Ekemini Ituen*,Abosede James,Onyewuchi Akaranta2,,Shuangqin Sun

1Materials Physics and Chemistry Research Laboratory,College of Science,China University of Petroleum,Qingdao 266000,China

2African Centre of Excellence in Oil field Chemicals Research,Institute of Petroleum Studies,University of Port Harcourt,Nigeria

3Department of Pure and Industrial Chemistry,University of Port Harcourt,Nigeria

1.Introduction

Corrosion inhibitors have been sourced from various materials including plant biomass.The effectiveness of extracts from various plant biomasses as corrosion inhibitors for metals in different aggressive media has received much attention because of their non-toxic nature[1–6].The corrosion inhibition efficiency of many plant extracts however declines at high temperatures owing to degradation of their complex phytochemicals[7].This limits their application in the field since hydrocarbon recovery from many rewarding pay zones requires high temperature operations. It would be desirable that corrosion inhibitors from plant extracts be optimized to be effective at high temperatures. Various groups of synergistic chemicals like halide ions, surfactants, solvents and polymers are getting increasing attention these days as intensifiers [8–10]. They are used to enhance performance of corrosion inhibitors sometimes at high temperatures. When the main corrosion inhibitor is blended with these additives, a corrosion inhibitor formulation (CIF) is obtained [11]. There are many reports on the use organic molecules as the main corrosion inhibitor [12–14] but some of these molecules are expensive and toxic..

In search for more sustainable,cost effective and eco-friendly source of corrosion inhibitors,elephant grass extract(EGE)has been blended with halide ions for investigation in this study.The blends from EGE would be cheap,readily available,renewable and non-toxic.Elephant grass grows abundantly almost everywhere in Nigeria.It is not edible,so using it as corrosion inhibitor source will not compete with food.Electron donor sites like O,N,and π electrons which are regarded as potential adsorption sites in other effective corrosion inhibitors are present in its phyto-compounds like anthocyanins,lignin and flavonoids[15–19].Commercial production of corrosion inhibitor from EGE would reduce export,generate internal wealth and contribute to local content development.The corrosion process is simulated in hydrochloric acid(HCl)using mild steel.Our study is also Significant to the industry because mild steel is used in construction of most hydrocarbon production,transportation and storage facilities which is vulnerable to corrosive attack.Likewise,HCl is usually employed as acidizing and well stimulation fluids in enhanced oil recovery and is very corrosive[20].With a concentration of 3.5%,EGE can be applied in fluids requiring intermediate acid concentration.

2.Materials and Method

2.1.Preparation of metal specimens

Mild steel sheet purchased from construction materials market in Uyo,Nigeria,was used for the study.The chemical composition of the mild steel was(wt%):C,0.13;Si,0.18;Mn,0.39;P,0.40;S,0.04;Cu,0.025 and balance is Fe.It was mechanically press-cut into coupons of dimensions 4 cm×4 cm,degreased in ethanol,abraded using different grades of silicon carbide papers and finished with CC-22F P1200 grade.The coupons were cleaned with acetone,air-dried,weighed and immediately immersed in appropriate test solution.Coupons for electrochemical studies were prepared by polishing about1.44cm2exposed area to mirror finishing.

2.2.Preparation of test solutions

The corroding medium was simulated using 3.5%HCl prepared by diluting 37%analytical grade HCl in distilled water.Mature elephant grass shoots harvested within University of Uyo community,Nigeria,was washed in distilled water,air-dried in the laboratory,grounded to powdery form and extracted in acetone by maceration,percolation and infusion[21].The main inhibitor from EGE was prepared to concentrationof 5.0 g·L?1in the acid.Ammonium chloride(AMC)and potassium iodide(PTI)at concentration of 0.1 mol·L?1 each were used as additives to obtain the blends.The blends are coded EGE/AMC for elephant grass extract blended with ammonium chloride and EGE/PTI for elephant grass extract blended with potassium iodide.

2.3.Weight loss technique

The procedures reported by Hussin and coworkers[22]according to NACE Recommended Practice RP-0775 and ASTM G-1&G-4 for weight loss experiment were employed in this study.However,different temperatures(30 °C,40 °C,50 °C,60 °C and 90 °C)were obtained using water bath.As a modification,the immersed coupons were retrieved after six(6)hours,washed in detergent solution,rinsed with distilled water,dipped in acetone to facilitate fast air-drying and then reweighed.All masses were measured using Sartorius CPA225D analytical balance of sensitivity±0.01 mg.Denoting the initial and final weights of the coupons with w1and w2respectively,corrosion rates(CR)(mg·cm?2·h?1)),percentage inhibitor effectiveness(inhibition efficiency),εWL,and degree of surface coverage(θ)was calculated respectively as follows:

where Rband Riare the corrosion rates(g·cm?2·h?1)in the absence and presence of the inhibitor respectively and A is the surface area(cm2)of the metal specimens.

2.4.Electrochemical impedance spectroscopy

Gamry Reference 600 Potentiostat/Galvanostat/ZRA REF600-18042 electrochemical workstation with conventional three electrode arrangement:saturated calomel electrode(SCE)as reference,platinium electrode as counter and mild steel as working electrode was used.The potential was stabilized for 30 min at room temperature((28±2)°C)at frequency range of 10 mHz to 100 kHz and amplitude voltage of 5 mV peak to peak using AC perturbation signal at Ecorr.Experiments were conducted by complete immersion without stirring. The inhibition efficiency was calculated from charge transfer resistance values(Eq.(4))which was obtained using Echem Analyst version Gamry Application software package and data fitting of Nyquist and Bode plots.

where RCTBand RCTIare charge transfer resistances in the absence and presence of inhibitors respectively.

2.5.Potentiodynamic polarization measurement

The electrochemical same set up was used to obtain Tafel polarization curves at (28±2) °C by changing electrode potential automatically from?250mV to+250mV vs. SCE at open circuit potential (OCP) with a scan rate of 1.0 mV·s?1.The inhibition efficiency(εPD)was calculated from corrosion current density values(Eq.(5))obtained from data fitting with the software mentioned.

where ICORRband ICORRiare the corrosion current densities in the absence and presence of the inhibitor respectively.

2.6.Linear polarization resistance

This measurement was conducted using an electrode potential of±20 mV vs.OCP maintained at a scan rate of 1.0 mV·s?1at(28 ±2)°C.Inhibition efficiency(εRP)was calculated from the polarization resistance obtained using Eq.(6).

where RPband RPiare the polarization resistances in the absence and presence of the inhibitor respectively.

3.Results and Discussion

3.1.Weight loss technique

The corrosion rates calculated from mass loss data were highest in 3.5%HCl at all temperatures but reduced in the presence of inhibitor solutions as shown in Fig.1.Addition of the blends reduces corrosion rate of mild steel at all temperatures studied.The extent of reduction is estimated using inhibition efficiency values(Table 1).Effectiveness of EGE alone declines markedly at high temperatures than when treated with halide additives.Similar results were obtained by Anupama and coworkers[1]and others[23,24].There are also cases where opposing trend is reported[25,26].The inhibition efficiency follows the trend EGE/PTI＞EGE/AMC＞EGE and decreases as temperature increases.

Fig.1.Variation of corrosion rates of mild steel in 3.5%HCl with temperature.

Table 1 Inhibition efficiency evaluated using weight loss technique

3.2.Electrochemical techniques

Fitting of EIS data into Nyquist(Fig.2)and Bode(Fig.3)plots produce imperfect single depressed semicircles,a behavior which[27]also observed and attributed to surface roughness or inhomogeneity of the mild steel.The single capacitive loop signifies that the corrosion process is mainly controlled by charge transfer process[28].It also indicates that presence of inhibitors does not change the mechanism of steel dissolution. Data obtained were also fitted into equivalent circuits and the best fit obtained is shown in Fig. 4. Surface roughness or heterogeneity is compensated by using the constant phase element(CPE),a non-integer element dependent on frequency,with magnitude given by Y0and n,related to impedance by:

Fig.2.Nyquist plot for the corrosion of mild steel in 3.5%HCl in the absence and presence of the different inhibitors.

Fig.4.Equivalent electrical circuit used for fitting of data for corrosion of mild steel in 3.5%HCl in the absence and presence of the different inhibitors.

where Y0is the CPE constant,w is the angular frequency,j is an imaginary complex number,(j2=?1)α is the phase angle of CPE and n=2α/π is the CPE exponent.

The values of n is used to predict the degree of heterogeneity or roughness of mild steel surface.The inhibitor is believed to act by diffusion from the bulk phase,condensation and adsorption on metal surface.Some results from electrochemical impedance studies provide considerable evidence that the inhibitor was adsorbed at the interface,and a thin insulating film was formed.Increase in n values on addition of inhibitor than free acid and EGE solution is an indication of possible reduction of surface roughness by adsorption of inhibitor[29].The coefficient,n,also indicates a shift to capacitive behavior initiated by interference of an insulator or dielectric at the metal/solution interface on addition of inhibitor.That insulator is considered to be the thin surface protective inhibitor film responsible for the corrosion inhibition.

The charge transfer resistances obtained increased in the presence of inhibitor than the free acid(Table 2),also pointing to presence of a resistive dielectric layer at the interface.Increase in peak heights of Bode plots also indicates more capacitive response of the interface caused by presence of adsorbed inhibitor layer.The magnitude of the capacitance is calculated from peak angular frequency(wmax)using Eq.(8).

Fig.3.Bode plot for the corrosion of mild steel in 3.5%HCl in the absence and presence of the different inhibitors.

Table 2 Parameters obtained from EIS measurement for mild steel in 3.5%HCl in absence and presence of different inhibitors

The Cdlvalues decrease more with the inhibitors than free acid and EGE perhaps due to decrease in local dielectric or an increase in thickness of the electrical double layer(adsorbed protective film),or both.Similar trend is reported in literature[30].The direction of decrease in Cdlalso coheres with the trend of inhibition efficiency implying that the thickness of the film is improved by the synergistic effects of halide compounds added.

Tafel plots (Fig. 5) also reveal that the corrosion current densities shifted to lower values (Table 3)which indicate that the inhibitors function by the adsorption mechanism and geometric blocking of active sites on the steel surface. The linear polarization resistances obtained decrease in the presence of inhibitor (Table 4). Calculated values of inhibition efficiency obtained using the various techniques are comparable(Table 5), with slight differences which can be attributed to differences in immersion time.

3.3.Thermodynamic and kinetic study

Stability of each of the adsorbed inhibitor film is important and was evaluated using thermodynamic transition state equation(Eq.(9)).Intercepts and slopes of linear plots(Fig.6)of）vs.1/T afford thermodynamic parameters,namely,enthalpy change(ΔHads)and entropy change(ΔSads)of the adsorption process.The free energy change of adsorption(ΔGads)is calculated from Eq.(1)0 for each temperature.Equilibrium constant of adsorption(Kads)which can give insights on the strength of interaction between the adsorbed film and surface ironmolecules is also estimated using Eq.(11).All these parameters are calculated and presented in Table 6.

Table 3 Parameters obtained from PDP measurement for mild steel in3.5%HCl inthe absence and presence of different inhibitors

Table 4 Parameters obtained from LPR measurement for mild steel in 3.5%HCl in absence and presence of different inhibitors

Table 5 Inhibition efficiencies obtained using different electrochemical techniques

where h (6.62 × 10?34J·s) is the Planck's constant,N(6.02×1023mol?1)is the Avogadro's number,R is the molar gas constant and T(K)is the absolute temperature.

Fig.5.Tafel plot for the corrosion of mild steel in 3.5%HCl in the absence and presence of different the inhibitors.

Fig.6.Transition state plot for the corrosion of mild steel in 3.5%HCl in the absence and presence of different the inhibitors.

The negative ΔGadsvalues show that the inhibitors are spontaneously adsorbed on mild steel surface while negative ΔH*shows that adsorption was exothermic,with the evolution of heat.The range of ΔGadsvalues(＜?20 kJ·mol?1)signifies physical adsorption mechanism[31].Adsorption becomes more spontaneous as temperature increases judged from the increasing magnitude of ΔGads.The spontaneous adsorption favors EGE/PTI at all temperatures more than EGE and EGE/AMC,hence the higher interactive strength.Also,Kadsvalues remain fairly constant in HCl implying no possibility of interaction due to the absence of an adsorbed film.However,the values change in each of the blends demonstrating the presence of adsorbed film.Trend of inhibitor interaction with metal surface(Kads)is similar to that of inhibition efficiency.

For desorption of inhibitors at high temperatures,Kadsvalues should decrease considerably as temperature increases[11].Our results show that Kadschanges rather ‘sluggishly’with temperature.This kind of behavior instructs that the decline in inhibition efficiency at high temperature is perhaps not due to desorption.The decline is therefore attributed to degradation of some phytochemicals of EGE by heat.The physically adsorbed inhibitors could form monolayer films that would be fairly thermally stabilized by the synergistic ions.Further study would be required to validate this claim.

The corrosion inhibition process was also examined kinetically to further explain its dependence on temperature.The idea of collision theory and activation energy is used to explain inhibitive effects of the inhibitors.The acid molecules was considered to first collide with the steel surface using a minimum energy(Ea)required to cause corrosion[32].This energy is obtained by fitting corrosion rate data into Arrhenius' kinetic model(Eq.(12)).

where A is the Arrhenius pre-exponential frequency factor,R and T retain their usual meanings.

Fig.7.Arrhenius plot for the corrosion of mild steel in 3.5%HCl in the absence and presence of different the inhibitors.

Table 7 Activation parameters obtained from Arrhenius plot

Activation parameters were calculated from slope and intercept of linear plots(Fig.7)of lg CR vs.1/T Addition of the halide ions increased the activation energy as shown in Table 7.Inhibitory action can be attributed to deactivation of some of the acid molecules by formation of a barrier by the adsorbed molecules.The barrier is higher in the presence of the halide ions than with EGE because the activation energy follows the trend EGE/PTI＞EGE/AMC＞EGE＞HCl.Similar explanations have also been given in previous report[33].

3.4.Synergistic halide ions effect

The degree of cooperative enhancement of the inhibition efficiency of EGE is estimated using synergism parameter(SP)equation(Eq.(13))[34]proposed by Amaraki and Hackermann.

where Iiis inhibition efficiency of EGE,Iais inhibition efficiency of the synergistic additive and Ii+ais inhibition efficiency of blend.According to the equation,there is no interaction between the additives and EGE when SP approaches or is unity;there is synergistic adsorption and interactions when SP is greater than unity and antagonistic adsorption when less than unity[12,35].Values of SP calculated using mass loss data are1.23and1.29for chloride and iodide ions respectively,meaning that the additives actually exhibited synergistic effect on adsorption of EGE.Iodide ions(radius 0.134 nm)give greater synergistic effect thanchloride ions(radius 0.090 nm)which may be attributed to differences in hydrophobicity due to larger ionic radius.Atoms and ions with large atomic radii are assumed to be more susceptible to adsorption and also promote cooperative adsorption of other molecules.

Table 6 Calculated thermodynamic parameters for mild steel in 3.5%HCl in the absence and presence of different inhibitors

The synergistic effect exhibited by both halides is attributed to either(1)initial adsorption of the halide anions on the steel surface followed by enhanced adsorption of EGE cations or(2)the halides initially modifies the chemical structures of some phyto-compounds in EGE and improve their affinity for the metal surface.The former has been reported for synergistic inhibition between halide ions and xanthan gum[12].There may also be a possibility of the later taking place by converting some EGE phytochemicals to composites. The chemically modified composites perhaps possess more adsorption sites with improved binding affinity for the mild steel surface.It is however difficult to ascertain the exact mechanism from results obtained and techniques used.Further studies with structural characterization techniques may give more insights on the chemical identities of these composites.

4.Conclusions

From the results of this study,it is concluded that corrosion inhibitor blends from EGE and halide ions inhibits mild steel corrosion in 3.5%HCl.The formulation with PTI is the most efficient with inhibition efficiencies of 94.9%and 57.6%at 30 °C and 90 °C respectively.Addition of PTI and AMC optimizes the effectiveness of EGE at high temperatures.The inhibitors inhibit mild steel corrosion by physical spontaneous adsorption with the release of heat.The formulations could be applied as effective alternative anti-corrosive additives for well acidizing and stimulationfluids.

Acknowledgements

The authors gladly acknowledge the financial support provided by World Bank Robert S.McNamara Fellowship Program 2015 to carry out this research abroad.The Materials Physics and Chemistry Research Laboratory of China University of Petroleum,Qingdao and members of the research group are also acknowledged for providing the facilities and assistance for this research.We also appreciate the assistance of African Centre of Excellence in Oil field Chemicals Research,Institute of Petroleum Studies,University of Port Harcourt,Nigeria and Ubong Jerome Etim of State Key Laboratory of Heavy Oil Processing,UPC,Qingdao.

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