosion or, in other words, reduces the erosion process. Matsumura and his colleagues [9] investigated the erosive corrosion process in a passive system and found that particles cause the wear of the passive protective film and accelerate the corrosion process. This corrosion flow decomposes the hardened layer on the surface of the material; hence, the erosion resistance decreases.

2.2.2.2 Creating a cavity on the surface layer

Preferential decomposition theory can explain the decrease in mechanical properties of composite materials, but it cannot be used to explain the decrease in mechanical properties of single-phase materials. Some researchers also researched the erosion-corrosion process of single-phase materials. Zhuo and his colleagues [31] investigated the reduction of mechanical properties of pure iron under anodic decomposition conditions in solution by in situ nano dentations and found that the surface hardness significantly decreased in the presence of a corrosion current compared with the conditions under cathodic protection. Matsumura et al. [9] reported that in a NaOH slurry, material loss from pure iron in the presence of a corrosion current was 20% higher than in the case without corrosion, although the corrosion current was measured to be very small. Jones et al. [32] proposed the vacancy generation theory to explain this effect for single-phase material. When a material is exposed to anodic decomposition, a supersaturated state of vacancies is formed on the surface of the material by anodic decomposition. A large amount of vacancies leads to the weakening of interatomic bonds on the surface of the metal. The interatomic bonds create a chemical potential gradient between the surface layer and the bulk material, which causes the induced decomposition of anodic and dislocation movement in the surface layer. During the electrochemical decomposition, the vacancies are attracted to the dislocations and increase their kinetic energy. This increase in the kinetic energy of the dislocation reduces the resistance of the surface layer against the change of plastic shape. Others [33] also found that the weakening of interatomic bonds on the surface of the metal leads to the deterioration of the mechanical properties of the material, such as tensile strength, modulus, and fatigue life; hence, it causes a decrease in erosion resistance.

Vacancies also cause corrosion and a chemical mechanical effect. A chemical mechanical effect occurs when a load is applied to the surface of a material while a chemical reaction is taking place. As a result, the mechanical properties are affected by this process. Gutman [34] found that on the surface of a metal, electrochemical or chemical decomposition can lead to a decrease in free energy and an increase in the kinetic power of the dislocations produced in this process. Therefore, the resistance to plastic deformation decreases. Jones et al. [32], and Zhu et al. [35] suggested that anodic decomposition can create a supersaturation of vacancies on the metal surface. These voids penetrate the grain boundaries and isolated voids, causing acceleration of infiltration creep and the ascent of dislocations.

2.3 Factors affecting the erosion-corrosion process

As mentioned above, erosion-corrosion is defined as the interaction of solid particles, fluid medium, and corrosion. This process is considered an interdisciplinary study between materials, hydrodynamics, and electrochemical properties. Therefore, the factors that affect each of these processes also affect erosion-corrosion.

2.3.1 Material properties

2.3.1.1 Microstructure

Resistance to erosion-corrosion is almost dependent on phase composition and particle size [36, 37, 38, 39]. Wang et al. [36] investigated the erosion-corrosion behavior of carbon steel and found that the lower bainite microstructure can increase the erosion-corrosion resistance of carbon steel. Patterson and his colleagues [37] investigated the effect of different microstructures on carbon steels in most of the impact angles and found that their erosion resistance is based on the increase of erosion speed in the following order:

Spherodized, pearlite, tempered martensite, and martensite.

Lindsley et al. [38] reported that in spheroidized Fe-C alloys, the wear resistance increased when the mean path between carbides and grain boundaries decreased. Berglozzi [39] reported that erosion resistance is also dependent on the particle size and the erosion resistance increased with decreasing particle size.

2.3.1.2 Composition

The resistance of metals and alloys against erosion-corrosion depends on their chemical composition, corrosion resistance, hardness, and metallurgical history. The corrosion resistance of metals and alloys is mainly determined by the chemical composition. If it is an active metal, or an alloy that consists of active elements, its corrosion resistance mainly depends on the ability to form and maintains a protective shell. If the metal is nobler, it has good inherent corrosion resistance. Therefore, if all other factors are equal, a metal with higher intrinsic resistance will be more resistant to erosion-corrosion [40].

In general, the increase of carbon in carbon steels increases the resistance to erosion, but the resistance to corrosion decreases [28]. For alloy steels, the addition of alloying elements such as Ni, Mn, Mo, and Cr improves the corrosion resistance of the alloy. Samy [41] reported that alloy steels with higher Cr have better erosion-corrosion resistance because increasing Cr increases both corrosion resistance and mechanical properties.

2.3.1.3 Surface shells

The nature and properties of surface protective shells that are formed on some metals and alloys are very important in terms of resistance to erosion-corrosion. The ability of these shells to protect the metal depends on quickly or easily forming them in the early stages of contact with the corrosive environment, their resistance against mechanical damage or erosion, and the speed of their re-formation in case of damage or destruction. A surface shell that is hard, dense, sticky, and continuous is better protection than when the shell is easily worn or peeled off. If the shell is brittle and cracks and crumbles under stress, it will no longer be protective. Sometimes, the nature of the protective shell that forms on the surface of the metal will depend on the corrosive environment in which the metal is located, which is a determining factor.

The changes in the corrosion rate of steel by static water at different pH are dependent on the nature and composition of the surface shells that are formed. Figure 3 shows the effect of the pH of distilled water at 50°C on the erosion-corrosion of carbon steel. Corrosion speed is low at pH 6 and 10, and corrosion speed is high at pH 8 and less than 6. At pH less than 5, the shell cracks, which is probably due to internal stresses, and the surface of the metal is exposed to the environment. In the areas where the corrosion rate is low, the corrosion products are Fe(OH)2 and Fe(OH)3, which are more protective because they have prevented the transfer of oxygen and ions. Erosion-corrosion tests in a boiler at 250°F using different equipment, as well as the experiences of power plants, confirm the results of high corrosion rate at pH = 8. The behavior of steel pipes and low-alloy steels against petroleum materials at high temperatures in refineries depends on the sulfide shell formed on the surface of the pipes. When the shell is worn, high corrosion occurs. For example, in organic systems, if cyanides are present, the sulfide shell, which is solid and integrated, became porous and will no longer be protected. The correct and effective use of inhibitors in reducing erosive corrosion in most cases depends on the nature and type of shell on the metal and, as a result, the reaction between the metal and the inhibitor [40].

Figure 3.

Effect of PH of distilled water on erosion-corrosion of carbon steel at 50°F (velocity 39 ft./sec) [40].

2.3.1.4 Mechanical properties

Mechanical properties such as hardness, toughness, strength, and strain hardness can influence the rate of erosion-corrosion by changing the erosion behavior. Many erosion models are mainly created by eliminating these mechanical features. Hardness is one of the important parameters that has attracted many researchers in the field of erosion-corrosion. It is generally said that increasing hardness leads to decreasing erosion. Many researchers who built erosion models have all shown that the erosion rate has an inverse relationship with the hardness of the material [42]. The following equation was created in this regard:

$$e_{°}=k_H{H}_v^{-n_H}$$E2

That $k_H$ and $n_H$ are laboratory constants that are related to hydrodynamic conditions. $H_v$ is the hardness of the bulk material. Oka and his colleagues [43] found that it is not the initial hardness of the material that affects the erosion process, but the hardness that affects it during the erosion process, that is, the effect of hardening. Hutchings [28] investigated the erosion behavior of a wide range of materials and found that the erosion rate is strongly dependent on the hardness difference between the target material and the impacting particles. Finnie [44] also investigated a range of materials and found that the erosion rate is inversely proportional to the hardness if the power constant is equal to 1.

When there is corrosion, the mechanical properties of the material change. Guo [31] and his colleagues investigated the degradation of the mechanical properties of the pure iron surface in a solution with current density of 0.5 mA/(cm²) and 1 mA/(cm²) and found that the mechanical properties of corrosion flow are reduced. Based on this, it can be concluded that the reduction of erosion resistance in the erosion-corrosion process is probably due to the loss of the surface mechanical properties of the material caused by the corrosion flow. Gottman [34] also performed the necessary experiments and theoretical analysis and found that the hardness and strength of the surface layer decrease with the increase in the density of the anodic decomposition current. Lu and Liu [45] have shown a theoretical prediction of the relationship between the increased erosion rate with the presence of a corrosion current and surface degradation. This relationship is as follows:

$$\frac{e_c}{e_{\circ }}=-n\frac{\Delta H_v}{H_v}$$E3

Where ${\mathit{\Delta {\rm H}}}_v$ is the difference between the hardness in the corrosion process and the hardness without the corrosion process, and $n$ is the laboratory constant, which is the difference between the erosion systems.

2.3.2 Hydrodynamic properties

2.3.2.1 Flow rate

The slurry is a two-phase fluid system including liquid and solid. Contrary to a single-phase system, the collision of particles should be considered. Meng and his colleagues [46] found that the speed of particle collision is an important parameter in determining the wear rate, and this parameter is directly dependent on the speed of the slurry. Postlewaite and his colleagues [47] have shown the relationship between the erosion-corrosion rate and the flow rate in Eq. (4):

$$w=m_1{M}_{p}f\left(\theta \right)U^{m_2}$$E4

Where $M_p$ is the weight of particles per unit volume of solution, $f\left(\theta \right)$ is a dependent function that determines the rate of erosion according to the angle of impact, and $m_1$ and $m_2$ are constants dependent on erosion-corrosion systems. Some researchers also found a power law between erosion-corrosion speed and flow speed and found that the power value of this law is between 1 and 4 [2].

Many metals and alloys have oxide films on their surface, such as ${\mathit{Al}}_2{O}_3$, ${\mathit{Cr}}_2{O}_3$, ${\mathit{TiO}}_2$, ${\mathit{Fe}}_2{O}_3$, ${\mathit{Fe}}_3{O}_4$, and $\mathit{NiO}$, and erosion leads to the removal of these films. If the flow rate is not high, the oxide films can reappear every time after being peeled off from the metal surface. If the flow rate exceeds some critical limits, there will not be enough time for the protective films to reestablish and remove the ions that will occur from the metal grid without the outer protective layer. In other words, the speed of removing the film is faster than the speed of its reproduction. Therefore, it may be easy to find different critical limits for water flow velocity for different metals and alloys on paper, but these limits must be used very carefully because the slightest change in the flow pattern, temperature, and chemical content of liquids and Alloys can lead to a change in the intensity of the critical flow rate [48].

In general, it can be said that increasing the flow rate, depending on its effect on the corrosion mechanism, may increase or decrease the corrosion rate. It can increase the rate of steel corrosion by bringing more oxygen, carbon dioxide, or hydrogen sulfide to the surface, and in the presence of corrosion inhibitors, increasing the flow rate can reduce the corrosion rate by bringing the inhibitors to the metal surface at a higher speed. It has been shown that to protect steel in drinking water at high flow rates, a small amount of sodium nitrite (inhibitor) is needed. Similar mechanisms have been proposed for other types of inhibitors.

The studies [40] of erosion-corrosion of aluminum and stainless steel alloys in fuming nitric acid have produced unexpected and interesting results, and with the increase of the flow rate, the corrosion of aluminum increased and the corrosion of stainless steel 347 decreased. The reason for this behavior was that the corrosion mechanisms in the two cases were different. Figure 4 shows the increase in the corrosion rate of aluminum with an increase in the flow rate. In nitric acid that fumes aluminum, it forms aluminum nitrate and aluminum oxide. In a static state or at very low speeds, the corrosion rate is low or zero. At moderate velocities of 1–4 ft./sec, the nitrate shell is removed but not enough to remove the sticky oxide shell. Velocities above 4 ft./sec also destroy most of the oxide shell, and erosion-corrosion occurs at a faster rate.

Figure 4.

Erosion corrosion of aluminum 3003 by fuming nitric acid at 108°F. Corrosion rate based on the average of four periods of 24 hours [40].

Figure 5 shows the reduction of the corrosion rate of 347 stainless steel with an increasing flow rate. Under static conditions, this steel is auto-catalytically corroded by nitric acid because the cathodic reaction forms nitro acid. Increasing the flow rate causes nitro acid to leave the environment and reduces corrosion. It means at higher rates, the corrosion rate reduces by preventing sludge deposition, which would cause crevice corrosion in the absence of high rates.

Figure 5.

Erosion corrosion of 347 stainless steel by white fuming nitric acid at 108°F. Corrosion rate based on the average of four periods of 24 hours [40].

Most stainless steels are prone to pitting and crevice corrosion in seawater and other chlorides, but some of these steels have been used successfully in seawater at high flow rates. This mode prevents the formation of sediments and prevents the initiation of cavities. Rate changes can also produce strange galvanic effects. In slow-moving seawater, the corrosion of steel does not change appreciably when in contact with stainless steel, copper, nickel, or titanium. At high flow rates, the corrosion of steel in contact with stainless steel and titanium is much less than when it is in contact with copper and nickel. This property is attributed to the more effective cathodic polarization of stainless steel and titanium at high rates [40].

2.3.2.2 Particle concentration

In low particle concentrations, an increase in the erosion rate has been observed linearly with increasing particle concentration. For high-concentration slurry, the erosion rate will also increase with increasing particle concentration, but it will be exponential and progressive rather than linear. Hutchings [28] found that this exponential movement in higher concentrations of particles is due to the interference of particles among each other. In lower particle concentrations, because the particles are almost independent of each other, the erosion rate increases linearly. He investigated the material loss behavior of mild steel in a slurry of silica particles and found that the lower the particle volume concentration from 12%, the material loss increases linearly.

When corrosion is present, the situation becomes more complicated. Zhu and his colleagues [18, 26] investigated the effect of particle concentration on corrosion current density in both active and inactive system states. For active systems [26], he studied the concentration effect on AISI 1045 low carbon steel at different rotation speeds with weight concentrations of 0, 20, and 35% and found that the current density significantly does not change with different concentrations of solid particles. For passive systems [18], the corrosion current density is highly dependent on the particle concentration, so the current density increases with the increase of the particle concentration.

2.3.2.3 Particle properties

The erosion-corrosion process depends on the properties of the particles. The rate of erosion is strongly affected by the hardness, size, and shape of the particles. Hutchings [42] found that particles that are 1.2–1.5 times harder than the surface of the target material produce a higher erosion rate than other particles. Oka and his colleagues [49] investigated the dependence of the particle size on the erosion rate of steel when its surface was hit by quartz particles and found that the erosion rate tends to increase up to a critical value, and after that, the erosion rate decreases to a relatively stable state.

2.3.2.4 Angle of impact

Finnie [50] investigated the erosion behavior of aluminum and its oxide at contact angles from nearly 0 to 90 degrees and found that for aluminum, as a soft material, the maximum erosion rate occurs between 15 and 30 degrees, while for aluminum oxide, as a hard material, the highest erosion rate occurs at 90 degrees. He explained that this angle change is due to the change in the erosion mechanism from soft to hard material. Erosive weight loss of soft materials is mainly done by grooving and cutting.

The reduction in the weight of hard materials is due to failure mechanisms because at 90 degrees, hard materials have the lowest resistance to failure; hence, we have the highest erosion rate.

Collision angles will also affect the corrosion process. Burstein and his colleagues [51] investigated the effect of collision angles on the increase of corrosion current, which is due to the breakdown or removal of the oxide layer on the surface.

2.3.3 Electrochemical parameters

2.3.3.1 The PH of the slurry

The PH of the slurry can greatly change the erosive corrosion behavior. As hydrogen ions are highly reactive, they can rapidly react with metal ions, resulting in severe surface erosion and reduced erosive resistance in anodic solutions. Guo and his colleagues [52] investigated the erosion-corrosion behavior of low carbon steel in abrasive slurries with pH 4, 7, and 10, and they found that the erosion rate was the highest in the solution with pH = 4 and the lowest in the solution with pH = 7 and at PH = 10, and our speed was between these two. Zhou et al. [53] also studied the effect of pH on the erosive corrosion characteristics of ductile cast iron under an open-circuit potential and obtained similar results. They explained that slurry with lower pH results in a higher corrosion current density under an open-circuit potential; hence, the erosion resistance will be lower.

2.3.3.2 Corrosion current density

Usually, the rate of erosion, in the case of corrosion, increases with the corrosion current density. Lu and his colleagues [45] investigated the effect of corrosion current density on the erosion-corrosion rate and found that the erosion rate increases linearly with the logarithm of the anodic current density. He also proposed an equation to calculate the theory of increased erosion rate due to corrosion with a given anodic current density.

$$\frac{e_c}{e_{°}}=zlog\left(\frac{i_a}{i_{\mathit{th}}}\right)$$E5

Where $i_a$ and $i_{\mathit{th}}$ are the actual anode current density and the threshold density that causes mechanical destruction of the surface, respectively. Z is a constant that varies between erosion-corrosion systems.

2.4 Erosion corrosion mechanism

If liquids and gases containing solid particles or gases containing liquid droplets flow on the surface of the metal, the abrasive agent causes mechanical wear of the metal. As a result, the metal decomposes in the form of ions or the formation of corrosion products.

The erosion-corrosion mechanism depends on the following components:

1. Flow speed and its characteristics: flow geometry (turbulent or smooth), the presence of obstacles in front of the flow, the angle of the flow to the metal surface, etc.

2. Environmental conditions (mechanical and physical characteristics): PH, presence of water droplets in steam flow, acidic gases, two-phase water or water-steam environment, etc.

3. The nature of metals and alloys: chemical composition, hardness, metallurgical factors (the presence of various phases such as ferrite, bainite, martensite, and austenite in steels), the type and structure of passive films on the surface of metals.

In general, there are two main reasons for the occurrence of erosion-corrosion. The first reason is “Erosion” due to the impact of certain materials or the impact of certain drops on the surface of metals. Since ancient times, humans have learned to cut “weak” materials such as wood and leather using “hard” materials such as stone and metal. The same phenomenon occurs on the surface of metals. If two solid materials that have hardness are different (metals and corroded particles) and come into contact with each other and move in opposite directions, the material with higher hardness will scratch the other surface.

The kinetic energy of certain materials (solid particles) and liquid droplets that move at high speed carries the necessary energy to cut or break the outer layer of metals. “Erosion” is a mechanical action of “wearing” metal. Corrosion is a chemical action of metal dissolution. Increasing the current speed increases the speed of transferring aggressive components to the metal surface and corrosion products in the opposite direction from the metal surface to the environment; as a result, the penetration speed of the components participating in the cathode and anode of corrosion reactions increases. The use of electrochemical methods for monitoring and controlling erosion-corrosion shows an aspect of the electrochemical mechanism in this complex phenomenon.

The second reason for the occurrence of erosive corrosion is cavitation, which is the formation and collapse of bubbles in the liquid (the first type of cavitation), or the condensation of vapor molecules (the second type of cavitation) on the metal surface during the flow. In the first type of cavitation phenomenon, the turbulent flow of liquids (strong flow under turbulence) causes a change in the pressure in the liquid flow near the metal surface. This condition can occur on the surface of a ship propeller or in centrifugal pumps. The collapse of vapor molecules forms local stresses on the surface of the metal due to the released shock wave. These stresses have higher energy and forces than the chemical electrostatic forces between atoms in the metal lattice. The surface of the metal is not uniform and forms surface pits such as cavities. These holes are called the pitting phenomenon. Chemical factors (if corrosive chemicals are present in the liquid or the vapor stream) can accelerate pitting [54].

Conflict of interest

“The author declares no conflict of interest.”