Synthesis of Disodium Salt of Sulfosuccinate Monoester from the Seed Oil of Terminalia catappa and Its Inhibitive Effect on the Corrosion of Aluminum Sheet in 1 M HCl
Oil was extracted from the seed of Terminalia catappa and used to synthesize disodium salt of sulfosuccinate monoester using simple reaction mechanism. The disodium salt of sulfosuccinate monoester was applied as corrosion inhibitor of aluminum sheet in 1 M HCl via weight loss method. The adsorption was found to obey Langmuir isotherm. The results presented disodium salt of sulfosuccinate monoester as an efficient inhibitor of aluminum sheet corrosion in 1 M HCl.
Corrosion is most commonly referred to as the degradation of a material due to its reaction with its environment. Such degradation may mean deterioration of the physical properties of the material which may be in form of weakening of the material due to loss of cross-sectional area, shattering due to hydrogen embrittlement, or cracking due to sunlight exposure. Corrosion is usually found in several materials but most especially in metals; these materials have both domestic and industrial uses but the existence of corrosion which can take place under acidic or alkaline medium has resulted in limitation to their use. Importance of protection against corrosion in acidic or alkaline solutions is known to be increased by the fact that metals are more susceptible to be attacked in aggressive media, most of which are the commonly exposed metals (such as mild steel) in industrial environments . The corrosion process is usually slowed down in various ways one of which is the use of corrosion inhibitors which when added in small amounts to a corroding environment decreases the rate of attack by such environment on material [2–4].
Being the third most abundant element and the most abundant metal, aluminum has found several industrial applications which may be due to its economical considerations and the fact that its corrosion falls into general attack . Thermodynamically, aluminum is expected to have a low corrosion resistance. The high corrosion resistance is due to the presence of a thin, compact film of adherent aluminum oxide on the surface which is formed on exposure to either air or water. This aluminum oxide dissolves in some chemicals, notably strong acids and alkaline solutions. When the oxide film is removed, the metal corrodes rapidly by uniform dissolution. So study of aluminum sheet corrosion phenomena has become important particularly in acidic media because of the increased industrial applications of acid solutions [6–9].
In the past time, use of inhibitors has been one of the most common different protective means used to control corrosion. Most inhibitors reported are synthetic organic compounds containing heteroatoms, such as O, N, and S, and multiple bonds [10, 11]; these heteroatoms have been established to have high electron density that contributes to the inhibitory capacity of such organic compounds. Although these synthetic organic compounds are widely used, their use as corrosion inhibitor has limitations such as being expensive, being nonrenewable, and being toxic to both plant and animal in the environment [12, 13]. Some efforts have been made to develop cheap and nontoxic corrosion inhibitors but quite a number of them have reduced inhibitory activity at low concentration or are toxic at high concentration when they get into the environment. Due to superb environmental stability, ease of sustainability, and low level of toxicity, several plant extracts have been considered and reported as corrosion inhibitors [14–16] but it has been established that their efficiency may be improved upon by simple modification in terms of chemical functionality. This has also shown the need for green corrosion inhibitors and their importance over synthetic chemical products. The use of plant extract with little modification is of much importance and economically viable because, aside from being ecofriendly, they are renewable, easy to modify, and inexpensive [17–20]. Thus, they can be used as feed stock for oleochemicals which can serve as green corrosion inhibitors. Terminalia catappa seed oil is an example of plant extract that can be utilized to achieve such purpose.
Terminalia catappa is a large tropical tree in the leadwood family, Combretaceae. It is commonly called almond, a small deciduous tree, growing 4–10 m (13–33 feet) in height, with trunk of up to 30 cm (12 inches) in diameter. The leaves are 3–5 inches long with a serrated margin and a 2.5 cm (1 inch) petiole. The flowers are white or pale pink, 3–5 cm (1-2 inches) diameter with five petals, produced singly or in pairs before the leaves in early spring . The antioxidant property of the solvent extract of the leaves has been reported . The almond fruit is about 3.5–6 cm long. The seed has been reported to contain 6% water, 31% lipid, 29% protein, 25% carbohydrate, 3% mineral, 2% vitamins, and 4% sugars; saturated fat (palmitic acid) was 6%, monosaturated fat (oleic acid) was 64%, and polyunsaturated fat (linoleic acid) was 26% while major minerals were Ca-14%, Mg-16%, P-27%, and K-42% . The oil has been reported to contain high levels of unsaturated fatty acids, especially oleic and linoleic; thus Terminalia catappa oil can be classified in the oleic-linoleic acid group .
Apart from the domestic use of plant products, they have also found wide application as sources of oleochemicals . Oleochemicals are completely biodegradable and so could replace a number of petrochemicals. Sulfosuccinate is an example of an oleochemical that is produced from a renewable source, biodegradable and environmentally friendly. Sulfosuccinates have been reported to exhibit compatibility with chromium and have no adverse effect on the textile strength of processed fibre with wide range of applications which includes household formulations, textiles, polymers, paints and coating, agriculture, and production of shampoos . Sulfosuccinates are known with excellent wetting properties which suggest them as possible corrosion inhibitors. At present, there is no report on their use as corrosion inhibitors. Since they are biodegradable, ecofriendly, and relatively cheap with the presence of heteroatoms in their structure, it will be important to determine their anticorrosion capacity.
In continuation of our search for cheap oleochemicals that can be used as corrosion inhibitors, the present study synthesized disodium salt of sulfosuccinate monoester from Terminalia catappa seed oil and investigated its inhibiting effect on aluminum sheet corrosion in strong acidic solution using weight loss method.
2. Materials and Methods
Seeds of Terminalia catappa were collected from the Botanical Garden, University of Ibadan. They were manually cracked, air-dried, and milled in a blender. The powdered seeds were finally extracted with hexane in a soxhlet extractor as described by Adewuyi and Oderinde .
2.2. Synthesis of Fatty Ethanolamide from the Seed Oil of Terminalia catappa
This was achieved as previously described by Adewuyi et al.  with little modification. Briefly, the oil of Terminalia catappa was reacted with diethanolamine in ratio 1 (oil) : 3 (diethanolamine) in a 250 mL round bottom flask equipped with a magnetic stirrer, a thermometer, and a condenser. The flask was placed in an oil bath while the reaction temperature was gradually increased and maintained at 140°C. The reaction mixture was continuously stirred for 10 h to form fatty ethanolamide. At the end of the reaction, the mixture was concentrated on a rotary evaporator after which the product formed was dissolved in a mixture of methanol and chloroform [50/50 (v/v)]. The solvent was later removed in a rotary evaporator. Then, acetonitrile was added to the resultant solid and the solution was cooled in an ice bath. The amide precipitated out and was subsequently recovered by filtration using Whatman filter paper . This is shown in Scheme 1.
2.3. Synthesis of Disodium Salt of Sulfosuccinate Monoester
The fatty ethanolamide synthesized was transferred into a round bottom flask and heated to 110°C while maleic acid anhydride (10 g, 0.1 mole) was gently added and stirred and the temperature was kept constant at 110°C. The reaction mixture was continuously stirred for 3 h while an aqueous solution of 30% sodium bisulphite (15.71 g, 0.1 mol) was added to the reaction mixture. The reaction temperature was gradually raised to 130°C with continuous stirring at this temperature for 1 h while the pH of the reaction mixture was adjusted using aq. NaOH. The product obtained was disodium salt of sulfosuccinate monoester as illustrated in Scheme 2. The synthesized disodium salt of sulfosuccinate monoester was purified by washing with petroleum ether for about 2 to 3 times. This removes any unwanted impurities and unreacted materials. The obtained product was filtered, dried, and analyzed using FTIR.
2.4. Corrosion Study
The corrosion inhibition study of disodium salt of sulfosuccinate monoester on aluminum sheet was carried out in 1 M HCl solution using weight loss measurement method. In this case, HCl was prepared to initiate the corrosion while disodium salt of sulfosuccinate monoester was used as the corrosion inhibitor. A cold rolled aluminum sheet of dimensions 5.0 cm by 5.0 cm with an area of 25.0 cm2 was washed, dried, and accurately weighed. After weighing accurately, the aluminum sheets were immersed in a beaker which contained 1 M HCl with and without addition of disodium salt of sulfosuccinate monoester. The solution of acid without the disodium salt of sulfosuccinate monoester was used as the control in this study while the concentration of disodium salt of sulfosuccinate monoester in the other solution varied from 0.50 g/L to 3.00 g/L. All the aggressive acid solutions were opened to air for a period of 6 h and at an interval of 1 h; the aluminum sheets were taken out of solution, washed, dried, and reweighed accurately. The experiments were carried out in duplicate, and the average weight loss of the cold rolled aluminum sheets was obtained and recorded.
3. Results and Discussion
3.1. Synthesis of Disodium Salt of Sulfosuccinate Monoester
Figure 1 shows the peaks for the FTIR analysis carried out on the oil of Terminalia catappa (a), fatty ethanolamide (b), and disodium salt of sulfosuccinate monoester (c) using Shimadzu FTIR-400S. It was observed that the Terminalia catappa oil, ethanolamide, and disodium salt of sulfosuccinate monoester showed characteristic absorption bands at 2924 cm−1 and 2852 cm1 corresponding to the C–H stretching of methyl (–CH3) and methylene (–CH2) functional groups, respectively. The absorption band present at 721 cm−1 in the oil, ethanolamide, and disodium salt of sulfosuccinate monoester spectra can be attributed to the rocking motion associated with –CH2 groups in an open chain while 1465 cm−1 suggests –CH2 of bending vibrations of alkanes. The absorption bands representing the C=O stretching of ester occurred at 1745 cm1 in Terminalia catappa oil. This C=O stretching band of ester disappeared in the ethanolamide and disodium salt of sulfosuccinate monoester with the appearance of a new peak at 1634 cm1 corresponding to the C=O stretching of amide functional group. The –OH functional group vibrational frequency in the ethanolamide was found at 3296 cm−1. The N–H stretching vibration was also observed at 3371 cm−1. The C–H bending vibration of alkane was observed at 1454 cm−1 while the O–H stretching vibration was absent in the sulfosuccinate monoester indicating the formation of the product.
3.2. Corrosion Study
The corrosion rate was determined using the following expression : where (g cm−2 h−1) is the corrosion rate, is the average weight loss after immersion, is the surface area of the aluminum sheet, and is the total time (6 h) of immersion. The inhibition efficiency (% ) was also calculated using the following equation : where and are corrosion rates of aluminum sheet with and without inhibitor, respectively.
The corrosion rate of the aluminum sheet immersed into the blank (solution without the inhibitor) was faster than that of the solution with the inhibitor; this is shown in Figure 2.
At the initial stage, the weight loss of aluminum sheet in the blank was almost double that of the solution with the inhibitor but as time went on the weight loss of aluminum reduced in the blank but was still higher than what was observed in the case of the inhibitor. This observation may be due to the fact that corrosion started immediately on exposure of the aluminum sheet to the aggressive HCl solution but with time there may have been the formation of a protective covering on the surface of the aluminum sheet which reduced the rate but later lost its protective capacity with time as shown in equation below with the formation of AlCl3 : Corrosion inhibitors or a mixture of corrosion inhibitors have been reported to form a protective film as a result of the reaction of the aggressive solution with the corroding surface of which inhibitors may impede the anodic, the cathodic, or both electrochemical reactions . Disodium salt of sulfosuccinate monoester used as inhibitor may have formed a protective covering on the surface of the aluminum sheet. As shown in Scheme 2, disodium salt of sulfosuccinate monoester has heteroatoms such as oxygen and nitrogen and also the presence of π (pie) electron systems which have been reported in the past to play active role in adsorption . These heteroatoms and the π electron systems are rich in electrons and may have interacted with the surface of the aluminum via this electron density to form the protective covering at the surface of the aluminum sheet.
Figure 3 presents the correlations between the inhibition efficiencies and the corrosion inhibition rates over a period of 6 h for the inhibitor. The inhibition efficiency of disodium salt of sulfosuccinate monoester was found to increase with time while the corrosion rate reduced with time in the presence of disodium salt of sulfosuccinate monoester. This observation must have been due to the fact that the inhibitor adsorbed on the surface of aluminum and was able to reduce the interaction between the aluminum surface and the aggressive acid solution .
The different values of corrosion rate, inhibition efficiency, and surface covering at various concentrations of disodium salt of sulfosuccinate monoester are presented in Table 1.
It was observed that the inhibition efficiency increased just as the concentration of disodium salt of sulfosuccinate monoester increased. This was also noticed in the surface covering of the aluminum sheet which increased as the concentration of disodium salt of sulfosuccinate monoester in solution increased; this trend has also been reported by Wang et al. . This may, apparently, be accounted for as the inhibitor interacting with the surface of the metal, thus blocking the active sites to form a barrier against infiltration of the aggressive electrolyte solution since the process of corrosion is considered to be electrolytic in nature [36, 37].
Attempt was made to fit the values of the surface coverage () into different adsorption isotherms but the best fit was obtained with Langmuir adsorption isotherm using the following equation as proposed by Langmuir : where is the concentration of the inhibitor and is the adsorptive equilibrium constant.
The essential characteristic of this isotherm can be expressed with the following equation: where is the equilibrium parameter, is the Langmuir constant, and is the inhibitor concentration. describes the type of the isotherm accordingly. If the process is unfavourable, if , the process is linear, if , the process is favourable, and when the process is irreversible [39, 40]. In the present study, value was found to be less than 1 and greater than 0 indicating that the adsorption process was favourable and reversible.
Disodium salt of sulfosuccinate monoester was synthesized from the seed oil of Terminalia catappa which was found to contain oleic acid as the most dominant fatty acid. The disodium salt of sulfosuccinate monoester had good inhibitive capacity against the corrosion of aluminum sheet in 1 M HCl with inhibition efficiency increasing as the concentration of disodium salt of sulfosuccinate monoester increased while the corrosion rate decreased.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors are most grateful to the Department of Chemistry, University of Ibadan, Ibadan, Oyo State, and also the Department of Chemical Sciences, Redeemer’s University, Mowe, Ogun State, for supplying chemicals, equipment, and laboratory space for this research work.
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