Copper corrosion inhibition by Azadirachta indica leaves extract

Copper corrosion inhibition by Azadirachta indica leaves extract

in 0.5M sulphuric acid

L.Valek ?,S.Martinez

University of Zagreb,Faculty of Chemical Engineering and Technology,Maruli ?ev trg 19,P .O.Box 177,HR-10000Zagreb,Croatia

Received 5July 2005;accepted 4April 2006

Available online 5May 2006

Abstract

Azadirachta indica leaves extract (AI)was investigated as a copper corrosion inhibitor in 0.5M sulphuric acid.Inhibition efficiency of AI was compared to that of the already proven good inhibitors 2-acetamino-5-mercapto-1,3,4-thiadiazole (AAMTDA)and 1,2,3-benzotriazole (BTAH).The inhibition properties were studied using electrochemical polarization and weight loss techniques.In the region of active copper dissolution,the highest inhibition efficiency was exhibited by AAMTDA (92.7%).AI exhibited somewhat higher efficiency (86.4%)than the widely used BTAH (85.5%),showing that the extract could serve as a effective substitute for currently preferred copper corrosion inhibitors in sulphuric acid.The weight loss results were interpreted by means of the Frumkin isotherm of adsorption on the metal surface.The values of ΔG ads equal to ?41.96kJ mol ?1for AAMTDA and ?35.22kJ mol ?1for BTAH indicate strong spontaneous adsorption while the surface coverage dependence on the log c following the Frumkin isotherm is suggestive of chemisorption in case of all three tested inhibitors.?2006Elsevier B.V .All rights reserved.

Keywords:Copper corrosion;Inhibitor;Azadirachta indica leaves extract;Thiadiazole derivative;BTAH

1.Introduction

Due to the currently imposed environmental requirements for eco-friendly corrosion inhibitors,there is a growing interest in the use of natural products such as leaves or seeds extracts.Some papers have reported the use of natural products for mild steel corrosion inhibition in various corrosive media [1–8],but only few examples of copper corrosion inhibition were reported [8,9].It has been found that AI is a very good inhibitor for mild steel corrosion [1,2],but copper corrosion inhibition by AI has not been investigated.AI extracts were reported to contain several thousands of compounds,among which are:azadirachtin,azadi-rone,gedunin,nimbin,nimbandiol,nimbinene,nimbolide,nimo-nol,nimbolin,salannin,margolone,melianol,vilasanin,flavanoids and structurally related compounds [10–14].Most of the compounds have complicated molecular structure,large molecular weight and significant number of oxygen atoms incorporated in the structure.Generally,compounds containing oxygen,nitrogen,or

sulphur have a tendency to act as good inhibitors in acid media [7,8,15,16],hence,the AI extract is expected to have great ability for inhibitory action thorough oxygen active centres [6,8].

The aim of this work was to investigate the potential of the AI extract to act as an inhibitor of copper corrosion in sulphuric acid.AI,as a cheap,environmental friendly substance,was compared to 2-acetamino-5-mercapto-1,3,4-thiadiazole (Scheme 1a)and 1,2,3-benzotriazole (Scheme 1b).BTAH is widely applied and very efficient,but ecologically questionable inhibitor.AAMTDA was chosen to be tested regarding that 2-methyl-5-mercapto-1,3,4-thiadiazole and 2-amino-5-mercapto-1,3,4-thiadiazole have been shown to have a significantly better inhibitive properties than BTAH on copper in chloride medium [17].2.Experimental

Investigation of copper corrosion inhibition by AI,AAMTDA and BTAH in sulphuric acid was performed using electrochem-ical polarization and gravimetric weight loss method.

Potentiodynamic polarization and linear polarization resis-tance measurements were conducted on manually

constructed

Materials Letters 61(2007)148–

151

?Corresponding author.Tel.:+38514597116;fax:+38514597139.E-mail address:lvalek@fkit.hr (L.Valek).

0167-577X/$-see front matter ?2006Elsevier B.V .All rights reserved.doi:10.1016/j.matlet.2006.04.024

copper electrode (d =7mm).Electrode was first mechanically abraded on 1200grade emery paper and polished on 1,0.3and 0.05μm Al 2O 3,then degreased with ethanol and rinsed in redistilled water.Counter electrode was Pt-foil and reference electrode was saturated calomel electrode (SCE).All potentials reported refer to SCE.Experiments were performed in 0.5M H 2SO 4without or with addition of the inhibitors in the fol-lowing concentrations:10?3M BTAH,5·10?4M AAMTDA and 1g L ?1AI.The inhibitor concentrations for potentiody-namic measurements were chosen based on the results of the surface coverage dependence on the inhibitor concentrations obtained from gravimetric https://www.360docs.net/doc/2e22748.html,mercial Azadir-achta indica leaves extract powder supplied by Rym Exports has been used.All the measurements were performed after half an hour immersion with scan rate 1mV s ?1for the poten-tiodynamic polarization and 2mV min ?1for the linear polariza-tion resistance measurements.

Weight loss measurements were carried out on the copper plate specimens (40×50×3mm).Plates were first mechanically cleaned with 600grade emery paper,degreased with ethanol,rinsed with redistilled water and then dipped in 7M HNO 3for 30s,rinsed in redistilled water and dried.The specimens were immersed in the test solution and left to stand for the 24h at the room temperature under the air atmosphere.Concentration ranges

of the test solutions were 2.5·10?6–5·10?4

M for AAMTDA,

2.5·10?6–0.05M for BTAH and 8.5·10?4–8.5g L ?1

for AI.

3.Results and discussion

Anodic polarization behaviour of copper in sulphuric acid with and without the addition of the inhibitors obtained in a wide polarization

range is presented in Fig.1.Curves exhibit a potential range of active copper dissolution ending with a current peak and followed by a current plateau that is indicative of a film formation process at the electrode surface,i.e.formation of the layer containing copper(I)oxide (Cu 2O)[18].

Addition of the inhibitors to the corrosive medium shifted the polarization curve towards lower current density values,i.e.lower peak current density and lower plateau current density pointing to the inhibitors'promotion of the passive behaviour of copper.The most pronounced passivating activity exhibited BTAH,which is known as a very good inhibitor in neutral media due to its pronounced ability to form a polymer film in the presence of the oxide layer at the copper surface [18,19].

Polarization curves in the polarization range of E corr ±150mV obtained with and without the addition of inhibitors are presented in Fig.2.A positive shift of the E corr by the AAMTDA can be observed,while AI and BTAH shifted E corr to the negative values.The presence of inhibitors suppressed the cathodic and especially the anodic current densities.During the cathodic polarization,oxygen reduction reaction expressed in Eq.(1),occurs at the electrode surface [20,21]:O 2t4H tt4e ??2H 2O

e1T

At highly cathodic polarization potentials,a behaviour charac-teristic of a diffusion limited process can be observed,

indicating

Fig.2.Potentiodynamic polarization curves (scan rate 1mV s ?1)of copper in 0.5M H 2SO 4solution without and with addition of the BTAH,AAMTDA and AI.

Table 1

Corrosion parameters obtained from the Tafel analysis of the polarization curves

b C /

mV dec ?1b A /mV dec ?1E corr /mV j corr /μA cm ?2r /μg cm ?2h ?1IE /%

0.5M H 2SO 4

?219838.6?3011.013.0–BTAH ?240.359.9?85 1.6 1.985.5AAMTDA ?532.168.0250.80.992.7AI

?181.4

59.7

?104 1.5

1.8

86.4

Table 2

Corrosion parameters obtained by linear polarization resistance measurements

R p /Ωcm 2

j corr /μA cm ?2r /

μg cm ?2h ?1IE /%0.5M H 2SO 4135712.014.2–BTAH 9934 2.1 2.582.5AAMTDA 16,702 1.6 1.986.7AI

10,623

1.8

2.1

85.0

that the transport of oxygen from the solution to the electrode surface is controlling the reaction rate.Consequently,high values of cathodic Tafel slopes are obtained.Corrosion parameters calculated from the data in Fig.2are presented in Table1.The presence of inhibitors lowers the cathodic Tafel slope values,implying the influence of the inhibitors on the cathodic reaction,probably by blocking the metal surface and slowing the oxygen reduction reaction[15,21].

During anodic polarization of the electrode,dissolution of copper takes place by the following two continuous steps[15,18,20]:

CuesT?e??k1

k?1

CuteadsTefast stepTe2TCuteadsT?e?Y k2Cu2tesolnTeslow stepTe3T

where Eq.(3)is the rate-determining step.Anodic Tafel slope value of 38.6mV dec?1confirms that diffusion of Cu2+ions from the metal surface has a little influence on anodic current[15].Presence of the inhibitors increases the anodic Tafel slope value yielding the corrosion rate decrease probably due to blocking of the active sites at the metal surface by the inhibitor molecules[7,21].

The best inhibitory efficiency was obtained by AAMTDA(92.7%). AI exhibited better efficiency than BTAH,exerting very good copper corrosion protection in the active region.The results show a dual nature of the BTAH inhibition mechanism.BTAH inhibits active copper corrosion,but its inhibitory action is more pronounced in the passive region,as it was shown by the anodic polarization measurements(Fig.

1).Inhibitory action of the AATDA and AI is,on the other hand,more marked in the range of active corrosion,while promotion of the pas-sivation is less pronounced.

Corrosion parameters obtained from the linear polarization resistance measurements(data not shown in figures)are presented in Table2.The results are in very good agreement with those obtained from potentiodynamic polarization measurements.

Performance of the tested inhibitors has been evaluated by the weight loss measurements in a wide range of concentrations.Since the inhibition works though the mechanism of adsorption of the inhibitor molecules on the metal surface,the use of adsorption isotherms pro-vides useful insight into the corrosion inhibition mechanism.The Frumkin adsorption isotherm[7,18]:

Bc?

h

1?h

e?2a he4T

has been found to give the best fit to the weight loss results of the three tested inhibitors(Fig.3).Where,c denotes the inhibitor concentration,θthe surface coverage,a the attraction constant and B the adsorption–desorption equilibrium constant linked to the free energy of adsorption,ΔG ads by the equation:

B?

1

55:5

exp

?D G ads

RT

e5T

Application of a nonlinear fitting procedure to the weight loss results yieldedΔG ads and a values for AAMTDA and BTAH.It was not possible to calculate those parameters for AI since the average molar mass of extract is unknown.The values ofΔG ads for the AAMTDA?41.96kJ mol?1and for the BTAH?35.22kJ mol?1 indicate strong spontaneous adsorption of the inhibitor molecules on the metal surface.Attraction constants were?1.620for the AAMTDA and?0.898for the BTAH.Negative values are pointing to the presence of the repulsive forces between the inhibitor molecules adsorbed on the surface of the metal[7,18].

Surface coverage dependence on the log c following Frumkin adsorption isotherm is suggestive of chemisorption of the tested inhibitors on copper surface[22].Since there is high possibility for the chemisorption to take place at the metal surface in the presence of organic compounds containing O,S,and N active centers.Due to the structural nature of the oxygen compounds in the AI extract,adsorption of these constituents may occur through their oxygen active centres[7,8,16].

4.Conclusions

A.indica leaves extract was proven as an excellent inhibitor of active copper corrosion in sulphuric acid.While AAMTDA revealed even better inhibitory properties,its ecological aspects and costs are of question.BTAH exhibited better inhibitory properties at high anodic polarization potentials,proving his passivating ability,while gravimetric measurements and polari-zation in the Tafel region revealed somewhat lesser inhibition efficiency of the active copper dissolution.The adsorptive behaviour of investigated inhibitors was approximated by Frumkin adsorption isotherm suggesting the chemisorption mechanism of inhibition.The results obtained on the inhibitory action of the A.indica leaves extract show that AI can be proposed as good,ecologically acceptable candidate for acid copper corro-sion inhibition.

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