Allylphenoxypiperidinium halides as corrosion inhibitors of carbon steel and biocides

A series of 1-(4-(2-allylphenoxy)butyl)piperidin-1-ium halides (4a-d) was synthesized and characterized via spectroscopic methods (FTIR, 1 H NMR). The corrosion inhibition of the synthesized halides on carbon steel in water-salt-hydrocarbon environment, saturated with H2 S, was investigated. For this purpose, a series of techniques such as gravimetric measurement, potentiodynamic polarisation, and scanning electron microscope (SEM) were used and some thermodynamic parameters of corrosion process (Δ Gads. , ΔH0ads. , Δ S0ads. ) were evaluated. The steel surface was checked by SEM, and the steel surface showed good surface coverage. The results showed that the synthesized compounds at the concentrations 125, 150 mg ×L-1 have corrosion inhibition activity of 78%–95% by gravimetric measurements and 81%–92% by potentiodynamic measurements at 100, 150 mg ×L-1. The biological activity was examined against sulphate-reducing bacteria (SRB). It was revealed that at the concentration of compounds 4c and 4d, 100 mg ×L-1, the antibacterial activity was 100%.


Introduction
The protection of different equipment from corrosion is one of the most important ecological and economic problems of petrochemistry, especially in environments containing hydrogen sulphide, CO 2 , salt, and other substances [1][2][3][4][5]. Moreover, the corrosion of metal constructions in oil, gas and gas condensate fields due to sulphate-reducing bacteria (SRB), which is mainly carried out with the formation of hydrogen sulphide and other sulphides [6][7][8][9][10], is a serious problem.
It is worth noting that a large number of organic compounds with anticorrosive, bactericidal, and other properties have been discovered [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. While using such compounds as corrosion inhibitors, the following can be observed: the formation of chemisorbed layers on the metal surface and a sharp decrease in the rate of corrosion. Although the various corrosion inhibitors comprising structure functional fragments and heteroatoms (O, N, S, P) are known [27], organic compounds with anticorrosive and bactericidal (SRB) properties are very limited [8,28]. Therefore, it is crucial to work towards discovering new corrosion inhibitors with these characteristics.
obtain photo-resisting materials [50,51]. Cationic forms of alkenylphenol derivatives are especially important as corrosion inhibitors and biocides, as they are highly soluble in water [28,44].
This paper aims to discover new corrosion inhibitors through the synthesis of new allylphenol derivatives containing nitrogen, halides, and unsaturated fragments in their structures. This will be accomplished by studying these new allylphenol derivatives as biocides and inhibitors of the hydrogen-sulphide corrosion of St.3 steel, employing gravimetric (25°C, 35°C, and 45°C) and electrochemical (25°C) methods in systems comprising a 3% aqueous solution of NaCl and hydrocarbons (kerosene) (water: kerosene = 9:1 vol.) saturated with hydrogen sulphide. The thermodynamic parameters ( ∆ G, ∆H, and ∆ S) of corrosion in identical conditions in the presence of synthesized inhibitors were determined.

Gravimetric corrosion tests
The gravimetric measurements were carried out at 25°C, 35°C, and 45°C for 5 h in a mixture of 3% NaCl water solution and kerosene (9:1 v/v), saturated with hydrogen sulphide; the volume of the mixture was 1 L and concentrations of the inhibitors used were in the rage of 50-150 mg ×L −1 . Further, the rectangular (3.5 cm ×2 cm ×0.2 cm) steel samples were grazed with emery paper, washed with bidistilled water, degreased with acetone and ethanol, and dried at room temperature. After tests, the steel samples were washed again with bidistilled water, degreased with acetone and ethanol, dried and then weighed.
The corrosion rate (K w (g ×m −2 ×h −1 )), inhibition effectiveness ( η w , %), and surface coverage ( θ) of the steel were determined both in the absence and presence of inhibitors 4a-d at various concentrations, using Equations (1), (2), and (3) [19,28]: where K W corr and K W corr(inh.) are the corrosion rates in the absence and presence of inhibitor respectively, and W 1 and W 2 are weight losses of carbon steel in the absence and presence of the inhibitors, respectively.

Electrochemical measurements
Electrochemical corrosion studies were carried out on the Autolab PGSTAT 30 (Eco-Chemie, Netherlands) potentiostat, with 0.7 cm 2 silver chloride electrode and platinum electrodes. The data obtained was processed using the GPES software. The above-described NaCl-water/kerosene-hydrogen sulphide mixture was used as a model solution at 25°C, without stirring [11], while the concentration of the tested compounds varied from 50 to 150 mg ×L −1 . The inhibition effectiveness ( η p, %) and surface coverage ( θ) of steel corrosion were calculated by Equations (4) and (5): where i corr.(inh.) and i corr. are the corrosion current density values in the presence and absence of inhibitor, respectively.

Scanning electron microscopy (SEM)
The surface morphology of the steel samples before and after the immersion was studied using the SEM-XL-30 microscope at the concentration of the inhibitors 4a-d 100 mg ×L −1 for 5 h in a mixture of 3% NaCl water solution and kerosene (9:1 v/v) saturated with hydrogen sulphide.

Biological activity tests
Antimicrobial activity of the synthesized compounds against SRB was studied at various concentrations (50, 75, 100, 150 and 200 mg ×L −1 ) of 4a-d using the method described in the literature [53]. Accordingly, compounds 4a-d were examined in water media containing SRB microorganisms (10 4 -10 6 cells in 1 mL), which were taken from the oilfield "Chirag" (Baku, Azerbaijan). 3 mL from the prepared samples of SRB water solutions and studied compounds 4a-d were added to Postgate medium and were incubated for 15 days at 32°C. After the incubation period, the concentration of hydrogen sulphide was determined. The control tests were carried out similarly, but without 4a-d. The degree of inhibition of SRB growth (S, %) was determined by Equation (6): where C 1 and C 2 are concentrations (mg ×L −1 ) of hydrogen sulphide in control and inspected samples before and after incubation period, respectively.
In our previous studies, it was shown that the length of the spacer (the length of the -CH 2 group in the structure of dibromoalkane) affects the yield of the product [44,52]. Therefore, in our future studies, the syntheses were carried out using 1,4-dibromobutane. The formation of precursor compound 2 -1-allyl-2-(4bromobutoxy)benzene and 3 -1-(4-(2-allylphenoxy) butyl) piperidine, and compounds 4a-d were confirmed by FTIR and NMR spectroscopy.  The results of the spectral characteristics confirmed the structure of the precursor compounds (2,3) and the synthesized (4a,b,d) compounds. In Figure 3, all the prepared compounds have the same absorption bands.
The only difference is in the absorption band of 4d, owing to the presence of methyl group bands and + NR 4 in the structure. While comparing the 1 H NMR spectra of compound 2 and 4d (Figure 1 and 4), it can be

Gravimetric measurements
Organic compounds containing nitrogen, sulphur, oxygen, aromatic rings, as well as various multiple bonds in their structures are effective corrosion inhibitors, especially in acidic environments [54,55]. These compounds, due to the functional groups in the structures, are well adsorbed on the metal surface. Among these compounds are functionally substituted organic compounds based on alkenylphenol, which have higher corrosion inhibition properties in aggressive environments [28,29,44]. Moreover, in the presence of quaternary ammonium fragments in their structures, their inhibitory properties increase due to their good solubility in water [28].
Considering all these points, it was interesting to study of the synthesized 4a-d as corrosion inhibitors of steel St.3 in a mixture of 3% NaCl water solution and kerosene (9:1 v/v) saturated with hydrogen sulphide. This can be explained based on the structure of the compound containing the chloride anion. Compounds 4b and 4d (150 mg ×L −1 ) , containing bromide and iodide ions in the structures, had the best corrosion inhibition effectiveness as expected. Moreover, as it can be seen, the protection of steel was 93%-95% at 25°C, 70%-72% at 35°C, and 66%-68% at 45°C. Compound 4c had a small difference in corrosion inhibition efficiency degree of protection (92% at the concentration 150 mg ×L −1 ) compared with 4b and 4d.
Furthermore, the results of the studies show that compounds 4b and 4d at the concentration 150 mg ×L −1 have high inhibition properties and that the studied inhibitors, due to their properties, are not inferior to corrosion inhibitors, which are known in the literature [8], and in some cases even surpassed them [1].

Potentiodynamic polarization studies
The potentiodynamic polarization studies ( Figure 5) revealed a shift in currents. With an increase in concentration of 4a-d, the corrosion potential shifts towards positive values. This can be the result of the adsorption of the tested compounds on the surface of the steel electrode. In this case, the rate of redox reactions slows down, and inhibition of corrosion process occurs. Notably, the comparison of the results of electrochemical and gravimetric studies conducted at 25°C makes it possible to identify the correlation.    Table 2. These data demonstrate that the studied inhibitors shift the corrosion potential ( E corr. ) into the cathodic direction. The cathodic Tafel slopes ( β c ) and anodic Tafel slopes ( β a ) values depend on the concentration of 4a-d. Among the investigated substances, high corrosion effectiveness was exhibited by substances 4b, 4c, and 4d at concentrations 100-150 mg ×L −1 with inhibition effectiveness as 68%-92%.
The results of anticorrosion studies showed that the studied inhibitors are not inferior to the inhibitors described in the literature in terms of anticorrosion properties [3][4][5].

Adsorption isotherm and thermodynamic data
The analysis of adsorption isotherms can provide basic information on the interaction of an inhibitor with a metal surface [11]. Therefore, the adsorption ability on the metal surface was studied. The surface coverage values ( θ) were determined at different concentrations of 4a-d from gravimetric measurements ( Figure 6).
The plots C / θ versus C ( Figure 6) are linear with correlation coefficients 0.9972, 0.9919, 0.9983, and 0.9991, respectively. In this case, the adsorption of indicators on the carbon steel surface obeys the Langmuir adsorption isotherm by Equation (7): where C is the inhibitor concentration, θ is the degree of coverage on the metal surface and K ads is the equilibrium constant for adsorption desorption process, calculated by the reciprocal of the intercept of isotherm line at various temperatures (Table 3).

Inhibitors
The degree (%) of inhibition of growth of SRB at the concentration of inhibitors, mg ×L − 1  50  75  100  150  200  4a  45  62  71  79  89  4b  81  90  96  100  100  4c  88  94  100  100  100  4d  91  95  100  100  100 The free energy ( ∆G ads ) of adsorption can be calculated by Equation 8: where ∆G ads is the free energy of adsorption, K ads is the equilibrium constant, T is the absolute temperature, and 55.5 is the molar concentration of water.
The positive values of ∆S ads (66.99, 71.66, 73.37, 124.89 J ×mol −1 ×K −1 for inhibitors 3a, 3b, 3c, and 3d, respectively) are possibly related to an increase in disorder due to the adsorption of the studied molecules and desorption of water molecules.

Scanning electron microscopy (SEM)
To test the inhibitory properties of organic compounds, the morphology of the metal surface is important [8].
For this purpose, the surface morphology of steel in the absence and presence of inhibitors 4a-d after immersion for 5 h was studied. The results are shown in Figures 7 and 8.
It was thus revealed that the surface was strongly damaged (Figure 7) in absence of the inhibitors 4a-d.

Antimicrobial activity
To prevent biocorrosion of equipment that is used in the oil industry, various compounds containing active atoms and functional groups in their structures are used [7][8][9][10]. Considering this, compounds 4a-d were studied as biocides for SRB microorganisms at the concentration 50-200 mg ×L −1 . The results are given in Table 4. According to the experimental results of the antimicrobial activity of 4a-d against SRB, all the compounds showed bactericidal activity. As shown earlier, the bactericidal activities of the studied compounds depend on their chemical structures. For example, compound 4a, consisting in the structure chloride ions, has 89% antimicrobial activity at the concentration 200 mg ×L −1 . However, 4b with bromide anions in the structure has 100% degree at 150 mg ×L −1 . The most actives are 4c and 4d. These compounds have 100% degree at 100 mg ×L −1 , probably due to the content of iodide ions in the structures of 4c and 4d. In general, the studied compounds have better bactericide effectiveness in comparison with those known [7,10,56]. the high corrosion inhibition effectiveness can be related to the adherent adsorption of these cationic surfactants on the steel surface and formation of a protective film.

Conclusions
Moreover, 4a-d were studied as reagents against SRB and their high bactericidal activity (100%) was revealed at concentration 100 mg ×L −1 .
Thus, our study allowed us to conclude that compounds 4a-d can be used as corrosion inhibitors and biocides. The results show that using such compounds can reduce the rate of metal corrosion and increase their service life.