3020208 Co2 Corrosion

H.M. Abd El-Lateef et al / Chemistry Journal (2012), Vol. 02, Issue 02, pp. 52-63

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Review Article

Corrosion Protection of Steel Pipelines Against CO2 Corrosion-A Review Hany Mohamed Abd El-Lateef 1, 2, Vagif Maharram Abbasov2, Leylufer Imran Aliyeva2 and Teyyub Allahverdi Ismayilov2 1

Mamedaliev Institute of Petrochemical Processes, National Academy of Sciences of Azerbaijan,Baku, Azerbaijan 2 Chemistry Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt *E-Mail: [email protected]

Abstract One of the serious problems of oil extracting industry is the corrosion process. The successful application of carbon steels in oil and gas pipelines and production tubular in Carbon Dioxide (CO2) containing environments depends mainly on either the formation of protective corrosion product film or the use of corrosion inhibitors. The mechanism of corrosion of carbon steel in media containing CO2 is complex, and in dependence on the prevailing conditions it may lead to general or local corrosion and corrosion cracking. The inhibition mechanism is attributed to the strong adsorption ability of the selected inhibitors on steel surface, forming a good protective layer, which isolates the surface from the aggressive environment. The current state of research in corrosion protection of steel pipelines against CO2 corrosion is surveyed. The review covers CO2 corrosion and its inhibition. The influence of inhibitors molecular structure on corrosion layers in CO2 corrosion is discussed. Keywords: CO2 Corrosion, Inhibition, Pipelines, Steel Inhibitors, Carbon Dioxide

1. Introduction Pipelines play an extremely important role through the world as a means of transporting gases and liquids over long distances from their sources to ultimate consumers. So that corrosion problems exist in the oil industry at every stage of production from initial extraction to refining and storage prior to use requiring the application of corrosion inhibitors (Migahed, 2005). Oilfield corrosion manifests itself in several forms, among which CO2 corrosion (sweet corrosion) and hydrogen sulfide (H2S) corrosion (sour corrosion) in the produced fluids and corrosion by oxygen dissolved in water injection are by far the most prevalent forms of attack found in oil and gas production (Villamizar et al, 2007). Corrosion of carbon steel is a significant problem in the oil & gas production and transportation systems, which causes significant economic loss (Song et al, 2004). As a result of corrosion, rupture of the pipe wall frequently causes failure of petroleum and gas pipelines. The breakdowns are

followed by large losses of the products, environmental pollution and ecological disasters (Mikhailovskii et al, 1997). The majority of oil and gas pipelines failures result from CO2 corrosion of carbon and low alloy steels (Lopez et al, 2003). It occurs at all stages of production from downhole to surface equipment and processing facilities (Fu el al, 1993). The mechanism of carbon dioxide corrosion is a complicated process that is influenced by many factors and conditions (i.e. temperature, pH, partial pressure of CO2, etc.) (Kermani and Morshed, 2003). These range from corrosion pipeline and composition of the solution to other environmental factors (Videm and Dugstad, 1989). However, significant progress has been achieved in understanding the mechanism of CO2 corrosion in the oil and gas industry (Kermani and Morshed, 2003), but an understanding of its inhibition mechanism and the kinetics of the inhibition process necessary to quantify formulations that will provide a desirable level of protection, remains incomplete.

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The use of chemical inhibitors has been acknowledged as one, and a very practical and most economical, method of combating CO2 corrosion (Ramachandran & Jovancicevic, 1999 and Durnie et al, 1999). The inhibiting molecule retards the rate of corrosion by acting at the metal – corrosive medium interface.

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Fe 2 + + CO32 − → FeCO 3 ………………………..(6)

2. The Background

During the process of CO2 corrosion, the corrosion products such as FeCO3 and Fe(HCO3)2 build up over time, partially passivating the corroding steel surface due to their limited solubility. The works of Hausler and Stegmann (1998) and Dugstad (1992) revealed that the corrosion film influenced significantly on the corrosion rate of carbon steel, which may also, influenced the efficacy of the corrosion control practice by altering the chemical nature of the steel surface. Obviously, it is significant to understand the microstructure and the characteristics of the surface film induced from carbon dioxide corrosion under precise conditions in order to achieve better protection of oil tube steels.

2.1. CO2 Corrosion

2.2. Effect of Corrosion Inhibition

CO2 corrosion is one of the most studied forms of corrosion in oil and gas industry. This is generally because of the crude oil and natural gas from the oil reservoir / gas well usually contains some level of CO2. The major concern with CO2 corrosion in oil and gas industry is that CO2 corrosion can cause failure on the equipment especially the main downhole tubing and transmission pipelines and thus can disrupt the oil/gas production.

The two most common sources of corrosion inhibition need to be considered:

The following topics will be covered in the text below:

• • •

CO2 Corrosion Effect of Corrosion Inhibition Inhibition of Carbon-Dioxide Corrosion

CO2 corrosion was reported as early as 1940 (Ikeda et al, 1983). The enhanced corrosion rate of oil tube steels under conditions in which carbon dioxide is present has been a serious problem in the oil industry for many years. Corrosion under these conditions occurs generally in oxygen-free environment. The type of CO2 corrosion varies according to the precise environmental conditions (Crolet, 1994). In general, the reactions involved in the CO2 corrosion of carbon steel in typical anaerobic aqueous solution are (De Waard et al, 1975 and Dayalan et al, 1995):

Fe 2 + + CO 32 − → FeCO 3 ………………......(1) H 2CO3 + 2e − ↔ H + + 2 HCO3− ……………….(2)

2 HCO3− + 2e − ↔ 2CO2− + H + ………………...(3) Fe → Fe

2+

−

+ 2e …………………………….…..(4)

……………...…(5a) 2HCO3− + Fe2+ → Fe( HCO3 ) 2 →

Fe( HCO3 )2 → FeCO3 + CO2 + H 2O ………....(5b)

a)

Inhibition by components present in the crude oil

b)

Inhibition by addition of corrosion inhibitors

2.2.1. Corrosion Inhibition by Crude Oil It has been known for a while that CO2 corrosion rates seen in the field, in the presence of crude oil are much lower than those obtained in laboratory conditions where crude oil was not used or synthetic crude oil was used. One can identify two main effects of crude oil on the CO2 corrosion rate. The first is a wettability effect and relates to a hydrodynamic condition where crude oil entrains the water and prevents it from wetting the steel surface (continuously or intermittently). Cai et al (2003) discussed this effect and its modeling in detail. The second effect is corrosion inhibition by certain components of the crude oil, which reach the steel surface either by direct contact or by first partitioning into the water phase. Little is known about the nature and the effectiveness of these components, which inhibit corrosion. In a recent detailed study on the subject, Hernandez et al (2003) declared that the degree of inhibition was quantitatively related to the chemical composition of the crude oil and specifically to the concentration of saturates, aromatics, resins, asphaltenes, nitrogen and sulphur (Nesˇic, 2007). Research studies performed with different Venezuelan crude oils, have shown the capability of some oils to inhibit significantly the uniform corrosion rates as well as the hydrogen permeation and sulfide stress cracking susceptibility in the presence of CO2 and /or H2S. For example, a reduction of over an order of magnitude in corrosion rate was found with the addition of only 1% of crude oil, in laboratory experiments at 72 psi CO2 and


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80°C. It was determined that protective corrosion properties of the evaluated crude oils were related to the adsorption of organic compounds, which modify the morphology, composition, and compaction grade of corrosion products. In addition, it was noticed that the presence of some sulfur in chemical analyses performed on the corroded coupons evaluated at those conditions (Mendez et al, 2001). The inhibiting effect of several crude oils on corrosion was studied at different crude oil /water ratios. A statistical analysis showed an important difference between paraffinic and asphaltenic crude oils from the mechanistic point of view, with new findings on the variables that mostly influence the corrosion inhibiting mechanism (Hernandez et al, 2002). 2.2.2. Corrosion Inhibitors Most corrosion inhibitors used in oilfields are organic compounds, containing nitrogen or sulfur functionalities. They belong to the surfactant category of molecules (surface-active agents), which preferentially adsorb onto any surface or interface in a system and alter the surface and interfacial free energies, even at low concentration (Hernandez et al, 2003). The surface-active properties come from their amphipathic, lipid like, molecular structure, which contains a polar head group having strong attraction to water, referred to as a hydrophilic head and a non-polar hydrocarbon chain having little attraction to water, called a hydrophobic tail. The way organic corrosion inhibitors inhibiting CO2 corrosion of carbon steel is related to their surface active properties can be described in three parts i.e. 1) adsorption onto the steel surface (diffusion or protective layer), 2) changing the wettability of the steel surface (so it is not wetted with water), 3) accumulation at the oil-water interfaces (changing the oil-water interfacial tension and making it easier for the oil to entrain the water). It is accepted that organic corrosion inhibitors adsorb onto the steel surface to inhibit corrosion processes. The types of adsorption can be distinguished by the mechanisms: physical (electrostatic) adsorption involves an electrostatic attractive force between ionic charges on inhibitor molecules and electric charged steel surface; chemisorption results from sharing free electron pairs or charge transfer to form strong chemical bonds between nonionic inhibitor molecules and steel (Papavinasam et al, 2007). The examples of ionic type and nonionic type inhibitors are quaternary ammonium chloride, RN (CH3)3+Cl- (Rosen, 2004) and imidazoline, R1N(CH2)2NR2 (Gusmano et al, 2006). Independent on the adsorption mechanisms, the attractions

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or chemical bonds are between the hydrophilic head of inhibitor molecule and the steel surface. In a layer of adsorbed inhibitor molecules, the hydrophobic tail on the inhibitor molecules face toward the bulk solution and have a strong tendency for self-assembly to form a hydrophobic barrier for corrosive water (Ramachandran et al, 1996). It is believed that by forming the hydrophobic barrier, surfactants can alter the surface wettability of carbon steel. The wettability of a steel surface in oil-water two-phase system, involves the interaction between oil-water, oilsteel and water-steel interfacial tension. Since liquid-steel interfacial tension is not easy to measure directly, the contact angle of a liquid droplet on steel surface has been used to indicate surface wettability (Rosen, 2004). The force balance of three interfacial tensions acting on the droplet is shown in Equation 7, called Young’s equation. An angle of 90° or greater indicates that the surface is hydrophobic and an angle less than 90° indicates that the surface is hydrophilic (Dobbs, 1999).

σ oil − water cos α = σ water − steel − σ oil − steel

…………(7)

Schmitt et al, (1998) and Li et al, (2008) conducted contact angle measurements by placing oil and water droplets on carbon steel specimens in a high pressure test apparatus. The tests were performed on temperature between 75 and 80 °C and under a pressure of 5 bar CO2. The testing fluids are different crude oils, a synthetic oil and brine with surfactants (inhibitors and deemulsifiers) added into the system. The contact angle measurements were made on the clean surface and pre-corroded (6, 24, 48 and 72 hours) surface. It was found that clean carbon steel surface and pre-corroded surface covered with FeCO3 scale both show hydrophilic wetting property. The addition of quaternary ammonium inhibitor under FeCO3 scale formation conditions, results in a hydrophilic surface, however fatty amine and imidazoline-based inhibitors produce a hydrophobic surface. Foss et al (2008) investigated the effect of corrosion inhibitors on the wettability of the steel surface covered by FeCO3 protective scale. The wettability tests were conducted with contact angle measurements in refined oilbrine system. The authors found that oleic imidazoline and phosphate ester promote oil wetting for clean steel surface and FeCO3 covered steel surface. The initially oil wetted steel surface becomes water wetted with the addition of cetyltrimethylammonium bromide (ionic surfactant). The corrosion tests on a stationary electrode in a 3-liters glass cell showed that the inhibition performances of oleic imidazoline and phosphate ester were greatly enhanced after a direct contact of the test electrode to the oil phase. Without a direct contact (i.e. the test electrode is wetted by the brine phase), the presence of an oil phase on the top of brine phase can also improve the inhibition performances of two inhibitors. The authors assumed that the structure of


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adsorbed inhibitor molecules layer was altered by the dissolved hydrocarbon in brine (micro-liter solubility) to have better protectiveness. It is a well-known fact that corrosion inhibitors partition between the oil and water phases. The concentrations of inhibitor in oil and water depend on the solubility of specific inhibitor. Some inhibitor molecules are also present at the oil-water interface, with their hydrophilic heads facing to water and hydrophobic tails orienting to oil. This behavior minimizes the contact area of oil-water two-phase, which results in a reduction of oil-water interfacial tension. The oil-water interfacial tension has a significant effect on the water entrainment in oil-water two-phase flow. With increasing surfactant concentration in a media, at one point, called Critical Micelle Concentration (CMC), colloidal-sized clusters (micelles) of surfactant molecules form (Carlota et al, 2005). A reduction break point of oil-water interfacial tension up to the CMC can be found for some corrosion inhibitors. Micelle formation of corrosion inhibitors in oil-water twophase flow promotes oil-in-water or water-in-oil emulsion, which is the stable dispersion of one liquid phase in another immiscible liquid phase. Moon et al (2002) measured the changes of a model oilbrine interfacial tension with five different corrosion inhibitors with a tensiometer. A great reduction of oilwater interfacial tension was found for even a low concentration of inhibitor. Above the CMC point the interfacial tension becomes stable and full dispersion begins to form. The emulsion stability tests were performed by mixing and separating oil and water in sampling tubes. It was found that inhibitors enhance stability of oil-water emulsions due to a lower interfacial tension. A dramatic increase of oil-water interface area during emulsion formation can trap appreciable amounts of inhibitor molecules. The authors claimed that loss of inhibitor at the oil-water emulsion interface can cause corrosion inhibition failure. Knag et al (2006) reported similar findings. McMahon (1991) studied adsorption, interfacial phenomenon and wettability by testing sodium alkylethoxyphosphate (water-soluble), oleic imidazoline and oleic amide (oil soluble) corrosion inhibitors. All the tested inhibitors have a significant affinity for the oil-brine interface. The reductions of interfacial tension measured by a Wilhelmy plate exist up to the CMC points. The author conducted adsorption tests by using iron powder. The adsorption of inhibitors from either the oil or the brine phase was proved fast enough to form a molecule monolayer fitting a Langmuir isotherm. The corrosion inhibition was proportional to the measured surface coverage. The contact angle measurements of oil-in-water and water-in-oil indicated that the surface became

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hydrophobic and oil wetting is enhanced after inhibitor adsorption. 2.2.3. The Influence of Inhibitors Molecular Structure on Corrosion Layers in CO2 Corrosion When the environment becomes highly aggressive or the scales formed on the steel are non-protective, corrosion inhibitors are added to the produced fluids in order to reduce corrosion failures. The most widely used inhibitors in the petroleum industry are nitrogen containing compounds such as amines, amides, qua-ternary ammonium salts and specially imidazolines and their derivatives. They usually adsorb to the metallic surface and can act by blocking the active sites or generating a physical barrier to reduce the transport of corrosive species to the metal surface. Despite their extensive use, their mechanism of action is generally unknown (López et al, 2005). Studies conducted by Rosenfeld et al (1982) established that inhibitors are incorporated to the corrosion product layer and form a protective barrier between the base material and the corrosive media. SEM results presented by French et al (1989) showed that the inhibitors use to modify the structure of the corrosion product layer. They suggested that the structure of the inhibitor must be the appropriate one in order to interact with the corrosion products, resulting in a selective behaviour by which they can be effective on iron carbonates or sulfides, but not effective on oxides. Regarding the microstructure of the material, Oblonsky et al (1995) studied the adsorption of octadecyldimethylbenzylammonium chloride (ODBAC) to carbon steel with two different heat treatments. They found that ODBAC physisorbs strongly to the Ferritic–Perlitic (F/P) microstructure and weakly to the martensitic microstructure. They attributed the differences to the persistency of the passive films on the two microstructures, with the more stable passive film on the martensitic steel preventing optimal adsorption of the inhibitor. The molecular structure of commonly used imidazolines and their derivatives have a significant role in their performance as corrosion inhibitors in chloride media containing CO2, although the information available is not conclusive. The molecular structure of imidazolines can be divided in to three different sub-structures: a nitrogencontaining five-membered ring, a long hydrocarbon chain (R1) and a pendant side chain with an active functional group (R2) (Figure 1). The functional groups in R1 and R2 can be variable (Xiuyu et al, 2006). According to Jovancicevic and Ramachandran (1999), the


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length of the hydrocarbon chain R1 has a key role in the inhibitive behaviour of the imidazolines. Other authors, however, consider that this is not a critical factor regarding the inhibitory properties of the compound [Ramachandran et al, 1996 and Cruz et al, 2001).

Figure 1. Scheme of a Typical Imidazoline Molecule

Even though it has great importance in field applications, the interaction between the molecular structure of the inhibitors and the microstructure of carbon steels in their performance is barely considered. Bearing in mind that imidazoline derivatives are widely used as inhibitors in the oil and gas industry it is, however, important to have a better understanding of the morphological and compositional properties of the corrosion layers that are formed during their usage in order to have a better comprehension of their inhibition mechanism. 2.3. Inhibition of Carbon-Dioxide Corrosion CO2 corrosion is a broad term that includes different types of corrosive failures; seen at pH = 2-10, within a broad range of temperatures in one-phase media, two-phase (hydrocarbon-electrolyte) media, and three-phase (hydrocarbon-electrolyte-vapour-gas phase) media; include general corrosion; local corrosion, (in the form of pitting & which takes place both in the absence of carbonateoxide deposits on the surface of the metal and under a layer of such deposits) corrosion-erosion; and a special type of failure – chalky fracture (Akhmadeeva and Zagidullin, 2008). The latter type of failure is a characteristic of CO2-corrosion exclusively. The main factors that influence the rate and intensity of corrosion processes and, ultimately, the type of corrosive failure, which occurs, are the temperature and pH of the medium and the partial pressure of the CO2. Temperature is the

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most important factor. The rate of corrosion of steel below 60 °C in solutions that do not contain O2, is controlled by the kinetics of the evolution of H2, which is in turn determined by the homogeneous and /or heterogeneous formation of H2CO3. Within the temperature range 60-100 °C, corrosion rate is controlled by the stage in which the layers of siderite undergo chemical dissolution and become permeable. Since the solubility of the corrosion, products and the protective properties of the deposits depend on the pH. This means, pH of the media largely determines the rate of corrosion i.e. a decrease in pH leads to an increase in corrosion rate. This relationship, however, is ambiguous because experience shows that the probability of local modes of fracture occurring under a layer of deposits is very high in carbonic-acid media at pH > 7 (Moiseeva et al, 2001). One of the important criteria during the creation of multifunctional inhibitors is their chemical structure e.g. for the creation of a inhibitor, it is important for its molecule to have diphilic structure-containing a hydrophilic functional group and a hydrophobic long chain radical in the molecule (Abdullayeva et al, 2011). In this case, hydrophilic functional group of molecule will influence into the different biochemical processes, running in the bacteria cells, causing their destruction and perishing. Hydrophobic radical, on the other hand, promotes inhibiting effect on the border of metalelectrolyte. The range of methods available for combatting CO2 corrosion is fairly broad. It is important to choose the optimum method, which depends mostly on the corrosion conditions and the mechanism of the corrosion process. One of the most efficient and economical methods of dealing with CO2 corrosion is the use of inhibitors. The corrosion of steel in aqueous media that contain dissolved carbon dioxide is an electrochemical process. As is known (Moiseeva and Rashevskaya, 2001), CO2 acts in two ways – it increases the amount of hydrogen formed on the cathode and forms the carbonate-oxide films on the surface of the metal. We can proceed on this basis to find the most efficient ways of preventing CO2 corrosion. The first approach involves using any available means to make the cathodic process efficiently inhibitive, while the second approach entails the formation of protective carbonate oxide films or deposits. To realize the second approach, it is necessary to regulate the pH of the medium and temperature by physical and chemical methods. The cathodic process can be slowed down effectively by using three types of inhibitors: Inhibitors that can displace depolarizers (HCO3ions and H2CO3 and CO2·H2O molecules) from the surface of the metal.


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Inhibitors, which can form films of the mixed type, those are impermeable to depolarizers.

Inhibitors that can form bonds with depolarizers (HCO3- ions and H2CO3 and CO2 ·H2O molecules). To varying degrees, inhibitors of all three types can also prevent the occurrence of the anodic process. However, inhibition of the cathodic reaction will predominate. In the first case, the most promising reagents will be organic substances that are close in chemical structure to carbonate ions i.e. carboxyl or carbonyl groups. Carbonic acids, ketones, and esters should be effective inhibitors of CO2 (when the dimensions of the molecules and their concentrations in the solution are optimal). Substances that contain these compounds however, are almost never used as inhibitors. The domestic oil and gas industry has made wider use of organic high-molecular weight compounds, which contain nitrogen: amines, amides, and imidazoles, nitrogen-bearing heterocyclic compounds, quaternary ammonium bases and their salts. All of these compounds efficiently inhibit mainly the anodic process (Rozenfel’d, 1977). The oil and gas industry continues its traditional use of nitrogen-bearing inhibitors despite the numerous instances of equipment having failed due to corrosion in media that contain CO2 and have only small or negligible amounts of hydrogen sulfide. These inhibitors efficiently prevent CO2 corrosion only when the concentration of hydrogen sulfide is substantial (Rozenfel’d, 1977). Thus, one of the most effective methods of combatting CO2 corrosion has not been put into practice. One group of inhibitors that is currently in wide use consists of organic substances or compounds that contain two or more heterocyclic atoms. Such atoms create (or help to create) a film on the surface of the metal during adhesion, and the film are impermeable to molecules and ions of depolarizers. It is a known fact that inhibitors, which simultaneously contain nitrogen, sulfur, and phosphorus, as well as nitrogen and oxygen or sulfur and phosphorus, can be introduced into oxide-carbonate films on metal and in the process form mixed films, which are impermeable to depolarizers (Lahogny-Sarc, 1985). Two approaches could be taken in forming protective carbonate-oxide films by regulating the pH and temperature of the medium. In practice, pH is controlled by the method of neutralization- with the use of reagents that shift the pH to the alkaline region (pH = 8-10). This helps to form a protective layer of siderite on the surface of the metal. Regulating the temperature regime by means of chemical reagents is possible theoretically but is difficult

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in practice (Moiseeva and Rashevskaya, 2001). Thus, the type of inhibitor of CO2 corrosion should be chosen based on the pH and temperature, as well as the initial condition of the metal surface (the type of film and /or deposits on the surface of the metal being protected). The addition of corrosion inhibitors is a standard practice in oil and gas production systems to control the internal corrosion of carbon steel structures. Nitrogen based organic inhibitors, such as imidazolines or their salts, have been successfully used in these applications even without an understanding the inhibition mechanism (Jovancicevic, 1999). The corrosion inhibition of organic compounds is related to their adsorption properties. Adsorption depends on the nature and state of the metal surface, on the type of corrosive environment, and on the chemical structure of the inhibitor (Bentiss, 1999). Studies report that the adsorption of the organic inhibitors mainly depends on some physico-chemical properties of the molecule, (related to its functional groups) to the possible steric effects and to the electronic density of donor atoms. Adsorption is also suppose to depend on the possible interaction of the π orbitals of the inhibitor with the d-orbitals of the surface atoms,-induces greater adsorption of the inhibitor molecules onto the surface of metal which leads to the formation of a corrosion protection film (Bentiss et al, 2001). Kuznetsov and Ibatullin (2002) studied the inhibitive effects of aliphatic carboxylic acids on steel corrosion in the liquid and vapour phases of carbonate media. The outcome of their study showed that the acids become more effective with an increase in their hydrophobicity and can inhibit both the cathodic and anodic processes on steel because of their high adsorbabilities. Lauric acid has declared to be the most effective inhibitor of carbon dioxide corrosion among the carboxylic acids studied. Although its molecule includes a relatively long alkyl fragment (C11H23), this acid is rather volatile. Caprylic acid, in a concentration of 3.7 m mol/L, inhibits steel dissolution in the temperature range from 30 to 100 °C and increases the apparent energy of corrosion activation. According to Kuznetsov and Ibatullin (2002), films containing fatty carboxylic acids with C10 to C12, cause the best adhesion to the steel. It remains, however, unclear that what is the role of the chemical structure of a carboxylic acid itself and as an option less hydrophobic i.e. relatively water-soluble, homologs can be used to protect steel from carbon dioxide corrosion. This question becomes increasingly topical since hydrophobicity, which often enhances the protective effect of chemical compounds, can make them environmentally dangerous. For example, hydrophobic substances with log P> 3 tend to accumulate in living organisms and hence their presence in water, even in low concentrations, is ecologically unsafe (Frenier, 2000).


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Demand for environmentally safe inhibitors of CO2 induced steel corrosion stimulated a search for new classes of organic compounds capable of functioning in this way. It turned out that some carboxylic acid anhydrides can both effectively inhibit corrosion and firmly chemisorb at the steel surface from carbonate solutions (Frenier, 2000). According to Schmitt et al (1993), this is due to the formation of a salt like product of an inhibitor with Fe2+ and its inclusion into a protective carbonate film. The inhibition activity of 2-mercaptopyrimidine, 2mercapto-4-methylpyrimidine hydrochloride, 2-mercapto4,6-dimethylpyrimidine and its hydrochloride upon CO2 corrosion of iron has been studied in a wide range of concentrations at 40-90 °C using electrochemical and mass-spectrometry methods. High activity has been found for all compounds (IE % = 80-99%) at very low concentrations (0.02–5 mg/L). Adsorption of inhibitor by metal not only blocks its surface but changes the reaction mechanism as well. Charge transfer is the limiting stage of both cathode and anode reactions (Reznik et al, 2008). Ortega-Sotelo et al (2001) studied CO2 corrosion inhibition of X-70 pipeline steel by carboxyamido by using electrochemical techniques. Good inhibition properties of carboxyamido imidazoline in salt water saturated with CO2, which increased with its concentration, reaching the highest corrosion protection with 8.1×10-5 mol L-1. Lower or higher carboxyamido imidazoline concentrations than 8.1×10-5 mol-1, increases the corrosion rate because the surface area covered by the inhibitor decreases. A mechanism, of charge transfer mechanism has been used, to explain the inhibitor desorption for concentration higher than the most efficient one. Carboxyamido imidazoline was a kind of anodic inhibitor. Farelas and Ramire (2010) studied the CO2 corrosion inhibition, of carbon steels, through Bis-imidazoline and Imidazoline Compounds. They found that 1-(2aminoethyl)-2(heptadec-8-enyl)-bis-imidazoline forms a compact inhibitor layer and therefore the corrosion protection was enhanced. The molecular structure of this inhibitor allows the usage of lower concentration (10 ppm) without loss of efficiency. Okafor et al (2009) studied the corrosion inhibition of mild steel by ethylamino imidazoline derivative in CO2 saturated solution. The findings of their study was that the presence of the inhibitor has greatly increased the corrosion potential (vs. Saturated Calomel Electrode (SCE) to a more positive region and the shifts are dependent on the inhibitor concentration. The large shift in the corrosion potential (vs. SCE) indicates that the inhibition for this system is probably due to the active sites blocking effect (Okafor et al, 2009). Secondly, the anodic

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reaction is slightly accelerated (above 0.5 V) by the presence of the inhibitor. This effect, however, may be due to the large change of the corrosion potential (vs. SCE) by the inhibitor. Lastly, a pronounced effect is exerted on the cathodic process. The limiting current for hydrogen evolution is greatly reduced, indicating that the inhibition is confined to the hindering of the hydrogen reduction reaction. At pH 4 or below, direct reduction of H+ ions i.e. 2H++2e-↔H2 is important particularly at lower partial pressure of CO2 and hindering this reduction process greatly inhibits the rate of the corrosion reaction. In addition, the corrosion current density is reduced to lower values. Moreover, the current density decreased with increasing concentration of the inhibiting molecules indicating an inhibiting effect. The polarization curves indicate (Figure 2) that the corrosion process in the presence of the inhibitor is under cathodic control. It should be noted that the parameters (pH 4, 25 °C and 30 minutes) were chosen to yield conditions where corrosion product film formation is unlikely or very slow. The kinetics of the inhibition of CO2 corrosion on high purity iron electrodes by cetyl trimethyl ammonium bromide, were investigated in order to elucidate the mechanism of inhibition. The inhibition was found to be a combination of two processes. First a rapid process (order of minutes) connected to diffusion limited adsorption of the inhibitor, resulting in the inhibition of the anodic part reaction and a second slower process (order of hours) leading to a reduction in the corrosion rate through the inhibition of the cathodic part reaction (Bílková and Gulbrandsen, 2008). In oil and gas industry, majority of modern corrosion inhibitors use nitrogen-containing compounds. Various amines (primary, secondary and tertiary both aliphatic and heterocyclic) from pyridines and imidazoline class, their salts, amine alcohols and triazines are applied as corrosion inhibitors (Abbsov et al, 2010). These reagents are practically feasible in application, stable and possess high degree of protection in corrosive mediums. However, under modern conditions the further improvement of their operational characteristics, expansion of functionalities and assortment is necessary that requires, in first place, research of perspective kinds of raw materials. Use of organic acids of various structure is expedient, (from oil and oil fractions), the synthetic greasy acids etc. Synthesis of N-derivatives oleic and isomer mono carboxylic acids and their anticorrosive ability in corrosive mediums containing CO2 was studding by Abdullayeva et al (2011). Their results by using Linear Polarization Resistance bubble tests (Figure 3) shows that 10 ppm dose is all highly effective with corrosion rates of less than 0.1 mm/yr. This was achieved in the case of inhibitor AI after 200 minutes and for inhibitor AII decreases the corrosion


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rate till 0.9 mm/yr after 80 minutes. They also found that, these inhibitors decreases the anodic and cathodic reactions, act as an inhibitor of mixed type of the steel electrode (Abbasov et al, 2005).

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3. Conclusions The following conclusions were drawn from this review:

Figure 2. Polarization Curves for N80 Carbon Steel in CO2-Saturated 3% NaCl Solution Containing 2-Undecyl-1-Ethylamino Imidazoline at 25 °C and at pH 4 After 30 Minutes of Immersion (Okafor et al, 2009)

Corrosion inhibitor was obtained on the base of olefins, extracted from light oil fractions. It has been extracted amino alcohols with two hydroxyl groups from olefin fraction at 160-180°C. Water-soluble corrosion inhibitor on the on the basis of active substance has been prepared which composes 5% of dosed inhibitor. Created inhibitor (T-1) has demonstrated high inhibition property and low corrosion rate. Reagent has been studied as corrosion inhibitor with the methods of Linear Polarisation Resistance (Figure 3) and Total Metal Loss. Influence of potassium salts, of nitro derivative of high α-olefins in 1% NaCl solution saturated with CO2 on steel corrosion, was study by Abbsov et al (2010) (Figure 4). Potassium nitronated, based on C8 olefin and propylene tetramer protects from corrosion, showed the following protective efficiency from corrosion. 1.

Potassium nitronated (C12)-98.52%.

2.

Potassium nitronated (C14)-97.78%.

3.

Potassium nitronated (C16-18)-99.74%.

1.

Carbon Dioxide corrosion is one the most studied form of corrosion in oil and gas industry. This is generally because of the crude oil and natural gas, from the oil reservoir/gas well, usually contains some level of CO2.

2.

CO2 acts in two ways - it increases the amount of hydrogen formed on the cathode and it forms carbonate-oxide films on the surface of the metal.

3.

The inhibitor for CO2 corrosion should be chosen on the basis of the pH and temperature, as well as the initial condition of the metal surface (the type of film and/or deposits on the surface of the metal being protected).

4.

Most corrosion inhibitors used in oilfields are organic compounds, containing nitrogen or sulfur functionalities.

5.

The percentage inhibition efficiency (η %) of the inhibitors increases by increasing inhibitor molecule size.


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3.5

3.0

Blank AI AII

Corrosion Corrosion Rate Rate (mm/Year) (mm/Year)

2.5

2.0

1.5

1.0

0.5

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Time (Hour) Figure 3. Aqueous Linear Polarization Resistance Bubble Test in Field Brine, 0.9 Bar CO2, at 50 °C (Abdullayeva et al, 2011)

3.0 Potassium nitronated (C14) Potassium nitronated (C12) Potassium nitronated (C16-18)

2.5

2.0

1.5

1.0

0.5

0.0 0

2

4

6

8

10

12

14

16

18

20

Time (Hour) Figure 4. Effect of Potassium Salts of Nitro Derivative of High α-olefins (50 ppm) on the Kinetics Corrosion of Steel in 1% NaCl Solutions Saturated with CO2 (Abbasov et al, 2010)


60


6.

The results indicated that the inhibitor molecules formed a good protective film on the steel surface.

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[in

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3020208 Co2 Corrosion

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