Ewemen Journal of Analytical & Environmental Chemistry ISSN 2488-913X
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Original Research
CORROSION INHIBITION AND ADSORPTION POTENTIALS OF ETHANOL EXTRACTS OF PTEROCARPUS SOYAUXII Taub LEAVES FOR MILD STEEL IN ACIDIC MEDIUM.
*1ONUKWUBE N. D., 1OKORO O. A., 2NWACHUKWU I.
1Department of Chemistry, Abia State Polytechnic, Aba. Abia State, Nigeria. 2 Department of Biology/Microbiology, Abia State Polytechnic, Aba, Abia State, Nigeria.
Ethanol extract of Pterocarpus soyauxii Taub, a species of Pterocarpus in the family Fabaceae, was investigated using gravimetric and thermometric techniques with the aim of determining its inhibition and adsorption potentials in acidic medium, effect of temperature on the corrosion of mild steel, effect of different concentrations of ethanol extract of P. soyauxii on mild steel corrosion at different temperatures, establish the kinetic and thermodynamic parameters, propose the mechanism of corrosion inhibition of mild steel in H2SO4 as well as identify the functional groups in the extract. From the results, inhibition efficiency was found to increase with increase in concentration of the inhibitor. The presence of catenated carbon compounds with heteroatoms and π electron structures as revealed by the FTIR and GC-MS studies are clear indication that P. soyauxii is a good inhibitor for the corrosion of mild steel. The adsorption behaviour of ethanol extract of P. soyauxii on the surface of mild steel was found to be consistent with exothermic and spontaneous processes and supported the mechanism of physical adsorption. Kinetic consideration revealed an increase in the half-life of mild steel in H2SO4 solution in the presence of the inhibitor, even as Langmuir adsorption isotherm was found to best suit the adsorption characteristics of the inhibitor on the surface of mild steel. Keywords: Pterocarpus soyauxii, adsorption, heteroatom, inhibition, catenated carbon.
INTRODUCTION
Corrosion as a naturally occurring phenomenon is commonly defined as deterioration of metal surfaces caused by the reaction with the surrounding environmental conditions. Mild steel, a ferrous material is largely used in acidic media in most industries including oil/gas exploration and ancillary activities. Acid solutions are commonly used in chemical industry to remove scales from metallic
surfaces and because steel is frequently used in contact with acidic solutions, its corrosion rate must be controlled. One of the useful methods of controlling the corrosion process is the addition of inhibitors. The use of inhibitors is one of the most practical methods for protection against metallic corrosion, especially in acidic media as a corrosion inhibitor is a substance which when added in small
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concentration to an environment, effectively reduces the corrosion rate of a metal exposed to it. However, separating the metal from corrosive environments has undoubtedly become the major tactic in mitigating metal corrosion. The use of chemical inhibitors has been limited recently, due to environmental challenges. Various inhibitors in use have been found to be hazardous to health and the environment at large. Thus efforts have been intensified towards formulation of modern environmentally safe inhibitors of which plant extracts have become important as eco-friendly, economical, readily available and renewable sources of effective corrosion inhibitors (Raja and Sethuraman, 2008). To be effective, an inhibitor must also displace water from the metal surface, interact with anodic or cathodic reaction sites to retard the oxidation and reduction corrosion reaction, and prevent transportation of water and corrosion active species on the surface (Obot et al., 2011). Several research groups have found plant materials such as Azadirachta indica (Oguzie, 2008), Occimum viridis (Oguzie, 2006), Strychnos nux-vomica (Raja and Sethuraman, 2009), Prosopis cineraria (Sharma et al., 2008), Hibiscus sabdariffa extract (Oguzie, 2008), Olives leaves (El-Etre, 2007), Datura stramonium (Raja and Sethuraman, 2007), Aloe vera extract (Abiola and James, 2009) as well as Phyllantus amarus extracts (Okafor et al., 2008) are effective inhibitors for metal in aggressive solutions. Some other research groups have equally reported on the corrosion inhibitive effectiveness of metals by Dacroydes edulis (Umoren et al., 2008a), Pachylobus edulis Umoren et al., 2008b), Vigna unguiculata (Umoren et al., 2008c), Gum Arabic (Umoren et al., 2006c), Raphia hookeri (Umoren et al., 2009), Ipomoea invulcrata (Obot et al., 2009) and Azadirachta indica (Okafor et al., 2010) as good corrosion inhibitors for different metals in various environments as optimum results were reported for all of them. The corrosion resistance of Steel is due to the formation of a passive film on its surface upon exposure to the corrosive media due to the presence of an inhibitor (Eddy et al., 2012). Similar work have been done by several research groups, even as most of them returned physiosorption as the mechanism of adsorption of
inhibitors on the surface of mild steel , while Langmuir isotherm was discovered as the best adsorption model (Obot et al., 2011; Eddy et al., 2012; Rajendran et al., 2013; Onuegbu et al., 2013). This work was designed as part of our contribution to the growing interest on environmentally benign corrosion inhibitors as we seek to investigate the corrosion inhibition and adsorption behaviour of ethanol extracts of Pterocarpus soyauxii leaves on mild steel in acidic medium. The genus Pterocarpus which is tropically and sub-tropically distributed belongs to the family Leguminosae. There are about 60 species of the genus of which 20 of these are found in Africa in countries such as Nigeria, Cameroon, Sierra Leone and Equatorial Guinea. The leaves of Pterocarpus soyauxii are used for soup making in the South Eastern part of Nigeria. Some tribes in the Eastern and Southern Nigeria use the leaf extracts in the treatment of headaches, pains, fever, convulsions and respiratory disorders and as antimicrobial agents (Onukwube et al., 2016). Notable phytochemical constituents of ethanol extract of P. soyauxii are saponins, phenols, alkaloids, flavonoids, tannins, cyanogenic glycosides, steroids (Ndukwe and Ikpeama, 2013). MATERIALS AND METHODS. Materials
Materials used for the study were mild steel sheets of composition (% w/w) Fe (98.86), Mn (0.6), P (0.36), C (0.15) and Si (0.03). The sheet was mechanically pressed cut into different coupons, each of dimension 4x3x0.11 cm. Each coupon was degreased by washing with ethanol, rinsed with acetone and allowed to dry in air before they were preserved in desiccators. Shimadzu FTIR-8400S Fourier transform infra-red spectrophotometer. GC clarus 500 Perkin Elmer system comprising AOC-20i auto sampler and gas chromatograph interfaced to a mass spectrometer (GC-MS) instrument. All reagents used for the study were Analar grade and double distilled water was used for their preparation. Extraction of plant
The leaves of Pterocarpus soyauxii were collected from several sites within Aba North L.G.A. of Abia State and were identified in the Biology/Microbiology Department of Abia State polytechnic Aba. The leaves
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were ground after air-drying. 100 g of the ground leaves were soaked in ethanol for 48 hours and subjected to heat. The Sample was cooled and filtered. The filtrate was further subjected to evaporation at 352K using thermo stated water bath in order to leave the sample free of the ethanol. The slurry extract so obtained was used in preparing different concentrations of the inhibitor by dissolving 0.1 g, 0.2 g, 0.3 g, 0.4 g and 0.5 g of the extract in 1 L of 1.0 M H2SO4 and 1 L of 2.5 M H2SO4 for use in gravimetric and thermometric analyses, respectively. Gravimetric Analysis
Gravimetric study using the ethanol extract of Pterocarpus soyauxii leaves was done by dipping the pre-cleaned mild steel into 20 mL of the test solution maintained at 303 and 333 K in a thermo stated bath. The weight loss was determined by retrieving the mild steel coupons at 1hr interval progressively for 7 hrs. Prior to measurement, each retrieved coupon was immersed in a solution of 20% NaOH containing 200 g/L of Zinc dust to terminate the corrosion reaction, scrubbed with brittle brush several times and dried in acetone. The difference in weight was taken as the weight loss of the steel. A control experiment was equally set up by considering the weight loss of mild steel coupons in various concentrations of H2SO4 (0.1 -0.5 g/L) in the absence of the inhibitor. From the weight loss, the inhibition efficiency of the inhibitor (I %), degree of surface coverage (θ) and corrosion rate (CR) of the steel were calculated using equations 1, 2 and 3 respectively
% 𝐼 = 1 − 𝑊2
𝑊1
∗ 100 …………………… . . (1)
Ө = %𝐼
100 ……………………………………… (2)
𝐶𝑅 = 𝑊
𝐴𝑡 ………………………………… . . (3)
Where % I is the inhibition efficiency of the ethanol extract of P. soyauxii, CR is the corrosion rate of mild steel in gcm-2hr-1, W1 and W2 are the weight losses of steel in the absence and presence of the inhibitor, respectively. W is the difference in weight in (g) before and after immersion, (i.e. W = W2 – W1), t is the period of immersion in hours and A is the area of the steel coupon in square cm.
Thermometric method.
This is a sensitive method for corrosion study, which involves the monitoring of temperature as corrosion progresses. A three necked reaction flask with provisions for the insertion of a thermometer and mild steel coupon into the test solution was used. To prevent heat loss to the surrounding, the flask was lagged. The temperature of the system was measured after every one minute until a steady value was obtained. (Odoemelam et al ., 2009; Eddy et al., 2008; Eddy et al., 2009, Umoren et al., 2008a; Odiogenyi et al., 2009). From the average of three replicate measurements obtained for the rise in temperature of the system per minute, the reaction number (RN) was calculated using Equation (4).
𝑅𝑁 ℃ 𝑚𝑖𝑛 = 𝑇𝑚 − 𝑇𝑖
𝑡 ……………… . ………… . . (4)
Where Tm and Ti are the maximum and initial temperatures, respectively, and t is the time (min) taken to reach the maximum temperature. The inhibition efficiency (% I) was evaluated from percentage reduction in the RN, using equation (5).
% 𝐼 = 𝑅𝑁𝑎𝑞 − 𝑅𝑁𝑤𝑖
𝑅𝑁𝑎𝑞
………… . . ………………… (5)
Where RNaq is the reaction number in the absence of inhibitors (blank solution) and RNwi is the reaction number for 2.5 M H2SO4 containing the inhibitor under study. Chemical analysis of samples.
FTIR analysis
FTIR analyses of ethanol extracts of P. Soyauxii sample and that of the corrosion products (particles scrubbed off from the surface of mild steel after the inhibition process at 0.5g/l of the inhibitor) were carried out using Shimadzu FTIR-8400S Spectrophotometer. The sample was prepared using KBr and the analysis was done by scanning the sample through a wave number range of 650 to 4000 cm-1. GC-MS analysis
GC-MS analysis was carried out on a GC clarus 500 Perkin Elmer system comprising a AOC-20i auto sampler and gas chromatograph interfaced to a mass spectrometer (GC-MS) instrument employing the following conditions: column Elite-1 fused silica
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capillary column (30 x 0.25 mm id x 1μm df, composed of 100% dimethyl poly siloxane), operating in electron impact mode at 70 eV; helium (99.999%) was used as carrier gas at a constant flow of 1 mL /min and an injection volume of 0.5 μL was employed (split ratio of 10:1) injector temperature 250 ºC; ion-source temperature 280 ºC. The oven temperature was programmed from 110 ºC (isothermal for 2 min), with an increase of 10 ºC/min, to 200 ºC, then 5 ºC/min to 280 ºC, ending with a 9 min isothermal at 280 ºC. Mass spectra were taken at 70 eV, a scan interval of 0.5 seconds and fragments from 40 to 450 Da. Total GC running time was 36min. Interpretation of mass spectrum GC-MS was conducted using the database of National Institute of Standard and Technology (2015) having more than 62,000 patterns. The spectrum of the unknown component was compared with the spectrum of the known components stored in the NIST library. The name, molecular weight and structure of the components of the test materials were ascertained. Concentrations of the identified compounds were determined through area and height normalization. RESULTS AND DISCUSSION
Inhibitor concentration and effect of temperature.
The variations of weight loss of mild steel with time for the corrosion of mild steel in the presence and absence of the test solution indicate a reduction in the weight loss of the mild steel in the presence of the inhibitor compared to the uninhibited solution (blank) as shown in Figures 1 and 2. Weight loss of mild steel was however discovered to have maintained a direct proportional relationship with the inhibitor concentration even at higher temperature, though with greater values. Thus, ethanol extract of Pterocarpus soyauxii inhibited the corrosion of mild steel in 0.1 M H2SO4. Variation of temperature with time for the corrosion of mild steel in 2.5 M H2SO4 containing various concentrations of Pterocarpus soyauxii in a controlled environment, (Figure 3) equally showed a reduction in the rate of change of temperature with time for the inhibitor solutions compared to the blank solution, suggesting that P. Soyauxii solution inhibited the corrosion of mild steel.
Figure 1: Variations of weight loss of mild steel with time for the corrosion of mild steel in 1.0 M H2SO4 containing various concentrations of Pterocarpus soyauxii at 303 K.
Figure 2: Variations of weight loss of mild steel with time for the corrosion of mild steel in 1.0 M H2SO4 containing various concentrations of Pterocarpus soyauxii at 333 K.
Figure 3: Variations of temperature with time for the corrosion of mild steel in 2.5 M H2SO4 containing various concentrations of Pterocarpus soyauxii.
0
0.2
0.4
0.6
0.8
1 2 3 4 5 6 7
We
igh
t Lo
ss (
g)
Time (hrs)
BLANK
0.1 g/L
0.2 g/L
0.3 g/L
0.4 g/L
0.5 g/L
0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4 5 6 7
We
igh
t Lo
ss (
g)
Time (hrs)
BLANK
0.1 g/L
0.2 g/L
0.3 g/L
0.4 g/L
0.5 g/L
0
5
10
15
20
25
30
35
40
1 3 5 7 9 11 13 15 17 19
Tem
pe
ratu
re (
ᵒC)
Time (mins)
Blank
0.1 g/L
0.2 g/L
0.3 g/L
0.4 g/L
0.5 g/L
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Inhibition Efficiency of the inhibitor.
Table 1 shows values of Inhibition efficiency (%I), Corrosion rates at 303 and 333 K obtained from Gravimetric method, Inhibition efficiency (%I) and Reaction number obtained from Thermometric method for the corrosion of mild steel in H2SO4 solution containing different concentrations of Pterocarpus soyauxii leaf extracts. The Table indicates that addition of small amount of P. Soyauxii solution to the H2SO4 solution containing mild steel resulted in a remarkable decrease in the corrosion rate of mild steel majorly at lower temperature, suggesting physical adsorption of the inhibitor molecules on the surface of mild steel and partial desorption with rise in temperature (Eddy and Ebenso, 2008; Eddy and Odoemelam, 2008; Eddy and Ekop, 2008, Namrata et al., 2015). For a physical adsorption mechanism, the inhibition efficiency, hence the extent of adsorption tends to decrease with increase in temperature but for a chemical adsorption mechanism, the reverse is the case (Eddy et al., 2012). Reaction number was equally found to maintain a downward trend with increase in the concentration of the inhibitor, confirming ethanol extract of Pterocarpus soyauxii as a good corrosion inhibitor for the corrosion of mild steel in acidic medium. Thus, corrosion rate and reaction number were found to be functions of the concentration of the acid and temperature. Table 1: Values of Inhibition efficiency (%I), Corrosion rates at 303 and 333 K obtained from Gravimetric method, Inhibition efficiency (%I) and Reaction number obtained from Thermometric method for the corrosion of mild steel in H2SO4 solution containing different concentrations of the leaf extracts.
Also, the inhibition efficiency (%I) increased with increase in the concentration of the added inhibitor. The inhibition efficiency may be affected by many factors, such as the adsorption of the inhibitor on mild steel surface, which in turn may depend on some physicochemical properties, such as the functional groups, stearic factors, electronic and the geometrical
configurations of the inhibitor (Zaafarany, 2006, Zaafarany et al., 2010). Values of inhibition efficiency from weight loss and thermometric methods showed no significant difference (p = 0.05) within the temperature range under review, indicating that the inhibitor can be used both for short and long term inhibitions. Kinetics of the corrosion reaction.
Data obtained from weight loss were treated kinetically by making attempts to establish the order of reaction for the corrosion of mild steel in the presence and absence of Pterocarpus soyauxii leaf extracts. The results obtained indicated that for all concentrations of P. soyauxii leaf extracts, the plots of –log(weight loss) versus time were linear. This indicates that the corrosion of mild steel and its inhibition by P. soyauxii leaf extracts is consistent with a pseudo first order kinetic, which can be written as shown in equation (6) (Eddy et al., 2012, Onukwube et al., 2016).
− log 𝑤𝑒𝑖𝑔𝑡 𝑙𝑜𝑠𝑠 = 𝑘1𝑡
2.303 ……………………… . (6)
Where k1 is the first order rate constant and t is the time in days. The kinetic plots for the corrosion of mild steel in 1.0 M H2SO4 and its inhibition by Pterocarpus soyauxii leaf extracts is shown in Figure 4. Similar plots were equally observed at 333 K.
Figure 4: Variation of –log (weight loss) versus time for the corrosion of mild steel in H2SO4 containing various concentrations of Pterocarpus soyauxii leaf extracts at 303 K
The kinetic parameters deduced from the kinetic plots are as shown in table 2. Also for a first order reaction, the rate constant is related to the half-life according to equation (7).
0
0.5
1
1.5
2
0 2 4 6 8
-Lo
g (W
eig
ht
Loss
)
Time (hrs)
0.1 g/L
0.2 g/L
0.3 g/L
0.4 g/L
0.5 g/L
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𝑡1/2 = 0.693
𝐾1
………………………………………… (7)
Calculated values of t1/2 are also presented in Table 2. The results show that Pterocarpus soyauxii leaf extracts increased the half-life of mild steel in solutions of H2SO4. Table 2: Kinetic parameters for the corrosion of mild steel in 1.0 M H2SO4 containing various concentrations of Pterocarpus soyauxii at 303 and 333 K.
303 K 333 K Conc. g/l
K1 t1/2(days)
R2 K1 t1/2(days)
R2
B I B I 0.1 0.14 2 5 0.963 0.07 4 10 0.950 0.2 0.15 2 5 0.875 0.07 4 10 0.963 0.3 0.17 2 5 0.837 0.07 4 10 0.975 0.4 0.17 2 5 0.915 0.07 4 10 0.968
Key: B = Blank, I = Inhibitor
Thermodynamic and Adsorption considerations
The activation energy for the corrosion of mild steel in the presence of Pterocarpus soyauxii leaf extracts was calculated using the Arrhenius equation, which can be written as shown in equation (8) (Eddy, 2008, Onukwube et al., 2016).
logCR2
CR1
= Ea
2.303R
1
T1
− 1
T2
………………………… (8)
Where Ea is the activation energy, CR1 and CR2 are the corrosion rates of mild steel at the temperatures T1 (303 K) and T2 (333 K) respectively. Values of Ea obtained from equation (8) are recorded in Table 3. These values ranged from 6.1 to 12.8 (kJ/mol) and less than 80kJ/mol, which is the threshold value required for chemical adsorption. Thus, the activation energies for the corrosion of mild steel in the presence of various concentrations of P. soyauxii are consistent with mechanism of physical adsorption. The heat of adsorption of P. soyauxii on the surface of mild steel was calculated using equation (9). For a corrosion inhibition reaction carried out at constant pressure, heat of adsorption should approximate the enthalpy change (Umoren et al., 2006a-c).
Qads =2.303R Log Ө2
1−Ө2 −Log
Ө11− Ө1
x T1T2
T2 −T1 kJ mol −1 …………..(9)
Where Ө1 and Ө2 are the degrees of surface coverage of the inhibitor at temperatures, T1 (303 K) and T2
(333 K) respectively and R is the gas constant. Calculated values of Qads are negative as recorded in Table 3 and less than the threshold value of -80 kJ/mol needed for chemical adsorption. Thus, the adsorption of P. soyauxii on mild steel surface is exothermic and consistent with physical adsorption mechanism. Table 3: Thermodynamic parameters for the adsorption of Pterocarpus soyauxii on mild steel surface in acidic medium.
Data obtained from degrees of surface coverage Ө at various concentrations of the inhibitor were used to fit into different adsorption isotherms (Eddy and Ebenso, 2011). All the isotherms can be represented as follows: 𝑓 Ө, 𝑥 𝑒𝑥𝑝 −2𝑎Ө = 𝐾𝐶 …………………………… (10)
Where f(Ө, x) is the configuration factor which depends upon the physical model and the assumptions underlying the derivation of the isotherm, Ө is the degree of surface coverage, C is the inhibitor concentration in the electrolyte, x is the size ratio, a is molecular interaction parameter and K is the equilibrium constant of the adsorption process. Langmuir adsorption model was found to best describe the adsorption of ethanol extract of P. soyauxii on mild steel surface and it is described by equation (11) (Onukwube, et al., 2016).
𝐿𝑜𝑔 𝐶
Ө = 𝐿𝑜𝑔𝐶 − 𝐿𝑜𝑔𝐾 ……………………… . (11)
Free energy values were equally calculated using equation (12), which relates K (Equilibrium constant of adsorption) to free energy of adsorption. Figure 5 shows the Langmuir isotherm plot for the adsorption of ethanol extract of P. soyauxii on mild steel surface. Langmuir adsorption isotherm confirms the formation of multi-layer adsorption, where there is no interaction between the adsorbate and adsorbent (Eddy and Ebenso, 2008). It was observed from the results that Langmuir adsorption isotherm was best applicable at 303 K, (R2 almost 1). Thus, the adsorption behaviour of the inhibitor is strongly
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influenced by temperature. The adsorption of the inhibitor can be said to be spontaneous and consistent with the mechanism of physical adsorption as exemplified in the values of ΔGoads which are negatively less than the threshold value of -40 kJ/mol. ∆𝐺𝑎𝑑𝑠 = −2.303𝑅𝑇𝑙𝑜𝑔 55.5𝐾 ………… (12)
Figure 5: Langmuir plots for the adsorption of Pterocarpus soyauxii on mild steel surface Table 4: Langmuir adsorption parameters for the adsorption of Pterocarpus soyauxii on the surface of mild steel in acidic medium
Model Temp
(K) Slope Log K ∆Gads
(kJ/mol) R2
Langmuir 303 333
0.721 0.693
0.05 -0.17
-10.41 -10.04
0.999 0.995
FTIR and GC-MS studies
The FTIR spectrum of Pterocarpus soyauxii leaves was analyzed and the characteristics parameters are presented in Table 5. Table 5: Frequencies, peaks and Functional groups of IR absorption by Pterocarpus soyauxii and the corrosion product of mild steel when the extract was used as inhibitor
Pterocarpus soyauxii Inhibited corrosion product Frequency (cm-1)
The OH stretch at 3313 cm-1 was shifted to 3037 cm-1, C-H stretch at 1441 was shifted to 1402 cm-1. Also the C=O stretch at 1032 cm-1, the NH2 wagging at 782 were shifted to 1009 and 678 cm-1. These shifts indicate that there is interaction between the metal and the inhibitor. Some bonds were found to be missing including C-Br stretch at 657 cm-1, C-H symmetric stretch at 1318 cm-1, C-N stretch at 1544 cm-1, C=C stretch at1620 cm-1, amide stretch at 1639 cm-1, C=O stretch at 1730 cm-1, OH stretch at 2085 cm-
1, CH stretch at 2851 and CH stretch at 2918 cm-1. These bonds must have been used for the formation of bonds with the mild steel surface. New OH bonds were formed at 3853, 3037, 3026 and 3019 cm-1. This suggests that these bonds were used for the adsorption of the inhibitor unto the metal surface. GC-MS study of Pterocarpus soyauxii The GC-MS spectrum of Pterocarpus soyauxi leaves was equally analyzed and the characteristics parameters presented in Table 6. The chromatographic spectrum of the extract of P. soyauxii is as shown in Figure 6 from where the various lines and their fragments were further identified using the Mass Spectrometer. Chemical structures of compounds suggested by reliable spectral library from National Institutes of Standards and Technology (NIST) are presented in Figure 7. Since the area (and height) under a chromatogram is proportional to the concentration of the active specie, data obtained from area normalization of the respective peaks were used in calculating the
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
-1.5 -1 -0.5 0 0.5
Log
C/Ө
Log C
303 K
333 K
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concentrations of the constituents of the inhibitor and the results obtained are also presented in Table 6. Table 6: Characteristics of suggested compounds identified from GC-MS of Pterocarpus soyauxii leaves.
Key: L/N = Line Number, MF = Molecular Formula, MM= Molar Mass, RT = Retention Time
Figure 6: The Chromatogram of Pterocarpus soyauxii extract
Figure 7: Chemical structures of compounds identified in the GCMS spectrum of Pterocarpus soyauxii (Numbering on the structures correspond to the line number in the GC-MS spectrum)
Table 6 also shows the chemical names of the compounds which were mainly carboxylic acids, the retention time and fragmentation pattern. The GC-MS spectrum of Pterocarpus soyauxii leaves revealed the presence of 0.94 % of nonanoic acid, tridecanoic acid,
1. Nonanoic acid 2. Tridecanoic acid
3. Pentadecanoic acid
4. 11-Octadecenoic acid
5. Octadecanoic acid 7. 9-Octadecenal
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pentadecanoic acid, 11- octadecanoic acid, octadecanoic acid and 9,-12-octadecanienoic acid.
Figure 8: IR spectrum of Ethanol extract of Pterocarpus soyauxii.
Figure 9: IR spectrum of the corrosion product (i.e. in the presence of ethanol extract of Pterocarpus soyauxii.) CONCLUSION
The results obtained from this study reveal that ethanol extract of Pterocarcus soyauxii is an efficient corrosion inhibitor for mild steel in solution of H2SO4. This is evident in the extension of the half life of mild steel corrosion in the test solution. The inhibitive properties of ethanol extract of P. soyauxii leaves are attributed to the presence of phytochemicals as well as long chain carboxylic acids and carbonyl compounds. Average inhibition efficiency obtained from Thermometric measurement is slightly lower than that obtained from weight loss measurements, suggesting that P. soyauxii is more effective over a range of time than instantaneous inhibition. The
extract inhibits corrosion of mild steel by being adsorbed onto the surface of the metal. Langmuir adsorption model was found to best suit the adsorption behaviour of the inhibitor on mild steel surface. The adsorption of ethanol extract of P. soyauxii on the surface of mild is spontaneous, exothermic and consistent with the mechanism of physical adsorption. From the above studies, the use of Ethanol extracts of P. soyauxii as a green inhibitor for mild steel corrosion in H2SO4 solution within the temperature range under review is therefore recommended. This can actually help in the elimination of toxicants associated with conventional inhibitors such as inorganic and synthesized inhibitors. CONFLICT OF INTEREST
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Article’s citation
Onukwube ND, Okoro OA and Nwachukwu I (2017). Corrosion inhibition and adsorption potentials of ethanol extracts of Pterocarpus soyauxii Taub leaves for mild steel in acidic medium. Ew J Anal & Environ Chem 3(1): 119 – 128.
