AbstractEnvironmental green compounds have emerged as powerful inhibitorsfor corrosion of metals and alloys. So, this article attempts to show thatbarbituric (BA) and thiobarbituric Acids (TBA) play as good green corrosioninhibitors for copper in presence of 3.5% or 0.6mol/L NaCl.

A combination ofquantitative and qualitative tools were used in this investigation such aselectrochemical impedance spectroscopy(EIS), potentiodynamic polarization,FT-IR spectroscopy, scanning electron microscopy (SEM) and energy dispersiveX-ray spectrometer (EDX). Polarization measurements indicate that thesecompounds can function as a mixed type of inhibitor. It was found that, theadsorption isotherm of these inhibitors on the copper surface obeyedFlory-Huggins isotherm. Also, the effect of temperature in presence and absenceof inhibitors was done according to Arrhenius isotherm. Some thermodynamicfunctions of dissolution and adsorption processes was calculated such as, Ea,?H*, ?Go and ?S*. The barbaturic andthiobarbaturic acids recorded high inhibition efficiency of copper metal in0.

6mol/L NaCl at concentrations 5×10-3mol/L and 1×10-3mol/Lrespectively.KeywordsBarbituric,Thiobarbituric, Potentiodynamic Polarization, Electrochemical impedancespectroscopy (EIS), Flory-Huggins isotherm. * Corresponding author [email protected]

com*Address: National Research Centre, 33 El Bohouth St.( El-Tahrir St.former) Dokki- Giza- P.O.12622  1. IntroductionCorrosionis a continuous and complicated problem dealing with the deterioration of metalas a result of chemical attack or reaction with the environments 1,2.

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Actually,corrosion cannot be eliminated completely but can be retarded or prevented tosome extent by using inhibitors. Recently, the world give more attentiontowards the protection of environment, by using of non-toxic green inhibitorsis the main concern of many researchers 3-6.Thesignificant effect of corrosion inhibitors depends on the complexation withmetal and the extent of dissolving them in water. However, the formation ofstable and insoluble complex between inhibitor molecules and hydrated metalions lead up to decrease the dissolution of metal, thus having an effect ofretarding the corrosion. Clearly, the corrosion inhibition efficiency iscontrolled by the existence of heteroatoms (e.

g. O, N, S) of high electrondensity 7-9, and aromatic rings with polar groups and ?-electrons in theinhibitor molecule 6, 10,11. The electron pairs of mercapto 12 and aminogroup 13 can support the adsorption of inhibitor molecules to the metalsurface. Naturally, these compounds can be adjusted to progress the strength oftheir adsorption bonds to the conversed metals. Also, these can be done byretarding the cathodic and/or anodic processes or adsorbing on metallic surfaceby forming adsorbed layer which act as compact barrier film 14. Pharmaceuticallyactive substances such as barbituric acid and its derivative such asthiobarbituric acid used as corrosion inhibitors for mild steel protection indifferent media 14-17. The adsorption of TBA on mild steel surface was foundto take place through mainly the electrostatic interaction 3.

Udhayakala et.al 17 used Fukui function (which provide information to relate to the atoms in a molecule thathave a higher tendency to either loose or accept an electron or pair ofelectrons) toshow the nucleophilic and electrophilic attacking sites in the inhibitors.Copperis suitable for making a wide range of application in submarine engineering18, seawater desalination, pipelines and heat exchanger 19. So, the inhibitingcopper corrosion is a continuing concern in industries. Films formed on thecopper surface not only prevent the surface from corrosion but also improve themorphology of the surface 20. Recently,investigators have examined the effect of some N-heterocyclic compounds oncopper with changing the placement or the number of substituent groups and alsoby inserting heterocyclic compounds such as triazole, tetrazole, pyrazole andimidazole 21-24.

Theaims of this work are to determine the efficiency of BA and TBA acids for usingas green corrosion inhibitors of copper metal in 0.6mol/L NaCl solution byusing various electrochemical techniques, also to illustrate the differencebetween the ability of BA and TBA to performance as corrosion inhibitors. Thechemical structures of BA and TBA are given in Fig. (1).                                        (a)                              (b)Fig.(1)The chemical structures of (a) barbaturic acid and (b) thiobarbaturicacid.    2. Experimental details2.

1. Electrochemical measurementsTheworking electrode is made of copper containing 99.5 wt % Cu, 0.002 wt% Ni,0.

018 wt% Al, 0.005wt% Mn and 0.115 wt% Si. The copper electrode was polishedwith a series of emery paper then washed with distilled water.

The corrosionpotential of the working electrode was measured against Ag/AgCl as a referenceelectrode and pure Pt-wire was used as counter electrode. The concentration ofsodium chloride which used as the aggressive medium in this work was 3.5% or0.6mol/L NaCl with different concentrations of the inhibitors (1×10-2,5×10-3, 2.5×10-3 and 1×10-3mol/L). Theelectrochemical measurements such as potentiodynamic polarization and impedancestudies were carried out using Autolabpotentiostat/galvanostat PGSTAT302N. Potentiodynamicpolarization curves were investigated by changing the electrode potential from0.2V to 0.

8V with potential scan rate of 1 mVs?1. For studying thetemperature effect, the same procedure of polishing the working electrode washappened and it was immersed in 0.6mol/L NaCl with and without selectedinhibitor concentration at different temperature. The corrosion parameters i.e.corrosion current density icorr and corrosion potential Ecorrwere evaluated from the intersection of the linear anodic and cathodic branchesof the Tafel plots. The values of inhibition efficiency of BA and TBA werecalculated according to the following equation: Where  are the uninhibited and inhibited corrosioncurrent densities, respectively.Theelectrochemical impedance spectroscopy (EIS) of the electrode surface after immersionin 0.

6mol/L NaCl with and without inhibitor has been carried out with Acvoltage amplitude of 10 mV using an electrochemical impedance system. Thefrequency range used in the study was 0.02–105Hz. All the electrochemicalimpedance measurements were carried out at open circuit potential.2.2. Surface MorphologyThesurface morphology of the copper specimens after 3 days of  immersion in 0.

6mol/L NaCl with and without  the ideal concentration of the inhibitor of BA (5×10-3 mol/L) and (1×10-3mol/L)of TBA was performed on scanning electron microscope JEOL-JSM-5600 equippedwith an energy dispersive X-ray spectrometer OXFORD Link-ISIS-300.2.3. FTIR StudyIRspectra were recorded for the powder of the inhibitors and the film of theinhibitors adsorbed on the copper metal with the aid of JASCO 4600 model FTIRspectrometer. 3.

Results and Discussion3.1 Polarization StudyCathodicand anodic polarization curves of copper in 0.6mol/L NaCl with and withoutdifferent concentrations of BA and TBA inhibitors were shown in Figs. (2, 3)The corrosion parameters predicted from Tafel polarization curves recorded forthe copper metal in 0.6mol/L NaCl solution with different concentrations of BAand TBA were listed in Table (1). It was observed that the corrosion rate ofcopper in 0.6mol/L NaCl decreased in presence of the inhibitors, the decreasein the anodic and cathodic current density may be due to formation of barrierfilm on the metal surface which is probably related to the adsorption ofinhibitor molecules on the electrode surface, beside elevating hydrogenevolution reaction or oxygen-reduction reaction which preventing the formationof soluble CuCl2 25. In other words at the first region of anodicpolarization the current density increases may be due to formation of solubleCuCl2 which after some times the current density decreasedattributed to formation of mainly CuCl film and then increased again because ofthe dissolution of the film to produce of Cu(II) ions26.

   In this study we notice that the open circuitpotential changed rapidly to less negative values and then become stablecorresponding to the free corrosion potential, Ecorr, of the metal.The change in Ecorr is about 65mV by at least 85mV that is anevidence of acting the inhibitors as mixed type 27. The preferentialadsorption of these compounds plays a role in blocking the active sites in bothanodic and cathodic reactions which leading to inactivation of part of thesurface to the aggressive attack 28.

This adsorption results in the formationof protective complex with copper ions and can be described as the followingmechanism 29Cu(S)+ BA(aq) Cu:BA(ads)+ H+(aq)   Noticeably, thevalues of inhibition efficiency for using TBA as inhibitor are higher than thatof BA. This may be related to the molecular structure of the inhibitor whichgives or accepts electrons to or from the metal surface leading to form of newbonds 30. The direction of inhibitor molecule is substantial where it gives achance to construct stronger bond, moreover due to the interaction of?-electrons of the ring with empty d orbitals of copper metal  resulted in parallel orientation of inhibitormolecule 31.  3.2 Impedance studies The Nyquist plots for copper in 0.6mol/L solution of NaCl inpresence and absence of different concentrations of TBA and BA was examined. Allthe experimental results investigated by using Randles equivalent circuit withone time constant shows in Fig.(4).

 Where Rs  is theresistance of the electrolyte, Qdl is the constant phase elementdisplaying the double layer and the Rct the charge–transferresistance representing the most suitable factors to observe the protectiveproperties of the film. The measured EIS parameters for adsorbed BA on coppersurface are deduced and outlined in Table (2), from it we can noticed that theRct values display increasing tendency with increasing theconcentration of BA. Furthermore, the values of Qdl whichcorresponding to the double layer capacitance Cdl decreased with anincrease in the concentrations of BA. This result can be related to theadsorption of the inhibitor molecules at copper/solution interface 32.

Thevalues of Cdl are estimated according to the following equation: Cdl= Yo(w)n-1 , where Yo is modulus of CPE refersto the capacitance  of the formed film, wis Warburg impedance and n is the phase shift. The presence of Warburg impedancein the electrochemical circuit pointed to the porous nature of the modulatedsurface 33,34. Fig.(5) For BA exhibits one capacitive loop conforming one timeconstant in Bode plots presented in Fig.(6). It is apparent that the diameterof the Nyquist plots is approximately amounting to the value of thecharge-transfer resistance (Rct) during the corrosion process. Inother words, the capacitive loop does not show ideal behavior increasing ordecreasing with addition of various concentrations of the inhibitors (BA and TBA)this result suggested that impedance dispersion take place. This result can beinterpreted that there is more than one electrochemical process occurred, alsomay be due to the rudeness of the electrode surface.

Obvious that the values ofthe phase angle registered much higher values nearly 63o in thepresence of the inhibitors (BA and TBA) indicated that the surface protected byformation of CuBA or CuTBA films. The deviations of the phase shift than theideal value (unity) in this case could be possible as a result ofirregularities and inhomogeneity of the surface 35. Thus the same situationhappened with using TBA under the same conditions which shown in Figs. (7,8)where their results listed in Table (3), except that the barbituric acidrecorded high inhibition efficiency of copper surface at concentration 5×10-3mol/Lbut TBA reported to some extent the same ratio of inhibition efficiency about94% at concentration 1×10-3mol/L. The difference in concentrationmay be attributed to the more solubility of TBA than BA due to the replacementof oxygen atom by sulfur atom in TBA 36. All the impedance parameters andalso the polarization measurements of the concentration 1×10-2mol/L werenot matching with the other concentrations, this may be due to the dissolutionof the formed film.

This result cleared from the value of the inhibitionefficiency at this concentration 3.3 Adsorption IsothermGenerally, the investigation ofadsorption processes are determined by adsorption isotherms, which providestructural information about the linkage between the additive inhibitor and thesurface of the metal which resulted in retardation of the corrosion rate byweakness the diffusion of corrosive species or increasing the resistance of themetal surface. Flory-Huggins isotherm was the best suitable model that fittedthe experimental data which used to describe the adsorption characteristics ofBA and TBA on copper surface in 0.6mol/L NaCl solution. Figure (9) offer the linearrepresentation of the function  against  37 according to Flory-Huggins equation,which is expressed as: Where, n is the number of ions occupyingadsorption sites; KFH is the equilibrium constant (L mol-1),Co is the equilibrium concentration and  is the degreeof surface coverage. The valuesof the regression coefficient (R2) are nearly close to one provedthat the adsorption of BA and TBA on copper surface obeys Flory-Hugginsisotherm. In addition, the values of theequilibrium constant KFH obtained from this isotherm and thechange of standard free energy (DGo) of adsorption can bespecified according to the following equation and listed in Table (4).

Where, R is the universal gasconstant (8.314mol-1K-1) and T is the absolutetemperature. Theadsorption processes are always classified into chemisorption or physicosorptionrelated to the values of DGo 38. The chemisorption case is consideredwhen DGo is equal toor more negative than (-40kJmol-1) where charge shared or transferred from the inhibitor molecules to thecopper surface forming coordinate covalent bond. While the physic-sorption type is regarded if DGo is equal to or less negative than (-20kJmol-1)in which an electrostatic interaction between charged molecules and chargedmetal surface is considered.According to Table (4), the valuesof DGo are in between -20 and -40kJmol-1, whichindicates that the adsorption of the inhibitor molecules include a mixed-type physicosorptionand chemisorption mechanism 39.   3.

4. Effect ofTemperature Activation variables for the adsorption features of the studiedcompounds on copper in 0.6mol/L NaCl were calculated by supposing a directrelationship between log icorr (the rate of constant for thecorrosion reaction) and 1/T according to the Arrhenius pattern asthe equation of the form: Where A is the frequency factor, R is universal gasconstant and T is absolute temperature. The relationship between logicorr and 1/T presented straight lines with good degreeof linearity as shown in Fig.(10).

 The activation energies concluded from the plots of copper immersedin 0.6mol/L NaCl without and with optimum concentration of BA and TBA arelisted in Table (5). The values of Ea for inhibited copperare higher than that for the uninhibited due to the adsorption of inhibitormolecules on the active sites on copper surface which increases the energybarrier connected with corrosion reaction.

Also, the increase in activationenergy may be explained as a result of the physical adsorption that happen atthe first step which is proved by decreasing the inhibition efficiency withincreasing the temperature 40,41.  Fig. (11) presents the plots of log icorr/T versus1/T depending on the transition state equation: Where N is Avogadro’s number and h is plank’sconstant.

This relationship gives straight lines with slope and intercept equalto and  respectively. The values of ?S* and DH* are listed inTable (6). The positive sign of enthalpy ?H* in presence of TBA andBA as inhibitor points to the nature of the reaction suggesting  endothermic one, also indicates that the dissolutionof copper is slow 42.

The change in entropy DS* is negative for the adsorption of BA and for the blank medium, proposing that the adsorption process isdominated by associative interaction between the copper and the inhibitormolecules rather than the disorder that taking place between the metal and thewater molecules, while the positive value of entropy have the opposite behavior43.    Table(6): Thermodynamic parameters for the adsorption of optimum concentration ofinhibitor molecules on copper surface in 0.6mol/L NaCl at differenttemperatures.           3.5.

FTIR Spectral Study   Different functional groups forcorrosion product on the copper in presence of BA and TBA were realized from ofFTIR spectra in Fig. (12) which clarify difference between the spectra of theadsorbed inhibitors and that of pure ones. That was noticeable from theshifting of the wavenumber from higher to lower values. The spectrum of pure BA and its adsorbed inhibitor display that thebroad peaks corresponding to ?(OH) and ?(NH) in pure BA appeared as sharpnature with CuBA associating with shifting from its position to lower value for?(OH) group (3500-3310cm-1) and higher value for ?(NH) group (3244-3277cm-1).    The stretching of CH group in BA appeared at 2900 cm-1exhibits lower wave number 2844 cm-1 for BA adsorbed on coppersurface Also, the peaks around 1780 and 1678 cm-1 which are assignedto ?(C=O) undergoes shift to lower wavenumber after adsorption on the metalsurface to become at 1733 and 1644 cm-1. This may be due to thecovalent bonding of the carbonyl group to empty hybrid orbital of copper atom44.This description confirmed the ability of the inhibitor molecules tocoordinate with Cu2+ ions.

Also, the same shifting appears in FTIR spectrum of TBA. Theoccurrence of peaks with sharp nature at wavenumber 955, 877, 833 and 778 cm-1pointed to the chemisorption of BA and TBA on copper surface 45.    The characteristic band of (C=O)group of TBA, adsorbed on copper surface, appear at the same wavenumber of pureTBA at 1706 and 1666 cm-1 but have a shape of medium peaks comparingwith weak peaks for pure TBA.

   The observed weak peak at1350 cm-1in the TBA spectrum assigned to ?(C=S) + ?(NH) whichshifted to higher wavenumber 1392 cm-1, that can be related tospecific molecular interactions thus leads to formation of coordination bondbetween sulfur hetero atom and unfilled d-orbital of copper atom46.3.6. Surface Morphology Investigation (SEM/EDX)Scanning electron microscopy and X-ray spectroscopy is one of themost ordinarily techniques for assessment and analyzing the surface andelemental constituents of corrosion products attached samples. SEM imagesappeared the interaction between metals and the corrosive media, so themorphology of the metal surface and collection of corrosion products can bedetermined by SEM analysis .Fig.

(13) exhibits the SEM micrograph of copperspecimens  after 4 hours of immersion in0.6mol/L NaCl in presence and absence of the studied inhibitors.The surface of copper sample returned from the solution withoutinhibitor is strongly deteriorated, offering deep cavern which is related todirect attack of copper by aggressive ions. While the SEM images of copperspecimens with the best concentration of TBA and BA offer relatively smoothersurfaces, which is due to the adsorption of the inhibitor molecules on metallicsurface. The protection of metallic surface was occurred by isolating surfacefrom the corrosive medium leading to less damaged and smoother surface.

The EDX spectra of copper surfaces are shown in Fig. (14), it isobvious that the intensity of oxygen in the EDX spectrum of blank copper sampleis referred to the formation of oxide film by oxidizing copper surface. Bycomparing the intensity of oxygen in the EDX spectra of copper surfaces inpresence and absence of inhibitors, increasing the intensity in presence ofinhibitors is noticeable which indicates the adsorption of inhibitor moleculeson the copper surface. The presence of nitrogen and sulfur in the EDX spectraof copper come from the inhibitor containing solutions definitely approve theadsorption of inhibitors on the metallic surface.

 ConclusionsBarbaturic (BA) and thiobarbaturic (TBA) acids act as mixed-typeinhibitors impede both the anodic and cathodic reactions on the corrosion ofcopper in 0.6mol/L NaCl solution, the inhibition efficiencies for BA and TBAhave maximum values at 5×10-3,    1×10-3mol/L respectively.According to electrochemical impedance spectroscopy (EIS) technique, morphologyanalysis and potentiodynamics polarization, the mechanism of adsorption ofinhibitor molecules on copper surface suggested to be physic-sorption andchemisorption modes.