**1. Introduction**

### **1.1. Wear and corrosion in oil and gas industry**

The advances in technology and higher demand for energy require enhanced rate of extraction and transportation of oil and natural gas fossil fuels. The use of pipelines for

transporting of oil and gas products is a safer method compared to other possible ways of transportation as statistical analysis have shown in Ref. [1]. However, leaks and ruptures can still occur with significant hazard for humans [2]. Beside the life-threatening aspects, failures caused by degradation of pipelines can lead to serious economic losses [3] and environmental disasters such as pollution and contamination of wildlife and sea creatures [4]. Corrosion and wear in components and pipelines can be considered as one of the main causes of failure and leakage in oil and gas industry [5, 6]. All the three major fossil fuels, crude oil, and oil-sand and natural gas are very corrosive. Crude oil contains the corrosive ingredients such as carbon dioxide (CO2 ), hydrogen sulfide (H<sup>2</sup> S), organic acids, dissolved gases, and salt water [7], while the oil sand comprises of CO2 and corrosive ions such as Cl<sup>−</sup> , HCO3 − , and SO4 − [8]. Natural gas is also corrosive due to the presence of CO2 , H2 S, and some calcium and chlorine compounds [9]. On the other hand, in some sequences of extraction of the petroleum products, erodant particles such as sand may be mixed with the flowing fluid to form multiphase solid-liquid mixtures. The flowing of these mixtures in pipelines and equipment in oil and gas industry may lead to solid particle erosion in addition to corrosion. The combination of wear and corrosion can extensively reduce the lifetime of the equipment due to the higher rate of material loss [10].

Although the corrosion and wear of components exposed to harsh erosive-corrosive environments cannot be thoroughly eliminated, protective coatings can be employed to improve the life time of equipment and prevent early and unpredicted failures. The selection of the proper coating material depends on the service condition, financial aspects, and fabrication processes. In large-scale applications, such as transportation pipelines, polymer-based protective liners are preferred owing to their relatively low cost and ease in fabrication. Among the polymeric protective liners, polyurethane (PU) elastomers have received great attention due to their ease in processability, excellent resistance to corrosion, erosive and abrasive wear, and comparatively low cost that allows for large-scale applications [11, 12]. PUs are organic polymers with the urethane group in their chemical structure that can be synthesized by the reaction of a diisocyanate and a polyol [13]. Although the PU fabrication process is similar to the methods typically used for polymers, its mechanical properties such as high elongation at break and minimal plastic deformation are comparable to that of vulcanized rubber that has a more complicated production process [14]. PU has better wear resistance than most polymers [15], rubbers [16], stainless steels [15], and even some hard-faced tungsten carbide-cobalt (WC-Co) coatings [17]. Excellent resistance to wear and corrosion together with ease in fabrication process and low cost has made PU an excellent option for use as protective liners in large-scale applications such as oil and gas pipelines [18].

### **1.2. Abrasive and erosive wear**

Abrasive and erosive wear are the two major wear mechanisms in conditions where the relative motion between the surface and hard erodant particles is responsible for damage and wear of the target material. In abrasive wear, the erodant particles are forced against the surface while moving along [13]. The erodant particles may slide or experience a combined sliding-rotating motion [19]. On the other hand, in erosive wear, the progressive loss of material occurs by the impact of hard particles that are moving in a gas or a liquid stream [13, 19]. The wear mechanisms of abrasive and erosive wear are discussed in the following.

transporting of oil and gas products is a safer method compared to other possible ways of transportation as statistical analysis have shown in Ref. [1]. However, leaks and ruptures can still occur with significant hazard for humans [2]. Beside the life-threatening aspects, failures caused by degradation of pipelines can lead to serious economic losses [3] and environmental disasters such as pollution and contamination of wildlife and sea creatures [4]. Corrosion and wear in components and pipelines can be considered as one of the main causes of failure and leakage in oil and gas industry [5, 6]. All the three major fossil fuels, crude oil, and oil-sand and natural gas are very corrosive. Crude oil contains the corrosive

[8]. Natural gas is also corrosive due to the presence of CO2

calcium and chlorine compounds [9]. On the other hand, in some sequences of extraction of the petroleum products, erodant particles such as sand may be mixed with the flowing fluid to form multiphase solid-liquid mixtures. The flowing of these mixtures in pipelines and equipment in oil and gas industry may lead to solid particle erosion in addition to corrosion. The combination of wear and corrosion can extensively reduce the lifetime of the equipment

Although the corrosion and wear of components exposed to harsh erosive-corrosive environments cannot be thoroughly eliminated, protective coatings can be employed to improve the life time of equipment and prevent early and unpredicted failures. The selection of the proper coating material depends on the service condition, financial aspects, and fabrication processes. In large-scale applications, such as transportation pipelines, polymer-based protective liners are preferred owing to their relatively low cost and ease in fabrication. Among the polymeric protective liners, polyurethane (PU) elastomers have received great attention due to their ease in processability, excellent resistance to corrosion, erosive and abrasive wear, and comparatively low cost that allows for large-scale applications [11, 12]. PUs are organic polymers with the urethane group in their chemical structure that can be synthesized by the reaction of a diisocyanate and a polyol [13]. Although the PU fabrication process is similar to the methods typically used for polymers, its mechanical properties such as high elongation at break and minimal plastic deformation are comparable to that of vulcanized rubber that has a more complicated production process [14]. PU has better wear resistance than most polymers [15], rubbers [16], stainless steels [15], and even some hard-faced tungsten carbide-cobalt (WC-Co) coatings [17]. Excellent resistance to wear and corrosion together with ease in fabrication process and low cost has made PU an excellent option for use as protective liners in large-scale applications such as oil and gas

Abrasive and erosive wear are the two major wear mechanisms in conditions where the relative motion between the surface and hard erodant particles is responsible for damage

), hydrogen sulfide (H<sup>2</sup>

S), organic acids, dissolved

, H2

,

S, and some

and corrosive ions such as Cl<sup>−</sup>

ingredients such as carbon dioxide (CO2

due to the higher rate of material loss [10].

HCO3 −

, and SO4

132 Aspects of Polyurethanes

pipelines [18].

**1.2. Abrasive and erosive wear**

−

gases, and salt water [7], while the oil sand comprises of CO2

In abrasive wear, the erodant particles are forced toward the target while sliding along the surface. As a result of this relative motion, small fragments can be detached from the surface by the cutting action of the sliding hard particles [20]. This wear mode that is one of the major types of abrasion is entitled as microcutting. The microcutting is usually the dominant mechanism of material removal in circumstances where the erodant particles are angular and harder than the target surface. Alongside with microcutting of the surface, the erodant particles may plough the surface by a combined action of cutting and plastic deformation to form groove shaped defects on the surface. The two wear modes of microcutting and ploughing are categorized as cutting mechanisms. On the other hand, in conditions where the grit media are blunt, the accumulation of residual strains together with fatigue mechanism caused by the repeated deformation of the surface is the major mechanism of material removal from the surface. The cracks formed from material defects will propagate by the repeated loading-unloading, leading to reduced strength of surface and loss of material. **Figure 1** shows a schematic of the material removal from a ductile surface caused by cutting and plastic deformation in abrasive wear. In hard brittle surfaces such as ceramics, the fracture of the surface, crack formation, and detachment of small pieces as a result of crack intersections is considered as the major abrasive wear mechanism.

The mechanism of material removal in erosive wear is not only a function of the properties of the target surface but also a function of the testing condition such as velocity and impact angle

**Figure 1.** Abrasive wear of ductile substrates: (a) cutting mechanism and (b) plastic deformation together with fatigue mechanism.

of the erodant particles [21]. In conditions where the erodant particles impact the surface at low angles with respect to the surface (10 to 30°), the cutting mechanism similar to abrasion will be dominant. At low angles, the particle's normal impact force is high enough to enforce the particle for partial penetration, while the tangential force slides the particle along the surface to microcut small pieces from the target surface. As the impact angle increases, the tangential force produced upon impact will not be high enough to cut pieces from the surface. Alternatively, at higher impact angles of 60–90°, ductile targets will mostly experience plastic deformation, and the material removal occurs due to the microforging and extensive deformation of the target surface. The chips formed will detach from the surface at subsequent impacts due to the further accumulation of the residual strains and final detachment of the formed ridges [20]. Since the chipping mechanism of material removal requires higher number of impacts compared to cutting mechanism at oblique impacts, the ductile substrates present a minimal erosion rate at high impact angles. In contrast, the brittle ceramics have the highest erosion rate at normal impact angles, since the particle's normal force is maximum leading to higher fracture, cracking, and damage of the surface of the brittle substrate.

In many engineering applications, such as slurry motion of particles in a pipe, the material removal mechanism can be considered as a combination of erosive and abrasive wear. The slurry particles flowing in a pipe may slide onto the pipe bottom surface while being pressed toward the pipe surface by gravity and fluid weight. The motion induced by the flowing fluid and the pushing force can lead to abrasive wear of pipe material. On the other hand, the particles that are freely moving along the fluid stream and suddenly impacting the surface due to the flow turbulences within the pipe represent the erosive wear at low impact angles. Slurry flow in an elbow can be mentioned as another example for conditions in which combined abrasive-erosive wear may occur. Some particles are sliding while being pushed toward the surface, whereas some other particles are freely impacting the surface due to their kinetic energy and inertial forces. Consequently, when studying the wear of protective coatings and liners, the resistance of material versus both abrasive and erosive wear should be evaluated.
