Conducting Polymers Films Deposited on Carbon Steel and Their Interaction with Crude Oil Conducting Polymers Films Deposited on Carbon Steel and Their Interaction with Crude Oil

The formation of scale/solids deposits inside the pipelines is a frequent problem in the petrochemical industry. These scales can be organic as the asphaltenes and inorganic as the accumulations of salts, which apart from blocking the inside of the pipes can also cause a change in the integrity of the steel. Therefore, it is necessary to avoid the conditions where deposition occurs, together with chemical and mechanical methods of remediation to mitigate the deposition. In this work we intend to use conductive polymers in order to inhibit the deposition of asphaltenes on carbon steel surfaces, by using polypyrrole (PPy) as material capable of conducting electrical current. The electrodeposition of PPy on carbon steel were performed by cyclic voltammetry (CV) and chrono- amperometry (CA). The results showed that under certain experimental conditions it is possible to make a PPy film with adequate characteristics. Important factors were the grip and electrochemical stability of the formed film on steel, which depends on the elec trosynthesis technique and in some cases favoured by a pre-treatment with a 10% HNO3 solution applied to the steel prior to electropolymerization. The PPy films deposited with pre-treatment completely covered the steel surface and showed better stability, adher - ence and generated a hydrophobic material.


Introduction
The formation of solid deposits is a frequent problem in the hydrocarbons transport due to asphaltenes precipitation where the crude oils flow. The asphaltenes are the fraction of higher molecular weight and polarity of the crude oil and they are responsible for these solid deposits, which usually give rise to several problems in the transport and processing of crude oil, mainly by the obstruction of the pipelines [1][2][3][4][5][6].
Common oilfield scales are crystalline deposits, resulting from the precipitation of mineral compound such as carbonates and sulfates present in the injected water, as method for enhanced oil recovery. These scale deposits may appear as a thick layer adhered to the inner walls of the pipes, they are often several centimeters thick with particles sizes up to 1 cm or more. One of the primary problems of scale formation in pipes is the reduction in flow rate by increasing the inner surface roughness of the pipe and reducing the flow ability area. This produces an increase in the pressure drop and consequently production decreases. By increasing the growth of deposited minerals, it becomes impossible to access deeper pipe sections, and finally the scale deposits end up blocking the flow of production. On the other hand, organic scales formation results from asphaltenes and waxes deposition. Usually, the localized corrosion can be developed beneath or around these deposits present in the steel surfaces, due to the presence of bacteria or sulfurous gas, which reduces the integrity of the metal.
The petroleum in its natural state is a mixture of organic compounds of varied structures and different molecular weights. In general, it is possible to group the oil constituents into four well-defined organic components: saturated, aromatic, resin, and asphaltenes [7], known as SARA. The study of the heavy oil fraction (asphaltenes) has increased in recent years due to the problems that they represent in the production and conversion processes. In general, the structure of asphaltenes is considered to consist of a condensed aromatic nucleus with side alkyl chains and heteroatoms incorporated into many of the cyclic structures. The condensed aromatic system may contain from 4 to 20 benzene rings [8]. Nowadays, there is a considerable debate about the structure of asphaltenes, particularly in the size of aromatic groups and how they are linked to other groups in the structure. The type and amount of deposits of heavy organic compounds varies depending on the hydrocarbons present, and the relative amount of each organic family involved. In general, asphaltenes deposition can be explained in detail on the basis of four effects or mechanisms: (1) effect of polydispersity, (2) colloidal steric effect, (3) aggregation effect, and (4) electrokinetic effect.
On the other hand, during crude oil transportation through pipes, there may be sludge deposits along the line and these are also called sediments. The deposits at the bottom of the storage tanks or crude transport lines are water, salts, sand, and heavy hydrocarbons, and their average concentration is about 25% water, 5% inorganic compounds, and 70% vol. of hydrocarbons [9]. In general, asphaltenes deposits cause problems in five points: extraction, transportation, processing, economic benefit of crude oil, and environmental pollution. There are several methods to prevent and/or remove asphaltenes deposits: mechanical methods [10], chemical cleaning, pressure [11,12], temperature and flow rate manipulations [13], additives, and chemical inhibitors [14,15]. The conductive polymers applications are highly diverse and several studies have pointed out the different variables that affect the properties and performance of polypyrrole (PPy) electrodeposition on the steel surface such as the conductivity, stability, roughness, adhesion, film thickness, and so on [16][17][18][19][20][21].
When an electroactive conducting polymer coats a metal surface, it can act as a membrane-like selective permeable layer, allowing the diffusion of certain ions and rejects others, depending on their chemical affinity, electronic structure, and inter-and intra-molecular arrangement. In this context, some works have been reported on the inhibition of corrosion and stability of conductive polymer films on steel substrates [22][23][24].

Conductive polymers
Conductive polymers are those synthetic polymers that are capable of conducting electrical current. These polymers may owe their conductivity to intrinsic properties of the material or modifications. Conductive polymers have a wide range of applications due to their physicochemical characteristics; many of these properties include electrical conductivity, electroactivity, electrochromism, environmental stability, chemical stability, among others [25,26]. The scientific and technological development has given rise to two types of electronic conductive materials with polymer matrix: the intrinsically conductive polymers and the extrinsically conductive polymers [27].
Several reviews in the literature focus on the corrosion protection by conductive polymers such as polyaniline (PANi), polypyrrole (PPy) and polythiophene (PTh). Although a number of possible protection mechanisms are proposed, the possible passivation of the metal by polymers such as polypyrrole or polyaniline is frequently indicated. In this work, we propose the use of PPy films to inhibit asphaltenes depositions. It is shown that the efficacy of conducting polymers for corrosion protection depends on the application modus and the experimental conditions, that is, depending on the suitable conditions, a conductive polymer may have excellent protection capacity or may lead to negative response of the coating. An important part of the deposition of PPy lies in the surface treatment of the electrode surface.
Intrinsically conductive polymers are organic polymer in which the electrical conductivity originates from the extended electrons conjugation along the polymer chain. The most common conductive polymers (polyacetylene, polyparaphenylene, polypyrrole, polythiophene, and polyaniline) have carbon atoms in the backbone with sp 2 hybridization. This hybridization creates covalent σ bonds between the carbons of the main chain and those of the branched chains. The sp 2 hybridization leaves an unbonded orbital p (usually pz); these orbitals overlap and form a bond, with a distribution of C = C double bonds alternating with single carboncarbon bonds along the chain.
Extrinsically conductive polymers are those that owe their conductivity to the inclusion of conductive materials such as metals, graphite, or charge transfer complexes in the polymer matrix, generally thermoplastic. In this case, above the percolation concentration, the conductive paths along the material give it electronic conductivity, while the polymer matrix allows the material to be processed in industrial operations to achieve different types of products and finishes [28]. The conductive polymers are formed from suitable monomers, and can be obtained either by chemical synthesis or by electrochemical methods ( Table 1).

Electrochemical synthesis of conductive polymers
The electrochemical synthesis of any conductive polymer has its particularities; however, there are a number of common factors in its synthesis that have been exposed and that must be taken into account at the time of its preparation. In principle, it is assumed that, except for the initiation step, the electrochemical polymerization will proceed by a similar mechanism as the thermal polymerization for the same monomer in a comparable environment.
Experimentation usually begins with the evaluation of the potential window in the selected electrolyte medium, which will allow defining the electrochemical parameters for the subsequent electrolysis. In the evaluation stage as in the synthesis itself, strict control of electrochemical variables (electrode potential, electrode nature, current density, solution conductivity, electric field, etc.) and aspects such as medium, presence or not of protonating agents, oxygen, inert atmosphere, and so on [35] are observed.
From a scientific point of view, the flow of an anodic current through an electrochemical system, formed by a monomer, a solvent, and an electrolyte, can initiate reactions such as the formation of an oxide layer on the electrode, oxidation of monomer on the metal, oxidation of the solvent, and oxidation of the electrolyte. But in this process, the polymerization develops an electrode coating the chemical nature of the electrode changes after a few seconds of polymerization from a metal electrode to a polymer electrode. So, the above reactions will

Conducting polymers General comments Refs
Polypyrrole (PPy) PPy is chemically and thermally stable. The conductivity of PPy strongly depends on the preparation technique. [19,20] Polyaniline (PANi) PANi is easy to synthesize and used as corrosion inhibitor and relatively inexpensive. [29,30] Polythiophenes (PTh) PTh poses high charge-carrier mobility due to their relative structural order, is soluble and demonstrates solvatochromism and thermochromism effect. [31,32] Polyacetylene Despite its discovery started the development of conductive polymers, to date, polyacetylene has no commercial applications. [33] Polyphenylenes Thermally stable up to 500-600°C but is quite insoluble in most solvent, with potential applications as light-emitting diode due to their electroluminescence properties.
[34] occur at different potentials on the new electrode, and a new metal-polymer interface appears in addition to that of the growth-dissolution polymer and then new reactions will occur, oxidation of the polymer and degradation thereof [27].

Applications of conducting polymers electrochemically synthesized
Conductive polymers have been widely used in the fields of electrochemistry, electroanalysis, electrocatalysis, batteries and capacitors, and so on [36]. In these applications, the electrochemical activity and the conductivity are two important properties of the conducting polymers, because they play fundamental roles. In addition, small ions and molecules are able to diffuse into matrices of conducting polymers, providing other advantages over conventional electrode materials. This intrinsic property allows electrochemical reactions to take place along the matrices of the conducting polymers and thus increase the active sites for the electrochemical processes by using a 3D electrode. However, in order to efficiently utilize all active sites and improve mass transport during the electrode process, the film thickness of the conductive polymer should be reduced to facilitate diffusion of the ion in the polymeric matrix. Considering these factors, conductive polymer nanomaterials show different characteristics that can provide advantages over other materials. In addition, nanostructures can produce conductive polymers with new surface properties and new functions [36]. Different applications of conducting polymers prepared nanomaterials by electrochemical techniques as been reported: sensors [37][38][39], electrochemical capacitors [40], fuel cell electrodes [41], batteries [42,43], electrochromic devices [44], and electrochemical actuators [45][46][47].

Inhibitors of asphaltenes deposition by using conducting polymer coatings
Polypyrrole (PPy) and polythiophene (PTh) are conductive polymers, which have been used as corrosion protection and have the characteristic that they can coat the steel [48,49]. This process can be carried out by electropolymerization in aqueous phase; in addition, PPy exhibits good mechanical properties, thermal stability, and high conductivity [49]. The performance of corrosion protection by bi-layered PPy coatings was investigated by Kowalski et al.; they used an inner PPy layer doped with molybdophosphate ions to protect steels from corrosion. The polymer layer maintained the passive state of the steel in an acid solution and a neutral NaCl solution for several days [22].
Warren et al. [16] reported that anion dopants containing sulfonates, such as dodecylbenzenesulfonic acid (DBSA), can be used to form PPy films with high conductivities, good stability, mechanical properties, and apparent order. However, when there is a chemical-physical mismatch at the metal-polymer interface, the adhesion between the PPy layer and the metal substrate is generally poor [50].
Beck and Michaelis [51] described anodic electrodeposition of black PPy films on steel electrodes from aqueous electrolytes containing the monomers and oxalic acid, with strong adherence and low surface roughness. Su and Iroh [52] investigated the electrodeposition mechanism of PPy coatings on steel substrates from aqueous oxalate solutions. Their results revealed the formation of a passive layer on the steel substrate before reaching the electropolymerization potential of pyrrole.
Tüken et al. [53] prepared multilayer coating of polypyrrole/polyphenol on mild steel by cyclic voltammetry (CV). The corrosion performance of this multilayer coating was investigated by electrochemical impedance in sulfate solution. Another study carried out by the same authors consisted of the coating of copper with a mixture of polymers (PPy/PTh) [31]. They observed that the electrochemical synthesis of stable and homogeneous PTh films could not be achieved on copper electrode by direct oxidation of thiophene in acetonitrile-LiClO 4 , due to insufficient surface passivation, and the copper dissolution in monomer oxidation potential region.
On the other hand, Pekmez et al. [32] informed that the electrochemical synthesis of an anticorrosive polybithiophene (PBTh) on stainless steel is feasible, and the obtained PBTh coatings strongly adhered to the working substrate.
Rocha et al. [14] carried out a study on the inhibition of asphaltenes precipitation in Brazilian crude oils using amphiphiles substances such as low molecular mass ethoxylated nonylphenols, vegetable oils (coconut essential oil, sweet almond, andiroba, and sandalwood oil), and organic acids (linoleic, caprylic, and palmytic). These compounds showed great efficiency in the asphaltenes precipitation inhibition, through a mechanism of asphaltenes stabilization as a function of its surfactant capacity.
Castellano et al. [15] performed a theoretical investigation of σ-π and π-π interactions on benzene, pyridine, and thiophene dimers; they observed the influence of these interactions with asphaltene stability on crude oil and concluded that chemical interactions between species with opposite polarities lead to an intermolecular association in the asphaltenes, which are responsible for the phenomenon of aggregation. This study was aimed to understand why these compounds tend to aggregate and then flocculate in oil operation processes.
The applications of the conducting polymers are highly diverse and rely on the final properties from the synthesis conditions [23,24]. Therefore, conducting electrochemical studies to understand the effect of the synthesis variables that affect electrodeposition of conductive polymers is necessary, in order to attain the required conditions in each specific application, as it is the case for application in corrosion protection [54]. When the electroactive conducting polymer is coated to the electrode surface, it may work as a selective permeable layer, which allows certain ions and molecules to pass, according to the degree of cross-linking of the films and supported molecules on the monomer. So the study of the surface properties of polymeric materials is justified, and the way in which the species present in the system can cause the material to deteriorate. In this context and considering the studies reported in the literature, the inhibitors of the asphaltenes deposit using PPy and PTh electrodeposited in carbon steel, is a novel application of this type of materials, in the literature there are few reports in this line of research.

Interfacial interaction (contact angle)
The deposition of a coating on a solid generates new interfaces between dissimilar materials and involves considerations of wettability, spreading, interface evolution, and adhesion.
The interaction at solid-liquid interface is determined by a balance between the adhesive and cohesive forces. Adhesive forces between a liquid and a solid cause a liquid drop to spread on the surface. The cohesive forces within the liquid cause the drop to maintain a stable position and avoid contact with the surface. This solid-liquid interaction at the interphase is called wettability. The wettability of electrochemically deposited conductive polymer films depends to a large extent on several parameters, such as deposition conditions (applied voltage, transferred charge, etc.), dopant, and working electrode roughness [55]. Mecerreyes et al. [56] carried out a study where they obtained a hydrophobic PPy film (water contact angle of >90°) using a perfluorinated dopant anion, and a hydrophilic film using a ClO 4 − doping anion. Controlling the wettability of a solid surface is important in many applications, for example, in self-cleaning surfaces, liquid lenses, smart fabrics, and in biomedicine [57][58][59].
It has been reported that the roughness of hydrophobic solid increases its hydrophobicity due to two different ways: roughness increases the surface area of the solid, which geometrically enhances hydrophobicity and due to the air that can remain under the drop [60,61]. It is important to note that a roughness at two or more length scales has been implicated as the cause of imitating the "lotus effect," which is the characteristic of a lotus leaf to promote water repellency and self-cleaning [62]. One method to surface wettability control is by oxidizing or reducing the polymer film by modifying thereby the surface morphology. Several research groups have produced films of superhydrophobic conductive polymers, creating micro-and nanostructured surfaces by tempering methods [63][64][65][66]. However, these methods have a disadvantage due to the complexity of manufacturing processes [67].
For both improved oil recovery and crude oil transport, it is necessary to develop chemical additives that modify the wetting behavior of reservoir rock (also known as a core), in order to facilitate the crude oil extraction, or to prevent it from wetting the pipe's inner wall and allow crude oil to easily flow through pipelines [68]. The oilfield scale formations are associated to interface activity of polar components of the crude oil. Asphaltenes are the most polar fraction of the crude and contain large amounts of active species [69] and because of this, asphaltenes are reported as the major fractions responsible for altering surface wettability through the interaction of polar functional groups with polar sites of solid surface [70,71]. A study by Kaminsky and Radke [72] indicates that low solubility asphaltenes can diffuse through water films to arrive at rock surfaces without significant wettability alteration; the rupture of the water film followed by direct deposition of crude oil onto rock allows explaining the wettability reservoir rock.

Reagents and chemicals
The electrolytes used were 0.1 M aqueous solutions of KNO 3 and KCl (both from J. T. Baker, reagent grade). Also a 0.1 M 1 of pyrrole (Py, Sigma Aldrich) solution was prepared with previous purification in a bed column packed with silica and activated carbon. All the solutions were prepared with deoxygenated water (Millipore, 18.2 MΩ) for 15 min with an atmosphere of pure nitrogen (Praxair, 99.99%) before each experiment. The film stability was evaluated in a KCl electrolyte according to a procedure reported [73].
The crude oil used in this work originates from the Gulf of Mexico and presents the following features: 15°API, 25% weight of asphaltenes, density 0.9647 g cm −3 , and a kinematic viscosity of 1.697 mPa s.

Materials
A conventional three-electrode cell was used for the electrodeposition of PPy over carbon steel (CS-1018) as the working electrode, graphite electrode as counter-electrode, and aqueous saturated calomel electrode, SCE (Tacussel), as reference electrode. Initially, the CS-1018 electrode was polished with different grain sandpaper to achieve a defined surface and subjected to ultrasonic baths (Branson 2510) for 5 min to remove contaminants from the surface. To improve the adhesion of the polymer to the steel surface, it was necessary to carry out a pretreatment of the electrode with a mordant layer to increase the roughness. For this purpose, some authors [23,24] immerse the metal electrode in acidic solutions of HCl or HNO 3 . In this work, the electrode was treated with acid solutions of 10% HNO 3 with an immersion time of 2 min. The treated surfaces were designated as treated in HNO 3 (T) and only polished surfaces without acidic treatment were designated as untreated in HNO 3 (NT).

Equipment
The polypyrrole electrosynthesis was performed by cyclic voltammetry and chronoamperometry (CA) techniques by using a Gamry Reference-600 potentiostat, in a three-electrode cell at room temperature (20±3°C). The electrodeposition by cyclic voltammetry was carried out at a scan rate of 100 mVs −1 , in a potential window of −0.8 to 1.0 V/SCE for 40 cycles of polymerization. The conditions for the chronoamperometry were at constant potential of 1.0 V/SCE, for 300 s. For the characterization of the polymeric films, a scanning electron microscope, SEM (Jeol, JSM-6390LV) coupled with energy dispersive spectroscopy (EDS) analysis (Oxford Instruments, INCAx-sight), an atomic force microscope, AFM (Veeco, Innova Scanning Probe Microscope), and an equipment to measure contact angle (Chem Instruments, CAM-plus) were used. The contact angle measurements of polymer surfaces were conducted with deionized water and crude oil, analyzing two areas on the sample and considering three measurements in each zone taken every 2 min for 10 min.

CS-1018 electrode characterization
The SEM and AFM micrographs of the CS-1018 substrates are shown in Figure 1, where the lines attributed to the mechanical polishing are observed. The images show that the treated substrates, CS-1018 T, present a rougher surface due to chemical attack with HNO 3 . The observed roughness measurements were of Rq = 0.0509 μm and Rq = 0.739 μm, for the untreated and treated surfaces, respectively. This difference in the roughness value will be important for the adhesion effect of the polymer material to be synthesized on it.

PPy film electrodeposition
In this work, the results obtained by two electrochemical techniques are presented: CV and CA. The electrodeposits were made in four different electrolytes, in order to find the best characteristics of the film. Figure 2a shows an example with the cyclic voltammograms of the polymerization of polypyrrole in electrolyte of KNO 3 . The voltammograms show the characteristic signals [74][75][76]. Figure 2b shows the signal of PPy deposited with applied constant potential method [77]. Figure 3 shows the deposited PPy films on the CS-1018 with and without treatment in the four different electrolytes, electrodeposited by CA technique. When KI and KF electrolytes were used, deposit films were not obtained, but when K 2 SO 4 and KNO 3 were used as electrolytes, the formation of PPy films was obtained. Figure 4a shows the SEM image of PPy deposited on CS-1018 NT, in which a homogeneous film is observed that completely covers the surface of the steel. However, it is possible to observe lines in said film due to the polishing process of the steel, indicating that the deposited film is thin. By contrast, in the PPy film deposited on AC-1018 T, non-uniform circular agglomerates with needle-shaped scales are observed, as can be seen in Figure 4b. This suggests that acid treatment to the steel affected the morphology of the deposited polymer. This observed morphologies show a characteristic topography of conductive polymers [78].

Contact angle characterization of the PPy surface
In order to analyze the surface of deposited PPy films, contact angle measurements were performed with and without water and crude oil on selected samples, and steel substrates with and without acid treatment. The measurements were performed in two areas of the sample    and in each zone, three measurements were taken every 2 min. The contact angle values were obtained from the averages of the measurements. Figure 5 shows the experimental setup, wherein a drop of water makes contact with the substrate CS-1018 NT and how its image is projected to measure the contact angle. Table 2 shows the values of the contact angle measurements with water and crude oil of the treated and untreated electrodes, and the respective formed polypyrrole films. It is noted that the contact angle of the CS-1018 NT surface is about 70° with water and crude oil. In other words, the untreated metal surface has the same affinity for both liquids. When the steel surface is subjected to acid treatment, CS-1018 T, the contact angle with water increases to 110°. This result indicates that nitric acid treatment induces a slightly hydrophobic behavior. On the other hand, the contact angle also increases slightly with crude oil and remains at a value close to 90°, that is, at the boundary between the oleophilic/oleophobic balances. Contact angle results show that the acid treatment to the metal surface provides hydrophobic properties to the surface; this measurement is in line with AFM results, showing greater surface roughness for electrodes exposed to acid medium. These results are consistent with those reported in the literature [52,60,74,79].
According to this result, the polypyrrole films deposited on CS-1018 T presented rougher surfaces than those synthesized on CS-1018 NT. In both cases, after the synthesis and coating with PPy of the electrodes, the contact angle with water exhibited an increase as the surface roughness increases. PPy deposition on CS-1018 NT has a slightly hydrophilic behavior contrary to the case when the polymer is synthesized on a treated surface.
The contact angle values with crude follow the same trend as the water. This contact angle value is lower in the untreated surface, with apparently less roughness. However, these contact Contact angle measurements show that the surface CS-1018 T PPy-KNO 3 forms a contact angle greater than the surface of CS-1018 NT PPy-KNO 3 . That is, that the greater the porosity of the metal surface, a higher roughness of the polymer deposited is obtained, and consequently the polypyrrole-oil interaction decreases.
The roughness plays an important role for the hydrophobicity of the polymer deposit. According to the literature [80], hydrophobicity increases as the roughness of the material grows. Figure 6 shows the average values of contact angle with oil as a function of roughness of the deposited   polymer films. The materials with greater roughness presented greater contact angle than those with low roughness. It is observed that the contact angle of the PPy-KNO 3 with acid treatment by CV increased around 20° with respect to CS-1018 NT. Therefore, this result shows that such polymer under certain conditions is efficient to reduce contact of asphaltenes with CS-1018.

Conclusions
Acid pretreatment modifies the roughness of the CS-1018 substrate generating an oxide layer that influences both the morphology and the stability of electrodeposited PPy films, creating surfaces with different arrangements, which depends on the electropolymerization technique employed. The roughness difference is directly related to the stability of the polymer film formed and its surface properties (wettability). CV electrodeposition is the most appropriate method for this type of application. The PPy materials deposited with KNO 3 by CV, with and without treatment in an acid medium, presented a greater homogeneity and roughness.
The roughness is directly proportional to the hydrophobicity of the PPy film, which was evidenced with an increase in contact angle values (lower affinity to crude oil). The inhibition of the asphaltene deposition is evidenced by obtaining contact angles of more than 90°. The results indicate that this methodology is cost-effective, versatile, and scalable for the synthesis of electrodes for applications to inhibit asphaltenes deposition.