Comparative study of green corrosion inhibition effect on mild steel in different acid medium by Laburnum waterier Vossiileaves extract

 

S. Perumal¹, R. Sayee Kannan², S. Muthumanickam², A. Elangovan², N. Muniyappan¹,

K.K. Mothilal¹*

¹ PG and Research Department of Chemistry, Saraswathi Narayanan College, Madurai-625022, India

²PG and Research Department of Chemistry, Thiagarajar College, Madurai-625009, India

*Corresponding Author E-mail: mothi63@yahoo.com

This paper deals with the comparative study of inhibitory effect of ethanol extract of Laburnum watereri Vossii  leaves on the corrosion of mild steel in 1 N HCl and 1N H₂SO₄ . Plant extracts are the rich sources of ingredients which have high inhibition efficiency on corrosion of mild steel. The leaf exarct suppressed the corrosion of mild steel in both acid media. The inhibition effect of leaf extract were studied at  different temperatures viz., 308 K, 313 K, 323 K and 333 K by weight loss method, Tafel polarization method, Nyquist impedance  measurements, FT-IR spectroscopy and  SEM analysis. The results of weight loss studies correlated well with those of impedance and polarization studies. The inhibition efficiency increased with increasing concentration of the inhibitor in both acid media. Further, the adsorption has been found to follow Freundlich adsorption isotherm in both acid media. Polarization studies indicate that leaf extract of LWV LE acts as a anodic type inhibitor in both acid media. The protective film formed on the steel surface was confirmed by FT-IR spectroscopy and SEM studies. The highest inhibition efficiency exhibited by LWVL extract is 94% .This plant extract can be considered as an eco-friendly and effective green corrosion inhibitor for mild steel in acid media.

 

1. INTRODUCTION:

Corrosion is the deterioration of materials their because of electrochemical  reaction with the environment. Most researchers insist there the definition should be restricted to metals but often the corrosion engineers must consider both metals and non-metals. From the context of corrosion science, Corrosion is defined as a reaction of solid with its environment whereas from the angle of corrosion engineering, it is a reaction of an engineering constructional metal with its environment with a consequent deterioration in properties of the metal [1,2].

 

It has been found that the organic inhibitors have O; N and S atoms in their structures donate electrons for bonding with the metal surface [6]. The inhibition by organic compounds is due to the adsorption on the metal surface. The adsorption may be electrostatic or chemisorptive adsorption resulting from p orbital interaction with the metal.  In recent years, owing to the growing interest towards the protection of the environment and hazardous effect of using chemicals as corrosion inhibitor, the traditrational approach on corrosion inhibitor has gradually changed. Due to the toxic nature and the high cost of chemicals as corrosion inhibiter, it is necessary to develop environmentally acceptable and less expensive chemicals as corrosion inhibitors. Natural products can be considered as good sources for this purpose due to their bio-degradability, non-toxicity, green friendly nature and easily availability.           

Most of the naturally occurring substances are safe and can be extracted by simple procedures [7]. Green inhibitors are usually added in a small amount in order to slow down the rate of corrosion through the mechanism of adsorption [8]. Green corrosion inhibitors are obtained from ethanol, aqueous, acid, methanol, or formaldehyde extracts of plant materials [9]. Natural products of plant origin contain different organic compounds (e.g. alkaloids, tannins, flavonoids, pigments, organic amino acids) are known to have inhibitive action [10].

 

Laburnum watereri Vossii belongs to family Fabaceae and will grow to be about 15 feet tall at maturity, with a spread of 10 feet [11]. It grows at a medium rate, and under ideal conditions can be expected to live for approximately 30 years. The leaves are trifoliate, somewhat like a clover the leaflets are typically 2–3 cm long in Laburnum watereri Vossii.  The fruit develops as a pod and is extremely poisonous, it can be used medicinally [12,13]. Laburnum watereri Vossii   belongs to family Fabaceae, so for no studies have been reported using LWVL extract as corrosion inhibition studies using on mild steel in acid media. The present study involves the study of corrosion inhibitory effect of the ethanol extract of the Laburnum watereri Vossii   leaf in 1N HCl and 1N H₂SO₄ media.

 

2. EXPERIMENTAL:                                                                                

2.1 Preparation of the specimens

The specimens were mechanically cut into sizes with 2.5 cm × 2.5 cm × 0.4 cm dimensions and abraded by emery paper of different grades and finally polished  with 4/0 grades emery paper to obtain mirror like finish.  mild The mild steel specimen composition was C: 0.13, Si: 0.18, P: 0.39, S: 0.04, Cu: 0.025 and rest Fe. All specimen was degreased by washing with acetone, dried at room temperature and preserved in a moisture-free desiccators.The concentration of both test solution (1 N HCl and 1N H₂SO₄) was prepared by using distilled water and AR grade hydrochloric acid and sulphuric acid.

 

2.2 Preparation of Laburnum watereri Vossii leafextract

Laburnum watereri Vossii leaves be composed in and around Madurai, Tamil Nadu, and India. The samples were dehydrated, grained and soaked in distilled water for 6 h. After 6 h, the crude extracts were boiled, cooled and triple filtered. The quantity of plant material extracted into solution was quantified by comparing the weight of dried residue with the initial weight of the dried plant material before extraction. From the individual stock solutions, inhibitor test solution was prepared in the concentration range from 450 ppm to 600 ppm.

 

2.3 Weight loss method

Weight loss experiments were done at various temperatures  range 308 – 333 K for 2 h in both 1N HCl and 1N H₂SO₄. Specimen’s were washed with distilled water then with acetone and dried using of stream of air. The specimens were immersed in 100 ml of the respective inhibitor and the test solution in a thermostated bath. The specimens were weighed before and after immersion. The difference in weight was taken as the weight loss of mild steel. From the weight loss (ΔW), corrosion rate (CR) and the percentage of inhibition efficiency (IE %) were calculated using the following equation :

                               

K= 534 to give CR in mpy and K= 87.6 to give CR in mm/yr                                                         

where W= weight loss (W_b-W_a), where  W_b and W_a are the specimen weights before and after immersion in the tested solution, D is the density of the specimen (g/cm³), A is the area of the specimen in inch² and T is the period of immersion in hours.

 

                                                                                  (1)

 

IE % = [(W₀ –W_l)/W₀]×100                                                (2)

 

The k value is constant and its magnitude depends on the system of units used. The CR expressed in terms of either mils (1/1000 inch) per year (mpy) or millimeters per year (m/yr). W₀ and W₁ are the weight loss of mild steel in the absence and presence of inhibitor respectively,

 

2.4 Electrochemical measurements

Tafel polarization curves and Nyquist impedance curves were recorded using H and CHI electrochemical workstation impedence analyzer model CHI 604D. A cell containing three compartments for electrode was used. The working polished mild steel electrode with exposed area of 0.5 cm² was immersed in a test solution. A platinum electrode and saturated calomel electrode were used as the counter and the reference electrode respectively. Before electrochemical impedance experiment, the electrode was allowed to corrode freely and its open-circuit potential was recorded. Potentiodynamic polarization curves were recorded from -300 to +300 m Vsce, (verses OCP) with a scan rate of 1 mVs^-1. Electrochemical impedance spectroscopy measurements were performed in the frequency range of 0.1 Hz to 100 K Hz. All electrochemical measurements were studied at 308 K using 100 ml of electrolyte (1 N HCl and 1N H₂SO₄) in stationary condition. Each experiment was repeated in triplicate to check the reproducibility of the data.

2.5 Surface analysis studies

2.5.1 Fourier Transform Infra-Red spectroscopy

One specimen was the Laburnum watereri Vossii leafextract. On the other hand, the mild steel was immersed for 2 h in 100 ml 1 N HCl and 1N H₂SO₄ solution containing 600 ppm Laburnum watereri Vossii leaf extract. After 2 h, the specimens were taken out and dried and then rubbed with the small amount of KBr powder and made into a disk and FT-IR spectra were recorded in Shimadzu-FTIR-8400S spectrophotometer.

 

2.5.2 Scanning Electron Microscopy Studies

The mild steel specimens were immersed in acid solutions in the absence and presence of optimum concentration of inhibitor for a period of 2 h. After 2 h, the specimens were taken out and dried. The nature of the surface film formed on the surface of the mild steel specimen was examined by using a JEOL (JSM 6390) scanning electron microscope.

 

3. RESULTS AND DISCUSSION:

3.1. Weight loss measurement

Corrosion rate and inhibition efficiency of mild steel in both acid media in the absence and presence of different concentrations of inhibitor at different temperatures (308-333K), obtained from weight loss measurements are given in figure 1 and 2.

 

Fig.1and 2. Corrosion rate and inhibition efficiency of mild steel specimens immersed in 1N HCl and 1N H₂SO₄ with and without LWVLE at 308, 313, 323, 333 K. (---) corrosion rate; (-) inhibition efficiency.

 

It is due to increase in the surface coverage, resulting retardation of metal dissolution [14]. The optimum concentration of LWV leaf extract was used in 600 ppm. The inhibition efficiency of LWV leaf extract is investigated at various temperatures such as 308, 313, 323, 333 K in the presence and absence of the inhibitor. Maximum inhibition efficiency is found to be 95.76% in 1N HCl at 313 K and 91.31 % in 1N H₂SO₄ at 308 K. It is also clear that the inhibition efficiency decreases with increasing temperatures from 308 to 333K in both media.

 

3.2. Electrochemical Measurement

3.2.a. Potentiodynamic polarization measurement

The potentiodynamic polarization curves formed steel in both acid media for prove LWV leaf extract with the studied inhibitor at different concentractions at 308K are shown in figures 3 and 4.   Table.1 gives the corrosion parameter such as corrosion potential (E_corr), corrosion current (I_corr), cathodic slope (β_c), anodic slope (β_a), Linear polarization resistance (LPR) and percentage of inhibition efficiency obtained from the tafel plot.

 

 

Table. 1 Potentiodynamic polarization parameters for the corrosion of mild steel in 1N HCl and  1N H₂SO₄ containing different concentrations of LWVLE.

 

 

 

 

 

 

 

The inhibition efficiency is calculated from potentiodynamic polarization curves by

 

IE % = (i⁰_corr—i_corr/i⁰_corr) × 100

 

Where i⁰_corr and i_corr are the corrosion current density values in the absence and presence of  inhibitor respectively. It is apparent that inhibition efficiency decreases with increasing concentrations of inhibitor. I_Corr value decreases from 5355 to 268.3 µA cm^-2 in 1N HCl and also decreases in 1N H₂SO₄ from 5437 to 1258 µA cm^-2. Based on the displacement of E_cor of inhibited system compared to the uninhibited system, the inhibitor can be classified into mixed type inhibitor. It inhibits both anodic and cathodic reactions by forming films on the steel surface. As shown in Table 1, the change in β_a values is more significant and suggestions that LWV leaf extract is the anodic type inhibitor in 1N HCl and 1N H₂SO₄ media. As shown in Table 1, the change in β_a values is more significant and suggestions that LWV leaf extract is the anodic type inhibitor in 1N HCl and 1N H₂SO₄ media.

 

 

 

 

 

Fig.3 and 4.  Tafel plots of mild steel immersed in 1 N HCl and 1N H₂SO₄ with and without LWVLE

 

3.2.b. Electrochemical impedance measurement

Fig. 5 and 6. Nyquist plots for mild steel immersed in 1 N HCl and 1N H₂SO₄  in different concentrations of LWVLE at 308 K.

 

Figure 5 and 6 shows Nyguist plot for mild steel immersed in the absence and presence of inhibitor in 1N HCl and 1N H₂SO₄ respectively.

 

The impedance parameters such as R_s, R_ct, C_dl and inhibition efficiency derived from Nyquist plots are given in Table 2. Inhibition efficiency was calculated from the following equation

 

% IE = 100 ( R_ct(inh)—R_ct / R_ct(inh) )

 

where R_ct and R_ct(inh) are the charge transfer resistance without and with various concentrations of  inhibitors, respectively. The formation of inhibitor film was confirmed by diameter of semicircle increased by increasing concentration of LWV leaf extract in both acid media [15,16].

Table 2  Electrochemical impedance parameters for mild steel in 1N HCl and 1N H₂SO₄  in the absence and presence of LWVLE.

Con (g/l)	Rs (Ώ cm-2)		Rct (Ώ cm-2)		Cdl (F/cm2)		% IE
	1N HCl	1N H2SO4	1N HCl	1N H2SO4	1N HCl	1N H2SO4	1N HCl	1N H2SO4
0	2.703	0.943	5.62	7.175	1.4 × 10-2	1.1 × 10-2
0.045	-1.155	3.020	50.793	21.447	2.3 × 10-4	8.4 × 10-4	88.90	64.54
0.050	0.942	2.198	56.098	23.587	1.5 × 10-4	7.4 × 10-4	89.93	69.58
0.055	-3.857	1.994	68.999	24.01	1.3 × 10-4	6.2 × 10-4	91.85	70.11
0.060	-10.672	2.742	123.758	34.45	4.5 × 10-5	3.1 × 10-4	95.45	79.17

 

 

From the Table 2, it is clear that increasing the concentration of inhibitor enhanced the charge transfer resistance (R_ct) [17,18]. This reveal that the adsorption of LWV leaf extract on the metal surface leading to the formation of electrical interface between the corrosive medium and the steel surface. These results could be attributed to decreased local dielectric constant and increased thickness of electrical double layer in both acidic media.

 

3.3. Thermodynamic Considerations

Generally, the adsorption of organic molecules on mild steel is important in corrosion study and the adsorption of inhibitor as a metal surface depends on  many factors such as  nature of metal surface, temperature and steric effect. The thermodynamic parameters such as the apparent activation energy E_a, the enthalpy of activatioΔH^* and the entropy of activation ΔS^* for corrosion of mild steel in 1N HCl  and 1N H₂SO₄ solutions in the absence and presence of LWV leaf extract at 308-333 K were calculated from the Arrhenius equation:

 

CR = A exp(-E_a/RT)                            (5)

 

Taking the logarithm of both sides of the above equation, we get

 

log CR = log A - E_a/2.303 RT                            (6)

 

The change of enthalpy (ΔH^*) and entropy (ΔS^*) for the formation of activated complex in the transition state was obtained from the transition state equation.

log CR/ T =

[ (log R/hN + (ΔS^*)/2.303 R)] - ΔH^*/2.303 RT   (7)

 

where CR is the corrosion rate, A is the pre-exponential factor, h is the planck’s constant, N is the Avogadro’s number, E_a is the apparent activation energy, R is the gas constant (R= 8.314 J mol^-1 K^-1) and T is the absolute temperature in Kelvin.

 

A plot of log CR vs 1/T of 1N HCl and 1N H₂SO₄ gave a straight line with a slope equal to (-E_a/2.303 R) and intercept to (log A) as shown in Fig 7 and 8.

 

 

Fig.7 and 8 Arrhenius plot of log CR versus 1/T at different concentrations of LWV LE in 1N HCland1NH₂SO₄.

 

The values of E_a were determined in solutions containing extracts of LWV leaf extract was could be interpreted as physical adsorption in 1N HCl and Chemical adsorption in 1N H₂SO₄ media. A plot of log CR/T vs 1/T  Fig 9 and 10 gave a straight line with slope of the line (-ΔH*/2.303R) and an intercept (log R/hN +(ΔS*)/2.303 R) from which the values of ΔH* and ΔS*were calculated and summarized in Table 3. From these data, it was found that the thermodynamic parameters, ΔH* and ΔS* of dissolution reaction of mild steel in 1N HCl and 1N H₂SO₄ in the presence of LWV leaf extract were higher than uninhibited solution. The negative signs of the enthalpies ΔH* reflected the exothermic nature of the steel dissolution process [19].

Table 3 :Corrosion kinetic parameters for mild steel in 1N HCl and 1N H₂SO₄ in the absence and presence of  LWVLE.

Con (g/l)	Ea  (K J/mol)		-ΔH* (K J/mol)		-ΔS*  (J/mol/K)
	1N HCl	1N H2SO4	1N HCl	1N H2SO4	1N HCl	1N H2SO4
0	57.28	55.18	187.56	173.09	197.01	197.03
0.045	37.52	65.80	138.31	209.46	197.22	196.94
0.050	33.45	72.31	126.92	232.06	197.26	196.88
0.055	28.33	79.40	113.82	256.18	197.32	196.80
0.060	53.38	84.89	196.25	279.54	197.07	196.75

 

 

Fig.9 and 10 Transition state plots log CR/T versus 1/T at different concentrations of LWVLE in 1N HCland 1N H₂SO₄

 

The values of ΔS* in the absence and presence of the inhibitor were negative, which indicated that the activated complex in the rate determining step represented an association rather than dissociation step [20-22].

 

3.4. Adsorption Isotherm

In general, the interaction between metal surface and the inhibitor can be understood by the various isotherms such as Frumkin, Langmuir, Temkin, Freundlich, Bockris-Swinkles and Flory Huggins isotherms. In our present study, the results were best fitted for Freundlich adsorption isotherm in both acid media [23,24].

 

The observed q value changes as function of concentration in 1N HCl and 1N H₂SO₄ is shown in Fig. 11 and 12 respectively.

 

 

 

 

 

Fig. 11 and 12.Freundlich adsorption isotherm for mild steel in 1N HCl and 1N H₂SO₄ containing different  concentrations of LWVLE at 308–333 K.

 

 

 

 

 

 

 

 

The Freundlich adsorption isotherm which was given by,

 = KCⁿ

Where 0< n <1  or

ln  = ln K + n lnC

 

C is the extract concentration and K the equilibrium constant for adsorption which is evaluated from the intercepts of the plots and is related to the standard free energy of adsorption, ΔG₀ ads by

 

ΔG_ads = - RT ln (55.5 K_ads)                   (9)

 

Here R is the gas constant, T is the absolute temperature and 55.5 is the concentration of water in the solution. The values of K_ads were found to increase with increasing temperature showing that the interactions between the adsorbed molecules and the metal surface are strengthened but the removal of inhibitor molecule from the steel surface is easy it was clear that in our weight loss measurement. Such data explained the decrease in the production efficiency with increasing temperature. In the present work, the negative value of ΔG_ads (Table 4) clearly indicated the spontaneous adsorption of LWV leaf extract on mild steel surface and strong interactions between inhibitor molecules and the metal surface.

 

 

 

 

Table 4 : Freundlich adsorption parameters of LWVLE as inhibitor on the surface of mild steel in 1N HCl and 1N H₂SO₄ .

Tem (K)	-ΔGads (K J/mol)		Kads (g/L)		R2  Value
	1N HCl	1N H2SO4	1N HCl	1 N H2SO4	1N HCl	1 N H2SO4
308	8.99	6.85	0.605	0.262	0.949	0.950
313	15.34	6.60	6.553	0.228	0.918	0.914
323	13.27	5.79	2.524	0.156	0.924	0.973
333	13.99	7.23	2.823	0.246	0.972	0.975

 

 

 

Generally the values of ΔG_ads up to – 20 K J/mol signifies Physisorption, which is consistent with electrostatic interaction between charged molecules and a charged metal.The values around –40 K J/mol or higher involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a co-ordinate type of bond [25-28]. In this study, the calculated values of ΔG_ads were around -14 K J/mol in 1N HCl and -7 K J/mol in 1N H₂SO₄, indicating that the adsorption of mechanism of LWV leaf extract on mild steel in 1N HCl and 1N H₂SO₄ solutions at the studied temperatures was physisorption[29].

 

3.5. Morphological Studies

3.5.1. FT-IR Spectra

The FT-IR spectroscopy is used to predict the type of bonding between organic inhibitors and the metal surface. The FT-IR spectra of the LMV leaf extract and their resulting solutions after mild steel immersion in both media are shown in Fig.13 a  and b.

 

 

Fig. 13 FT-IR Spectrum for (a) LWVLE leaf extract and after immersion 1 N HCl,  (b) LWVLE leaf extract and after immersion  1N H₂SO₄

 

 

 

 

 

A broad peak is obtained at 3382 cm^-1 corresponds to N-H / O-H stretching vibration. The frequency at 2930 cm^-1 assigned to C-H stretching frequency. Absorption of strong peak at 1622 cm^-1 corresponds to C=C or C=N stretching or N-H bending vibrations. Absorption band at 1460 cm^-1  assigned to C-H bending in –CH₃ or O-H bending vibrations. Peaks obtained at 1028 cm^-1 and 1266 cm^-1 are observed due to C-N and C-O stretching vibrations. On the basis of results, LWV leaf extract has (N-H, N=C=S, C=N, C-N, OH, C=O, C-O) various functional groups and aromatic rings, which make the extract as inhibitor in the corrosion mild steel.

 

The N–H stretching shifted from 3382 to 3743 cm⁻¹ indicated the coordination of inhibitor with Fe²⁺ through N atom of the N–H group in both media. The shifting of C=O stretching from 1622 to 1740 cm⁻¹ might confirm that there is a strong interaction between LWV leaf extract and the mild steel surface.

 

3.5.2. Scanning Electron Microscopic Studies

SEM images Fig .14 were taken to study the effect of corrosion on the morphological properties of mild steel. The SEM image of polishe mild steel shows smooth surface [Fig.14A]

 

Fig. 14. SEM images of mild steel: (A) plain mild steel; (B) mild steel in 1 N HCl ; (C) mild steel in 1N HCl  with LMV  leaf extract; (D) mild steel in 1N H₂SO_4; (E) mild steel in 1N H₂SO₄ with LMV leaf extract.

Fig.14 B and C show the SEM images of mild steel surface after immersion in 1N HCl in absence and presence of LWV leaf extract for 2 h respectively, Fig. 14 D and E show the SEM images of mild steel surface after immersion in 1N H₂SO₄ absence and presence of  LWV leaf extract for 2h respectively.  SEM images showed that the uniformity of surface of the inhibited sample of mild steel specimens were better than the uninhibited sample in both media. This examination indicated that the corrosion rate was reduced in the presence of inhibitors. The film formed on the metal surface is found to be more compact in 1N HCl compared to 1N H₂SO₄ medium. This might be due to the adsorption of inhibitor molecules on the metal surface as a protective layer [30].

 

4. CONCLUSION

1.         The inhibition efficiency of mild steel  increases with increasing the concentration of LWV leaf extract in both acid media(1 N HCl and 1N H₂SO₄) and the inhibitor efficiency decreases  with increase in temperature.

2.         Potentiodynamic polarization measurements reveal that LWV leaf extract acts as an anodic type inhibitor in both acid media.

3.         EIS measurement reveals that charge transfer increases with increase in concentration of LWV leaf extract in both acid media, indicating that the inhibition increases with increase in concentrations.

4.         The adsorption of the inhibitor obeys the Freundlich adsorption isotherm at all investigated temperatures in both acid media.

5.         The adsorption is spontaneous and the inhibition of corrosion by LWV leaf extract is due to the formation of physisorption on the mild steel surface in both media.

6.         Protective film formation against acid attack has been confirmed by SEM studies.

 

5. ACKNOWLEDGEMENTS:

One of the authors (S. Perumal) thanks to UGC , New Delhi for granting the Teacher Fellow ship (FDP) and the Management of Saraswathi Narayanan college, Madurai,India for encouragement to carry out this work. The authors are thankful to Head, PG and Research Department of Chemistry, Thiagarajar College, Madurai, India for providing instrumental facilities.

 

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Received on 23.09.2017         Modified on 09.11.2017

Accepted on 28.12.2017         © AJRC All right reserved