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9Chemical Journal of Kazakhstan
ISSN 1813-1107, elSSN 2710-1185 https://doi.org/10.51580/2021-1/2710-1185.32
Volume 2, Number 74 (2021), 104 - 112
UDC 620.197.3
INVESTIGATION OF VARIOUS PHOSPHATE CORROSION INHIBITORS IN CARBON DIOXIDE
A.B. Niyazbekova, T.A. Shakirov, G.N. Urinbaeva
Zhangir Khan West-Kazakhstan Agrarian-Technical University, Uralsk, Kazakhstan E-mail: abnyazbekova@mail.ru
Abstract: Corrosion leads to huge losses every year, and solving this problem is an important task. One of the effective methods of corrosion protection of equipment and pipelines in the oil industry is the use of corrosion inhibitors.Inhibitory protection is the most technological and effective way to control corrosion of oilfield equipment.The article deals with phosphate corrosion inhibitors of complex action in a carbon dioxide environment.The method of corrosion testing is generally accepted. Quantitative indicators of corrosion processes were calculated using formulas, and the measurement uncertainty was estimated using an algorithm using the Student's coefficient with a confidence probability of 0.95. In the course of the work, potentiometric determination of the pH of corrosive media using a combined glass electrode and an ionomerand photocolorimetric determination of the content of iron (III) with potassium rhodanide, as well as methods of infrared spectroscopy and electron microscopy were carried out.
Keywords: corrosion of metals, corrosion inhibitors, carbon dioxide environment, oil and gas fields.
1. Introduction
During oil production and transportation, corrosion causes great damage to oilfield equipment and pipelines.One of the most common causes of premature failure of oil-producing equipment is carbon dioxide corrosion [1].
Carbon dioxide corrosion occurs when the metal surface interacts with a carbon acid (H2CO3) formed by dissolving CO2 in water according to the following total reaction:
CO2 (gas) + H2O (liquid) ~ H2CO3 (liquid) (1)
Therefore, the presence of CO2 and water in oil-producing media is a necessary condition for the occurrence of carbonate corrosion. The main corrosion process is described by cathodic (2-4) and anodic (5-6) reactions [2]:
Citation: Niyazbekova A.B., Shakirov T.A., Urinbaeva G.N. Investigation of various phosphate corrosion inhibitors in carbon dioxide. Chem. J. Kaz., 2021, 2(74), 104-112. DOI: https://doi.org/10.51580/2021-1/2710-1185.32
2H2CO3 + 2e- ^ H2 + 2HCO3- (2)
2HCO3 + 2e- ^ H2 + 2CO32- (3)
2H+ + 2e- ^ H2 (4)
Fe ^ Fe2+ + 2e (5)
Fe + H2CO3 ~ FeCO3 j + H2 (r) | (6)
As a result of these reactions, deposits of corrosion products - iron carbonate FeCO3-are formed on the surface of the corroding steel.Hydrogen depolarization takes place in a carbon dioxide environment.
In the presence of gaseous oxygen in the solution and the impossibility of the corrosion process with hydrogen depolarization, the main role of the depolarizer is played by oxygen. Corrosion processes in which cathodic depolarization is carried out by oxygen dissolved in the electrolyte are called metal corrosion processes with oxygen depolarization. This is the most common type of metal corrosion in an aqueous medium, in neutral and even weakly acidic salt solutions, in sea water, in the ground, in the air. The general scheme of oxygen depolarization is reduced to the reduction of molecular oxygen to a hydroxide ion, and iron molecules are oxidized to a two- and trivalent state, which then interacts with ions of an aggressive medium:
At the anode: Fe - 2e- ^ Fe2+ (oxidation) Fe - 3e- ^ Fe3+ (oxidation) At the cathode: 2H2O + O2 + 2e ^ 4OH- (reduction)
This leads to the formation of deposits on the surface of the steel, which will influence the development of corrosion processes.
One of the most effective methods of anticorrosive protection of field equipment and pipelines in the oil and gas industry is the use of corrosion inhibitors.
Corrosion inhibitors are the most technologically advanced and effective way to combat corrosion of oilfield equipment [3]. In this regard, they have been widely used in the oil industry. By changing the dosage of the inhibitor or using inhibitors with various anticorrosion properties, it is possible to achieve a reduction in the corrosion rate to an acceptable level without a fundamental change in the existing technological schemes [4].
2. Research methodology
The aim of the work was to study the effect of phosphate corrosion inhibitors in a carbon dioxide environment.
The following inhibitors were taken: sodium dihydrogen phosphate NaH2PO4, sodium hydrophosphate Na2HPO4, sodium phosphate Na3PO4, sodium dihydrodiphosphate Na2H2P2O7 and sodium diphosphate Na4P2O7. Carbon dioxide was used as the medium.
The method of corrosion tests was generally accepted . Rectangular steel plates with a size of 30^20x3 mm were used. The duration of the experiments is 24-480 hours. The corrosion rate was estimated by the mass loss of the samples through 24, 48, 72, 96, 120, 240 hours.
Quantitative indicators of corrosion processes were calculated using formulas, the uncertainty of measurements was estimated using an algorithm using the Student's coefficient with a confidence probability of 0.95.
In the course of the work, electrochemical studies were carried out using well-known methods. The identification of corrosion deposits in the systems under consideration was carried out using infrared spectroscopy and electron microscopy.
Table 1 - Results of corrosion tests in model systems
Inhibitor Е.мВ Corrosion rate. mg /m2-h Depthindicator. •10-3mm/year
NaH2PO4 94±2.14 с a = 2.52 Е = 3,7 % 0.95±0.03 1.06
Na2HPO4 114±2.09 с a = 2.52 Е = 3,45 % 1.13±0.7 1.26
Na2H2P2O7 105±1.52 с a = 2.52 Е = 3.55 % 1.06±0.8 1.18
Na3PO4 78±1.87 с a = 2.52 Е = 3.25 % 0.27±0.08 0.30
Na4P2O7 38±1.36 с a = 2.52 Е = 4.1 % 0.09±0.05 0.10
Without inhibitor 102±1.25 с a = 2.52 Е = 3.6 % 0.27±0.35 0.30
The curve of the anode current density of corrosion shows the oxidation of the metal with its further destruction and the transition of iron ions from the plate to the solution. The cathode curve shows the reduction of oxygen in the solution.
0.04
1 2 3 4 5 6 7 8
i. A/m2
Anodic density of metal corrosion L A ! m2 Cathode density of metal corrosion i. A ! m2
Figure 1 - Polarizing diagram for the systemwith Na4P2O7 as an inhibitor
The curve of the anode current density of corrosion shows the oxidation of the metal with its further destruction and the transition of iron ions from the plate to the solution. The cathode curve shows the reduction of oxygen in the solution. In this case, oxygen is an oxidizing component of an aggressive corrosive environment. The presence of oxygen in the solution is due to the presence of partial pressure and the contact of the electrolyte with the environment. A sharp jump in the cathode and anode curves with their further intersection indicates a slow down in the transition of the metal to the solution and its further oxidation, as well as a slow down in the corrosion process.
The rate of corrosion processes with oxygen and hydrogen depolarization is affected by pH. Changing the values of the hydrogen index causes a decrease in the rate of hydrogen depolarization. An increase in the concentration of OH-ions also reduces the rate of oxygen depolarization. Both in this and in the other case, the supply of iron ions from the anode sites decreases, thus, the dissolution of the metal slows down.
The inhibitory effect of phosphates in an acidic environment in the presence of dissolved oxygen plays the role of a passivator, which promotes the adsorption of oxygen on the metal surface and converts it to a passive state. The anodic process of metal dissolution slows down due to the formation of hard-to-dissolve protective films of various types.
Phosphate inhibitors, undergoing hydrolytic degradation, causing the reduction of iron and the transition to a solution in the form of ions. The speed of the cathodic process increases. With further oxidation of iron, chemisorption processes take place on the surface of the metal plate, which causes a decrease in the rate of the cathode process. In the future, the inhibitor enters into a chemical process with chemisorption iron ions to form a stable protective layer on the surface.
According to the results of the tests, the EMF of the Na4P2O7 E < 80 inhibitor was determined for the system, these values correspond to category A (Table 2).
Table 2 - Categories of inhibitors and their corresponding EMF values
Category Inhibitor E, mB
A Na4P2O7 38±1.36 c a = 2.52 E = 4.1 %
Category A - the use of inhibitors in model solutions of produced water is permissible, the probability of corrosion and metal destruction is lowest, EMF values up to 80 mV [5].
The limits of the thermodynamic possibility of an electrochemical reaction and the composition of corrosion products are presented in the diagram of the state of metal-water systems (Purbe diagram).
The work on electrochemical research shows the active and passive level of dissolution of the metal, and also leads to the formation of a protective film in accordance with the EMF, the concentration of the metal and the pH value of the medium.The results of these studies arepresented in Figure 2.
lis 0.6 0.4 0.2 Е 0 -02 -0.4
-0.6 -0.5
"" - - Д 1 г - -
\ - - —■ -_ _
п N. Ш IV
\
1 ~ - -
a __ __ E
e I С ^ ""
6.45
6.4 6.35 6.3
. 6.25 . 6.2 - 6.15 . 6.1
6.05 6.0 5.95 5.9
й 10 12 14
рН
Figure 2 - Purbe diagram for a system with Na4P2O7 as an inhibitor: I - region of thermodynamic stability; II, IV - region of corrosion; III - region of passivity.
As shown in Figure 2, four main areas have been identified by the experimental value of the EMF, the change in concentration, and the change in the medium pH.
The first region is the region of thermodynamic stability, the second and fourth is the region of melting, the third is the passive region. When the pH value is 1-7, thermodynamic stability occurs at a negative EMF value. In the second area, it undergoes hydrolysis in accordance with an increase in the EMF value, i.e., it dissolves and, accordingly, turns into an active area.
3. Results and discussion
Based on the further course of the process, the pH value increases, the process shifts towards a slightly acidic environment. There is a passive process going on here. As a result of the research work, corrosive products were collected and studied by physicochemical methods (IKS, SEM).The spectra of the product showed that compared to the spectra of the starting materials, the height, distance of the main characteristic peaks of the starting materials changed, peaks corresponding to some fluctuations. From here, in conclusion, it can be said that a complex protective layer was formed on the surface of the iron plate.
Research work is well consistent with thermodynamic parameters and values of electromotive forces.The lower the Gibbs energy in the system, the higher the rate of the corrosion process.
In the course of the study, the thermodynamic parameters of the inhibition process (AG°, AH°, AS°) were calculated. Along with this, the stability constants of the formed corrosion deposits were calculated. All these indicators are reflected in table 2 for a model sodium phosphate solution as an inhibitor, which showed
the highest efficiency. A high negative Gibbs energy value indicates a high inhibitory effect of the system [6, 7].
Table 3 - Basic thermodynamic characteristics of linear phosphate systems
Inhibitor Instability constant AG" kJ/mol AH°-10-4kJ/mol AS° kJ/mol
Na4P2O7 Ki = 1.910-21 -354.05±4.25 -2.54±3.48 556.43±4.89
To study the composition of the protective film, an analysis of corrosion deposits was carried out on a Shimadzu IR Prestige-21 IR spectrometer.
Figure 3 shows an analysis of the IR spectrum of corrosion residues Na4P2O7. An atlas of infrared spectra was used to identify the compounds. Thus, for the compound of sodium diphosphate Na4P2O7, according to the literature, the bands 1270 cm-1 correspond to asymmetric v(P=O), and the band 1092-990 cm-1 -symmetric v(P=O) fluctuations. Bands 884 cm-1 correspond to asymmetric v(P-O-P), and bands 792-595 cm-1 correspond to symmetrical v(P-O-P), oscillations.The figure shows that in the IR spectrum of the corrosion deposit peaks are formed within the range of 960-670 cm-1, as well as 3100-2970 cm-1. Based on the fact that the values of the peaks of the corrosion deposition spectrogram do not coincide with the literature data, the compound formed on the plate surface is not sodium diphosphate. This means that another compound has been formed which forms a protective film.
Figure 3 - Corrosion deposition spectrogram of the system with sodium diphosphate.
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Figure 4 - Spectral scale of elements for corrosion deposition of a system with sodium diphosphate.
The protective film was also analyzed on a raster electron microscope (Figure 4).The analysis was carried out on a modern instrument - a raster electron microscope JOOLJSM - 6490 LV.It was revealed that the corrosive deposits of this system contain oxygen - 25.78%, iron - 21.84%, phosphorus - 18.21%, carbon - 7.34% and sodium - 19.56%.
4. Conclusion
1. Electrochemical studies of a model solution of formation water with a phosphorus-containing inhibitor in an aggressive environment containing carbon dioxide showed that the inhibitory effect of a diphosphate inhibitor is effec-tive.This inhibitor slows down the reaction rate by 100 times, the depth corrosion index of this inhibitor is 0.1060 mm/year, the inhibition coefficient y is 0.095.
2. The resulting protective film prevents the corrosion process.Thermo-dynamically, this system will be stable, as confirmed by the Purbe diagram. The passivity region in which the stable compound is formed corresponds to pH values from 6.82 to 11.2.
3. Analysis of corrosion deposits by IK spectrometry and also using a raster electron microscope proves that there is a change in the initial composition of phosphate, peaks within the range of 960-670 cm-1, as well as 3100-2970 cm-1 are formed in the IR spectrum of corrosion deposition. Proceeding from the fact that the magnitudes of the peaks of the spectrogram of corrosion deposits do not coincide with the literature data, the compound formed on the surface of the plate is not sodium diphosphate.This means that another compound has formed, which forms a protective film.
Funding: The research was carried out with the financial support of the Republic of Kazakhstan for fundamental research on the topic "Development of scientific foundations for obtaining corrosion inhibitors based on modified phosphates».
Acknowledgements: The authors express their gratitude to the reviewers for their recommendations when publishing the article.
Conflict of Interest: In the process of scientific research and solving the tasks set, all legal and ethical standards were observed, including ethical management procedures, which are maintaining high standards of intellectual honesty and preventing fabrication and falsification of scientific data, plagiarism and false co-authorship. Individual participants in collective research are not allowed to use the data and conclusions obtained in the research, without agreement with other participants, as well as for personal purposes.
Information about authors:
Niyazbekova A.B. - Cand. of chemical sciences, Associate Professor; e-mail: abnyazbekova@mail.ru; ORCID ID: https://orcid.org/0000-0001-9388-9715
Shakirov Т.А. - magister of engineering and technology, senior lecturer; shakirov_1985@mail.ru; https://orcid.org/0000-0002-2504-1357
Urinbaeva G.N.- master's student of the Higher School of oil, gas and chemical engineering; e-mail: zhaskairatova97@list.ru; ORCID ID: https://orcid.org/0000-0003-1048-2210
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ИНГИБИТОРЛАРЫН ЗЕРТТЕУ
А.Б. Ниязбекова, Т.А. Шакиров, Г.Н. Уринбаева
«БЦАТУ им. Жэцгiр хан», Орал, Цазацстан E-mail: abnyazbekova@mail.ru
Коррозия жыл сайын Yлкен шыгындарга алып келедi жэне б^л мэселеш шешу мацызды мшдет болып табылады. М^най eндiрiсшдеri жабдьщтар мен к^бырларды
