STUDY OF THE CORROSION INHIBITION PROPERTIES OF A COMPOUND SYNTHESIZED FROM DIPHENYLAMINE

ABSTRACT

This article presents a study on the corrosion inhibition properties of 1-(diphenylamino)butan-2-one synthesized on the basis of diphenylamine. The inhibition efficiency of 1-(diphenylamino)butan-2-one for St.20 steel was investigated using the gravimetric method in 1 M hydrochloric acid solution within the temperature range of 35–65 °C. The results showed that the inhibition efficiency increased with increasing inhibitor concentration, reaching a maximum protection degree of 91.5 % at a concentration of 350 mg/L. It was established that the inhibitor forms a stable protective film on the steel surface through both physical and chemical adsorption, which reduces the corrosion rate and enhances the stability of the metallic surface.

АННОТАЦИЯ

В данной статье исследованы коррозионно-ингибирующие свойства 1-(дифениламино)бутанона-2, синтезированного на основе дифениламина. Эффективность ингибирования 1-(дифениламино)бутанона-2 в отношении стали марки Ст.20 была изучена методом гравиметрического анализа в 1 М растворе соляной кислоты при температурах 35–65 °C. Результаты показали, что эффективность ингибирования возрастает с увеличением концентрации ингибитора, достигая максимальной степени защиты 91,5 % при концентрации 350 мг/л. Установлено, что ингибитор образует на поверхности стали стабильную защитную плёнку за счёт физической и химической адсорбции, что приводит к снижению скорости коррозии и повышению стабильности металлической поверхности.

Keywords: diphenylamine, corrosion inhibitor, steel protection, hydrochloric acid, adsorption, inhibition efficiency, chemical technology.

Ключевые слова: дифениламин, ингибитор коррозии, защита стали, соляная кислота, адсорбция, эффективность ингибирования, химическая технология.

Introduction

In order to reduce the risk of corrosion in metallic pipelines and storage tanks, corrosion inhibitors are widely used to slow down the corrosion process. Most organic inhibitors contain heteroatoms such as oxygen, nitrogen, and sulfur, as well as aromatic rings and multiple bonding structures, which enhance adsorption by directing their lone pair electrons toward the metal surface. As a result, such inhibitors interact with the metal surface either physically or chemically, forming a stable protective film that shields the metal from an aggressive environment [1,2]. Previous studies [3] have systematically analyzed the free-radical reactions occurring during the spontaneous combustion of coal, and the mechanism by which diphenylamine reduces these radicals has been investigated from three aspects using quantum chemical methods. Therefore, the presence of a nitrogen atom in the structure of diphenylamine makes it a promising compound to be studied as a corrosion inhibitor. In our recent research, we have examined the addition reaction between diphenylamine and vinylacetylene.

Purpose of the study:

Investigation of the dependence of the Inhibitive properties of 1-(diphenylamino)butan-2-one (IK-DFA) on temperature and concentration.

Experimental methods

Continuing our previous research [4,5,6], 1-(diphenylamino)but-2-yne was synthesized and subsequently hydrolyzed in an acidic medium in the presence of Hg²⁺ salts to obtain 1-(diphenylamino)butan-2-one (IK-DFA). The overall reaction can be represented as follows:

The corrosion inhibitor 1-(diphenylamino)butan-2-one (IK-DFA) was synthesized, and its inhibitive properties were investigated. Due to the presence of heteroatoms, the corrosion inhibitor (IK-DFA) possesses several active adsorption centers. The conjugated bonds containing the unshared electron pairs of oxygen and nitrogen atoms, as well as hydrophobic alkyl chains connecting different adsorption sites, contribute to the inhibitor’s ability to interact with the metal surface. The inhibition efficiency of IK-DFA for St.20 steel samples was studied and analyzed using the gravimetric method in 1 M HCl solutions with a pH value of 1.

Research method:

Gravimetric method

Studies were carried out to determine the corrosion rate of steel plates using the gravimetric method under various conditions, including different inhibitor concentrations and temperature ranges. The steel specimens were immersed for 24 hours in both inhibited and uninhibited solutions. After exposure, the samples were cleaned, and the gravimetric corrosion rate (K_grav) as well as the degree of protection (Z) were calculated by comparing the corroded metal with the blank sample (metal corrosion in the solution without inhibitor).

where:

m₁ — mass of the metal specimen before immersion, g;

m₂ — mass of the metal specimen after immersion, g;

W₀ — mass loss of the metal in the uninhibited solution;

W₍ing₎ — mass loss of the metal in the inhibited solution;

S — surface area of the steel specimen, cm²;

τ — exposure time, h.

The chemical composition of St.20 steel samples (in weight percent) was as follows: C — 0.14–0.22; Si — 0.05–0.17; Mn — 0.35–0.65; P — 0.03; Cr — 0.25; Ni — 0.30; Cu — 0.30; and the remainder Fe. The surfaces of the St.20 steel specimens were polished with emery paper, then washed with ethanol and distilled water, dried, and subsequently used for experimental testing.

Results and discussion

The effect of IK-DFA concentration on the corrosion inhibition process was studied within the temperature range of 35–65 °C in 1 M HCl solution. Data on the weight loss of steel samples, both in the presence and absence of the inhibitor, at various concentrations of IK-DFA are presented in Table 1.

Table 1.

The corrosion rate values obtained for steel in 1.0 M HCl solution at various temperatures in the presence of the IK-DFA inhibitor are presented below

According to the obtained data, the protective efficiency of the synthesized inhibitor decreases slightly after reaching a certain concentration threshold. The experimental results demonstrated that all inhibitors exhibited the highest efficiency at an optimal concentration of 350 mg/L, while no significant improvement in inhibition efficiency was observed at higher concentrations. At 30 °C and 350 mg/L of IK-DFA, the corrosion rate (Kt) was determined to be 0.151 mg·cm⁻²·s⁻¹, whereas in the absence of the inhibitor, Kt reached 1.781 mg·cm⁻²·s⁻¹. Furthermore, the corrosion protection degree and inhibition efficiency of IK-DFA gradually increased with the increase in inhibitor concentration, achieving a maximum inhibition efficiency of 91.5%. This indicates that as the inhibitor concentration increases, more IK-DFA molecules are adsorbed onto the steel surface, thereby forming a barrier that effectively isolates the metal from the corrosive medium.

Figure 1. Dependence of the corrosion rate on temperature in the presence (a) and absence (b) of IK-DFA

It is evident from the data that the corrosion rate increases with temperature, both in the presence and absence of inhibitors. The increase in temperature may cause desorption of the inhibitor molecules from the steel surface (Figure 1.). However, Table 1 indicates that the corrosion rate in the absence of the inhibitor rises more sharply compared to that in the presence of IK-DFA. For instance, without the inhibitor, the corrosion rate increased from 1.781 mg·cm⁻²·s⁻¹ at 30 °C to 3.624 mg·cm⁻²·s⁻¹ at 60 °C, demonstrating a pronounced acceleration of the corrosion process with temperature. In contrast, in the presence of the inhibitor, the corrosion rate increased only slightly with temperature, indicating that IK-DFA provides effective anticorrosive protection by forming a mixed-type adsorption film and coordination compounds on the steel surface.

Conclusion

The groups containing inhibitors possess higher activation energy compared to the uninhibited system. This phenomenon is attributed to the adsorption process that occurs through the unshared electron pairs present in the inhibitor molecules, leading to the formation of a physico-chemical protective layer on the metal surface. Such a layer impedes the transfer of corrosion charges and increases the energy barrier for the dissolution of St.20 steel in HCl solution, which plays a significant protective role. Similarly, the activation entropy in the presence of inhibitors is found to be negative and of higher magnitude, indicating that the rate-determining step involves the formation of an associative, activated complex with a more ordered structure.

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