SYNTHESIS OF CORROSION INHIBITOR BASED ON POLYSULFIDE AND MONOETHANOLAMINE

ABSTRACT

This study focuses on the synthesis and application of a zinc-polysulfide complex (MEA-S_x-ZnS_x) as a corrosion inhibitor. The inhibitor was obtained through the reaction of sodium polysulfide (Na₂S_x), monoethanolamine (MEA), and zinc oxide (ZnO) in an aqueous medium. During the reaction, MEA facilitates the stabilization of polysulfide species, while ZnO hydrolyzes to form Zn(OH)₂, which further reacts with polysulfides to generate a ZnS_x complex. The resulting MEA-S_x-ZnS_x system exhibits excellent corrosion inhibition properties due to the synergistic effects of MEA coordination, ZnS_x precipitation, and polysulfide reactivity. The presence of NaOH as a byproduct also contributes to the pH regulation of the medium, enhancing the inhibitor’s efficiency. Preliminary tests indicate that this novel inhibitor effectively mitigates metal surface degradation, particularly in environments containing hydrogen sulfide (H₂S) and other corrosive agents. This research provides valuable insights into the development of effective corrosion inhibitors for industrial applications. Based on the experimental results, its effectiveness was evaluated using electrochemical methods.

АННОТАЦИЯ

В статье рассмотрен синтез и применению цинк-полисульфидного комплекса (MЭA-S_x-ZnS_x) в качестве ингибитора коррозии. Ингибитор был получен в результате реакции полисульфида натрия (Na₂S_x), моноэтаноламина (MЭA) и оксида цинка (ZnO) в водной среде. В ходе реакции MЭA способствует стабилизации полисульфидных видов, в то время как ZnO гидролизуется с образованием Zn(OH)₂, который далее реагирует с полисульфидами с образованием комплекса ZnS_x. Полученная система MЭA-S_x-ZnS_x проявляет превосходные свойства ингибирования коррозии благодаря синергетическим эффектам координации MЭA, осаждения ZnS_x и реакционной способности полисульфида. Присутствие NaOH в качестве побочного продукта также способствует регулированию pH среды, повышая эффективность ингибитора. Предварительные испытания показывают, что этот новый ингибитор эффективно смягчает деградацию поверхности металла, особенно в средах, содержащих сероводород (H₂S) и другие коррозионные агенты. Это исследование дает ценную информацию о разработке эффективных ингибиторов коррозии для промышленного применения. На основе экспериментальных результатов его эффективность была оценена с использованием электрохимических методов.

Keywords: polysulfide, monoethanolamine, corrosion inhibitor, hydrogen sulfide, gravimetric and electrochemical methods.

Ключевые слова: полисульфид, моноэтаноламин, ингибитор коррозии, сероводород, гравиметрические и электрохимические методы.

Introduction

Corrosion inhibitors protection of metals against corrosion in various corrosive environments [1–3]. A corrosion inhibitor is a compound that is added in low concentrations to a corrosive solution to reduce and/or minimize the corrosion rate [4-6]. If we talk about the economic damage of this corrosion process, as an example, we can cite the following figures, for example: according to the results of international research conducted by NACE (IMPACT 2016), the annual economic damage of the corrosion process worldwide is 2.5 trillion US. It is concluded that, if we analyze this figure in each country section, it is about 3.4% of the average gross domestic product (GDP) of each country [7]. The results of many years of scientific research carried out by world scientists show that the environment should be taken into account when choosing corrosion inhibitors, and that the use of compounds containing nitrogen and sulfur and substances based on them is more effective for acidic environments [8]. In addition, such as aldehydes, thioaldehydes, including various alkaloids, such as papaverine, strychnine, quinine, and nicotine, have been proven to be highly effective corrosion inhibitors and meet the requirements for corrosion inhibitors. Many researchers show that the use of corrosion inhibitors based on benzoates, nitrites, and inhibitors based on them, as well as chromates and phosphates, have a high inhibition efficiency for alkaline and acidic solutions [9-11]. In this paper, study of the obtaining corrosion inhibitors based urea, thiourea and orthophosphoric acid and determining their inhibition efficiency by gravimetric and electrochemical methods. The main aims of present work are as follows:

(i). Determination of the structure of synthesized corrosion inhibitors by physicochemical methods.

(ii). Determination of the inhibition efficiency of this corrosion inhibitor by gravimetric and electrochemical methods in different corrosion enivoriments.

Research Methodology

Polysulfide (Na₂S_x), Monoethanolamine (MEA), Hydrochloric acid (to create an acidic pH environment), steel samples (St20 steel) and distilled water, ZnO 99%. The inhibition efficiency of this corrosion inhibitor was determined by the gravimetric method using the device at atmospheric pressure.

Our researched PMZN-1brand corrosion inhibitor was tested by gravimetric method. This method is used to determine the corrosion rate for corrosion control purposes and to evaluate the protective ability of corrosion inhibitors. The gravimetric method is based on measuring the difference in the mass of control metal samples before and after exposure to a corrosive environment. A limitation of the use of this method is that it characterizes the average corrosion rate without taking into account the unevenness of the corrosion.

Modification of polysulfide with an amine

For this reaction, a sodium polysulfide solution was prepared at a concentration of 1 M. MEA was prepared in a separate solution, HCl was added to balance the pH, and the proton exchange between MEA and HCl occurred as follows. Hydrochloric acid (HCl) dissociates into the following ions:

HCl→H⁺+Cl⁻

MEA can accept a proton (H⁺) in solution because its amine nitrogen has a lone pair of electrons. As a result, a proton exchange occurs:

HOCH₂CH₂NH₂+H⁺→HOCH₂CH₂NH₃⁺

Also, the Cl⁻ anion in HCl forms an ionic bond with the protonated form of MEA, forming monoethanolamine hydrochloride:

HOCH₂CH₂NH₃⁺+Cl⁻→HOCH₂CH₂NH₃⁺Cl⁻

The pH effect.

MEA is essentially a weak base, with a pKa value of approximately 9.5. When MEA is exposed to acidic conditions, it forms MEA hydrochloride, a strong electrolyte, which increases its solubility in water and its ionization ability. This process lowers the pH of the solution and brings it to an optimal environment. In addition, MEA has the ability to react with polysulfides in its protonated form (MEA H⁺), which helps to increase the effectiveness of the corrosion inhibitor. The general reaction equation for monoethanolamine with polysulfides can be written as follows:

Na₂S_x+HOCH₂CH₂NH₂→(MEA-S_x)+2NaOH

The resulting mixture is deeply modified with zinc oxide.

Na₂Sx+HOCH₂CH₂NH₂+ZnO+H₂O→(MEA−S_x-ZnS_x)+NaOH.

The formation of the system significantly increases the inhibitory efficiency of this compound. The following reasons can be cited for this:

Polysulfide-MEA complex: forms a strong corrosion inhibitor

Reaction of ZnO and polysulfide: promotes the formation of a protective layer in the form of ZnS or ZnS_x.

The presence of NaOH: increases the pH of the solution, which ensures the effectiveness of MEA.

We named the resulting corrosion inhibitor PMZn-1.

Results and discussion. According to the study, a new corrosion inhibitor was developed based on amino compounds and polysulfides. This inhibitor was tested at various concentrations—200, 400, 600, and 1000 mg/L. The tests were carried out three times, each lasting 72 hours, under atmospheric pressure in a dedicated test rig. The exposure time was calculated from the moment the samples were introduced into the testing environment. To closely replicate the actual operating conditions of equipment in two-phase systems, the inhibitor tests were conducted in laboratory setups featuring intensive medium stirring. A typical laboratory apparatus used for such experiments is shown in Figure 1. The environment under study is saturated with oil products, and it becomes a bubble through the introduction of inert gas. The flow rate of the liquid that washes the metal samples of corrosion is determined using a tube lowered into the liquid stream.

Figure 1. Experimental setup used for corrosion inhibitor testing at atmospheric pressure conditions

1 – U-shaped vessel; 2 – mixer; 3 – thermometer; 4 – electric motor; 5 – metal samples; 6 – test medium; 7 – tripod; 8 – reflux condenser.

The U-shaped two-chamber vessel (1) is designed to create circulation of the test medium using a mixer (2), which is driven through a water seal by an electric motor (4). Metal samples (5) are placed inside the vessel, which is also equipped with an internal thermometer (3) for monitoring temperature and a reflux condenser (8) to maintain system integrity during testing.

Table 1.

Values of inhibition coefficient (γ), complete surface coverage (θ), protection level (Z) of PMZN-1brand corrosion inhibitor as a result of concentration and temperature

The concentration of PMZn-1brand corrosion inhibitor is 200; 400; 600; 1000 mg/l It was carried out in a condensate environment. As a result of the tests, the level of protection was 81.1, 89, 98.5, 98,6 percent, respectively.

Table 2.

Corrosion rates, protection levels and surface coverage coefficient values at different mass ratios of PMZn-1brand corrosion inhibitor.

As a result of the test research, we can see with the help of table 1 that the best mass ratio of amine compounds and fatty acid is 1:2, and the level of protection in it is 89%.

Conclusion. The various properties of this corrosion inhibitors were identified, and the following main points were found:

(i). The inhibition efficiency of this inhibitor was studied in different concentrations (200; 400; 600; 1000) and using the gravimetric method. As a result of the tests, the level of protection was 83.3, 90.6, 98.5, and 98.6 percent, respectively.

Our researched and tested PMZn-1corrosion inhibitor can be used in various environments such as 1 M HCl and 0.5 M HCl.

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