Keywords

Aminated soybean derivatives, Mild steel, Green corrosion inhibitors, Weight loss method, Chemical analysis method

Introduction

The soybean seeds are composed of 40% protein and 20% oil content, and most importantly soybean oil accounts for approximately 30% of the world's vegetable oil supply [1]. Although soybean oil is primarily used as edible oil, there is a continuing demand for its use in industrial applications. The fatty acid composition of soybean oil “10.6% palmitic acid; 2.4% stearic acid; 23.5% oleic acid; 51.2% linoleic acid; 8.5% linolenic acid; and 3.8% other” by weight [2]. Soybean oil derivatives containing a Schiff base (SOS-B) were prepared and evaluated as microbial corrosion inhibitors against sulfate-reducing bacteria using the gram-positive Desulfosporosinus orients bacteria as a representative bacterium [3].

Corrosion is the damage to metal caused by reaction with its environment. "Damage" is specified purposely to exclude processes, such as chemical milling, anodizing of aluminum, and bluing of steel, which modify the metal intentionally. Rusting is a type of corrosion but it is the corrosion of ferrous metals (irons and steels) only, producing that familiar brownish-red corrosion product, rust. The environment that corrodes a metal can be anything; air, water, and soil are common but everything from tomato juice to blood contacts metals, and most environments are corrosive. Corrosion is a natural process for metals that causes them to react with their environment to form more stable compounds [4].

Corrosion inhibitor is a chemical added to the corrosive environment in small amounts to reduce the corrosion rate. Some inhibitors interfere with the anode reaction, some with the cathode reaction, and some with both. They are usually used to prevent general corrosion but most are not effective in preventing localized attack, such as crevice corrosion, pitting, or SCc. Inhibitors generally interact with the metal surface in some way: to form a barrier film of adsorbed inhibitor that may be only a monolayer or less, or to form a thick barrier layer of reaction products or inhibitor [5-12]. Organic Adsorption Inhibitors that coat metal with an oily surface layer will protect it. The inhibition property of these compounds is attributed to their molecular structure. The planarity and the lone pairs of electrons in the heteroatoms are important features that determine the adsorption of these molecules on the metallic surfaces. They can adsorb on the metal surface, block the active sites on the surface and thereby reduce the corrosion rate [13-16]. These inhibitors are commonly used in acids, although a few function in neutral or alkaline solutions. In adsorbing on the metal, they replace adsorbed water molecules and prevent water from solvating metal ions or prevent H+ ions from adsorbing at cathode sites where reduction to H₂ (g) could occur [17,18].

In the present work Gabriel method was used for synthesizing of four different types of green aminated soybean derivatives. The prepared compounds were characterized using FT-IR, and 1H-NMR. The prepared compounds were evaluated as corrosion inhibitors for mild steel using 0.5 N HCl by weight loss method and chemical analysis method. The effect of temperature was also studied.

Materials and Methods

Experimental

Brominating of soybean oil

One mole of soybean oil (87.8 g) was placed in round bottom flask, and six moles of bromine (95.88 g) were placed in quick fitted dropping funnel. The bromine solution was allowed to react drop by drop, until the end of bromine addition, with continuous stirring in an ice bath. The obtained product is mentioned at Scheme 1.

Scheme 1: Brominated soybean.

Synthesis of novel green corrosion inhibitors based on soybean oil

Synthesis of corrosion inhibitor (S₁)

In 250 mL round bottom flask, about (18.368 g) of brominated soybean oil, (7.2 g) of urea, 10 mL of acetone and 3 mL of NaOH were placed [19]. The flask was connected with quick fitted condenser and enabled to reflux using heating mantel at 75°C, for 1 h, Scheme 2.

Scheme 2: Aminated derivatives of soybean oil.

Synthesis of corrosion inhibitor (S₂)

In 250 mL round bottom flask, about (18.368 g) of brominated soybean oil, (9.13 g) of thiourea, 10 mL of acetone and 3 mL of NaOH were placed. The flask was connected with quick fitted condenser and enabled to reflux using heating mantel at 75°C, for 1 h, Scheme 2 [19].

Synthesis of corrosion inhibitor (S₃)

In 250 mL round bottom flask, about (18.368 g) of brominated soybean oil, (16.45 g) of 3-amino benzoic acid, 10 mL of acetone and 3 mL of NaOH were placed. The flask was connected with quick fitted condenser and enabled to reflux using heating mantel at 75°C, for 1 h, Scheme 2 [19].

Synthesis of corrosion inhibitor (S₄)

In 250 mL round bottom flask, about (18.368 g) of brominated soybean oil, (12.85 g) of p-toluidine, 10 mL of acetone and 3 mL of NaOH were placed. The flask was connected with quick fitted condenser and enabled to reflux using heating mantel at 75°C, for 1 h, Scheme 2 [19].

Elucidation of chemical structure of the prepared compounds

Using FT-IR

The prepared compounds were elucidated using Nicolet iS10-FT-IR Spectrophotometer, KBr, Jazan University, Saudi Arabia.

Using ¹H-NMR

The structure of the prepared corrosion inhibitors were elucidated using Proton Nuclear Magnetic Resonance "1H-NMR" spectra using a 500 MHz (JEOL NMR ECA-500) using DMSO-d6 as a solvent, NRC, Dokki, Egypt.

Corrosion studies

Weight Loss Measurements

Mild steel coupons having chemical composition mentioned in Table 1, with dimensions 7.0 × 5.0 × 0.3 cm were abraded using different grades of emery papers grades (310, 410, and 610), washed with bidistilled water, degreased with acetone, dried and kept in a desiccator. Weigh accurately using a digital balance with high sensitivity. The coupons were immersed in 0.5 N HCl solution with and without various concentrations (250, 500, 1000, 2000, and 3000) ppm of the prepared inhibitors, separately for 3 hours, at different temperatures (308, 318, 328 and 338 K). Weight loss experiments were carried out according to the ASTM standard procedure described in reference [20]. In brief, mild steel specimens in triplicate were immersed in 100 mL 0.5 N HCl containing various concentrations of the studied inhibitors. The mass of the specimens before and after immersion was determined using an analytical balance accurate to 0.01 mg. Reading were taken after each one hour, the coupons were washed with bidistilled water, dried and weighed accurately [21-28]. The investigations carried out in the open air. For further data processing, the average of the three replicate values was used. The pre-cleaned and weighed coupons were suspended in beakers containing the test solutions using glass hooks and rods. The weight loss was calculated by the following equation:

Table 1: Constituents of mild steel alloy.

ΔW=W₁-W₂ (1)

Where W₁ and W₂ are the weight of coupons before and after immersion.

The surface coverage area (θ) for the different concentrations of the prepared corrosion inhibitors in 0.5N HCl was calculated according to the following equation:

θ=1-W₁/W₂ (2)

The inhibition efficiency according to weight loss method IE_w (%) was determined by the following equation:

IEw%=(W_corr/W`_corr) × 100 (3)

Where W_corr and W`_corr are the corrosion rates of coupon with and without corrosion inhibitors, respectively.

The corrosion rate (CR) was calculated by the following equation:

CR=(W₁-W₂)/At (4)

Where W₁ and W₂ are the weight of coupons before and after immersion in test solutions. A is the total area of the coupon (cm²), and t is the immersion time (h).

Chemical analysis of solution according to weight loss method

When a metal undergoes corrosion in an electrolyte of a fixed volume, cations of corroding metal will accumulate in solution. Accordingly, the solution becomes more concentrated in the dissolved cation with the progression of time [29]. Thus, chemical analysis of withdrawn aliquots of the solution as a function of time allows determination of the corrosion rate according to equation (5).

Results and Discussion

Elucidation of the prepared Corrosion inhibitors

The prepared brominated soybean (S) was elucidated using Fourier transform infrared spectroscopy (FT-IR), Figure 1. There was a different peak than soybean oil located at 758 cm^-1 corresponding to C–Br, and disappearance of peak at 1638 cm^-1 corresponding to C=C bond was noticed. FT-IR spectra corresponding to the four prepared corrosion inhibitors (S₁-S₄), showed the disappearance of C–Br at 758 cm^-1 and appearance of one peak at 3446 cm^-1 corresponding to secondary –NH group. The presence of peak at about 1740 cm^-1 insures the stability of ester group toward reaction with amines. ¹H-NMR analysis of the brominated soybean (S) and the aminated soybean derivatives (S₁-S₄) are mentioned at Table 2.

Figure 1: FT-IR of soybean oil, brominated soybean, and S₃ aminated derivative of soybean.

Table 2: ¹H-NMR data of the prepared (S) and (S₁-S₄) compounds.

Corrosion tests

Weight loss method

The inhibition efficiency of the organic compounds depends on many factors including the number of adsorption sites and their charge density, molecular size, heat of hydrogenation, mode of interaction with the metal surface and formation of metallic complexes [8]. Weight loss parameters were mentioned at Table 3.

Table 3: Weight loss parameters of mild steel (CS-37) treated without and with the prepared corrosion inhibitors (S₁-S₄) at 308 K.

Effect of (S₁-S₄) concentrations

The variation of inhibition efficiency (IEw%) from weight loss measurements with (S₁-S₄) concentrations is shown in Figure 2. It was found that the weight loss parameters; “corrosion rate CR, surface coverage θ, and percentage of inhibition efficiency” increases with increasing (S₁-S₄) concentration and record the maximum value (93.63% S₁, 94.78% S₂, 95.46% for S₃ and 96.68% for S₄) of inhibition efficiency at 3000 ppm concentration. The inhibition efficiency of the prepared additives increases in the order S₄>S₃>S₂>S₁. The highest inhibition efficiency of inhibitor S₄ may be attributed to the presence of (methyl group) electron donating group.

Figure 2: Variation of inhibition efficiency with different concentrations of the prepared inhibitors (S₁–S₄) in 0.5 N HCl at 308 K .

Effect of temperature

In order to calculate the activation energy (Ea) of the corrosion process and investigate the mechanism of inhibition, weight loss measurements were performed in the temperature range of 308-338 K in absence and presence of 3000 ppm of the prepared inhibitors (S₁-S₄) at 0.5 N HCl. It was found that the corrosion rate of mild steel increases with increasing temperature. The results indicates that increasing temperature leads to a decrease of RT, hence increasing the corrosion rate of carbon steel as shown in Figure 3. A plot of ln corrosion rate (ln k) against the reciprocal of absolute temperature (1/T) was drawn graphically to obtain activation energy Ea, according to Arrhenius equation:

Figure 3: Variation of RT values of carbon steel with 3000ppm of (S₁–S₄) inhibitors at different temperatures

ln K=-Ea/RT+constant (6)

Where Ea equal to slope of this equation.

Adsorption isotherm

To understand the corrosion inhibition mechanism, the organic compound’s adsorption behaviour on the carbon steel surface must be known. The plot of C_inh/Θ vs. C_inh, Figure 4 yielded a straight line, proved that the adsorption of the prepared corrosion inhibitors from the hydrochloric acid solution obeys Langmuir adsorption isotherm, which is presented by equation (7) [27].

Figure 4: Langmuir isotherms for the adsorption of the prepared corrosion inhibitors on carbon steel surface in 0.5 N HCl at 308 K

C_inh/θ=(1/K_ads)+C_inh (7)

Where C_inh is the inhibitor concentration and K_ads is the equilibrium constant for the adsorption/desorption process. From the intercepts of the straight lines on the C_inh / Θ - axis, one can calculate K_ads, which is related to the standard free energy of adsorption, ΔG_ads, as given by Eq. (8) [28]:

ΔG_ads=-RT (ln⁡ K_ads × 55.5)

The calculated free energy of adsorption (ΔG_ads) is given in Table 4. The negative values of ΔG_ads indicated that the adsorption of the inhibitors on the metal surface is spontaneous. Generally, values of ΔG_ads around 20 kJ mol-1 or lower are consistent with the electrostatic interaction between charged molecules and the charged metal surface (physisorption and chemisorption); It can be seen from Table 4 that, calculated ΔG_ads values indicated that the adsorption mechanism of the prepared corrosion inhibitors on carbon steel in 0.5 N HCl solution is physical and chemical adsorption. The large values of ΔG_ads and its negative sign are usually characteristic of strong interaction and a highly efficient adsorption. The aminated derivatives of soybean oil were generally chemisorbed at the metal surface and displace the adsorbed water and electrolytes from the surface. It is assumed that these N-containing functional groups act as electron pair donors to the electron depleted dehydrated metal surface. This interaction between the inhibitor and the metal surface is due to the formation of a bond between the N electron pair and the electron cloud at the surface.

Table 4: Adsorption parameters for the prepared corrosion inhibitors (S₁-S₄) on mild steel in 0.5 N HCl at 308 K.

Conclusion

Four green corrosion inhibitors were prepared via reaction of brominated soybean oil with different types of amines "urea, thiourea, 3- amino benzoic acid and p- toluidine". The aminated derivatives were elucidated using FT-IR and ¹H-NMR, and they were evaluated as corrosion inhibitors for mild steel at 0.5N HCl, using weight loss method and chemical analysis method at different temperatures "308-338" K. It was found that the inhibition efficiency "IE_w%" increases with increasing the inhibitor concentration, while decrease with rising of temperature and the order of increasing the inhibition efficiency "IE_w%" using the prepared corrosion inhibitors was as fellow: S₁<S₂<S₃<S₄. The adsorption of the prepared inhibitors on metal surfaces from 0.5 N HCl solution obeys Langmuir adsorption isotherm. The high value of adsorption equilibrium constant suggested that the prepared organic corrosion inhibitors were strongly chemically and physically adsorbed on the mild steel surfaces. Chemical analysis method was used for the determination of weight of the corroded iron according to weight loss calculations and it was found that the amount of the corroded iron decreases with increasing concentration, and the order of increasing the corroded iron was as fellow: S₁>S₂>S₃>S₄.

Acknowledgment

I would like to thank the management of faculty of science and arts, Samtah, Jazan University for providing me with chemicals and analysis.

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