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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.50 no.4 Concepción Dec. 2005

http://dx.doi.org/10.4067/S0717-97072005000400008 

 

J. Chil. Chem. Soc., 50, N° 4 (2005), págs: 685-690

 

ADSORPTION OF METHIONINE ON MILD STEEL

 

OLUSEGUN K. ABIOLA

Department of Pure and Industrial Chemistry, University of Port Harcourt, P.M.B. 5323, Port Harcourt, Nigeria. (phone: +234-8033360504; e-mail: abiolaolusegun@yahoo.com)


ABSTRACT

The adsorption of methionine at the mild steel surface from acidic solutions is studied using gravimetric technique. The adsorbability of methionine (values of surface coverage) depends on the nature of the acid and the concentration of methionine. The surface coverage with the adsorbed methionine is used to calculate the free energy of adsorption, DG0ad of methionine using Bockris - Swinkel isotherm. The dependence of free energy of adsorption, DG0ad with surface coverage, q is ascribed to surface heterogeneity of the adsorbent. The effect of methionine is discussed from the viewpoint of adsorption model. The adsorption of methionine molecules on the surface occurs without modifying the kinetic of corrosion process.

Key words: Methionine; Mild steel; Adsorption; Kinetics; Free energy of adsorption; Bockris - Swinkel isotherm.


INTRODUCTION

Studying adsorption of methionine from acidic aqueous solutions on a mild steel surface is a part of an ongoing research into the development of non-toxic and eco-friendly (green inhibitors) corrosion inhibitors for industrial metals. When developing corrosion inhibitor for metal and its alloys in corrosive medium, it is of important to know mechanism of their adsorption on metal surface [1-4]. The polar function is frequently regarded as the reaction centre for the adsorption process establishment, since the adsorption bond strength is determined by the electron density and polarizability of the functional group [5]. In continuation of our effort [6-7], in development of eco-friendly corrosion inhibitors for metals, the adsorption of methionine on mild steel surface is reported here.

The development of green corrosion inhibitors, which do not contain heavy metals, has been regarded as important; due to environmental restrictions on toxic corrosion inhibitors [9]. Consequently, there has been an increased interest in employing naturally occurring substances, and ecologically harmless chemicals as corrosion inhibitors for metals in several media [10-11]. Several reports [12-13] have shown that methionine is widely distributed in leaves, seeds, fruits, and fruit shells of many plants. Previous work [18] has shown that methionine actually inhibits corrosion of mild steel in acid solution.

The aim of this investigation is to study the adsorption of methionine, a biodegradable product on mild steel in acidic solutions.

The type of interaction of inhibitor on metal surface during corrosion has been deduced from its adsorption characteristics [14-16] by using gravimetric technique. Previously [17], using chemical techniques, we reported the adsorption of vitamin B1 derivative; (4-amino-2-methyl-5-pyrimidinyl methylthio) acetic acid (AMMPTA). The thermodynamic parameters of adsorption obtained with Bockris-Swinkels adsorption isotherm reveals a strong interaction of this compound on mild steel electrode surface.

EXPERIMENTAL PART

Material Preparation

The material studied is a mild steel sheet of 0.04cm in thickness provided by World Bank Engineering Workshop of University of Port Harcourt, Port Harcourt. The chemical composition and preparation of the mild steel coupons are described in detail in earlier report [18].

The inhibitor methionine, HCl and H2SO4 were products of BDH laboratory, England and were used without further purification. Sacar pues se repite en el párrafo que sigue a continuación.

The concentrations of the additive methionine were chosen as 1 x 10-3, 2 x 10-3, 3 x 103, 5 x 10-3, 7 x 10-3 M and were prepared in the electrolytes (0.1MHCl and 0.1M H2SO4 solutions). All reagents were of analar grade and doubly distilled water was used for the preparations of all solutions. In all the experiments, the temperature of solutions was controlled using a water thermostat.

The structure of the compound used as additive is given below:

Weight loss determination

Rectangular specimens (4cm x 5cm x 0.04cm) of mild steel were used for the determination of the corrosion rate. The coupons were weighed and their initial weight recorded prior to immersion in 250-ml open beakers containing 200ml of 0.1M HCl as corrodent and then with addition of different concentration of additive to the corrodent at 300C. The variation of weight loss was monitored at 36h interval progressively for 216h per coupon at 300C. This experiment was repeated using 0.1M H2SO4 as corrodent.

The procedure for weight loss determination was as previously reported [17]. The corrosion rates were calculated for 216h immersion periods from weight loss using the relation [27].

where W is the weight less (mg), D the density of the specimen (gcm-3). A the area of specimen (cm2) and T the immersion time (h).

RESULTS AND DISCUSSION

Figure1 depicts the dependence of material loss as corrosion rate (mm/year) on concentration of methionine in the supporting electrolytes (0.1MHCl and 0.1MH2SO4) at 300C. The corrosion rate (mm/year) of mild steel coupon in the electrolytes decreases with increasing concentration of methionine in the electrolytes; suggesting that methionine is a corrosion inhibitor for mild steel in these electrolytes. And this result is in agreement with previous report [18].


Fig. 1: Variation of corrosion rate (mm/yr) with concentration of methionine in 0.1M HCl and 0.1M H2SO4 at 30°C

The surface coverage values q at electrode are calculated from the rate of dissolution of mild steel in the electrolytes [19]:

Where q is the surface coverage, r1 and r0 are the uninhibited and inhibited corrosion rates, respectively as determined from the mass loss measurements. The results of adsorption measurements in the electrolytes are presented in Figs. 1, 2, 3, 4 and 5. The degrees of coverage q (adsorption) increases with increase in concentration of methionine in both electrolytes and reaches a maximum value which depends on the nature of the acid (Fig. 2). At the 7 x 10-3M methionine concentration, values of about 0.76 and 0.42 were obtained in HCl and H2SO4 solutions, respectively.


Fig. 2. Variation of surface coverage (q) with concentration of methionine in 0.1M HCl and 0.1M H2SO4 at 30°C

The result (Fig. 2) indicates that the order of the adsorbate to adsorb and protect the metal surface in the two electrolytes is HCl > H2SO4. It has been demonstrated that anions of acids formed as a result of dissociation of acids determine the mode of adsorption and the degree of inhibition [20-22].

The protonation reaction of methionine (scheme 1) has been rationalized as follows [23, 24]; in acidic medium N of the NH2 group; the probable adsorption site is completely protonated as in scheme 1. In HCl solution, chloride ion (Cl-) probably forms a bridge between the protonated site of methionine and the positive mild steel surface. In H2SO4, this bridge is not formed because of the low electron density of SO42- due to its low charge to mass ratio compared to that of Cl- ion. Thus, the adsorption of methionine on the metal surface in H2SO4 gives lower q value compared to the adsorption of methionine on the metal surface in HCl, where its adsorption is facilitated by the chloride ion bridge between the methionine and the positive metal surface.

Scheme 1. The protonation reaction of methionine in acidic solutions
Scheme 2. Adsorption model for Methionine in HCl solutions.
Scheme 3. Adsorption model for Methionine in H2SO4 solution.

The surface of the electrode in aqueous solution is considered to be covered with water dipoles and for adsorption of organic molecules to occur, these water dipoles must be replaced by organic molecules in a reaction that is equivalent to a chemical reaction as follows [25].

nH2O electrode + Organic solution organic electrode + nH2O solution

The thermodynamic of the substitution process depends on the number of water molecules (n) removed by the organic molecules. The values of the apparent free energy change (DG0ad) for adsorption process can be evaluated from q values with Bockris - Swinkels equation [17, 25-26] which is written in the form:

Where q the surface coverage, n is the number of water molecules being replaced and C0 is the concentration of the organic compound in the electrolyte.

Using the above equation, based on substitutional adsorption process for the space filling model [25 -26] of organic molecules on metal surface; n = 9 for flat adsorption of methionine on the surface and n=3 in the perpendicular direction to the surface.

Figs 3 and 4 show the dependence of DG0ad of methionine with q in 0.1M HCl and 0.1M H2SO4 solutions respectively. Although the same trend in the variation of DG0ad is observed for different n values, the result as shown (Figs 3 and 4) indicates that there is a slight change in the DG0ad value with change in the n value from 9 to 3. In Fig. 3 the negative DG0ad decreased with increase of q but increased significantly at q = 0.48 in 0.1M HCl. In 0.1M H2SO4 (Fig. 4) the negative of DG0ad also decreased with increasing value of q but increased markedly at q = 0.25.


 
Fig. 3: DG0ad for methionine on mild steel as a function of surface coverage. (q) in 0.1M HCl as the suporting electrolyte.   Fig. 4: DG0ad for methionine on mild steel as a function of surface coverage. (q) in 0.1M H2SO4 as the suporting electrolyte.

Figures 2, 3 and 4 show vividly that the behaviour of methionine on the mild steel surface in the two acids is very similar: the curve for methionine in the two electrolytes are analogous in shape. The DG0ad dependences of the coverages of the mild steel by methionine obtained in HCl solutions (Fig 3) resemble those obtained in H2SO4 solutions (Fig.4) The coverages with methionine in HCl solutions are lower than those in H2SO4 solutions under the same condition. This may be associated with the tendency of chloride (Cl-) ion to form a bridge between the metal and the cationic form of methionine in HCl solution; methionine exists in cationic form in acidic solutions due to protonation [21-22].

The fact that the DG0ad versus q curves obtained in acidic solutions of methionine are clearly similar suggest that the mechanisms, through which the methionine interact with the electrode surf`ace, are similar in these electrolytes.

The dependence of free energy of adsorption DG0ad of methionine on coverage (Figs. 3 and 4) is due to the heterogeneous nature of the electrode. For solid electrode such as mild steel, platinum and gold, all sites are not equivalent on the surface due to heterogeneity [27 - 28], there will be a hierarchy of sites and hierarchy of adsorption energies as observed experimentally in Figs. 3 and 4.

The calculated values of DG0ad are low; less than -40kJmol-1 (Figs. 3 and 4) and negative suggesting that the nature of the methionine adsorption is mainly physisorption and spontaneous on the surface of mild steel in these solution.

On the basis of experimental result the following models for the adsorption of methionine on mild steel surface in the acidic solutions are proposed.

Methionine exists as cathionic species R+ in solutions of pH less than 7, scheme 1, the cationic species of methionine can be adsorbed on previously adsorbed anions (chlorides and sulphate ions), since organic compounds are known to be adsorbed on the metal surface even at high concentration of anions [28].

In model 1, schemes 2 and 3, at low concentrations of methionine [1 x 10-3M] methionine cations (R+) from solution cluster around the anions (Cl- and ) vicinity by coulombic attraction on the metal surface where anions are previously adsorbed, then as a result methionine molecules are weakly bound to the anions, leading to low surface coverage, 0.20 for HCl solutions and 0.08 for H2SO4 solutions. In [17] similar model for adsorption of Vitamin B1 derivatives in HCl solution has been proposed. Rudresh and Mayanna [26], earlier had proposed similar model on the adsorption of n - decylamine on zinc surface in acidic solution.

Model 2, schemes 2 and 3, is for a situation of weak adsorption of chloride ions or sulphate ions, R+ at high concentration withdraw the anions on the surface into the solution, coadsorption of cations and anions is possible as in model II, on increasing the concentration of methionine from 1 x 10-3 to 2 x 10-3M.

The degrees of desorption of anions from the surface depends on the degree of adsorption of C+ which is a function of the concentration of methionine. In a situation where the concentration of methionine is increased above 2 x 10-3M in the electrolytes, the anions (Cl- and SO42-) on the surface provide a better electrostatic condition, which promotes a direction adsorption of cations on the surface through its polar group as depicted in model III, schemes 2 and 3. The values of DG0ad obtained in this study are in agreement with electrostatic interaction between methionine cations and the charge metal surface.

The kinetics of iron corrosion by the supporting electrolytes (HCl and H2SO4) containing methionine at the studied temperature was investigated with the help of Fig. 5 This figure depicts the plots of logarithm of final weight (log Wt) against time at 300C in HCl and H2SO4 solutions without and with 7 x 10-3M methionine. A linear variation obtained confirms a first - order reaction kinetics with respects to mild steel in HCl and H2SO4 solutions in absence and presence of methionine. A similar observation was reported by Abiola and Oforka [17] for adsorption of vitamin B1 derivative on mild steel surface using HCl solution as the supporting electrolytes.


Fig. 5: Variation of Log wt for mild steel coupons in both 0.1M HCl and 0.1M H2SO4 solutions with and without Methionine at 303K: (A) 0.1M H2SO4 (blank); (B) 0.1M H2SO4 + 7 x 10-3 methionine; (C) 0.1M HCl (blank); (D) 0.1M HCl + 7 x 10-3M methionine.

The anodic dissolution reaction of iron in acid solution is as follows:

Fe ¾® Fe2+ + 2e- (3)

Fig. 5 reflects the reaction order with respect to mild steel, i.e. the anodic reaction. From the result (Fig. 5) it could be said that methionine adsorption on the mild steel surface in the supporting electrolytes does not influence the anodic reaction order.

CONCLUSIONS

1. The corrosion rate of mild steel in the supporting electrolytes decrease with the increase in adsorbability of methionine (values of surface coverage, q).

2. Methionine molecules in cationic form adsorb on the mild steel surface through electrostatic interaction with the mild steel surface.

3. The negative values of free energy of adsorption, DG0ad suggest the spontaneous adsorption of methionine on the mild steel surface.

4. The dependence of DG0ad on surface coverage q is due to heterogeneous nature of the mild steel surface.

5. Methionine adsorb on mild steel surface in the supporting electrolytes without modifying the kinetics of corrosion process.

6. Adsorption of methionine on to mild steel surface gives a first order kinetics.

 

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