Carbon dioxide corrosion inhibitors: current state of research and development

Introduction

Oil and gas industry equipment is subject to aggressive influence of the external environment. One of the main factors of accidents (up to 80 %) and failure of oilfield equipment is corrosion of external and internal pipe walls. When the metal surface is in contact with a medium (a mixture of liquid and gaseous hydrocarbons), the corrosion process is significantly accelerated in the presence of hydrogen sulfide, carbon dioxide, water, oxygen, and mechanical impurities [1].

Factors having maximum influence on the mechanism and intensity of corrosion of field oil and gas pipelines [2, 3]: high water content of transported products; high content of corrosive-aggressive gases (carbon dioxide and/or hydrogen sulfide); high temperature of transported products; high content of mechanical impurities.

Previously, carbon dioxide was not considered as an active corrosive agent; hydrogen sulfide and oxygen posed a significant threat. However, the situation has changed dramatically with the development of deep-lying seam. The simultaneous presence of bicarbonate and carbonate ions in the aqueous phase causes carbonic acid corrosion, which leads to failure of oil and gas field equipment. [4]. The simultaneous presence of bicarbonate and carbonate ions in the aqueous phase causes carbonic acid corrosion, which leads to failure of oil and gas field equipment [2]. At 25-35 % content of the water phase in the oil-water mixture, the surface of steel equipment is wetted by water, which causes corrosion stimulated by CO₂ dissolved in the mixture [5]. The rate of corrosion damage is 3-4 mm/year and in some cases reaches 6-8 mm/year. When predicting corrosion rates, it should be assumed that the carbon dioxide content in the aqueous phase is closely related to the partial pressure of carbon dioxide (pCO₂ > 0.2 MPa – severe corrosion, pCO₂ = 0.02-0.2 MPa – moderate, pCO₂ < 0.02 MPa – insignificant [6]), and the partial pressure is temperature dependent [7].

One solution to the environmental problem of carbon dioxide emission and decarbonization of the industrial sector is the use of carbon capture, utilization and storage (CCUS) technology. However, carbon dioxide corrosion is a serious problem that has hindered the widespread development and application of this technology [8, 9].

Three temperature areas can be distinguished in the carbon dioxide corrosion process [10]:

• low temperature area (below 60 °С) – corrosion is mostly uniform and its rate increases with increasing temperature; small amounts of FeCO₃ are formed on the metal surface;
• medium temperature area (about 100 °С) – the formed iron carbonate film is uneven in thickness; local corrosion intensifies, the rate of which reaches a maximum;
• high temperature area (above 150 °С) – corrosion rate decreases (by an order of magnitude or more) due to the formation of a dense coating of iron oxide (Fe₃O₄) with high adhesion.

Carbon dioxide can cause both uniform and more dangerous localized (moss-like and meso-like pitting) corrosion of steel. In the process of localized corrosion, an irregular and/or unstable protective film of corrosion products is formed on the metal surface (FeCO₃, Fe₃O_4, etc.).

Methods of corrosion control in the oil industry:

• technological methods of protection, i.e. creation and maintenance of corrosion-safe conditions of equipment operation;
• use of corrosion-resistant/non-metallic materials [11];
• application of corrosion inhibitors (CI);
• use of protective coatings.

Among the listed methods of corrosion control in the oil and gas production industry in Russia and abroad, the leading place is currently occupied by inhibitor protection, as it does not require significant capital investments and a serious restructuring of the technology of oil production, gathering and treatment [10]. CI are fast acting and cost effective, so no field containing aggressive components is operated without the use of corrosion inhibitor protection. The advantage of this method is its simplicity and cost-effectiveness, possibility to use it both on new and operated wells, which allows in the process of field development to easily replace the existing inhibitor with a more effective one without disturbing the production technology at the fields. In the Russian Federation, corrosion inhibitors are used during the development of oil and gas fields as prescribed by Gosgortechnadzor [2].

Inhibitors are supplied to pipeline systems with periodic or continuous dosing. During periodic dosing, contact of the pipeline metal surface with the commercial form of inhibitor or its concentrated solution is organized. At the same time, inhibitors should have an effect of after-action, i.e. the protective layer formed by their application should retain its integrity for a long time. At constant dosing the formation and maintenance of the protective layer is realized due to diffusion of the active component of the inhibitor from the liquid volume on the metal surface of the pipeline wall. The effectiveness of pipeline protection with corrosion inhibitors depends on the chemical composition, adsorption capacity and the amount of inhibitor injected [12-14].

Carbon dioxide corrosion inhibitors

Corrosion inhibitors are chemical compounds that reduce the rate of corrosion without significantly changing the concentration of any corrosion reagent. The ability of inhibitors to retard corrosion is mainly due to the following properties: adsorption of CI on the metal surface; change in the rate of anodic/cathodic reactions; slowing down the rate of CI diffusion to the metal surface; decrease in the electrical resistance of the metal surface.

The corrosion inhibitor acts at the metal/solution interface to form a film of various types – passivating, precipitating and adsorptive.

The most widespread use of passivators is for corrosion control in neutral or near-neutral pH environments. The chemical composition and structure of inhibitors largely determines their mechanism of action. Among inhibitors one can find inorganic substances with oxidizing properties (nitrites, molybdates, tungstate, chromates), which are able to create protective oxide films on the surface of the corroding metal. Currently, the use of inhibitors of this type has been abandoned due to toxicity; they are used only in the aviation industry and construction.

Precipitating film inhibitors are chemicals that form insoluble protective films by reacting with soluble substances in the environment (e.g., phosphonates and polyphosphates that form protective films with calcium ions in solution) or with the protected medium – metal ions (e.g., copper-benzotriazole salt film (BTA) [1].

Inhibitors that form adsorptive protective films are mainly organic substances (surfactants), which often have a surfactant molecular structure with a hydrophilic group capable of binding to the metal surface and a hydrophobic part of the molecule protruding towards the solution volume. The adsorbed inhibitor molecules restrict oxygen diffusion and water access to the metal surface, which reduces the corrosion rate.

Modern adsorption-type corrosion inhibitors are usually a solution of one or more organic compounds with high inhibitory properties (active base) in a hydrocarbon or water-alcohol solvent.

There are a number of requirements for corrosion inhibitors used in the oil industry:

• protective effect in a wide range of temperatures and pressures (from normal to high), as well as in conditions of high flow velocities and the presence of abrasive particles;
• low solidification temperature (not less than –50 °С);
• good solubility and/or dispersibility in working media (water-soluble, hydrocarbon-soluble, hydrocarbon-soluble-water-dispersible, insoluble in neither water nor hydrocarbons);
• absence of influence on the stability of oil-water emulsions;
• fire and explosion safety, compliance with the requirements of sanitary norms;
• compatibility with other reagents used in the technological process and absence of influence on the quality and processes of subsequent oil refining, etc.

Also, when developing effective corrosion inhibitors, it is worth considering the environmental impact, the possibility of use at low concentrations (100-200 mg/l), and chemical stability in corrosive environments.

All commercially available corrosion inhibitors have an optimal area of application depending on the industry segment, composition of corrosive media and technological features of the protected objects [15]. Table 1 shows the most common composition of corrosion inhibitors used in various fields.

Table 1

Composition of inhibitors used in various segments of the oil and gas industry

The main volume of reagents is used in the processes of production (for protection of pressure, oil gathering, in-field pipelines and water lines) and transportation of crude oil (Fig.1). Therefore, the most urgent is the development of new and improvement of currently used corrosion inhibitors, including carbon dioxide, used in the oil industry.

Organic corrosion inhibitors

Organic compounds containing heteroatoms (O, P, N, S) are being actively studied as corrosion inhibitors [16, 17]. Structures of the main phosphorus-, oxygen- and sulfur-containing compounds used in industry as active bases of carbon dioxide corrosion inhibitors:

• esters of phosphoric acid [16]

• bis(2-ethylhexyl) phosphate [18]

• carboxylic acids [19]

• amino acids and their derivatives [20]

• rhodanine and derivatives [21]

where

• ω-mercaptoalcohols [22]

Phosphorus-containing compounds

The mechanism of adsorption of inhibitor molecules on the metal surface can be considered as donor-acceptor: heteroatoms containing π-electrons act as donors, and free d-orbitals of metal surfaces act as acceptor [16]. This interaction leads to the formation of a protective layer on the metal surface. The ability of phosphorus atoms to form adsorption bonds at the expense of π-electrons and vacant d-orbitals of transition metals determines the chemisorption and inhibitory properties of phosphorus-containing compounds, which are used as corrosion inhibitors individually or in mixtures.

Mono- and diethers of phosphoric acid form sparingly soluble salts with ions Fe²⁺ and Ca²⁺, which form a protective layer on the metal surface, thereby reducing the rate of corrosion.

Corrosion inhibitors based on organic phosphorus compounds, such as N-heterocyclic alkylphosphonic acid HtC(R₁R₂)P(O)(OH)₂, where Ht – is a heterocyclic fragment with a nitrogen atom in the ring – pyrrolidine or morpholine, showed a high degree of protection against local corrosion; R₁, R₂ – are hydrogen atoms (alkyl radicals СН₃ or СН₃ and C₂H₅) [23]. The presence of the acid residue Р(O)(ОН)₂ in the molecule provides the formation of a nanoscale self-organizing layer on the metal surface at the inhibitor concentration in aqueous solution from 30 to 60 mg/l.

In the research [18] the possibility of using bis(2-ethylhexyl)phosphate to inhibit carbon dioxide corrosion is reported. It is shown that P–O–Fe and P–Fe bonds are formed on the metal surface, due to which the corrosion inhibitor shows high efficiency – 93 % at a concentration of 500 ppm.

Oxygen-containing compounds

Carboxylic acids show high protective ability (above 90 %) in conditions of carbonic acid corrosion [19], while the inhibition efficiency is primarily associated with adsorption of carbonyl groups –C=O on the metal surface and significantly increases with increasing their amount. Tricarboxylic acids exhibit maximum efficacy even at low inhibitor concentrations. The presence and number of hydroxyl groups in the structure of the inhibitor molecule has a significant effect on its efficiency – succinic and maleic acids, which have no hydroxyl groups in their composition, showed better inhibition efficiency than malic and tartaric acids.

Amino acids, particularly glutamic acid, are effective corrosion inhibitors, but the concentration should be carefully selected depending on the environmental conditions, as the opposite effect can be achieved [20]. Amino acids are environmentally friendly compounds that are completely soluble in aqueous media, non-toxic and cheap, so more and more research has been directed towards the synthesis of amino acids and their more complex derivatives to improve the effectiveness of corrosion inhibitors [24].

In the research [25], the esterification reaction of natural petroleum acids with allylic alcohol in the presence of an ionic liquid, N-methylpyrrolidone hydrosulfate, was studied. Based on of the obtained allyl alcohol of petroleum acid nitro derivatives were synthesized, their salts and complexes were obtained and the influence of solutions of these compounds on the kinetics of the corrosion process was investigated. The nature of salts and complexes was found to affect the effectiveness of СО₂ corrosion inhibitors and varies in a series: potassium salt > monoethanolamine complex > diethanolamine complex. Potassium salt and monoethanolamine complex based on nitro product of natural petroleum acids showed the highest anticorrosion efficiency at a concentration of 300 ppm, which amounted to 98.4 and 98.9 %, respectively.

Sulfur-containing compounds

The authors of the source [21] suggest the use of rhodanine and its 3- or 5-derivatives of general formula I or II as a corrosion inhibitor. It was found that inhibitors exhibit high protective properties (up to 99 %) in carbonic acid corrosion of iron at very low concentrations (0.1-2.5 mg/l).

Decantiol can be an effective inhibitor of localized corrosion in carbonate media due to the formation of adsorption monolayer [26] and reduction of adsorption/desorption processes of intermediate compounds on the metal surface resulting from the slowing down of cathodic and anodic reactions. It was found that the introduction of 10-400 ppm decanthiol can reduce the local corrosion of carbon steel. Investigations of the metal surface by physicochemical methods made it possible to establish that corrosion protection is provided by physical adsorption of the inhibitor on the metal surface rather than by chemisorption, since no Fe–S bonds were detected.

The relationship between the structure of a number of mercaptoalcohols and their anticorrosion efficacy as corrosion inhibitors was studied in [22]. While using molecular dynamic modeling, it was found that the high efficiency of mercaptoalcohols is due to their ability to adsorb on the Fe (110) surface with both –SH- and –OH-groups. As the carbon chain lengthens, the corrosion resistance properties improve and the optimum chain length is С₁₁.

When straight, branched, cyclic or heterocyclic alkylene, arylene, alkylarylene, arylalkylene or hydrocarbon fragments containing from 1 to 30 carbon atoms of carbon-containing radicals with the general formula (HS)n–R–(OH)m (n, m = 1-3), are introduced into mercaptoalcohols, the anticorrosive efficiency increases [27]. The best reduction in both uniform and localized corrosion rates was achieved in the presence of 2-mercaptoethanol, 2-mercaptopropanol, 1-mercapto-2-propanol and 2-mercaptobutanol.

Nitrogen-containing compounds

Inhibitors based on nitrogen-containing compounds are the most widespread among all organic compounds containing heteroatoms [17, 28].

Structures of the main nitrogen-containing compounds used as active bases of carbon dioxide corrosion inhibitors:

• imidazolines

• primary amines R–NH₂;
• diamines

• amidoamines

• dimerized amidoamines

• quaternary ammonium bases (QAS)

• oxyethylated primary amines

• polyethoxyalkyldiamines

• alkylpyridines

Amines, amides, imidazolines and their derivatives are used as the active base of corrosion inhibitors. Analysis of corrosion inhibitors used on the territory of the Russian Federation and produced by domestic companies shows that complex mixtures of nitrogen-containing compounds are predominantly used as the active base. Effectiveness of active bases of carbon dioxide corrosion inhibitors of domestic manufacturers: mixture of nitrogen-containing surfactants – 90 %; reaction product of tall oil with tetraethylenepentamine – 90 %; mixture of alkylimidazolines – 91 %; amido-imidosoline of fatty acids – 90 %; condensation product of fatty acids of tall oil with aminoethyl-ethanoamine – 91 %; mixture of alkylimidazolines of fatty acids С₈-С₁₂ – 90 %; alkyldimethylbenzylammonium chloride – 92 %.

Amines and QAS

Primary, secondary, tertiary amines and tertiary derivatives containing a positively charged nitrogen atom covalently bonded to four organic radicals and ionically bonded to an anion are used as corrosion inhibitors. The radicals commonly used are fatty acid residues, alkyl (С₈-С₃₀), phenyl, oxy- and polyoxyalkyl.

QAS are cationic type surfactants. This means that in aqueous solutions, QAS dissociate into a positively charged radical and an acid anion. QAS have been used for a long time and widely in the oil and gas industry. They are used as individual active bases or in mixtures with other classes of compounds. Dimethyldodecylbenzylammonium chloride is an example of QAS-based CI. N-alkyl quaternary ammonium compounds with different anions have the structure of amphiphilic cationic surfactants and are therefore potential metal corrosion inhibitors.

In the research [29] a series of pyrrolidine-based quaternary ammonium salts containing hydrophobic C₁₂-C₁₆ alkyl moieties and hydrophilic oxygen-containing substitutes (propargyl, dodecyl and their combination) were synthesized. High inhibitory properties of the studied ammonium compounds are primarily due to the presence of several adsorption centers. Hydrophilic fragments are chemisorbed on the metal surface to form a protective film (coating), and hydrophobic substitutes displace the adsorbed water layer to form a protective layer on the metal surface. As the alkyl chain length increases, the inhibition efficiency increases. Aliphatic hydrocarbon radicals contained in ammonium cations as well as in organic acid anions provide adsorption interaction with the metal surface.

In addition, it is known that the simultaneous presence in aqueous solutions of a mixture of inhibitors exhibiting the properties of both cationic (containing ammonium cation) and anionic (containing hydrocarbon anion) surfactants can provide special properties that can significantly increase the effectiveness of anticorrosion action.

The length of alkyl substitute of dimethylbenzylammonium affects the effectiveness of corrosion inhibitor based on it [30]. The molecular structure of the model compounds includes the same polar group, dimethylbenzylammonium, and four different hydrophobic moieties with different lengths of alkyl tails (butyl (–C₄H₉), oktyl (–C₈H₁₇), dodecyl (–C₁₂H₂₅) and hexadecyl (–C₁₆H₃₃). It was found that the longer the length of the alkyl moiety, which has good adhesion to the metal surface, the higher the corrosion reduction efficiency for the homologous series evaluated. The main role of the leading group of model quaternary ammonium compounds is to form bonds of the inhibitor molecule with the metal surface, while the hydrophobicity of the alkyl tail plays a determining role in the inhibition process by displacing water molecules from the surface.

Ammonium salts of O, O-dialkyl dithiophosphoric acids (Fig.2) are effective inhibitors of carbonic acid corrosion, in the presence of which a protective effect of up to 99 % is achieved [31]. In the case of carbon dioxide corrosion, the inhibitory activity is independent of the nature of the amine and the length of the alkyl substitute in the ester group. An important feature is that the inhibitory effect of these compounds increases with increasing temperature.

Imidazolines

Imidazolines and their derivatives (Fig.3) are widely used as organic corrosion inhibitors in the oil and gas industry worldwide to effectively suppress CO₂ corrosion.

Imidazoline can be readily synthesized from natural fatty acids and ethylenediamines, as well as from aldehydes and diamines [20]. The use of natural fatty acids make imidazolines environmentally friendly. Imidazolines can also be derived from plant material. Thus, in [32] a hydroxyethyl imidazoline derivative was synthesized based on coffee oil and found to be highly effective in reducing carbon dioxide corrosion rate, 99.9 % at a concentration of 10 ppm. In the research [33] a novel N-(3-(2-fatty-4,5-dihydro-1H-imidazol-1-yl)propyl) – fatty amide-type surfactant was derived from fatty acids (С₁₆-С₁₈), contained in mango seeds and showed high performance under corrosive conditions of CO₂. It was previously determined that adsorption of olein-substituted imidazoline on steel surfaces creates a fully hydrophobic surface, forming a watertight barrier between the corrosive aqueous phase and the steel surface, which provides effective corrosion protection. To improve the efficiency of corrosion inhibitors, the authors [34] synthesized a number of imidazoline derivatives from oleic acid and various amines (ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, hydroxyethyl ethylenediamine, and polyethylenepolyamine) and showed that the efficiency of CI based on them is 97-98 %. The inhibition efficiency of imidazolines obtained by condensation of soybean oil fatty acid and various amines (DETA, TETA, TEPA) was 50-90 % [35]. Imidazoline derivatives derived from semi-purified rice bran oil exhibited high inhibitory properties up to 99.69 % for carbon steel [36].

Table 2

Structures of the main imidazoline compounds used as active bases of inhibitors of carbonic acid corrosion

Compound	Structure	Substitutes	Reference
Bis-imidazoline		R = С17Н33	[37]
Alkylimidazoline		R = C17H33R1=HC2H4NH2C3H6NH2C2H4NHС2H4NH2(C2H4NH)2С2H4NH2(C2H4NH)xС2H4NH2 (x ≥ 3)C2H4OH	[34], [38]
		R = C2H5, C18H37R1= C2H4OH	[38]
		R = C17H33	[39]
		R = C17H33	[40]

The presence of several nitrogen atoms in the structure of compounds increases their anti-corrosion ability. The authors of [37] developed a method of obtaining a corrosion inhibitor based on ethylene oxide and bis-imidazoline obtained by interaction of tetrapropylene pentamine and oleic acid, which demonstrated high efficiency of corrosion reduction in carbonate medium – 97-98 % at a concentration of 30-50 mg/g.

A number of researches are aimed at studying the adsorption and anticorrosion properties of alkylimidazolines depending on the nature of substitutes R and R¹ (Table 2). Substitutes exhibit different properties and perform different functions. The hydrophilic part provides orientation and bonding of the corrosion inhibitor to the metal surface, while the hydrophobic part provides formation of a protective adsorption barrier between the metal surface and the corrosive medium. It is still debated which substitute has a greater influence on the effectiveness of corrosion inhibitors. For example, a number of alkylimidazolines have been synthesized in the research [38] by varying the nature of both hydrophilic and hydrophobic substitutes, the most representative examples are shown in Table 2.

It was established that at the same R (C₁₇H₃₃) replacing the hydrogen with aminoethylene (C₂H₄NH₂) in the position R¹ significantly increases the CI efficiency, from 77 to 92 %. Increasing the chain length of the amine substitute does not affect the efficiency as much, and in some cases (C₃H₆NH₂) decreases it. The introduction of a hydroxyethyl group into the imidazoline ring weakens the inhibitory ability of imidazoline due to an increase in its hydrophilicity, which leads to an increase in water solubility with a decrease in the insulating ability of the inhibitor film. Whereas, for the same R¹ (C₂H₄OH) replacing the alkyl chain С₁₇ with С₂ decreases the CI efficiency sharply from 90 to 13 %, confirming the important role of the hydrophobic part length. The chain branching slightly changes the effectiveness of the inhibitor.

The authors [34] synthesized a series of imidazolines using oleic acid and various nitrogen- and oxygen-containing groups as substitutes (Table 2) and showed that the introduction of hydroxyethyl group increases the hydrophilicity of imidazoline, while aminoethylene increases the hydrophobicity. The effectiveness of a corrosion inhibitor is directly related to the combination of its hydrophobic and hydrophilic properties. Another important factor affecting the properties of corrosion inhibitors is the operating conditions, particularly the flow rate. Thus, at low speeds (0.3-0.6 m/s), alkylimidazolines with side substitutes –C₂H₄NH₂ and –C₂H₄NHС₂H₄NH₂ showed the highest performance, whereas at higher speeds (5.5 m/s), alkylimidazoline without side substitute R¹ had the highest efficiency.

In the research [41] a synergistic effect was found – when oleic imidazoline and mercaptoethanol were used in a 3:1 ratio, the efficiency reached 96.56 %.

In early researches it was shown that olein-substituted imidazoline is an effective base for corrosion inhibitors, and in order to improve the anticorrosion properties, the authors [39] introduced a sulfhydryl substitute into olein-substituted imidazoline (Table 2), which led to an increase in inhibition efficiency up to 95.58 %. This difference is due to the presence of an additional active adsorption center.

An inhibitor of carbon dioxide corrosion based on imidazoline derived from polyamine (triethanoltetramine, polyethylene polyamine) with carboxylic acid (oleic acid, stearic acid, acetic acid) was proposed in the research [42]. An important feature of this inhibitor is the addition of copper-containing carbon nanostructures to the obtained imidazoline, which makes it possible to reduce the working concentration of the corrosion inhibitor from 40 to 15 mg/l without loss of efficiency.

In studying the effectiveness of imidazoline-containing inhibitor of uniform and localized corrosion in the medium CO₂ [43] two main factors affecting the inhibition efficiency, i.e., inhibitor adsorption/film formation and the presence of corrosion products, were identified. The presence of corrosion products had a significant effect on both the adsorption of the inhibitor and the depth of the corrosion pit. This effect was most pronounced in the formation of a layer consisting predominantly of FeCO₃, while the corrosion inhibitor provided only ~25 % of the total protection.

Imidazolinthioureidooleic acid is widely used in the oil and gas industry as the basis of an effective carbon dioxide corrosion inhibitor for carbon steel, but due to the increasing depth of oil fields, improvement of its anticorrosion characteristics is required. One way of improvement is the modification of imidazolinathioureidooleic acid. For example, in [40] a modified imidazolinthioureidooleic acid (Table 2) was obtained by reaction with formaldehyde and propargyl alcohol c, which showed improved anticorrosion properties.

It was found that due to the additional adsorption center and higher hydrophobicity, the modified imidazoline forms a more stable and effective adsorption film, increasing the efficiency of the corrosion inhibitor based on it. In the research [44] branched tetraimidazoline derivatives that contain four adsorption centers were synthesized (Fig.4).

It was found that the efficiency of the corrosion inhibitor based on the synthesized compound increases with increasing concentration and temperature and reaches 98.29 %. Scanning electron microscopy data confirm the formation of dense dendrimer-like protective films that ensure the absence of contact between the aggressive medium and the metal surface.

The pyridine Schiff base derivatives, namely 3-pyridinecarboxaldehyde-4-phenylthiosemicarbazide (3-PCPHC) and 4-pyridinecarboxaldehyde-4-phenylthiosemicarbazide (4-PCPHC) (Fig.5), showed efficacy as carbonic acid corrosion inhibitors [45]. At the same concentration, the inhibition efficiency of 3-PCPHC is higher than that of 4-PCPHC, increasing sharply with increasing concentration and remaining constant at different temperatures. According to the results of molecular dynamic simulations, both protonated and non-protonated 3-PCPHC and 4-PCPHC adsorb on Fe (110) almost parallel to the surface, forming an adsorption layer.

Pyrimidine-based compounds can exhibit high corrosion resistance activity. Thus, in [46] 5-(4-hydroxy-3-methoxyphenyl)-2,7-dithioxo-2,3,5,6,7,8-hexahydropyrimido[4,5-d]pyrimidin-4(1H)-one was synthesized (Fig.6) and used as a carbon dioxide corrosion inhibitor.

It was found that the high efficiency (up to 90 % at a concentration of 20 ppm) was mainly due to two factors, namely, the large number of adsorption centers (N, S and O, as well as π-electrons in the aromatic ring) and the selectivity of adsorption on the Fe (110) surface, which significantly reduced the local corrosion rate on the steel surface during sweet corrosion. This is consistent with earlier studies showing that the initiation of localized corrosion occurs in the sequence of iron crystal faces (110) > (100) > (111) given that the propagation of uniform corrosion occurs in the following order of iron crystal faces: (100) > (110) > (111) [47].

Polymer-containing compounds

Polymeric corrosion inhibitors have a number of advantages over inhibitors with small organic molecules – easy formation of multilayer adsorption films on the metal surface and a greater number of molecule sites involved in adsorption and higher temperature resistance. According to a review [48], most polymer-containing corrosion inhibitors are polyamines modified with carboxylate and heterocyclic moieties, thiols, disulfides and phosphorus-containing functional groups.

Structure of polymers used as corrosion inhibitors:

• poly(maleic acid-co-N-[3-(dimethylamino)propyl]-methacrylamide [49].

• poly(urethane-semicarbazides) containing thiadiazoles,

In the research [50] poly(maleic acid-co-N-[3-(dimethylamino)propyl]-methacrylamide) synthesized by polymerization in aqueous solution was used as the base of the corrosion inhibitor. The results showed that the polymeric corrosion inhibitor exhibited high corrosion inhibition efficiency (90.1 % at a dosage of 200 mg/l) and acted as an anodic type inhibitor by forming an adsorptive polymeric film on the metal surface.

Polysemicarbazides have good thermal and mechanical properties due to hydrogen bonding. Poly(urethane-semicarbides) containing a chain of 1,3,4-thiadiazoles were synthesized. It was found that corrosion inhibitors based on them show high efficiency, forming a strong adsorbed film. The polymer containing aromatic rings showed more activity, which is probably due to its higher stiffness and stability [51].

In [49] polyacrylamide, poly(2-methoxyaniline) and copolymer of polyacrylamide and poly(2-methoxyaniline) were synthesized and investigated as corrosion inhibitors polyacrylamide, poly(2-methoxyaniline) and copolymer of polyacrylamide and poly(2-methoxyaniline) were synthesized and investigated as corrosion inhibitors. Among all the polymers studied, poly(2-methoxyaniline) showed the highest anticorrosion efficiency, which was 80 % versus 63-74 %. The higher adsorption capacity may be due to the presence of more donor groups, which improves the coordination of the polymer to the metal surface.

The authors [52] compared the effectiveness of polypropylene glycol and polymethacrylic acid based corrosion inhibitors in the concentration range of 50-1000 ppm. The polymers were found to be mixed type inhibitors and the effectiveness of polypropylene glycol was higher, with corrosion activity increasing with increasing inhibitor concentration and decreasing with increasing temperature.

In recent years, inorganic porous materials and polymers have been widely studied for encapsulation of active substances, i.e., under certain environmental conditions, the capsules are destroyed with the release of a corrosion inhibitor [53]. For example, in work [54] the polymeric material was combined with classical organic corrosion inhibitors (Fig.7).

The polymer can be used as a shell for a capsule covering the corrosion inhibitor concentrate (Fig.7, а). When exposed to the medium, slow uniform diffusion of the corrosion inhibitor through the organic polymer membrane of the capsule occurs. Additional components such as weighting agent and capsule wall modifier can be used to provide the required density, resistance to external influences and reagent diffusion rate through the membrane. Polymer cross-linked surfactants are polymer corrosion inhibitor molecules that have specific sites (spacers) that form an additional stable bond between them, reinforcing the film (Fig.7, b).

These approaches allow, on the one hand, to reduce the concentration of corrosion inhibitor without loss of efficiency, and on the other hand, to achieve a high aftereffect due to the creation of a stable film.

“Green” corrosion inhibitors

Another class of compounds actively used as environmentally safe corrosion inhibitors are substances of plant origin, which are promising due to biodegradability, availability and non-toxicity. The use of plant extracts can achieve inhibition efficiencies in excess of 80 %, making them attractive targets for research. The use of plant extracts as a crude corrosion inhibitor has been widely reported in the literature [55-57] as an alternative to classical corrosion inhibitors. The anticorrosive activity of many of the studied extracts may be related to the presence of heterocyclic components such as alkaloids, flavonoids, etc., which contribute to the formation of adsorption layer. A review of the literature on “green” corrosion inhibitors shows that the main criterion for inhibitor selection is the presence of heteroatoms (N, O, P, and S) in the composition, since hydroxyl, carboxyl and amino groups are mainly responsible for chelation and adsorption efficiency.

For example, alcoholic extracts of plants Lycium shawii, Teucrium oliverianum, Ochradenus baccatus, Anvillea garcinii, Cassia italica, Artemisia sieberi, Carthamus tinctorius and Tripleurospermum auriculatum were used as corrosion inhibitors. It is confirmed that these plant extracts exhibit high efficacy, which is 62-91 %. The extract of water hyacinth, a type of water weed that is often a problem for hydroelectric power plants, was also used, its effectiveness increases with increasing concentration, the optimum value being 50 ppm.

Biopolymers, especially water-soluble ones, are effective corrosion inhibitors in various aqueous environments [58]. Due to their massive functional groups, biopolymers are able to form complexes with a large area of the metal surface, providing a high degree of protection. The effectiveness of biopolymers used as corrosion inhibitors varies depending on their molecular weight, aromaticity, and the presence of groups forming bonds and adsorption centers.

Biopolymers such as lignin [59], inulin [60], cellulose [61], starch, pectin, chitosan [62], and their mixtures are considered as promising materials for “green” corrosion inhibitors for various environments.

Natural polymers can act as additives to traditionally used nitrogen-containing corrosion inhibitors, resulting in improved efficacy through synergistic effects [63].

Natural polysaccharides can be modified with different groups to obtain highly effective corrosion inhibitors. For example, two carboxymethyl chitosan derivatives were synthesized and used in [64], which exhibited maximum efficiency (87.97 and 93.95 %) at a concentration of 100 mg/l.

When considering plant extracts and biopolymers as corrosion inhibitors, it is necessary to take into account points related to the preparation of raw materials (drying, dehydration), to the conditions and reagents for extraction, as well as to the utilization of solvents, which are often highly acidic and alkaline media. Factors such as solvent to solid ratio, solvent polarity, extraction time and temperature can significantly affect the chemical composition and physical properties of the samples and, consequently, the efficacy of the corrosion inhibitors. In addition, development of mathematical models including kinetic and mechanistic studies is required in predicting the corrosion inhibitory effect from plant extracts to increase their efficiency.

Conclusion

Modern adsorption-type corrosion inhibitors are usually a solution of one or more organic compounds with high inhibitory properties (active base) in a hydrocarbon or water-alcohol solvent. All commercially available corrosion inhibitors have an optimal area of application depending on the industry segment, composition of corrosive media and technological features of the protected objects.

The efficiency of corrosion inhibitor action is mainly determined by its adsorption properties, which depend on physical and chemical properties, functional groups, aromaticity, steric effect, electron density on donor atoms. Metal surface composition, microstructure and temperature also affect the adsorption and hence the effectiveness of the inhibitor.

Organic compounds containing heteroatoms (O, P, N, S) are currently the most studied as corrosion inhibitors to effectively suppress CO₂ corrosion. The most widely spread in Russia and in the world are corrosion inhibitors, where alkylimidazolines and other nitrogen-containing compounds act as an active base, which show high anticorrosion properties in the range of conditions. The effectiveness of commercial inhibitors from different manufacturers varies between 90-92 %. It is shown that the anticorrosion properties can be significantly improved by selecting substitutes of nitrogen-containing compounds by varying their nature, chain length, structure, etc. This approach allows not only to increase the protection efficiency up to 98-99 %, but also to reduce the corrosion inhibitor concentration up to 10 ppm. In addition, one of the effective approaches to increase the anticorrosion properties of the bases used is to increase the number of adsorption centers mainly by introducing heteroatoms and aromatic structures. Corrosion inhibitors based on sulfur-containing compounds are used less frequently, but can also exhibit high corrosion protection properties, e.g. the use of rhodanine and its derivatives can achieve protection efficiencies of up to 99 % at very low concentrations (0.1-2.5 mg/l). Phosphorus-containing corrosion inhibitors show comparable anticorrosion properties at higher concentrations (up to 60 mg/g).

One of the actual directions of research in the field of corrosion inhibitors is the use of polymeric materials, which have advantages over inhibitors with small organic molecules, which include easier formation of multilayer adsorption films on the metal surface, a greater number of adsorption sites and higher temperature resistance. The polymer-containing components are mostly polyamines modified with carboxylate and heterocyclic moieties, thiols, disulfides and phosphorus-containing functional groups and are used as an active base, exhibiting efficacy above 90 % at dosages up to 200 mg/l. Also polymeric materials can be used in combination with classical organic compounds, where they act as a shell for more uniform diffusion of the corrosion inhibitor, which allows to increase the efficiency of the anticorrosion process.

In recent years, substances of plant origin, which are promising due to biodegradability, availability and nontoxicity, have been actively investigated for use as environmentally safe corrosion inhibitors. The possibility of using biopolymers and polysaccharides, such as lignin, cellulose, starch, pectin, chitosan, etc., as well as their mixtures, the use of which allows achieving effective protection above 93 %, is considered. This class of compounds is used as an additive to traditionally used nitrogen-containing corrosion inhibitors, which increases the effectiveness due to synergistic effect, and as a main component, due to massive functional groups, which, adsorbed on the larger surface of the metal, provide a high degree of protection. Plant extracts also exhibit high anti-corrosion activity (above 91 %), which is attributed to the presence of heterocyclic components such as alkaloids, flavonoids, etc., which contribute to the formation of a strong adsorption layer.

Two main approaches have been proposed to improve corrosion protection in the petroleum industry. The first one is based on modification of currently used corrosion inhibitors based on nitrogen-containing compounds, which has advantages as this class is well studied in a wide range of conditions and has been successfully applied in the fields for many years. At the same time, “classical” corrosion inhibitors can be supplemented with new polymeric, biological, and nanoscale additives. Another approach is aimed at using alternative corrosion inhibitors, including those based on plant extracts and polymers, which requires more directed research and testing in real conditions, but is attractive from the point of view of the environmental performance of the applied objects.

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