A novel green inhibitor for C-steel corrosion in 2.0 mol·L−1 hydrochloric acid solution

A.Y.El-Etre*,A.I.Ali

Chemistry Dept.,Faculty of Science,Benha University,Benha,Egypt

1.Introduction

Iron and its alloys are widely used in a lot of industrial processes such as petroleum industry.This industry has been required mineralacid solutions(hydrochloric acid and sulphuric acid)for several purposes such as pickling,industrial acid cleaning,acid descaling,oil well acidizing,etc.[1,2].The acidic applications could cause corrosion of iron and its alloys during working.Corrosion damage of metals and alloys causes both economic and hazard problems in the work place.Therefore,the corrosion rate of metals or alloys must be decreased.Adsorption of an effective coating forms a barrier between the metallic surface and the corrosive environment and causes the protection to the metal from corrosion.The usage of organic molecules as corrosion inhibitors has been used extensively to reduce the acid corrosion damage on metallic surfaces.Various classes of organic inhibitors containing different active functional groups,allowing its adsorption on the metal surface,are successfully used as corrosion inhibitors of carbon steel in acid media up to date[3–7].

The choice of optimal inhibitor should be based on their structures;ease synthesis,inexpensive raw materials and its toxicity in the environment must be negligible.The eco-friendly naturally corrosion inhibitors are very identical due to many benefits such as;plant extracts are rich valuable sources of natural compounds that could be extracted simply without economic cost;most of the natural products are easily biodegradable,have no toxicity,and have continuous feed sources of materials.Many plant extracts have been used as effective corrosion inhibitors of C-steel in acidic medium[8–14].

MeliaazedarachL.(MA)is a botanical species belonging to the family Meliaceae.It is native to Asia but is now found in parts of Northern Australia,Africa,North America,tropical South America and Southern Europe.Chemical constituents of the seeds were reported in the literature[15–17].

No reported literature is available for using the MA seed extract as corrosion inhibitor for C-steel in hydrochloric acid medium.The scope of this work is studying the inhibitive effect of the new aqueous green MA seed extract to control the carbon steel corrosion in hydrochloric acid.Gravimetric,potentiodynamic methods,EIS and SEM surface analysis were used for the characterization the inhibitive behavior of MA seed extract.

2.Experimental

2.1.Materials preparations

The ASTM composition of the studied A573 Grade 70 low C-steel is given in Table 1.

The tested samples were previously worked in the Cairo Co.for petroleum refining as a tank wall samples.The specimens were cut into small samples having a certain form according to the tested technique as described below.

2 mol·L−1HCl was prepared as aggressive solutions by diluting 37%HCl(Merck)by double distilled water.

2.2.Preparation and characterization of MA extracts

Fresh intact parts of MAseed materials were dried at45°C overnight priorto grinding them into fine powder.Extract of the MA seed was carried out by soaking 25 mg of dried pulverized seed in 250 ml of bidistilled water and refluxed over a hot water bath at60°C for6 h.After cooling,the solution was filtered and the mother liquor evaporated in a drying oven at80°C.The solid residue was used to prepare a stock aqueous solution from which the desired concentrations were prepared by dilution.

The MA extract was demonstrated by FTIR spectroscopy using Nicolet ISO 10 model spectrophotometer in the frequency range of 400–4000 cm−1using KBr pellet technique.

Table 1ASTM chemical composition of C-steel

2.3.Experimental methods

Different techniques were examined to clarify the behavior of the tested C-steel in the presence of aqueous MA seed extract.The corrosion experiments were carried out in open air solutions at 298 K.

2.3.1.Gravimetric measurements

Rectangular steel specimens having lengths of(1.2×1.1×0.75)cm with total surface area,6.09 cm2were used.Every sheet specimen was fully cleaned using silicon carbide polishing papers of(#180–2000)grades and washed with bidistilled water.The residual polished parts were taken away in acetone bath for 10 min.The polished C-steel specimens were kept in dried container for corrosion test performance.Before each experiment,the specimens were strictly weighed,immersed in 50 ml of tested media using inert hook bar.The duration time of immersion ranged between 3 and 48 h.The temperature of the environment was maintained by thermostatically controlled water bath with an accuracy of±0.2 °C under aerated condition.At the end of tested time interval,the specimens were pulled out for washing,drying,and strict weighing.For accuracy,experiments were done three times in each case prior to reporting the average loss in weight.The corrosion rate was calculated using the loss in weight in milligrams for each square centimeter with respect to time(mg·cm−2·h−1).The surface coverage(θ)and inhibition efficiency,IE(%),were determined according to the following equation:

wherewoandwiare the rate of corrosion for carbon steel samples in hydrochloric acid free and containing MA extract,respectively.

2.3.2.Electrochemical experiments

Conventional three electrode cell was used in all electrochemical measurements.C-steel specimen acts as working electrode against platinum sheet electrode.The cell potential was measured with respect to reference SCE.The working electrode was shaped in the form of cylindrical C-steel rod covered by araldite with bottom surface area of 0.38 cm2exposed to the corrosive solution.Prior to each experiment,the working electrode was fit as mentioned before(Section 2.3.1.).The working electrode was immersed for 30 min in hydrochloric acid containing different concentrations of MA seed extract to attain its steady state potential prior to start each experiment.Meinsberger potentiostat/Galvanostat with software zum PS6 remote,was used to carry out potentiodynamic polarization and cyclic voltammetry.

Potentiodynamic experiments were performed by scanning the electrode potential from −1800 to 600 mV at 10 mV·s−1sweep rate.Tafel lines were extrapolated to the corrosion potential prior to calculating the electrochemical kinetics parameters.From cyclic voltammograms,the charge densities under each anodic branch for C-steel in aggressive solutions containing different concentrations of MA extract were calculated by current integration using PS6 software.The IE and θ were obtained by using Eqs.(3)and(4):

whereiocorrandicorrare the corrosion current densities of steel specimen(mA·cm−2)in the absence and presence of different concentrations of the MA extract,respectively.

EIS was carried out using PARSTAT 4000 teamed with the Versa Studio software package to calculate the values of polarization resistance(Rct)and double layer capacitance(Cdl).EIS study was carried out at potential amplitude of 10 mV,peak-to-peak(AC signal)inEcorr,with 10 points per decade and the frequency ranging from 0.1 Hz to 100 kHz under potentiodynamic conditions.The impedance diagrams were plotted in Bode and Nyquist plots.

The inhibitive efficiency of the extract on C-steel corrosion,IE%,was determined according to Eq.(5):

whereRct(o)andRct(i)are charge transfer resistance of C-steel in the absence and presence of inhibitors,respectively.

2.3.3.Scanning electron microscopy(SEM)

Samples of dimension(1.2×1.1×0.75)cm were prepared according to Section 2.3.1.procedure.After immersion in 2 mol·L−1HCl solutions without and with addition of 600 mg·L−1MA seeds for 3 h,the specimens were cleaned with distilled water,dried and then examined by SEM model QUANTA FEC 250 SEM microscope—Holland.

3.Results and Discussion

3.1.Fourier transforms infrared(FTIR)spectroscopy of MA extracts

FTIR spectrum of MA seeds was showed in Fig.1.The figure depicts that;–OH stretching of alcohol or phenol appears at 3416.15 cm−1,while 3009.64 cm−1is assigned to C–H of strong stretching in aromatic ring.The peaks at 2926.29 and 2854.76 cm−1can be assigned to stretching mode of aliphatic and aromatic C–H groups,respectively[18].The peak at 1745.64 cm−1is probably corresponded to the strong stretching mode of C=O in esters or saturated aliphatic.Peak at 1632.88 cm−1is ascribable to vibrations of–C–C–,in aromatic ring or alkenes,while the C–Hbending in–CH2is found at1402.08 cm−1.Peak at 1100.8 cm−1is usually ascribed to C–O strong stretching of alcohols,esters or ethers.

Fig.1.FTIR of aqueous crude extract of Melia azedarach L.seed.

Finally,the FTIR results confirm that the MA seed extract contains aromatic rings,besides varied functional groups(O–H,C=C,C=O),all of them meet the standard specification of inhibitive compounds.

3.2.Gravimetric measurements

3.2.1.Effect of immersion time

Fig.2 shows the effect of exposure time,in hours,on the corrosion rate of C-steel sample immersed in HCl solutions containing different concentrations of MA seed extract.It can be noted that,the corrosion rate increase with increasing the immersion time in all tested solutions.In the case of free HCl,this behavior is found to be sharp and could be attributed to the formation of the loosely adherent porous rust layer Fe2O3·H2O on C-steel surface[19].This layer does not form the protective barrier that can isolate the metal from the corrosive environment.On the other hand,in HCl containing a certain concentration of MA extracts,the corrosion rate increases slowly compared with the pure acid media.This behavior could be ascribable to the inhibition of specific parts of the steel surface that is covered by adsorbed MA extract molecules which in turn could isolate the metal from the corrosive environment.This leads to decrease in the corrosion rate of the steel coupons.

SEM con firms this output interpretation.Fig.3 shows SEM micrograph of C-steel immersed for 3 h in 2 mol·L−1hydrochloric acid,either free or has 600 mg·L−1of MA seed extract.The morphology in Fig.3a shows a rough surface,characteristic of the uniform corrosion of C-steel in acid solution.Fig.3b shows that smooth surface can be observed,due to the formation of green inhibitor's protective film on the metal surface without visible polishing lines,indicating that the seed extract provides high protection.Also Fig.3b shows that,discontinuous green inhibitor’s protective film on the metal surface which leads to decrease in the corrosion rate of the C-steel depending on the extent of the film continuity.

Fig.2.Relation between time of immersion and mass loss in 2 mol·L−1 HCl solutions avoid of and containing different concentrations of MA seed extract.

3.2.2.Effect of concentration

The effect of MA seed extract concentration on the inhibition efficiency(IE)was studied in 2 mol·L−1HCl solution.It was found that,IE was increased with increasing the inhibitor concentration.Fig.4 showed that,the maximum inhibition efficiency is achieved in the presence of 600 mg·L−1of seed extract,94.23%approximately at 298 K.

Fig.4.Relation between extract concentration and inhibition efficiency.

3.2.3.Effect of temperature

Gravimetric experiments were performed in free HCl solution and that containing 600 ppm of MA seed extract at different temperatures after a 3 h exposure time for evaluating both the stabilization of adsorbed film of the extract molecules on the tested metal surface and the activation parameters of the corroding process in acid media.

Fig.3.SEM images of C-steel sheets immersed in 2 mol·L−1 HCl(a)free(b)containing 600 mg·L−1 of MA seed extract.

The obtained data are recorded in Table 2.Generally,it is obvious that the corrosion rate increases with increasing temperature in the absence and presence of the MA extracts.However,the presence of MA extracts effectively retards the corrosion rate of C-steel as a result of protective film formation as discussed previously.It is clear that,the inhibition efficiency remains slightly constant during the studied temperature range.This result reflects the adsorption stabilization of MA extracts at the tested metal surface.

The activation energies(Ea)of the corrosion process were determined according to Arrhenius equation[20]:

Table 2Corrosion behavior of tested steel alloy in 2 mol·L−1 HCl in the absence and presence of 600 mg·L−1 of MA seed extract at different temperatures

where CR is the corroding rate(mg·cm−2·h−1)andAis the frequency factor.

The plot of the logarithm of corrosion rate of the C-steel in both free and inhibited HCl solutionsversusthe reciprocal absolute temperature gives straight lines(seefig.5a).Multiply the slope of Fig.5a curves by the molar gas constant(R)identify the activation energy as 67.94 and 62.51 kJ·mol−1in corrosive solution free and containing MA compounds.Aqueous active ingredient of MA seed clearly decreases apparent activation energy.Also,it is worthy that;MA extract has good inhibitive effect along the studied temperature range(25 °C–70 °C).These results may be interpreted by a chemisorption process of inhibitor on the steel surface[21,22].

Thermodynamics of C-steel corrosion in 2 mol·L−1HCl(at examined concentrations of MA extract,0 and 600 mg·L−1)are calculated using an alternative formula of the Arrhenius equation:

whereh,N,ΔH*,and ΔS*refer to Planck's constant,Avogadro's number,enthalpy,and entropy of activation respectively.

Plotting logarithmic form of Eq.(7),as shown in Fig.5b,allows the calculation of ΔH*and ΔS*using the slope and intercept of resulted lines.Calculated ΔH*values for free and inhibited solution are 65.64 and 59.89 kJ·mol−1,while ΔS*values are −12.6 and −35.54 respectively.The obtained positive(ΔH*)values indicate the endothermic behavior of the dissolution process related to C-steel[15].It is also noticed that,the values of activation enthalpy are lesser than the corresponding values of ΔE*meaning that the dissolution process should involve the hydrogen evolution reaction.On the other side,the negative entropy accounts at examined concentrations of inhibitor indicate the decrease in disorder on going from reactant to the activated complex.Therefore the activated complex association at rate determining step is preferable rather than dissociation.The more negative values of the ΔS*in solution containing inhibitor mean the participation of MA extract molecules in making more ordered systems due to the demolishment of corrosion activated complex.

3.2.4.MA extract inhibition isotherm

Inhibition properties of organic compounds are result of their adsorption to form protection layer onto the metal surface.Study of the inhibition mechanism implies application of some widely used adsorption isotherms.Trials were done for fitting the experimental results to suitable isotherm.Langmuir adsorption isotherm gives the best fitting for MA extract inhibition on C-steel surface as Fig.6 demonstrates where linear correlation coefficient and slope of the obtained line are unity.

Fig.6.Langmuir isotherm.

Langmiur isotherm is formulated as[18,19]:

Fig.5.Effect of temperature on C-steel corrosion in 2 mol·L−1 HCl containing 600 mg·L−1 of MA seed extract(a)Arrhenius plot(b)transition state plot.

Cinh,Kad,and θ are the tested inhibitor concentration,the equilibrium constant of adsorption process,and surface coverage of the steel respectively.

The calculated value ofKadwas found to be 40 L·g−1.This considerable level ensures good adsorption stability for seed extract on the steel surface.

where ΔGoadrefers to standard free energy related to adsorption process when water molecule is exchanged by another inhibitor,Csolventis the liquid concentration in solution and has the account of 1.103g·L−1.

The calculated ΔGoadfor adsorption process of seed molecule extract was −26.25 kJ·mol−1.This high negativity account of ΔGoadmeans both the spontaneous and the stable nature of the adsorption process.In addition,the seed extract is adsorbed onto C-steel surface according to physico-chemical adsorption mechanism[22].

3.3.Potentiodynamic polarization

Tafel plots of C-steel electrodes in 2 mol·L−1HCl solution at different concentrations of seed extract are presented in Fig.7.

Fig.7.Polarization curves of C-steel in different MA seed extract concentrations.

The related corrosion outcomes extracted from resulted curves are given in Table 3.These data illustrate the decrease inicorras a result of MA extract addition.This behavior is a consequence of the increased covered portions of the electrode surface(θ).Also,the values of anodic and cathodic Tafel section slopes are observed to decrease in the case of extract addition compared to the blank slopes.Moreover,Ecorrvalues show slight change in the positive direction in the presence of inhibitor than those in the blank.This outcome demonstrates both the anodic dissolution of C-steel and the cathodic reduction of hydrogen ion depression by adsorption of MA seed extract onto anodic and cathodic sites of the steel surface,and means that the tested seed compounds behave as mixed type inhibitor[23].

Table 3Corrosion outcomes of C-steel in 2 mol·L−1 HCl at different inhibitor concentrations at 298 K

3.4.Cyclic voltammetry

The sweep voltammetry for tested steel electrode in 2 mol·L−1HCl at various MA extract concentrations at a sweep rate of 10 mV·s−1is depicted in Fig.8.All cyclic voltammograms were swept from−1200 mV up to oxygen evolution.As seen in Fig.8,C-steel electrode in all studied solutions has short passive region in the anodic section.The passive film formed on the metal surface is due to formation of self-double layer at metal/solution interface which retards the electron transfer[24].

The trans-passive formation started in all solutions approximately at the beginning potential of oxygen evolution.However,during the current rises,an abroad anodic peak is formed in the presence of MA seed extract,as well as increasing the crude extract concentrations could decrease the area under anodic section.This behavior can be interpreted by adsorption of active function group of the seed extract to plug the pores of self-double layer on the metal surface,consequently,decrease the charge transfer.

The charge magnitude of the film grown on the steel surface in free acid media is higher than that on C-steel surface in the presence of MA extract as noticed in Fig.9,indicating higher oxidation rate of C-steel in the free acid.This behavior is attributed to the real inhibition of MA extract.

3.5.Electrochemical impedance spectroscopy(EIS)

Electrochemical impedance spectroscopy constitutes a rapid way to estimate the characteristics of the adsorbed surface films formed by inhibitor molecules.

Fig.10(a,b)represents the Nyquist and lgf–lgZimpedance plots of C-steel in 2 mol·L−1HCl at different concentrations of MA seed extract.It is obvious that the obtained impedance spectra show a single moderate depressed semi-circle and only one time constant appears in lgf–lgZplot,which points out that the tested metal behavior in corrosive media is controlled by a charge transfer process[25].

The deviation of Nyquistplots with respect to the ideal case is mainly due to frequency dispersion,adsorption of inhibitive compounds,polycrystallinity and inhomogeneity levels of the surface[26,27].According to Fig.10,the presence of the seed extract in the corrosive solution does not affect the shape of impedance plots,which suggests a similar mechanism for the C-steel corrosion.Fig.10a clears that,diameters of Nyquistplots increase with increasing the inhibitive molecule concentration to the corrosive media,due to their enhancement of C-steel protection.

The linear region of the lgf–lg│Z│ plots should have a slope(α)of−1 for the ideal capacitors.The experimental α values are calculated and given in Table 3.It is seen that,α-values for MA extract are less than 1.This could be related to the non-ideal structure of the double layer as a result for the heterogeneities of the metal surface.

According to the EIS,Randles electrical equivalent circuit has better fitting for the experimental results.Fig.11 shows the Randles circuit model that is composed of solution resistance(Rs),charge transfer resistance(Rct)and constant phase element(CPE)which is related to the capacity of the electrical double layer(Cdl).

The impedance of a CPE element is described according to:

whereQis a proportional factor called admittance(sα·Ω−1·cm−2),j2= −1 is an imaginary number,and ω is the angular frequency in rad−1.The“double layer capacitance”values(Cdl)were calculated using the Hsu and Mansfeld formula[14].

Fig.8.Cyclic voltammograms of C-steel electrode in 2 mol·L−1 HCl in different concentrations of MA seed extracts.

Fig.9.Charge density on C-steel surface immersed in 2 mol·L−1 HCl as a function of MA extract concentrations at 298 K.

Table 4 shows the EIS parameters associated with the Nyquist and Bode impedance diagrams.Inspection of Table 4 reveals that,the values ofRctincrease with increasing the MA seed extract concentration which indicates the enhancement of surface coverage by the inhibitor molecules and the increasing inhibition of steel corrosion.Furthermore,the rates ofCdldecrease with increasing of the inhibitory molecule concentration.This behaviorcan be attributed to the adsorption ofthe inhibitory molecules on the steel causing the thickness increase of protective layer at the metal/solution interface[28].

3.6.Mechanism of corrosion inhibition

MA seed extract contained different natural organic constituents.The main constituents of MA seeds are reported[15–17].Fig.12 submits their corresponding chemical composition.

Fig.10.Nyquist plots(a)lgƒ–lg│Z│ plots(b)for CS electrode in 2 mol·L−1 HCl solutions containing different concentrations of MA seed extract.

According to Fig.12,such compounds fit the general particular properties of different corrosion inhibitors due to their oxygenated function groups and heterocyclic rings that can be protonated in acidic medium to enable their adsorption onto the C-steel surface.Generally,the magnitudes ofEcorr–Eq=0set the charge of the metal surface[29].The magnitudes of both corrosion potential(Ecorr)and zero charge potential(Eq=0)related to iron in 2.0 mol·L−1HCl medium were −480 mV[as set in this work]and−530 mV[30]respectively against standard calomel electrode(SCE).Therefore,steel surface becomes positively charged in 2.0 mol·L−1HCl medium asEcorr–Eq=0＞ 0.This situation causes the specific adsorption of excess negatively Cl−anions to the steel surface,that in turn creates an intensive final negatively charged metal surface at the solution medium[31],which performs electrostatic interaction towards the cationic inhibitor molecules[32].This authorizes the physical adsorption of protonated seed molecules onto the steel surface through electrostatic interactions and causes the increased protective effect in halide-containing solution[33].In addition,the adsorbed protonated compounds of MA seed extract on the tested metal surface can be coordinated through the partial transfer of electron lone pairs from their oxygen atoms to the unoccupied iron d orbits and/or may be combined with freshly produced Fe2+ions at the metal surface forming the metal-inhibitor complexes that might adsorb to steel surface across Van der Waals forces keeping the protected C-steel far away behind corrosion.This discussion may con firm the physicochemical adsorption type on C-steel deduced from Langmiur calculation.

Fig.11.Randles circuit model.

4.Conclusions

According to the obtained results,several conclusions can be attached:

(1)Melia azedarach L.(MA)act as good eco-friendly green inhibitor to protect C-steel in 2 mol·L−1HCl solution.

(2)Inhibitive effect of MA extract increases with increase in the concentration of the inhibitor and almost independent on studied temperature.

(3)MA extract is mixed type inhibitor blocking both cathodic and anodic steel reactions.

Table 4EIS factors and inhibition efficiency of C-steel in 2 mol·L−1 HCl media at different inhibitor concentrations

Fig.12.Molecular structures of some organic compounds in MA seed extract.

(4)The adsorption of MA extract on the surface of C-steel follows Langmuir adsorption isotherm.

(5)Energy of activation value suggests that MA seed extract undergoes physic-chemical adsorption on the surface of C-steel.

(6)SEM images suggest that,the addition of inhibitor to aggressive solutions results in the formation of protective film on the C-steel surface.

Acknowledgments

The authors are greatly thankful to Cairo Co.for petroleum refining,for providing the C-steel coupons.

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