Synergistic effect on corrosion inhibition of copper by sodium

Available online at Materials Chemistry and Physics109 (2008) 281–286Synergistic effect on corrosion inhibition of copper by sodiumdodecylbenzenesulphonate(SDBS)and2-mercaptobenzoxazoleH.Tavakoli a,T.Shahrabi a,∗,M.G.Hosseini ba Department of Materials Engineering,Tarbiat Modares University,Tehran,Iranb Electrochemistry Research Laboratory,Department of Physical Chemistry,Chemistry Faculty,Tabriz University,IranReceived21December2006;received in revised form8June2007;accepted15November2007AbstractThe inhibition effects of sodium dodecylbenzenesulphonate(SDBS)and2-mercaptobenzoxazole(2-MBO)on corrosion of copper in sul-phuric acid solution have been studied using electrochemical impedance spectroscopy(EIS)and Tafel polarization measurements.For2-MBO, a monotonous increase in inhibition efficiency was observed as a function of concentration.For SDBS,however,an optimum in the inhibition efficiency was observed for a certain concentration,which is ascribed to the formation of hemi-micellar aggregates that provoke inhibitor desorption from the metal/solution interface at higher concentrations.Upon mixing2-MBO and SDBS inhibitors,concentrations range showing synergistic inhibition behaviour were identified,and it is concluded that electrostatic interactions between the adsorbed ions.Different adsorption isotherms were tested for describing the adsorption behaviour of both2-MBO and SDBS.© 2007 Elsevier B.V. All rights reserved.Keywords:Inhibitors;Sodium dodecylbenzenesulphonate;2-Mercaptobenzoxazole(2-MBO);Copper;Synergistic effect1.IntroductionAcid solutions are widely used in different industries.The most importantfields of application are acid pickling,industrial acid cleaning,acid descaling and oil well acidising[1].Copper and copper-based alloys are widely used in a great variety of applications,such as industrial equipment,building construc-tion,electricity,electronics,coinages,ornamental parts,water treatment,etc.[2].The use of inhibitors is one of the most practical methods for protection against corrosion especially in acidic media[3]. Inhibitors are commonly used in those processes to control the metal dissolution as well as acid consumption.Organic corrosion inhibitors have been extensively investi-gated during the last three decades as they provide an effective and inexpensive means of reducing the degradation of metals and alloys in manyfields of applications[4].The role of heteroatoms such as N or S in organic compounds showing an inhibitive effect towards corrosion of iron and copper alloys in acidic solutions is generally acknowledged[5,6].∗Corresponding author.E-mail address:tshahrabi34@modares.ac.ir(T.Shahrabi).At present,it is agreed that mercapto-substituted het-erocyclic compounds, e.g.2-mercaptobenzothiale(2-MBT), 2-mercaptobenzimidazole(2-MBI)and2-mercaptobenzoxazole (2-MBO),are the most effective inhibitors for copper[7–10]. The inhibition action of2-MBI on corrosion of brass(60/40) in ammonia solutions has revealed that,while2-MBI provide excellent corrosion inhibition,its polymericfilm,poly(2-MBI) was not very protective[11].We have also studied the inhibition effect of2-MBI and the synergistic effect of2-MBI and surfactant SDBS on corrosion of copper in acidic solutions[12,13].The inhibition efficiency of2-MBO and its synergetic effect with cyanide ions for the corrosion inhibition of copper in aque-ous chloride media is investigated using electrochemical and surface-enhanced Raman spectroscopic techniques[14].The study of surfactants adsorption on metal surfaces is extremely important in a variety offields such as corrosion inhibition and metallic electrodeposition.The latter approach offers further advantages to inhibition enhancement such as detergency,dispersibility and improved wetting power of the solutions,and has already been found in several industrial appli-cations[15,16].In a pervious paper[9]the inhibition effects of sodium dodecylbenzenesulphonate(SDBS)and hexam-ethylenetetramine(HA)on corrosion of mild steel in sulphuric0254-0584/$–see front matter© 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.018282H.Tavakoli et al./Materials Chemistry and Physics 109 (2008) 281–286acid solution has been studied.It has been shown that SDBS acts as a corrosion inhibitor and presents a synergistic effect on inhibitive action of HA.Recently,villamil et al.[17]have observed a synergistic effect of sodium dodecylsulphonate (SDS)and Benzotriazole (BTAH)on corrosion inhibition of 304stainless steel in 2.0mol dm −3sulphuric acid solution at several BTAH–SDS concentration ratios.In the present paper,the effects of combining 2-MBO with the anionic surfactant SDBS upon copper corrosion in 0.5mol dm −3sulphuric acid solution are investigated.The corrosion rates,experimentally determined using electrochemical impedance and Tafel polarization measurements,are interpreted in terms of what likely occurs on a microscopic scale at the metal/electrolyte interface.Several isotherms are tested for their potential rel-evance in describing the adsorption behaviour of the studied compounds.The interaction between the inhibitors upon mixing is analysed by calculating synergism parameter values.2.Experimental 2.1.MaterialsFor polarization and EIS measurements,copper specimens with composi-tion (in wt.%)Al 0.003,Fe 0.02,Pb 0.02,Sn 0.004,Zn 0.014,Mg 0.02,P 0.001,Mn 0.001,S 0.002,Sb 0.05and Cu (balance)and surface area of 1cm 2were used.The samples were polished to mirror finish using emery paper fol-lowed by aqueous alumina suspensions with particle sizes decreasing down to 0.05␮m,degreased by sonication in analytical reagent grade ethanol,and blown dry with nitrogen.Perfectly uniform wettability with water after this treatment was considered a good indication for surface cleanliness.All experiments were carried out at a constant temperature of 30◦C,with the electrolyte solutions in equilibrium with the atmosphere (i.e.,aerated solutions).All chemicals with the exception of SDBS were of analytical reagent grade (Merck)and were used without further purification,and solutions were prepared using double distilled water.SDBS was obtained as a solid containing 80%active constituent (Merck),the remainder being sodium sulphate.Importantly,surfactant impurities were reported to be absent in the product.The present study being conducted in sulphuric acid medium,the presence of a slight additional amount of sulphate ions was believed to have no effect on the results.In preparing SDBS solutions,the actual surfactant concentrations in the starting material was taken into account by weighing 1/0.8=1.25times the theoretical mass of the pure SDBS.The concentrations range of inhibitors employed was 1×10−5to 4×10−4M in 0.5M sulphuric acid solution.Fig.1shows the molecular struc-tures of the investigated organic compounds,which have been labelled 2-MBO and SDBS.2.2.Electrochemical impedance spectroscopy (EIS)Impedance measurements were carried out at the open circuit potential (Eocp/V(SCE)),using a computer-controlled potentiostat (PAR EG&G Model 273A)and (Frequency Response Detector 1025).In a conventional three-electrode assembly,a Pt foil auxiliary electrode and a saturated calomel reference electrode (SCE)were used.After immersion of the specimen,prior totheFig.1.Schematic molecular structure of investigated 2-mercaptobenzoxazole (2-MBI)and sodium dodecylbenzenesulphonate (SDBS).impedance measurement,a stabilisation period of 30min was observed,which proved sufficient for Eocp/V(SCE)to attain a stable value.The ac frequency range was extended from 100to 10mHz,with 5mV peak-to-peak sine wave being the excitation signal.The data processing was based on a non-linear least squares fitting procedure as described elsewhere [18,19].For transforming con-stant phase element parameter values into values of idealized capacitances,a procedure outlined in the same reference was employed [18].Inhibition effi-ciencies η%Z in this case were calculated through the following expression:η%Z =R ct −R ct0R ct×100(1)where R ct0and R ct are values of the charge transfer resistance ( m 2)observed in absence and presence of inhibitor,respectively.2.3.Tafel polarization measurementsThe same equipment was used as for the impedance measurements,leav-ing the frequency response analyser out of consideration.Quasi-potentiostatic polarization curves were obtained using a sweep rate of 1mVs −1.All potentials were reported versus saturated calomel electrode (SCE),unless mentioned oth-erwise.Corrosion current densities were obtained from the polarization curves by linear extrapolation of the Tafel curves at points ±50mV than the open cir-cuit potential value.Correction of the curves for IR-drop compensation was not required due to the high electrical conductivity of the strongly acidic solutions.The corrosion inhibition efficiencies η%P ,for this case is η%P =i corr0−i corri corr0×100(2)where i corr0and i corr are the corrosion current densities (Am −2),respectively.3.Results and discussion 3.1.Inhibition effect of 2-MBOIt can be seen from Graph 2(curve a),that one capacitive loop at the higher frequency range is followed by the Warburg impedance at lower frequency values.This diffusional behaviour can be attributed to the cathodic process of oxygen reduction.The other curves in Graph 2shows only one depressed capacitive loop.Instead of fitting a capacitance to behaviour that is obvi-ously not satisfactorily described by a capacitance,as is usually done in corrosion science,we prefer using a constant phase ele-ment (CPE),which has an impedance function of thefollowingGraph plex plane plot for copper in 0.5M sulphuric acid solution with the presence of the following 2-MBO concentrations (in mol dm −3):(a)with-out inhibitor ions,(b)1×10−5,(c)5×10−5,(d)1×10−4,(e)2×10−4,(f)3×10−4,and (g)4×10−4.H.Tavakoli et al./Materials Chemistry and Physics 109 (2008) 281–286283Graph3.Equivalent electrical circuits of copper corrosion in0.5M sulphuric acid solution and corresponding impedance diagrams:(a)experimental data;(b)simulated.R s,medium resistance;R ct,charge transfer resistance;CPE1, constant phase element.form[4]:Z CPE=1Y0(jω)n(3) where Y0and n are frequency-independent parameters and −1,≤n≤1.Equivalent electric circuits are shown in Graph3.It is easily verified that this CPE shows purely resistive,capacitive or inductive behaviour respectively when n=0,1or−1.The extraction details of EIS measurements have been shown in Table1.The representative examples of Tafel polarization curves are shown in Graph4,whereas relevant parameter val-ues are summarized in Table3(corrosion current density i corr, corrosion potential E corr,anodic and cathodic Tafel slopesβa,βc).The approximate values of Tafel slopes(βa andβc)in Table3shows that the addition of2-MBO affects the mech-anism of the proton discharge and metal dissolution reactions. This result indicates that2-MBO exhibits both cathodic and anodic inhibition effect.This suggests that2-MBO acts mainly as a mixed-type inhibitor in0.5M sulphuric acid solution.Comparison with the data in Tables1and3learns that satisfactory agreement is found with the inhibition efficien-cies as obtained through EIS and Tafel measurements.Table1 shows that as the2-MBO concentrations increase the R ct values increase but the C dl values decrease.The decrease in the C dl values is due to the adsorption of2-MBO on the metalsurface.Graph4.Tafel curves of copper in0.5M sulphuric acid solution with the pres-ence of different concentrations of2-MBO(in mol dm−3):(a)without inhibitor ions,(b)1×10−5,(c)5×10−5,(d)1×10−4,(e)2×10−4,(f)3×10−4,and (g)4×10−4.Graph5.The double layer capacitance for copper in0.5M sulphuric acid solu-tion containing different concentrations of2-MBO.According to the study undertaken by EIS,we can establish the curves of R ct−1and C dl versus the inhibitor concentration of2-MBO.We can notice that the shape of these two curves (Graphs5and6)is the same,so we can conclude that more the concentration of inhibitor increases,the more the charge transfer resistance increases and the more the double layer capac-itance decrease up to a concentration of10−4mol dm−3,where it remains almost constant.3.2.Inhibition effect of SDBSThe inhibition effect of SDBS was investigated by Impedance Spectroscopy technique.The extraction details of EIS measure-ments have been shown in Table2.The behaviour resultingTable1Impedance parameters for the corrosion of copper in0.5M sulphuric acid solution with the presence of different concentrations of2-MBOC2-MBO(mol dm−3)×105R ct( cm2)Y0×106( −1cm−2)n dl C dl(␮F cm−2)η%z 01202620.8212414381050.897370 5984940.856486 101900770.886093 202678630.95295 302870370.872795 402967310.862295284H.Tavakoli et al./Materials Chemistry and Physics109 (2008) 281–286Graph 6.Relation between R t and immersion time for copper in 0.5M sulphuric acid solution containing 10−4mol dm −32-MBO.Graph 7.Inhibition efficiency for copper in 0.5M sulphuric acid solution con-taining different concentrations of SDBS.from the addition of SDBS is somewhat more intricate (Graph 7),as a maximum in the inhibitive efficiency of around 76%is observed for concentrations around 4×10−4mol dm −3.The corrosion protection is diminished as the concentration of SDBS inhibitor increases beyond this value.This phenomenon is likely to be ascribed to the formation of so-called hemi-micelles.At concentrations of the surfactant close to or beyond the critical micelle concentration (cmc),the withdrawal of adsorbate backinto the bulk solution probably becomes thermodynamically favorable.For studying the joint effect of SDBS and 2-MBO,the syn-ergism parameter,s ,should be calculated as initially proposed by Murakawa et al.[20]for describing the combined inhibition behaviour of amines and halide ions.Generally,for the interac-tion of inhibitors A and B,this synergism parameter is defined as follows:s =1−ηA −ηB −ηA ηB1−ηAB(4)where ηA and ηB are the inhibition efficiencies observed with compounds A and B,respectively acting alone,and ηAB is the experimentally observed inhibition efficiency for the mixture AB (obviously,c A and c B in the mixture should be the same as in the corresponding separate situations).The expression actually compares the theoretically expected corrosion rate (numerator),based on the condition when either A or B are present or on the condition that they do not inter-act,with the experimentally observed rate in the presence of the inhibitor mixtures (denominator).Consequently,in the case where inhibitors A and B have no effect on each other and adsorb at the metal/solution interface independently,s =1as in that case the predicted behaviour is experimentally confirmed.Alterna-tively,the effect would be synergistic if s >1or antagonistic if s <1.Graph 8presents values of s for all investigated concentra-tions of SDBS and 10−4mol dm −32-MBO inhibitors.From this plot,it becomes clear that a partial conclusion based on Graph 4is to be extended:antagonism occurs if either SDBS is present in concentrations below around 5×10−5mol dm −3,irrespective of the concentration of the other compound.For the concentration of SDBS between 1×10−4andTable 2Impedance parameters for the corrosion of copper in 0.5M sulphuric acid solution with the presence of different concentrations of SDBS C SDBS (mol dm −3)×105R ct ( cm 2)Y 0×106( −1cm −2)n dl C dl (␮F cm −2)η%z 01202620.82128101542360.8413022202862210.8514058304831300.899375405021350.888876503311500.848764603101800.859361Table 3Tafel polarization parameters for the corrosion of copper in 0.5M sulphuric acid solution with the presence of different concentrations of 2-MBO C 2-MBO (mol dm −3)×105E corr (mV vs.SCE)i corr (␮A cm −2)βc (mV decade −1)βa (mV decade −1)η%p 0−3118138.137.7–1−18 4.37847.4765−8 3.66642.6801018 2.55838.1862025.5 2.160.235.4883026.4 2.154.240.8884025.92.158.343.488H.Tavakoli et al./Materials Chemistry and Physics 109 (2008) 281–286285Graph8.Synergism parameter s for the combined effect of SDBS and 10−4mol dm−32-MBO on the corrosion of copper in0.5mol dm−3sulphuric acid solution(key:s>1,synergism;s<1,antagonism).5×10−4mol dm−3moderate synergism(1<s<1.4)and for above5×10−4mol dm−3antagonism effect is observed.The slight antagonistic effect observed for low concentrations of SDBS(below10×10−5mol dm−3)can simply be due to the overlay formation of the minor inhibitor,as electrostatic inter-action between the oppositely charged ions most likely prevents their co-adsorption on the metal surface.At higher concentrations,principally the same interaction may be responsible for the observed synergism,as in the pres-ence of higher amounts of inhibitors the overlay formation is no longer sufficient to compensate the attractive electrostatic interaction.The forces involved in that case enable interfacial adsorption to occur at a higher extent(resulting in a higher value ofθfor the same concentration)than in the case if only the anionic or cationic inhibitor is present.Finally this fact deserves consideration that upon2-MBO addition in sufficient amounts,the decrease in s for C SDBS above 5×10−4mol dm−3as seen when SDBS acts alone,is no longer observed,the change in behaviour may well be due to the pre-vention of the hemi-micellar formation.The co-adsorption of MBO n+n(1≤n≤4)and SDBS will hinder the aggregation of SDBS molecules or anions so that the formation of hemi-micelles is prevented and consequently the inhibitor desorption which leads to a higher corrosion rate does not take place.3.3.Adsorption isotherm of2-MBOIf the adsorption of this inhibitor follows the Langmuir adsorption isotherm,the degree of surface coverage(θ)is given byθ=bC inh1+bC inh(5)where b designates the adsorption coefficient.A simple model allows to connect C dl to the degree of surface coverage(θ)by[1]:C dl,θ=(1−θ)C dl(θ=0)+θC dl(θ=1)(6) where C dl,θis the double layer capacitance with inhibitor, C dl(θ=0)is the double layer capacitance without inhibitor and C dl(θ=1)is the double layer capacitance of an entirely covered surface.After rearrangement,theθfor differentconcentrations ngmuir adsorption plots for copper in0.5M sulphuric acid solution containing different concentrations of2-MBO from the double layer capacitance. of the inhibitor in acidic media isθ=C dl(θ=0)−C dl,θC dl(θ=0)−C dl(θ=1)(7) The surface coverage values(θ)were tested graphically for fitting a suitable adsorption isotherm.The plot of C inh/θvs.C inh yielded a straight line.This observation clearly proves that the adsorption of the2-MBO from0.5M H2SO4solutions on the copper surface obeys the Langmuir adsorption isotherm as shown in Graph9.The expected linear relationship is well approximated in the case of2-MBO(with correlation coefficient R2equals to0.998),and the line has a slope of0.957.The devi-ation of the slope from unity is often interpreted as a sign that the adsorbing species occupy more or less a typical adsorption site at the metal/solution interface[20].The isotherm’s parameters and the free energies of the inhibitor adsorption calculated from equation[21]:b=155.5exp− G0adsRT(8)where R is the universal gas constant,b designates the adsorption coefficient and T is the thermodynamic temperature,a value for the free energy of adsorption G0ads for2-MBO equal to −23.72kJmol−1.A value of−40kJmol−1is usually adopted as a threshold value between chemi-and physisorption[21].The value found for2-MBO on copper thus clearly indicates that the adsorption is of a physical–probably electrostatic–nature, and that no covalent bond between inhibitor molecule and metal surface is established.4.Conclusions2-MBO inhibits the corrosion of copper in0.5M sulphuric acid solution.The2-MBO molecule is found to affect both the anodic and cathodic processes by simple blocking of the active sites of the metal,i.e.,2-MBO is a mixed-type inhibitor.The inhibition efficiency increases with increasing inhibitor concen-trations and reaches a highest value at2-MBO concentration of 2×10−4mol dm−3.The results of electrochemical impedance spectroscopy techniques and Tafel polarization measurements are in reasonably good agreement.The adsorption of2-MBO inhibitor on the copper surface in0.5M sulphuric acid solution286H.Tavakoli et al./Materials Chemistry and Physics 109 (2008) 281–286obeys a Langmuir adsorption isotherm.The adsorption of inhibitor is a physical–probably electrostatic–nature,and that no covalent bond between inhibitor molecule and metal surface is established.When SDBS is used alone as corrosion inhibitor,the inhibition efficiency shows a maximum at a concentration of about5×10−4mol dm−3.The decrease of inhibition efficiency at higher concentrations is ascribed to the formation of hemi-micellar aggregates that promote inhibitor desorption from the metal/solution interface.Upon mixing SDBS and2-MBO inhibitor,a slight antagonistic effect is observed if one of the additives is present in a concentration below5×10−5mol dm−3.For higher concentrations,this trend is reversed and a moderate synergism is found.Both observa-tions can be understood through a single phenomenon,which is the electrostatic interaction between the generated DBS−and MBO n+n(1≤n≤4)ions,leading to overlay formation for SDBS concentrations less than5×10−5mol dm−3and to interfacial co-adsorption for SDBS concentration more than 5×10−5mol dm−3.References[1]F.Bentiss,M.T.L.Gengembre,grenee,Appl.Surf.Sci.161(2000)194.[2]J.R.Davis,ASM Specialty Handbook:Copper and Copper Alloys,ASMInternational,2001.[3]D.R.Crown,Principles and Application of Electrochemistry,Blackie Aca-demic&Professional,1994.[4]G.Trabanelli,V.Carassiti,in:M.G.Fontana,R.W.StaehIe(Eds.),Advances in Corrosion Science and Technology:p.,vol.1,Plenum,New York,1970,p.147.[5]L.S.Nandeesh,B.S.Sheshadri,Bri.Corros.J.23(4)(1988)239.[6]A.Raman,bine,Reviews on Corrosion 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