Accepted 5 July, 2011

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1 African Journal of Pure and Applied Chemistry Vol. 5(8), pp , August 2011 Available online at ISSN Academic Journals Full Length Research Paper Comparative study of corrosion inhibition of aluminium alloy of type AA3003 in acidic and alkaline media by Euphorbia hirta extract. L. A. Nnanna 1 *, I. U. Anozie 1, A. G. I. Avoaja 2, C. S. Akoma 1 and E. P. Eti 3 1 Department of Physics/Electronics, Abia State Polytechnic, P. M. B. 7166, Aba, Abia State, Nigeria. 2 Department of Chemistry, Abia State Polytechnic, P. M. B. 7166, Aba, Abia State, Nigeria. 3 Mechanical Engineering Department, Abia State Polytechnic, P. M. B. 7166, Aba, Abia State, Nigeria. Accepted 5 July, 2011 The inhibitive effect of leaf extract of Euphorbia hirta on aluminium corrosion in HCl and NaOH solutions were studied using gravimetric technique at 30 and 60 C. Results obtained showed that the leaf extract functioned as effective and excellent inhibitors in the acidic and alkaline media. Also, the leaf extract suppressed the corrosion reaction in both media. Inhibition efficiency of the extract was found to vary with concentration, temperature and exposure time. The leaf extracts of E. hirta demonstrated better inhibition efficiency in the acidic medium than in the alkaline medium. The adsorption characteristics of the leaf extract were best described by the Langmuir isotherm. Values of the free energy of adsorption G ads were determined from the isotherm parameters and they showed that the adsorption was spontaneous. The molecular interaction parameter (α) between the molecular species of the inhibitors and the metal surface they cover was also determined. The heat of adsorption (Q ads ) and apparent activation energy (E a ) were estimated from the temperature dependence of inhibition efficiency. Physical adsorption mechanism is proposed from the obtained thermodynamic parameters. Key words: Adsorption isotherm, corrosion inhibition, aluminium, leaf extracts, thermodynamics. INTRODUCTION The study of the corrosion behaviour of aluminium in different aggressive environments has continued to attract considerable attention because of the many applications of the metal. Aluminium relies on the formation of a compact, adherent passive oxide film for its corrosion immunity in various environments. This surface film is amphoteric and dissolves substantially when the metal is exposed to high concentrations of acids or bases (Hurlen et al., 1984; Oguzie, 2007). Bared metal surface sites become exposed to the corrodent after the breakdown of the oxide film, leading to a sequence electrochemical reaction as the metal dissolves. Corrosion is accompanied by many conjugate cathodic processes in which hydrogen gas evolution is *Corresponding author. lebennanna@yahoo.com. Tel: often predominant in strong acid and alkaline solutions (Gomma, 1998; Obot et al., 2009). Hydrochloric acid solutions are normally used for pickling of aluminium and for its chemical and electrochemical etching, processes that normally lead to substantial loss of the metal to corrosion (Oguzie, 2007; Nnanna et al., 2010). In efforts to mitigate aluminium corrosion, the main strategy is to effectively isolate the metal from corrosive agents. This can be achieved by the use of corrosion inhibitors. Inorganic substances such as phosphates, chromates, dichromate and arsenates have been found effective as inhibitors of metal corrosion, but a major disadvantage is their toxicity and as such their use has come under severe criticism (Oguzie, 2007). Among the alternative corrosion inhibitors, organic substances containing polar functions with nitrogen, oxygen and/or sulphur atoms in a conjugated system have been reported to exhibit good inhibiting properties(oguzie, 2007; Nnanna et al., 2010; Abdallah, 2004; Maayta et al., 2004; Oguzie, 2005;

2 266 Afr. J. Pure Appl. Chem. Oguzie, 2006; Popova et al., 2004; Rosliza et al., 2008; Umoren et al., 2008). Corrosion inhibition occurs via adsorption of their molecules on the corroding metal surface and efficiency of inhibition depends on the mechanical, structural and chemical characteristics of the adsorption layers formed under particular conditions. Some investigations have in recent time been made into the corrosion inhibiting properties of natural products of plant origin, which have been found to generally exhibit good inhibition efficiencies (Abiola et al., 2007; El-Etre, 2006; Eddy et al., 2008; Ebenso et al., 2009; Oguzie, 2008; El-Etre, 2003; Zucchi et al., 1985; Nnanna et al., 2010). This area of research is of much importance because in addition to being inexpensive, readily available and renewable sources of materials, plant products are environmentally friendly and ecologically acceptable. Plant products are organic in nature and some of the constituents including tannins, alkaloids, saponins, essential oils, flavones, organic and amino acids are known to exhibit inhibiting action (Okwu et al., 2003; Olabiyi et al., 2008; El-Sayed, 1997; Chetouani et al., 2005; Umoren et al., 2005; Bouklah et al., 2006). As a contribution to the current interest on environmentally friendly, green, corrosion inhibitors, the present study investigates the inhibiting effect of leaf extract of E. hirta on aluminium corrosion in acidic and alkaline solutions using the gravimetric technique. The plants, which are popularly called asthma weed (E. hirta), belongs to the family of Euphorbiaceae (Spurge family) and are known to grow in Sub-Sahara Africa and Asia (Okwu et al., 2003; Olabiyi et al., 2008). The effect of temperature on the inhibition efficiency has also been studied. Kinetic and activation parameters that govern metal corrosion have been evaluated. MATERIALS AND METHODS Aluminium sheets of type AA3003 having weight percentage composition of : Si-0.362%, Fe-0.54%, Cu-0.077%, Mn-1.219%, Ti %, Pb-0.063%, Zn-0.004% and the remainder being Al were used in this study. The test coupons were prepared having a dimension of 2 x 2 cm and thickness 1.32 mm, were polished mechanically with Sic emery papers of grade nos 220, 400 and 600 degreased with ethanol and acetone having been washed with distilled water. The test coupons were air dried before being immersed in the acidic and alkaline media. All chemicals and reagents used were of analytical grade. The blank corrodents were, respectively, 0.5 M HCl and 0.25 M NaOH solutions. Stock solution of the leaf extract was prepared by boiling weighed amount of the dried and ground leaves of E. hirta for 3 h in the 0.5 M HCl and 0.25 M NaOH, respectively. The solutions were cooled and then filtered and stored. From the respective stock solutions, inhibitor test solutions were prepared in the concentration range 0.1 to 0.3 g/l. The cleaned and dried test coupons were weighed before in the respective test solution using electronic balance of series JA 1003A with accuracy of ± Total immersion time interval was 5 h. Tests were conducted with different concentration of inhibitors, after which, the specimen were carefully washed in absolute ethanol having used nitric acid to quench further corrosion from taking place, and then reweighed. Triplicate experiments were performed in each case and the mean values reported. RESULTS Gravimetric technique and corrosion rates The weight loss of the aluminium alloy in 0.5 M HCl and 0.25 M NaOH with and without the various concentrations of leaf extract was determined after 5 h of immersion at 30 C and the corrosion rate values were evaluated using Equation 1. The corrosion rate values were plotted against the concentration values of the leaf extract (Figure 1). From the bar chart, the extract showed higher corrosion rate values in the alkaline medium than in the acidic medium. This depicts a better corrosion inhibition in the acidic environment than in the alkaline medium. Inhibition efficiency The characterization of the corrosion rate of aluminium alloy in the inhibitor/corrodent solution was carried out by an assessment of the inhibition efficiency (I %) using Equation 2 below. The bar chart representing the inhibition efficiency with respect to the concentration of inhibitor for E. hirta in the acidic and alkaline environments was plotted (Figure 2). In all the concentrations, considering the approximate rise in values, the acidic media exhibited better inhibition efficiency than the alkaline with as much as about 10% efficiency. DISCUSSION In accounting for the observed protective effect, it should be noted that the extract comprise mixture of organic and resinous matter, some of which have good corrosion inhibiting abilities. The complex chemical compositions make it rather difficult to assign the inhibiting action to a particular constituent or group of constituents. Nevertheless, the net adsorption of the extract organic matter on the metal surface creates a barrier to charge and mass transfer, thus protecting the alloy surface from corrodent attack. The degree of protection varies for different extracts, with notable sensitivity to the inhibitor concentration. Tables 1, 2 and 3 are a listing of the phytochemical components of the extract, which include tannins, alkaloids, and essential oil. The inhibitive properties of tannins have been attributed to the reaction (1) (2)

3 Nnanna et al. 267 Figure 1. Corrosion rates of various concentrations of E. hirta leaf extract in 0.5 m HCI and 0.25 m NaOH on aluminium alloy AA3003. Figure 2. Inhibition efficiencies of various concentrations of E. hirta leaf extract in 0.5 m HCI and 0.25 m NaOH on aluminium alloy AA3003. of the polyphenolic fraction of the tannins moieties, which ensures effective protection of the metal surfaces. The triterpeniod and other constituents also posses functional groups which are capable of chelating with aluminium ions and thus facilitate strong coordination on the alloy surface (Umoren, 2009). Having ascertained the effectiveness of these leaf materials in the present study, subsequent studies to enable precise interpretation of the nature and composition of the absorbed inhibitor layer will be achieved from in depth studies of the corrosion morphology and characterization of the active materials in the absorbed layer is envisaged. Langmuir adsorption isotherms Further elucidation of adsorption mechanism from the experimental data requires estimation of the adsorption

4 268 Afr. J. Pure Appl. Chem. Table 1. Constituents of E. Hirta leaf. S. no. Constituents of E. hirta leaf 1 Triterpenoids 2 Alkaloids 3 Tannins 4 Glycosides 5 Sterols 6 Essential oil 7 Flavones 8 Phenols Table 2. Calculated values of inhibition efficiency (I%), apparent activation energy (E a) and heat of adsorption (Q ads) of Euphorbia hirta extract on AA3003 aluminium alloy in 0.25M NaOH at different temperatures. Extract concentration Inhibition efficiency (I %) at 30 C at 60 C E a (kjmol -1 ) Q ads (kjmol -1 ) Blank Table 3. Calculated values of inhibition efficiency (I%), apparent activation energy (E a) and heat of adsorption (Q ads) of E. hirta extract on AA3003 aluminium alloy in 0.5 M HCl at different temperatures. Extract concentration Inhibition efficiency (I%) at 30 C at 60 C E a(kjmol -1 ) Q ads(kjmol -1 ) Blank modes of the inhibiting species (whether molecular or ionic). The predominant adsorption mode will be dependent on factors such as the extract composition, chemical changes to the extract and the nature of the surface charge on metal. A negative surface charge will favour the adsorption of cations whereas anion adsorption is favoured by a positive surface charge. The ability of Cl and OH ions in hydrochloric acid and sodium hydroxide to be strongly adsorbed on the metal surface and hence facilitate physical adsorption of inhibitor cations is an important consideration. The plot of the ratio of concentration to surface coverage (C/θ) against concentration (g/l) displayed a straight line for the tested inhibitor in the acidic and alkaline media (Figures 3 and 4). The linear plots, with high correlation coefficient and slope of about unity in both media, clearly reveal that the surface adsorption process of E. hirta on the aluminium alloy surface obey the Langmuir adsorption isotherm. Therefore, one can infer that adsorption occurred through physisorption mechanism. Temkin isotherm Temkin isotherm assumes a uniform distribution of adsorption energy which decreases linearly with increase of the value of the surface coverage (θ) (Figure 5). This model is often used for the description of chemisorptions (Praus et al., 2008). In the theory, θ and C are related through (Oguzie, 2008).

5 Nnanna et al. 269 Figure 3. Langmuir isotherm for E. hirta adsorption on AA3003 aluminium alloy in 0.5 m HCI. Figure 4. Langmuir isotherm for E. hirta adsorption on AA3003 aluminium alloy in 0.25 M NaOH. (3) Where C is the concentration, K is the equilibrium constant and f is a coefficient of inhomogeneity connected with the range of inhomogeneity C by, Which implies that? (5) (4) The values obtained by the linear plot of correlation

6 270 Afr. J. Pure Appl. Chem. Figure 5. Temkin isotherm for E. hirta adsorption on AA3003 aluminium. coefficient for the leaf extract in the acidic medium is far from the assumption for chemisorption, but that of the alkaline medium showed a good factor of near unity. This is an assumption that the inhibition is of chemical adsorption. With the deductions from the Langmuir and Temkin adsorption isotherms, the acidic medium showed pure physical adsorption mechanism while the alkaline medium showed a mixed reaction. Effect of temperature Two main types of interaction often describe adsorption of organic inhibitors on a corroding system viz: chemical adsorption and physical adsorption. It has been suggested (Oguzie et al., 2005; Ebenso, 2003; Popova et al., 2003) that physisorbed molecules are attached to the metal at the cathodes and essentially retard metal dissolution by stifling the cathodic reaction whereas chemisorbed molecules protect anodic areas and reduce the inherent reactivity of the metal at the sites where they are attached. The apparent activation energies (E a ) for the corrosion process in absence and presence of inhibitor were evaluated from Arrhenius equation. Whereas estimates of the heats of adsorption (Q ads ) were (6) (7) obtained from the trend of surface coverage with temperature as follows (21): ρ 1 and ρ 2 are the corrosion rates in respective temperatures T 1 and T 2, respectively while θ 1 and θ 2 are the degrees of surface coverage at temperature T 1 and T 2 and R is the molar gas constant. The standard free energy of adsorption, G ads, which can characterize the interaction of adsorption molecules and metal surface, was calculated using Equation 9. The negative values of G ads ensure the spontaneity of adsorption process and stability of the adsorbed layer on the aluminium surface. Generally, the values of G ads around -20 kj/mol or less negative are consistent with physisorption, while those around -40 kj/mol or more negative involve chemisorptions (Ebenso, 2003; Popova et al., 2003; Oguzie, 2008). Analysis of the temperature dependence of inhibition efficiency as well as comparison of corrosion activation energies in the presence of inhibitor gives some insight into the possible mechanism of inhibitor adsorption. A decrease in inhibition efficiency with rise in temperature, with analogous increase in corrosion activation energy in the presence of inhibitor compared to its absence, is frequently interpreted as (9) (8)

7 Nnanna et al. 271 being suggestive of formation of an adsorption film of physical (electrostatic) nature. The effect, corresponding to an increase in inhibition efficiency with rise in temperature and lower activation energy in the presence of inhibitor, suggests a chemisorption mechanism (Oguzie, 2007; Nnanna et al., 2010; Eddy et al., 2008). From the foregoing, the trend for the extract of E. hirta leaf suggests a predominant effect physisorption of inhibiting species in acidic environment. This is not the same in the alkaline medium, where a mix reaction was obtained. Conclusion The extract from the studied leaf material was found to be effective green inhibitor of aluminium alloys corrosion in both acidic and alkaline environments of 0.5 M HCl and 0.25 M NaOH, respectively. The corrosion process in acidic medium was inhibited by adsorption of the extract organic matter on the aluminium alloys surface and the blocking of active sites. Analysis of all the results yielded some interesting observations. The extract inhibited the corrosion of aluminium alloys in acidic media by means of hindering both cathodic and anodic electrode processes, because the greater the number of bonds in the extracts, the higher the inhibition efficiency. It is found that the inhibitive action was basically controlled by temperature, exposure time and concentration of the inhibitor. The inhibitor was found to obey Langmuir adsorption isotherm, in the acidic and alkaline medium, which is based on physisorption phenomenon, and also confirmed Temkin adsorption isotherm in the alkaline medium. REFERENCES Abdallah M (2004). Guar Gum as Corrosion Inhibitor for Carbon Steel in Sulphuric Acid Solution. Port. Electrochim. Acta., 22: Abiola O, Okafor N, Ebenso E, Nwinuka N (2007). Eco-Friendly Corrosion Inhibitors: Inhibitive Action of Delonis regia Extract for the Corrosion of Aluminium in Acidic Medium. Anti-Corros. 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