Commiphora pedunculata gum as a green inhibitor for the corrosion of aluminium alloy in 0.1 M HCl Paul Ocheje Ameh & Nnabuk Okon Eddy
Research on Chemical Intermediates ISSN 0922-6168 Res Chem Intermed DOI 10.1007/s11164-013-1117-0
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Author's personal copy Res Chem Intermed DOI 10.1007/s11164-013-1117-0
Commiphora pedunculata gum as a green inhibitor for the corrosion of aluminium alloy in 0.1 M HCl Paul Ocheje Ameh • Nnabuk Okon Eddy
Received: 16 June 2012 / Accepted: 12 February 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract The effect of Commiphora pedunculata (CP) gum on the inhibition of the corrosion of aluminum alloy AA 3001) in solutions of HCl was investigated using gravimetric and thermometric methods of monitoring corrosion. The results obtained indicated that CP gum is a good adsorption inhibitor for the corrosion of aluminum in solutions of HCl. The inhibition efficiency of CP gum was found to increase with an increase in concentration but to decrease with increasing temperature. The adsorption of CP gum on the surface of aluminum was found to be endothermic, spontaneous and to support the mechanism of physical adsorption. The Langmuir adsorption model has been used to describe the adsorption characteristics of CP gum on aluminum surface. Keywords Corrosion Green inhibitors Commiphora pedunculata gum Aluminum
Introduction Although corrosion is only nature’s method of recycling, or of returning a metal to its lowest energy form, it is nevertheless a serious problem in the oil, fertilizer, metallurgical and other industries, where contact between metal and aggressive solutions are inevitable [1]. The rate of corrosion of a given metal can be affected by environmental variables such as temperature, pH, concentration of the aggressive medium, oxidizing power of the metal, etc. [2].
P. O. Ameh (&) N. O. Eddy Department of Chemistry, Ahmadu Bello University, Zaria, Kaduna State, Nigeria e-mail:
[email protected] N. O. Eddy e-mail:
[email protected]
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In order to tackle the menaces due to corrosion, several measures (including anodic and cathodic protection, lubrication, painting, and electroplating) are in use. However, one of the best options accessible for the protection of metals against corrosion involves the use of inhibitors. An inhibitor is a compound that has the potential to reduce the rate of corrosion of a metal through the mechanism of adsorption [3]. Corrosion inhibitors that are currently in use are either synthesized from cheap raw materials or are organic compounds that have centers for donation of p-electron(s). The electron-donating ability of such inhibitors is often enhanced by the presence of hetero atoms and suitable functional groups and the initial mechanism associated with corrosion inhibition involves the adsorption of the inhibitor on the metal surface [4, 5]. The adsorption of the inhibitor on the surface of the metal may proceed through the transfer of charge from the charged inhibitor to the metal surface (i.e. physical adsorption) or the transfer of electrons from the inhibitor to the metal surface (chemical adsorption). In spite of the high efficiencies recorded for most of these compounds, environmental requirements have been considered, in recent times, as a focus point for accepting corrosion inhibitors [6]. Some corrosion inhibitors are toxic and are rich in heavy metals, some are too expensive, and some are non-biodegradable [7]. Hence, eco-friendly corrosion inhibitors (green corrosion inhibitors) are preferred [8]. The search for green corrosion inhibitors has extended into the utilization of extracts of plants and other natural products, polymers, drugs, ionic compounds, amino acids, carbohydrates, etc. [9–11]. Of significant interest is the use of gums because they possess a large adsorption surface [12]. Gums are cheap, biodegradable, easily available, and eco-friendly [2]. A few studies have been reported on the corrosion inhibition potentials of some gums [13–15], but the literature is scanty on the use gum exudates from Commiphora pedunculata (CP) as corrosion inhibitors for aluminium alloy. Therefore, the present study is aimed at investigating the potentials of CP gum for the corrosion of aluminium alloy in solutions of HCl.
Materials and methods Aluminum alloy sheet of composition (wt%, as determined by quantiometric method) Mn (1.28), Pb (0.064), Zn (0.006), Ti (0.029), Cu (0.81), Si (0.381), Fe (0.57), and Al (96.65 %) was used. The sheets were mechanically pressed cut into different coupons, each with dimensions 5 9 4 9 0.11 cm. Each coupon was degreased by washing with ethanol, cleaned with acetone, and allowed to dry in the air before preservation in a desiccator. All reagents used for the study were analar grade and double-distilled water was used for their preparation. Thermometric method In order to determine the corrosion rate in the different reagents, a three-neck flask with provisions for the introduction of chemicals and for insertion of thermometer was used for thermometric study. For each study, the aluminum coupon was introduced into the flask. Enough quantities of each solution were in turn transferred
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into the flask until the aluminum coupon was completely immersed. Once this was done, the temperature of the reacting solution was read at 1-min intervals until a constant temperature was obtained. The reaction number (RN) of each system was calculated by dividing the difference between the highest and lowest temperature attained by the time interval. From the reaction number, the inhibition efficiency (%I) of the inhibitor was calculated using Eq. 1 [16]. %I ¼
RNaq RNwi 100 RNaq
ð1Þ
where RNaq is the reaction number in the absence of inhibitors (blank solution) and RNwi is the reaction number of 2 M HCl containing the studied inhibitor. Gravimetric method A clean, dried, and previously weighed aluminum alloy coupon was completely immersed in 250 ml of the test solution in a beaker. The beaker was covered with aluminum foil and inserted into a water bath maintained at 303 K. After every 24 h, the corrosion product was removed by washing each coupon (withdrawn from the test solution) in a solution containing 50 % NaOH and 100 g l-1 of zinc dust. The washed coupon was rinsed in acetone and dried in the air before re-weighing. The experiment was repeated at 333 K. In each case, the difference in weight for a period of 168 h was taken as the total weight loss. From the average weight loss (mean of three replicate analysis) results, the inhibition efficiency (% I) of the inhibitor, the degree of surface coverage (h), and the corrosion rate of aluminum (CR) were calculated using Eqs. 2–4, respectively, W1 %I ¼ 1 100 ð2Þ W2 W1 h¼ 1 ð3Þ W2 w2 w1 ð4Þ CR ¼ At where W1 and W2 are the weight losses (g) for aluminum in the presence and absence of the inhibitor, h is the degree of surface coverage of the inhibitor, and t is the period of immersion (in hours).
Results and discussions Effect of CP on the corrosion of aluminum Figure 1 shows the variation of weight loss with time for the corrosion of Al alloy in solutions of HCl containing various concentrations of CP gum at 303 and 333 K. The figure reveals that weight loss of Al alloy increases with increase in the period of contact but decreases with increasing concentration of CP gum, indicating that
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CP gum inhibited the corrosion of Al alloy in solutions of HCl. Weight loss of Al alloy was also found to increase with increase in temperature indicating that the corrosion rate of Al alloy in the presence of CP gum increases with increase in temperature, hence its inhibition efficiency also decreases with increasing temperature. This also indicates that CP gum is an adsorption inhibitor for the corrosion of Al alloy in solutions of HCl. Trends for the variation of temperature of Al alloy with time and with concentration of CP gum were similar to those observed for weight loss, and the results obtained from weight loss measurements correlated very well (r = 0.9871) with those obtained from the thermometric study. However, inhibition efficiencies obtained from thermometric measurements were relatively larger than those obtained from weight loss measurements, indicating that the instantaneous inhibition efficiency (as measured by thermometric methods) of CP gum is better than its average inhibition efficiency (as measured by weight loss).
Fig. 1 Variation of weight loss with time for the corrosion of aluminum alloy in solutions of HCl containing various concentrations of CP gum at at 303 and 333 K
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Values of corrosion rates of aluminium and inhibition efficiencies of various concentrations of Commiphora pedunculata (CP) gum are presented in Table 1. The results indicate that the corrosion rate of aluminium alloy increases with increasing temperature but decreases with the increase in the concentration of the inhibitor, indicating that these compounds inhibited the corrosion of aluminium alloy in solutions of HCl. The inhibition efficiencies of CP gum were found to increase with increasing concentration, also indicating that CP gum is a good adsorption inhibitor for the corrosion of aluminium alloy in solutions of HCl. For an adsorption inhibitor, the inhibition efficiency is expected to increase with increasing concentration [17]. The results obtained also indicated that the inhibition efficiency of CP gum tends to decrease with the increase in temperature, indicating that these compounds are adsorbed on the surface of aluminium through the mechanism of physical adsorption. According to Eddy et al. [16], the initial mechanism in any corrosion inhibition process is the adsorption of the inhibitor on the metal surface, and for a physical adsorption mechanism, the inhibition efficiency is expected to decrease with increasing temperature, but for a chemical adsorption inhibitor the inhibition efficiency is expected to increase with increasing temperature.
Effect of temperature According to Nnanna et al. [18], the comparison of corrosion activation energies in the presence of inhibitor and analysis of the temperature dependence of inhibition efficiency can give some insight into the possible mechanism of the inhibitor’s adsorption. Consequently, the Arrhenius equation (Eq. 5) was used to calculate the activation energies for the inhibited and uninhibited corrosion reactions of Al alloy. Ea 1 logCR ¼ log A ð5Þ 2:303R T where CR is the corrosion rate of Al alloy, A is the Arrhenius or pre-exponential constant, Ea is the activation energy, and T is the temperature. Considering a corrosion reaction proceeding between two temperatures, T1 and T2 (where T2 [ T1), then Eq. 5 can be simplified to the following form,
Table 1 Corrosion rate (CR), reaction number (RN) of Al alloy and inhibition efficiency (% I) of CP gum CR (g/h/cm2) (303 K)
R (g/h/cm2) (333 K)
3.24E-05
5.63E-05
1.49E-05
3.27E-05
59.24
0.7340
54.50
1.43E-05
2.56E-05
63.54
0.6566
58.72
55.56
1.34E-05
0.000025
65.87
0.6146
0.4
60.55
57.67
1.28E-05
2.38E-05
69.55
0.5484
0.5
64.22
57.95
1.16E-05
1.46E-05
73.03
0.4857
C (g/l)
% I (303 K)
% I (333 K)
0.1
54.13
41.80
0.2
55.96
0.3
Blank
% I (Therm)
RN (°C/min) 1.801
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CR2 Ea 1 1 log ¼ CR1 2:303R T1 T2
ð6Þ
Calculated values of Ea are presented in Table 2. The activation energies (which were below the threshold value of 80 kJ/mol required for the mechanism of chemical adsorption) tend to increase with increasing concentration indicating an enhancement in the inhibition process, with increasing concentration. The results obtained also indicate that the adsorption of CP gum is consistent with the mechanism of charge transferred from charged inhibitor to charged metal surface (physiosorption) [3]. The heat accompanying the adsorption of CP gum on the surface of Al alloy is significant in predicting the feasibility of the adsorption and inhibition process. This parameter was calculated by substituting values obtained for degrees of surface coverage of the inhibitors (at the studied temperatures) into an established equation (Eq. 7), developed from the Langmuir adsorption model,
Qads
h2 h1 T 1 T2 ¼ 2:303R log log kJ mol1 1 h2 1 h1 T 2 T1
ð7Þ
where Qads is the heat of adsorption of CP on Al alloy surface, h1 and h2 are the degrees of surface coverage of CP gum at temperatures, T1 (303 K) and T2 (333 K), respectively. Qads values are also presented in Table 2. The results reveal that Qads values are negative, hence the adsorption of CP gum on the Al alloy surface is exothermic.
Adsorption isotherm Basic information on the interaction between the inhibitor and the alloy surface can be provided by the adsorption isotherm and this was achieved by fitting data into different adsorption isotherms including Langmuir, Fruendlich, Temkin, Frumkin and El awardy adsorption isotherms. The tests revealed that the best isotherms for the adsorption of CP gum on Al surface are the Langmuir and Frumkin adsorption isotherms. The Langmuir adsorption isotherm can be represented as follows,
Table 2 Activation energy and heat of adsorption of CP gum on aluminium surface
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C (g/l)
Ea (kJ/mol)
Qads (kJ/mol)
Blank
50.27
0.1
54.43
0.2
54.55
-1.24367
0.3
54.83
-2.70747
-10.4154
0.4
54.51
-2.4989
0.5
54.56
-5.54202
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log
C ¼ logC logb h
ð8Þ
where C is the concentration of the inhibitor in the bulk electrolyte, h is the degree of surface coverage of the inhibitor, and b is the equilibrium constant of adsorption. Figure 2 shows the Langmuir isotherm for the adsorption of CP gum on the Al surface. Adsorption parameters deduced from the isotherm are presented in Table 3. The results indicated that the degrees of fitness of the data are 99.88 and 98.98 % respectively, while the slopes were very close to unity. These indicate that the Langmuir model is applicable to the adsorption of CP gum on the Al surface [17]. On the other hand, the assumptions establishing the Frumkin adsorption isotherm can be expressed as follows, h log ½C ¼ logb þ 2ah ð9Þ 1h where h is the degree of surface coverage, C is the concentration of the adsorbate, b is the adsorption coefficient, which represents the adsorption–desorption equilibrium constant, and a is lateral interaction parameter. Using Eq. 9, a plot of h log 1h ½C versus h yielded a straight line with R2 values tending to unity. The Frumkin isotherm for the adsorption of CP gum is presented in Fig. 3. In Table 3, adsorption parameters calculated from the plots are also presented. From the presented results, it can be seen that the interaction parameters, a, are positive, indicating that there is attraction between the molecules of CP gum and the Al alloy surface.
Fig. 2 Langmuir isotherm for the adsorption of CP gum on the surface of aluminum alloy
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Temperature (K)
Slope (a)
log b
R2
DG0ads (kJ/mol)
303
0.8996
0.1734
0.9988
-11.13
333
0.8013
0.1587
0.9898
-12.13
303
4.7334 (2.3667)
-2.7746
0.9101
-5.98
333
7.6891 (3.8446)
-4.7130
0.9232
-18.93
Fig. 3 Frumkin isotherm for the adsorption of CP gum on the Al surface
The equilibrium constant of adsorption obtained from the Langmuir and Frumkin adsorption isotherms is related to the standard free energy of adsorption according to the following equation [7], logb ¼ 1:744
DGads 2:303RT
ð10Þ
where DG0ads is the standard free energy of adsorption, R is the gas constant, b is the equilibrium constant of adsorption, and T is the temperature. Values of free energy calculated from Eq. 10 are presented in Table 3. The free energies (which ranged from -5.98 to -12.13 kJ/mol) are less than the limit (-40 kJ/mol) expected for the mechanism of chemical adsorption. Hence, the adsorption of CP gum on Al alloy surface is spontaneous and supports physisorption [19]. Conclusions From the results and findings of the study, it can be concluded that Commiphora pedunculata gum is an effective adsorption inhibitor for the corrosion of Al alloy in acidic media. Commiphora pedunculata gum inhibited the corrosion of Al alloy in acidic media through the mechanism of adsorption. The adsorption of the gum is
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exothermic and spontaneous and is best described by the Langmuir and Frumkin adsorption models. Hence, utilization of Commiphora pedunculata gum as corrosion inhibitor for Al alloy in acidic media is recommended. Acknowledgment The authors are grateful to the technical staff in the Department of Pharmaceutical Sciences for supporting this study.
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