Xianghong Li*,Shuduan Deng ,Xiaoguang Xie

1 Faculty of Chemical Engineering,Southwest Forestry University,Kunming 650224,China

2 Faculty of Materials Science and Engineering,Southwest Forestry University,Kunming 650224,China

3 School of Chemical Science and Technology,Yunnan University,Kunming 650091,China

Keywords:Steel Sulfuric acid Red tetrazolium Corrosion inhibitor Adsorption

A B S T R A C T The corrosion inhibition of cold rolled steel(CRS)in 7.0 mol·L-1 H2SO4 solution by red tetrazolium(RTZ)was carefully investigated using both experimental procedures and theoretical techniques.The results show that RTZ acts as an effective inhibitor for the corrosion of CRS in 7.0 mol·L-1 H2SO4,and the maximuminhibition efficiency is higher than 95%with a RTZ concentration of 2.0 mmol·L-1.The adsorption of RTZ on CRS surface follows Langmuir isotherm.RTZ effectively retards both cathodic and anodic reactions,and acts as a mixed-type inhibitor.EIS exhibits two capacitive loops,and their resistances increase drastically in the presence of RTZ.SEM and AFM confirm that the addition of RTZ could significantly retard the corrosion of CRS surface.A series of characterizations like FTIR,RS,XRD and XPS reveal that the corrosion CRS surface is composed of the corrosion products of iron sulfates,iron oxides and iron hydroxide,as well as inhibitor.Theoretical results of quantum chemical calculation and molecular dynamics(MD)indicate that the adsorption center of RTZ+(organic cationic part of RTZ)mainly relies on its tetrazole ring,and the adsorption of RTZ+on Fe(001)surface is in a nearly flat orientation mode.

1.Introduction

N-heterocyclic compounds have been widely considered as the most effective inhibitors for steelin acids[1–3],and the inhibitive mechanism can be attributed to their adsorption on steel surface or steel/solution interface.In general,the N-heterocyclic ring is the main adsorption center for the N-heterocyclic inhibitor,and the adsorption enhances with increasing the number of N atoms in the N-heterocyclic ring.In the view of molecular structure,tetrazole compounds can strongly adsorb on steel surface through the five-membered tetrazole ring containing four N-hetero atoms.Till now,there have been only a few reports on using tetrazole compounds as acid inhibitors for iron or steel corrosion.The corrosion inhibition of iron in 1.0 mol·L-1HCl by 1-phenyl-5-mercapto-1,2,3,4-tetrazole(PMT),1-H-tetrazole(TTZ)and 1-methyl-5-mercapto-1,2,3,4-tetrazole(MMT)has been comparatively investigated,and inhibition efficiency at 2 × 10-3mol·L-1is 98%for PMT,but only-1.5%and-0.5%for TTZ and MMT,respectively[4].Besides HCl,PMT acted as an efficient corrosion inhibitor for steel in 0.5 mol·L-1H2SO4and 1/3 mol·L-1H3PO4,and the optimum inhibition efficiency value can reach as high as 98%at a PMT concentration of 10-3mol·L-1[5].Another tetrazole derivative of 5-mercapto-1-tetrazoleacetic sodium salt(MTAc)exhibited moderate inhibition performance for steel in 1.0 mol·L-1H2SO4,and the highest inhibition efficiency of 200 mg·L-1was only 68.2%[6].Obviously,the inhibitive action of a tetrazole compound is mainly decided by the substitution group to the tetrazole ring,and the phenyl group(--C6H5)stands out the best performance among various chosen substitution groups.To screen the effective tetrazole inhibitors,phenyltetrazole derivatives may be more suitable.

Over the years,our research group has investigated the adsorption property and inhibition performance of some phenyltetrazole derivatives on steel surface in acid solutions,such as red tetrazolium(RTZ)with three substituted phenyl groups[7–9],blue tetrazolium(BT)with six substituted phenyl groups[10,11],nitrotetrazolium blue chloride(NTBC)with six substituted phenyl groups[12,13]and triazolyl blue tetrazolium bromide(TBTB)with two substituted phenyl groups[14].The results reveal that RTZ is a good inhibitor in HCl[7]solution,while exhibits moderate performance in H2SO4[8]and H3PO4[9]solutions.Other three tetrazole derivatives of BT[10,11],NTBC[12,13]and TBTB[14]act as effective inhibitors in both HCl and H2SO4media.Commonly,higher acid concentration leads to deteriorated inhibitive performance[13].However,for the studied tetrazole compound of RTZ,the inhibition performance improved with the increasing HCl concentration from 1.0 to 5.0 mol·L-1[7],while fluctuated slightly with increasing H2SO4concentration from 1.0 to 5.0 mol·L-1[8]and H3PO4from 1.0 to 10.0 mol·L-1[9].In addition,the cost of RTZ is rather low compared with other phenyltetrazole derivatives of BT,NTBC and TBTB.Taking advantages of these merits,RTZ might be the first to be chosen as an effective potential inhibitor for steel in high concentrated acids.

Concentrated sulfuric acid(H2SO4)is one of the most produced strong acids;however,the steel corrosion by concentrated H2SO4has received little attention[15].Panossian et al.[15]presented a review report of carbon steel in concentrated H2SO4solution,and the corrosion mechanism together with accelerating factors were outlined.Wang et al.[16]proposed that the addition of rare earth element of Y was an effective method to control the corrosion of ND steel in 85 wt.%H2SO4.Despite a substantial number of published papers on selecting various organic inhibitors to retard steel corrosion in H2SO4solution,they deal with dilute H2SO4.On the one hand,many inhibitors unfortunately lose their inhibitive ability in concentrated H2SO4media owing to stronger corrosive attack on steel surface.On the other hand,the oxidizing ability of concentrated H2SO4strengthens with continually increasing H2SO4concentration,and then the inhibitor would be oxidized.Consequently,the effective inhibitors for steel in high concentrated H2SO4solution are very scarce.Amin and Ibrahim[17]synthesized a new glycine derivative of 2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride that can effectively retard mild steel corrosion in 4.0 mol·L-1H2SO4.Abdel Renhim et al.[18]reported that adenine together with KI was an effective inhibitor for low carbon steel in 4.0 mol·L-1H2SO4.Noticeably,steel can be chemical passivated by further increasing the concentration of concentrated H2SO4.In other words,there is a critical concentration which is the break point from active corrosion to passivation.The corrosion rate of steel reaches the maximum value at this concentration,but it decreases sharply once the concentration of H2SO4cross this critical concentration.After an extensive survey of published papers,there is almost no report on the effective inhibitor for steel in the critical concentrated H2SO4.

Recently,we have found a unique and interesting result that RTZ can exhibit good inhibitive performance when H2SO4concentration is further increased to the critical concentration of 7.0 mol·L-1.To fundamentally understand the mechanism of this phenomenon,herein,the adsorption and inhibition effects of RTZ on the corrosion of CRS surface in 7.0 mol·L-1H2SO4solution have been systematically investigated through weight loss measurements,potentiodynamic polarization curves and electrochemical impedance spectroscopy(EIS).New inhibition results of RTZ as well as reduction reactions of undissociated H2SO4molecule in concentrated 7.0 mol·L-1H2SO4solution were obtained and discussed by comparing those in dilute H2SO4solution.The microstructure of corroded steel surface was examined by scanning electron microscopy(SEM)and atomic force microscopy(AFM).The composition of corrosion CRS surface was systematically characterized by a series of advanced analytical techniques including Fourier transform infrared spectroscopy(FTIR),Raman spectroscopy(RS),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS).Besides the experimental methods,the adsorption of RTZ+(organic cationic part of RTZ)was theoretically elucidated by quantum chemical calculation and molecular dynamics(MD)simulations.

2.Experimental

2.1.Inhibitor and preparation of test solutions

The chemical structure of the investigated inhibitor of RTZ(C19H15ClN4)is shown in Fig.1,and it was purchased from Sinopharm Chemical Reagent Co.Ltd.of China.A certain amount of RTZ was dissolved in distilled water to obtain 20.0 mmol·L-1inhibitor stock solution.This stock solution and H2SO4(98%,AR grade)were subsequently used to prepare the test H2SO4solutions without and with different concentrations(0.1–2.0 mmol·L-1)of RTZ.

Fig.1.Chemical molecular structure of the investigated inhibitor of red tetrazolium(RTZ).

2.2.CRS sample preparation

CRS(0.07%C,0.3%Mn,0.022%P,0.010%S,0.01%Si,0.030%Al,and balance Fe)specimens of 2.5 cm×2.0 cm×0.06 cm were used for weight loss measurements.CRS of 1.0 cm×1.0 cm was embedded in polyvinyl chloride holder using epoxy resin as working electrode(WE)in the electrochemical tests.CRS coupons were successively abraded by 320,500,and 800 emery papers,then washed with distilled water,degreased with acetone and dried with a stream of cold air.

2.3.Weight loss measurements

Three parallel sheets of CRS specimens were weighed accurately by digital balance(±0.1 mg)to obtain the initial mass,and completely immersed in 250 ml H2SO4test solution at 20°C.After immersing for 6 h,the CRS sheets were taken out,and were thoroughly scrubbed using brush under running water,and washed with ethanol to remove corrosion products.Then,they were washed by distilled water,dried and weighed accurately again.The average weight loss of three parallel CRS sheets before and after immersion was obtained,and then corrosion rate(v)and inhibition efficiency from weight loss(ηw)values were calculated[14].

2.4.Electrochemical measurements

All electrochemical tests were conducted on a PARSTAT 2273 advanced electrochemical system(Princeton Applied Research)which was controlled by a computer with PowerSuite software.For the electrochemical measurements,a three-electrode cell consisting of a WE,a counter electrode(large platinum plate of 2.0 cm×2.0 cm)and a reference electrode(saturated calomel electrode(SCE)coupled to a fine Luggin capillary)were used.The electrolyte was the corrosive media of250 ml7.0 mol·L-1H2SO4aerated solution maintained at20 °C without any stirring.The tip of Luggin capillary was located about 3 mm away from WE to minimize the ohmic drop,and the exposed surface of WE was placed right against the platinum plate to form uniform electric field.Open circuit potential(OCP)-time curves were recorded immediately when the test cell was ready right.Before potentiodynamic polarization and EIS measurements,WE was immersed in the test electrolyte for 2 h at OCP to reach a steady state(OCP fluctuates within±3 mV).The polarization curves were obtained in the potential range from-250 to+250 mV versus OCP at a scan rate of 0.5 mV s-1that is widely setup forthe steel corrosion in acid solutions.The EIS was carried out at a stable OCP level with the employed frequency range from 100 kHz to 10 mHz,and the signal amplitude was 10 mV root mean square.

2.5.SEM and AFM examinations

The CRS samples of 1.5 cm×1.0 cm×0.06 cm were immersed in 7.0 mol·L-1H2SO4solutions without and with 2.0 mmol·L-1RTZ at 20°C for 6 h,then cleaned with distilled water,and dried with a cold air blaster.The SEM and AFM examinations were conducted by a XL30 ESEM-TMP scanning electron microscope(Holland)and a SPA-400 SPM Unit atomic force microscope(Japan),respectively.

2.6.FTIR,XRD,RS and XPS characterizations

After immersing in 7.0 mol·L-1H2SO4test solution at 20 °C for 6 h,the steel specimens were taken out and cleaned with distilled water,then dried with a cold air blaster.The thin corrosion layer formed on the steel surface was scratched using a glass knife to obtain the powder specimen.Because there was only a little amount of scratched power for one steel specimen,several steel specimens at same experimental conditions were scratched to collect the desired amount of power sample.FTIR,XRD,RS and XPS characterizations were performed using AVATAR-FTIR-360 spectrophotometer(Thermo Nicolet Company,USA),TTR III target X-ray diffractometer(Rigaku Corporation),INVIA Raman spectrometer(Renishaw,UK)and X-ray photoelectron spectrometer(PHI-5500 ESCA,USA),respectively.

2.7.Quantum chemical calculations and MD simulations

Quantum chemical calculations and MD simulations were respectively performed with DMol3and Discover in Materials Studio 4.0 software from Accelrys Inc.[19],and the corresponding calculation procedures were reported in our previous work[20,21].Geometrical structure of RTZ+was optimized using density function theory(DFT)at the level of GGA(generalized gradient approximation)/BLYP(Becke exchange plus Lee-Yang-Parr correlation)/DND(double numerical plus d-functions)/COSMO(conductor-like screening model).For the MD simulations,the adsorption system containing one RTZ+molecule and Fe(001)surface with 3.153 nm×3.153 nm×1.530 nm of total 1331 Fe atoms was optimized using COMPASS(condensed phase optimized molecular potentials for atomistic simulation studies)force field,and performed at 293 K using NVT(constant volume and temperature)ensemble for a simulation time of 1000 ps with the time step of 1.0 fs.The adsorption energy(ΔEads)of inhibitor molecule on Fe(001)surface was defined according to the following equation[22–24]:

where Einhand Esurfare the energies of free inhibitor molecule and Fe(001)plane,respectively.Etotalrepresents the total energy of Fe(001)plane together with the adsorbed inhibitor molecule.

3.Results and discussion

3.1.Weight loss measurements

To obtain the corrosion rate of steel in acid solution,weight loss method is simple and of high reliability.In the present study,all relative standard deviation values of three parallel experiments are less than 3.5%,indicating good reproducibility(higher than 95%)for both corrosion rate and inhibition efficiency from weight loss method.

3.1.1.Effect of H2SO4concentration on the corrosion rate of CRS

The concentration of H2SO4is an important accelerating factor on the corrosion of steel in sulfuric acid,and so the corrosion rates of CRS in a wide H2SO4concentration range of 1.0–10.0 mol·L-1with an interval of 1.0 mol·L-1have been measured using weight loss method.Fig.2 shows the relationship between corrosion rate(v)and H2SO4concentration(C)at 20°C with the immersion time of 6 h.Obviously,the corrosion rate increases with the concentration of H2SO4from 1.0 to 7.0 mol·L-1,and reaches a peak at 7.0 mol·L-1,but then drops drastically from 8.0 till to 10.0 mol·L-1.In the experiment,it was observed that CRS suffered from hydrogen evolution corrosion in 1.0–7.0 mol·L-1H2SO4,but it was naturally passivated in 8.0–10.0 mol·L-1H2SO4owing to strong oxidization ability of concentrated H2SO4.7.0 mol·L-1H2SO4is an important transition turning point for the corrosion of CRS in sulfuric acid from active corrosion to passivated state,and is thus chosen for the investigated media.Also,it is reasonable to deduce that during the corrosion process of CRS in the transition concentration of 7.0 mol·L-1H2SO4,the hydrogen evolution together with the oxidization ability should be considered.

Fig.2.Relationship between corrosion rate(v)of CRS and H2SO4 concentration(C)at20°C obtained from weight loss method with immersion time of 6 h.(Each error bar is determined by relative standard deviation value of three parallel data).

3.1.2.Inhibition effect of RTZ on CRS corrosion in 7.0 mol·L-1H2SO4

Fig.3.Relationship between corrosion rate(v)and inhibition efficiency(ηw)with the concentration of RTZ(c)in 7.0 mol·L-1 H2SO4 at 20 °C obtained from weight loss method with immersion time of 6 h.(Each error bar is determined by relative standard deviation value of three parallel data).

Fig.3 shows the plots of mean values of corrosion rate(v,g·m-2·h-1)and inhibition efficiency(ηw,%)of three parallel specimens against RTZ concentration(c,mmol·L-1)for CRS corrosion in 7.0 mol·L-1H2SO4solution at 20 °C.Obviously,CRS surface is strongly corroded by 7.0 mol·L-1uninhibited H2SO4,and v reaches a high value to 138.17 g·m-2·h-1.When RTZ is added into 7.0 mol·L-1H2SO4,the corrosion rate decreases significantly with the increase of the concentration of RTZ.Upon addition of 2.0 mmol·L-1RTZ,the corrosion rate of CRS is reduced to as low as 2.65 g·m-2·h-1.On the other hand,the corresponding ηwreversely increases with the concentration of RTZ,while fluctuates slightly within the inhibitor concentration range from 1.0 to 2.0 mmol·L-1.The maximum ηwof 2.0 mmol·L-1RTZ is as high as 95.6%,which indicates that RTZ can be used as an effective inhibitor for CRS in 7.0 mol·L-1H2SO4.According to our previous study[8],ηwof 250 mg·L-1RTZ maintained a moderate value of 61%in a wide H2SO4concentration range from 1.0 to 5.0 mol·L-1.Further increasing H2SO4to 7.0 mol·L-1,the inhibitive ability of RTZ reversely becomes better,which implies that the adsorption of RTZ on steel surface in 7.0 mol·L-1H2SO4solution is of particularity.

In 1.0 mol·L-1H2SO4solution,the adsorption of RTZ on CRS surface obeys Freundlich isotherm[8].However,in 7.0 mol·L-1H2SO4solution,Freundlich isotherm is not suitable to elucidate the adsorption of RTZ.Herein,the adsorption of RTZ can be simulated by Langmuir adsorption isotherm[25,26]:

where c represents the concentration of inhibitor(mmol·L-1),K is the adsorptive equilibrium constant(mol-1·L),θ refers to the coverage of inhibitor on steel surface,and approximately equals to inhibition efficiency.Fig.4 illustrates the straight line of c/θ versus c with both linear correlation coefficient(0.9993)and slope(0.9691)values are also nearly equal to unity,which confirms the adsorption of RTZ from 7.0 mol·L-1H2SO4media onto steel surface fully follows Langmuir adsorption isotherm.K(7.18 × 103mol-1·L)is calculated from the intercept value(0.1392)of straight line of c/θ versus c.From the obtained value of K,the thermodynamic parameter of standard adsorption free energy(can be calculated according to the following Relation(3)[27]:

Fig.4.Langmuir isotherm adsorption mode of RTZ on the CRS surface in 7.0 mol·L-1 H2SO4 at 20°C using weight loss method with immersion time of 6 h.

where csolventis the concentration of H2O in solution,and a value of 55.5 mol·L-1has been commonly used for aqueous dilute acid solution[28,29].However,in studied concentrated 7.0 mol·L-1H2SO4solution,the concentration of H2O is approximately equal to 34.5 mol·L-1.The corresponding ΔG0adsvalue is-31.4 kJ mol-1,which probably suggests that the adsorption of RTZ involves both physisorption and chemisorption[27–29].

3.2.OCP-time curves

Fig.5 shows the relationship between OCP and immersion time(t)of CRS electrode in 7.0 mol·L-1H2SO4solutions without and with 2.0 mmol·L-1RTZ at 20 °C.For the blank solution,OCP gradually increases with the immersion time to about 1000 s,but sharply decreases till 1500 s,thereafter fluctuates slightly along with the increase of immersion time.In the presence of2.0 mmol·L-1RTZ,OCP decreases with immersion time,and then reaches a relatively stabcole level at 3600 s.Upon addition of RTZ into 7.0 mol·L-1H2SO4solution,the adsorption of RTZ participates at electrode/solution interface,which changes the characteristics of OCP-t curves,and takes longer to reach stable state.Moreover,OCP in the presence of 2.0 mmol·L-1RTZ moves positive comparing with that in the absence of inhibitor.When polarization curves and EIS measurements were carried out,the immersion time of 2 h for WE was enough for OCP to reach stable state.

Fig.5.Relationship between OCP and immersion time(t)for CRS electrode in 7.0 mol·L-1 H2SO4 solutions without and with 2.0 mmol·L-1 RTZ at 20 °C.

3.3.Potentiodynamic polarization curves

Fig.6 shows the potentiodynamic polarization cathodic and anodic curves for CRS electrode in 7.0 mol·L-1H2SO4solutions without and with 0.6,1.0,2.0 mmol·L-1RTZ at20°C after2 h immersion.Comparing with polarization curves of blank solution,the addition of RTZ causes both anodic and cathodic branches to move lower corrosion current densities,which means RTZ can effectively inhibit both anodic and cathodic electrochemical reactions.Thus,RTZ acts as a mixed-type inhibitor for the corrosion of CRS in 7.0 mol·L-1H2SO4media.

The cathodic polarization curves appear Tafel regions,which reflects that the cathodic reduction reactions are actively controlled.The main cathodic reaction for steel corrosion in concentrated 7.0 mol·L-1H2SO4solution is still the hydrogen evolution reaction represented as following:

Unlike dilute H2SO4solution,undissociated H2SO4molecules existin concentrated H2SO4media where sulfuric acid is not completely dissociated to H+and SO42-[30].Undissociated H2SO4molecule is of strong oxidation ability,thus it may undergo following cathodic reduction[31]:

Fig.6.Potentiodynamic polarization curves for the corrosion of CRS in 7.0 mol·L-1 H2SO4 solutions without and with different concentrations of RTZ at 20°C in the potential range from-250 to+250 mV versus OCP with a scan rate of 0.5 mV·s-1(immersion time is 2 h).

For the anodic polarization branch,three distinct regions can be observed:(i)active dissolution region,(ii)transition region(AA',BB',CC',DD')and(iii)limiting currentregion.This unique appearance of anodic branch has not been observed for CRS in 1.0 mol·L-1H2SO4[8,11]or steel in 4.0 mol·L-1H2SO4[17,18].

In the first region of active dissolution,the potential almost linear increases with logarithmic current density(lg i),which exhibits apparent Tafel region.The main anodic reaction in this region is listed as following Reaction(6):

Noticeably,Fe2+can be further oxidized to Fe3+by undissociated H2SO4molecule through the following reaction:

Furthermore,Fe3+iron can directly oxidize Fe to Fe2+:

For the second transition regions of AA′,BB′,CC′and DD′,i drastically decreases with the increase of positive shifting potential.When the corrosion products of FeSO4and Fe2(SO4)3covered on the freshly steel surface,it was relative difficult for H+to penetrate cross the corrosion layer to the steel substrate.As a result,the pH of steel/corrosion product interface was increased,and then some iron oxides would be formed on steel surface through following Reactions(9)and(10):

These iron oxides of Fe2O3and Fe3O4would dramatically restrain the reaction rate of anodic dissolution,and then i values decrease to A′,B′,C′and D'.

For the third limiting current region,i values began to increase after the limiting current of A′,B′,C′and D′.Accompanied with the continuously aggressive attack ofH2SO4,the iron oxides formed on steelsurface can be dissolved by H+through Reactions(11)and(12):

It should be noted that the anodic polarization curve does not display an extensive Tafel region for the first active region.Thus,it is rather difficult to obtain accurate evaluation of the anodic Tafel slope by Tafel extrapolation of the anodic branch.Amin et al.[32]proposed that the corrosion parameters could be obtained by firstly extending the cathodic polarization curve,and then fitting the anodic region of about 50 to 150 mV(vs.Ecorr).Namely,it is possible to calculate the anodic Tafel line from the experimental data.This method could be acceptable owing to good consistent with other methods of corrosion rate determination[32].In the present system,this method is used to obtain the polarization parameters.

The electrochemical corrosion parameters such as corrosion current densities(icorr),corrosion potential(Ecorr),cathodic Tafel slope(bc)and anodic Tafel slope(ba)were obtained,and then the inhibition efficiency from polarization curves(ηp)was calculated based on icorr[14,25].These electrochemical corrosion parameters and ηpare summarized in Table 1.Comparing with blank solution,the presence of RTZ shifts Ecorrto more positive position,and the positive shifting Ecorrtrend increases with RTZ concentration,which suggests the addition of RTZ prominently strengthens the anodic polarization degree and then retards the anodic dissolution to more extent.The value of icorris as high as 4.92×104μA cm-2for the blank solution,but it decreases considerably to only 41.1 μA cm-2with the addition of 2.0 mmol·L-1RTZ,suggesting RTZ drastically retards the corrosion of CRS in 7.0 mol·L-1H2SO4.From the results presented in Table 1,ηpincreases with the inhibitor concentration,and reaches ideal values(higher than 99%)at 1.0 and 2.0 mmol·L-1RTZ concentrations.Therefore,RTZ performs excellent inhibitive activity for CRS in 7.0 mol·L-1H2SO4solution.

According to ourearlier reports about CRS in 1.0 mol·L-1H2SO4solution without inhibitor[8,11],Tafel slopes of bcand bawere around-120 and 60 mV·dec-1,respectively.However,for CRS corrosion in uninhibited 7.0 mol·L-1H2SO4solution,Table 1 shows that bcis as more negative as-443 mV·dec-1,while bais as large as 114 mV·dec-1.The high absolute values of Tafel slopes could further confirm that the electrochemical corrosion mechanism of steel in concentrated 7.0 mol·L-1H2SO4is quite different from that in dilute 1.0 mol·L-1H2SO4.The adsorption of RTZ participates in the corrosion process at electrode/solution interface,and subsequently lead to the changeable Tafel slopes as compared with blank solution.

Table 1Potentiodynamic polarization parameters derived from polarization curves of Fig.6 for the corrosion of CRS in 7.0 mol·L-1 H2SO4 solutions without and with different concentrations of RTZ at 20°C

3.4.Electrochemical impedance spectroscopy(EIS)

Fig.7.Nyquist plots of the corrosion of CRS in 7.0 mol·L-1 H2SO4 solutions without and with different concentrations of RTZ at 20°C and OCP with the immersion time of 2 h(solid lines show fitted results).

Fig.7 shows the Nyquist diagrams of EIS result for CRS in 7.0 mol·L-1H2SO4solutions containing different concentrations of RTZ at 20°C and OCP with the immersion time of 2 h.The Nyquist spectra consists of two capacitive loops,and thus there are two time constants.In previous reported EIS result for steel 1.0 mol·L-1H2SO4[8]and 4.0 mol·L-1H2SO4[17]solutions,Nyquist spectra exhibits a large capacitive loop at high frequency(HF)and followed by a small inductive loop at low frequency(LF)for steel corrosion.Thus,this EIS result for CRS in 7.0 mol·L-1H2SO4is also quite unique.

The capacitive loop at HF region can be assigned to the charge transfer and double layer behavior of the corrosion process at steel/solution interface,while the second capacitive loop at LF region arises from the dissolution process of corrosion layer,oxidization film or adsorbed film of inhibitor.These two capacitive loops are depressed semicircles with the center under the real axis owing to the frequency dispersion effect of roughness and inhomogeneous of the electrode surface or interfacial origin[33,34].Furthermore,in the tested concentrations of RTZ,the curve shape is similar to that of the blank solution,which indicates that the electrochemical corrosion mechanism does not vary with the addition ofRTZ[35,36].Clearly,the capacitive loop at HF region has larger size in the presence of each concentration of RTZ,indicating there is a more significant inhibition effect with RTZ.

The corresponding Bode modulus and phase angle plots versus logarithmic frequency(lg f)for CRS in 7.0 mol·L-1H2SO4solutions are illustrated in Fig.8(a)and(b),respectively.From the Bode modulus shown in Fig.8(a),the absolute impedance of lg f within-2.0 to 1.0 increases with the RTZ concentration,which confirms that the corrosion resistance enhances with the increase of additive dosage[37,38].As shown in Fig.8(b),there is one phase peak at102–103Hz with phase peak angle is lower than 90°,which again confirms that there is frequency dispersion on electrode surface[39,40].Based on two time constants for two capacitive loops mentioned above in Nyquist plots,it should have two phase peaks in phase angle plots,but actually only one phase peak is observed.This result can be explained on that the second capacitive loop at LF shown in Fig.7 is only a small section of whole capacitive loop,and so there should have a phase peak for the frequency below 0.01 Hz.Correspondingly,the second phase peak at LF cannot be observed in Fig.8(b).

A conventional equivalent

circuit with two-time constants as shown in Fig.9 was applied to fit the EIS results.Rs,Rtand Rfrepresent the solution resistance,charge transfer resistance and adsorbed film resistance,respectively.CPE1and CPE2are the constant phase elements that correspond to the first and second capacitive loops,respectively.It can be observed that the fitted solid curves shown in Figs.7 and 8 are in good agreement with the experimental data.The inhibition efficiency from EIS(ηR)is calculated from Eq.(13):

Fig.8.Bode plots of the corrosion of cold rolled steel(CRS)in 7.0 mol·L-1 H2SO4 without and with different concentrations of RTZ at 20°C and OCP with the immersion time of 2 h:(a)Bode modulus;(b)Bode phase angle plots(solid lines show fitted results).

where Rt(0)and Rt(inh)are the charge transfer resistances in the absence and presence of inhibitor,respectively.

Fig.9.Equivalent circuit used to fit the EIS results.

Table 2EIS parameters derived from EIS fitted solid curves of Figs.7 and 8 for the corrosion of CRS in 7.0 mol·L-1 H2SO4 solutions in the absence and presence of different concentrations of RTZ at 20°C

The impedance parameters of the EIS results are listed in Table 2.The chi-squared(χ2)values are low,which indicates that the fitted data have good agreement with the experimental data.The small Rsvalue confirms that the IR drop between WE and SCE is negligible in 7.0 mol·L-1H2SO4solution.Higher Rtvalue,more resistance of steel to be corroded.As seen from Table 2,Rtis only 3.6 Ω·cm-2in blank solution,while it increases remarkably with the addition of RTZ.The value of Rtreaches 209.8 Ω· cm-2with 2.0 mmol·L-1RTZ,which means the steel electrode corrosion is effectively retarded by RTZ.Meanwhile,another resistance of Rfis much higher for the inhibited solution than uninhibited solution.In the absence of RTZ,Rfisjust0.7Ω·cm-2,which indicates that it is easy for the corrosion product to be dissolved at LF.In contrast,when RTZ is added to the medium,Rfis drastically increased to 138.8 Ω·cm-2,which confirms it is rather difficult for the adsorptive film of inhibitor to be desorbed from electrode surface.In other words,RTZ molecules can tightly adsorb on electrode surface to improves the corrosion resistance.Both CPE1and CPE2decrease with the increase of RTZ concentration,which is probably due to the decrease of local dielectric constant in the double layer through the replacement water molecules by inhibitor molecules[34].The results from Table 2 reveal that ηRincreases with increasing the RTZ concentration,and the maximum ηRvalue reaches up to 98.3%,which again confirms that RTZ can be used as an effective inhibitor to retard steel corrosion in 7.0 mol·L-1H2SO4solution.Thus,the inhibition efficiency data from EIS are in good agreement with that from polarization curves or weight loss method.

EIS experiments for steel electrode with different immersion time in 7.0 mol·L-1H2SO4solutions containing 2.0 mmol·L-1RTZ at 20 °C were carried out.Different fresh samples were used for each immersion time.Nyquist plots at different immersion times from 2 to 16 h are shown in Fig.10.It can be seen that all the curves have similar characteristic appearance,and the diameter of the first capacitive loop increases with the immersion time.Thus,the charge transfer resistance increases with the immersion time,and also exhibits more inhibitive ability.This result infers that the adsorptive film may become more thicker along with the immersion time.

3.5.SEM and AFM

Fig.10.Nyquist plots of the corrosion of CRS in 7.0 mol·L-1 H2SO4 solutions containing 2.0 mmol·L-1 RTZ at 20 °C with different immersion time.

Fig.11 shows the SEMmorphologies of CRS surface after exposure to tested solutions without and within the inhibitor.It can be found in Fig.11(a)reveals that the CRS surface is severely damaged after the immersion in uninhibited 7.0 mol·L-1H2SO4for 6 h,and covered uneven and ratherrough scaly shaped corrosion products.On the contrary,as shown in Fig.11(b),the micro topography of CRS surface appears to be less corroded in inhibited 7.0 mol·L-1H2SO4solution of 2.0 mmol·L-1RTZ,and there are relative smoother substances coated with the inhibited CRS surface.It should be noted that there are some cracks on the thick adsorptive film,which could be caused by the dryness in the air due to ex-situ SEM examinations.In inhibited system,this adsorptive film can protect steel surface from corrosion.

AFM provides high resolution 3D images as well as surface roughness parameters[41].Fig.12 shows 3D-AFM micrographs of CRS surfaces after 6 h of immersion in 7.0 mol·L-1H2SO4solution without and with 2.0 mmol·L-1RTZ at20 °C.As shown in Fig.12(a),CRS surface was intensively damaged in 7.0 mol·L-1H2SO4without any inhibitor,there were uneven convex of corrosion products on steel surface in uninhibited 7.0 mol·L-1H2SO4.In the presence of 2.0 mmol·L-1RTZ,Fig.12(b)shows that the relative smaller sized aggregates were distributed on whole steel surface,and the corrosion degree reduced to more extent.In order to quantitatively evaluate the surface roughness of AFM image,the roughness parameters of average roughness(Ra),root-mean-square roughness(RMS)and maximum peak-tovalley(P–V)are discussed.For the CRS surface immersed in uninhibited 7.0 mol·L-1H2SO4solution of Fig.12(a),the corresponding roughness parameters of Ra,RMS and P-V are 37.24 nm,46.80 nm and 316.6 nm,respectively.In contrast,for CRS surface immersed in inhibited 7.0 mol·L-1H2SO4solution by 2.0 mmol·L-1RTZ shown in Fig.12(b),Ra,RMS and P–V are reduced to 16.45 nm,20.87 nm and 256.7 nm,respectively.Thus,the CRS surface is smoother with the addition of RTZ.

3.6.FTIR,RS,XRD and XPS

In order to fully characterize the compositions of the corroded CRS surface,a series of analytical techniques,like FTIR,RS,XRD and XPS,were used.Fig.13 shows the FTIR spectra of the corrosion CRS surface in 7.0 mol·L-1H2SO4solution containing 2.0 mmol·L-1RTZ after immersing for 6 h.The Fe--O bending appears at two weak bands of 3858 and 3761 cm-1[25,42].According to FTIR characteristic absorption bands of iron oxides and iron hydroxoides[42-44],FeOOH is observed at 881 and 781 cm-1,Fe2O3at 665 and 467 cm-1.The strong peak at 3445 cm-1corresponds to O--H stretching,which further confirms the existence ofH2Oororganic compound in the protective film.Around 3000 cm-1,there are two weak bands at 2971 and 2920 cm-1,which represent the aliphatic C--H asymmetric and symmetric stretching vibrations,respectively.The absorption bands at 1628,1460 and 1394 cm-1may be attributed to the framework vibrations of aromatic rings.In addition,the S=O stretching frequency of SO24-absorbs at 1088 and 1024 cm-1,which evidently comes from the corrosion products of FeSO4and Fe2(SO4)3.The wavenumber at 586 cm-1may be attributed to Fe--N bond[45],which probably implies that the N heteroatom of RTZ can chemisorb on steel surface.

Fig.14 exhibits Raman spectra(RS)of the corrosion CRS surface exposure to inhibited solution.The Raman shift at 494 cm-1has been reported due to the vibration of Fe3+–OH2in the products of hydrolysis of Fe3+[46],which should be resulted from the corrosion product of Fe2(SO4)3·x H2O.Anotherpeak at419 cm-1suggests the presence of FeSO4.The peak bands at215 and 267 cm-1could belong to the Fe--Nbending vibrations.According to the similar RS for steel inhibited by triazole compound[47],the peaks at 614 and 661 cm-1may be assigned to tetrazole and benzene rings torsion.Furthermore,the peaks at 1021,1092 and 1194 cm-1are attributed to in-plane tetrazole ring stretching vibrations,--N=N--linkage stretches and C--H in plane bending,respectively.All these functional groups of Raman shifts further confirm that RTZ can strongly adsorb on steel surface.

Fig.11.SEM micrographs of CRS surface:(a)after6 h of immersion in 7.0 mol·L-1 H2SO4 solution at20 °C;(b)after6 h of immersion in 2.0 mmol·L-1 RTZ+7.0 mol·L-1 H2SO4 solution at 20°C.

Fig.12.3D-AFM micrographs(μm)of CRS surface:(a)after 6 h of immersion in 7.0 mol·L-1 H2SO4 solution at20 °C;(b)after6 h of immersion in 2.0 mmol·L-1 RTZ+7.0 mol·L-1 H2SO4 solution at 20°C.

Fig.13.FTIR spectra of the corrosion CRS surface after 6 h immersion in 7.0 mol·L-1 H2SO4+2.0 mmol·L-1 RTZ at 20 °C.

The corrosion products formed on the steel surface immersed in 7.0 mol·L-1H2SO4without and with 2.0 mmol·L-1RTZ were also analyzed by XRD.As shown in Fig.15(a),the substances formed on steel surface after immersing in uninhibited 7.0 mol·L-1H2SO4are mainly composed of FeSO4·H2O,FeSO4·4H2O,Fe2O3,FeOOH,Fe2(SO4)3,Fe3O4,FeO and Fe.Similar XRD results were previously reported for the corrosion of ND steel in concentrated H2SO4(85 wt.%)solution[16],and mild steel in 1.0 mol·L-1H2SO4solution[48].For the CRS immersed in H2SO4containing 2.0 mmol·L-1RTZ(Fig.15(b)),the corrosion product is also composed of FeSO4·H2O,FeSO4·4H2O,Fe2O3,FeOOH,Fe2(SO4)3,Fe3O4and FeO.Thus,the main corrosive reactions of CRS in concentrated 7.0 mol·L-1H2SO4are not affected by the addition of RTZ.

Fig.16 shows the XPS survey scan of CRS immersed in 7.0 mol·L-1H2SO4inhibited by 2.0 mmol·L-1RTZ for 6 h at 20 °C.It contains Fe,S,O,C and N elements,and the presence of N1s apparently confirms that RTZ has adsorbed on steel surface.The high resolution spectra of the core elements of S2p,O1s,Fe2p,C1s and N1s are summarized in Fig.17.From the simulated results for S2p shown in Fig.17(a),two binding energy(BE)peaks are observed at 168.7 and 169.9 eV,which are associated with sulfates[49].The peak of 168.7 eV corresponds to FeSO4,while another peak at 169.9 eV to Fe2(SO4)3.The O1s signal shown in Fig.17(b)is composed of three components:iron oxide(530.6 eV),iron hydroxide(531.2 eV)and iron water salts(532.0 eV)[49–51].Fig.17(c)presents four analytical XPS peaks of the main element Fe2p:710.4 eV of Fe2p3/2 can be assigned to FeSO4,FeO or Fe3O4[52].The second fitted BE at713.8 eV of Fe2p3/2 can be attributed to FeOOH[53]or Fe2(SO4)3.The fitted peak at 724.0 eV of Fe2p1/2 indicates the presence of Fe2O3[54,55],whereas the highest BE at 727.9 eV may be presumed to the complexes of Fe ion(Fe2+/Fe3+)with organic inhibitor.

Fig.14.RS spectrum of the corrosion CRS surface after 6 h immersion in 7.0 mol·L-1 H2SO4+2.0 mmol·L-1 RTZ at 20 °C.

Fig.15.XRD of adsorbed layer on CRS surface:(a)after 6 h of immersion in 7.0 mol·L-1 H2SO4 solution at 20 °C;(b)after 6 h of immersion in 2.0 mmol·L-1 RTZ+7.0 mol·L-1 H2SO4 solution at 20°C.

Fig.16.Survey spectra of XPS analysis for CRS surface exposed to 7.0 mol·L-1 H2SO4+2.0 mmol·L-1 RTZ solution for 6 h of immersion at 20 °C.

As shown in Fig.17(d),four independent fitted peaks of C1s spectrum are found at 283.6,284.2,285.2 and 287.6 eV,which are associated with C--H,C--C,C--N and C=N[56],respectively.For N1s simulated peaks as shown in Fig.17(e),BE at 400.0 eV is for C--N,and 401.8 eV is for the protonated nitrogen of C--N+.These carbon and nitrogen chemical bonds evidently come from the inhibitor of RTZ,thus XPS provides the direct evidence of the adsorption of RTZ onto steel surface.

3.7.Quantum chemical calculations of RTZ+

In water solution,RTZ can be easily ionized through the following reaction:

The organic part of RTZ+(C19H15N4+)as shown in Fig.18(a)can be considered as the vital effective component for RTZ,and it involves one tetrazole ring and three benzene rings of A,B and C.Quantum chemical calculations of RTZ+were conducted to elucidate the active adsorption sites on steel surface at the molecular level.Fig.18(b)shows the optimized RTZ+molecule calculated by Dmol3software based on DFT.It can be seen from Fig.18(b)that the tetrazole ring and substituted benzene ring of A are almost in one plane,while B and C benzene rings are in another two planes.

It is more likely for an atom with more negative charge to donate electrons to the empty d-orbital of Fe atom[57–59].Mulliken charges of C and N atoms in RTZ+molecule are summarized in Table 3,and it is found that N8 and N11 atoms as well as some C atoms possess the excess negative charges.Noticeably,these carbon atoms connect the H atoms,and so the reactivity of these C atoms drops to some extent owing to the passivation effect of H atoms[60,61].Accordingly,N8 and N 11 atoms in the tetrazole ring would be the most active adsorption sites.

Fig.17.High resolution spectra of XPS analysis for CRS surface exposed to 7.0 mol·L-1 H2SO4 with 2.0 mmol·L-1 RTZ solution for 6 h at20 °C:(a)S2p;(b)O1s;(c)Fe2p;(d)C1s;(e)N1s.

According to quantum chemical calculations for corrosion inhibitors[62],HOMO(highest occupied molecular orbital)is related to the adsorptive site of inhibitor molecule to donate electrons to empty orbital of metal,whereas LUMO(lowest unoccupied molecular orbital)represents the active site of the inhibitor molecule to accept electrons from met al.Fig.19 shows the electric/orbital density distributions of HOMO and LUMO,and EHOMO(energy of HOMO)is-6.047 eV,ELUMO(energy of HOMO)is-3.940 eV,with the energy gap(ΔE,ELUMO-EHOMO)of 2.107 eV.As seen in Fig.19,the electron densities of both HOMO and LUMO are mainly localized on the entire tetrazole ring,which indicates that the tetrazole ring could be the main adsorption site as both acceptor and donor of electron pairs.For the substituted benzene ring of A,HOMOis well located but LUMO is absent,which reflects that Abenzene ring is not the acceptor of the electron pairs,but the donor of electron pairs or π electrons to unoccupied d orbital of Fe atom.For either B or C benzene ring,both HOMO and LUMO are weakly localized on almost three carbon atoms,which means there might be donor and acceptor interaction simultaneously for these two benzene rings.

In addition,Fukui function(f）)is applied to evaluate the local direction of electron transfer between adsorbed inhibitor molecule and steel surface[63],and it is commonly determined by nucleophic attack Fukui index(f)and electrophilic attack Fukui indexcan be derived as follow equations[64].Higher values ofandmore susceptible sites to capacity of the atom to gain and lost electrons,respectively.Based on the quantum chemical indices in Table 3(the values of f（and fwith Fig.20(Fukui function distributions of fand fit can be found that the most nucleophic attack sites are four nitrogen atoms(N8,N9,N10,N11)of the tetrazole ring,and these active nitrogen atoms can accept electrons from steel surface to form back-donating bonds.The most susceptible electrophilic sites are located on C1,C4,N10 and N11 atoms that would denote electrons to steel surface to form coordinate bond.

Fig.18.RTZ+molecular structure:(a)RTZ+;(b)optimized RTZ+molecule using density function theory(DFT)method at theoretical level of GGA/BLYP/DND/COSMO.

In summary,RTZ+can directly adsorb on CRS surface via the formed chemical bonds(coordinate bonds and back-donating bonds)in between.Additionally,the physical adsorption of RTZ+would also occur.The dipole moment(μ)has been calculated as 4.1998 Debye.The large value of dipole moment means there probably was physical adsorption via electrostatic force[64,65].In inhibited system,Cl-(the anionic part of RTZ)and SO42-could firstly adsorb on steel surface through following steps:

Through electrostatical attraction,the positive RTZ+could adsorb on the negatively charged species of(FeCl-)adsand(FeSO42-)adsas follows:

Table 3Quantum chemical parameters of Mulliken charge,f（r→）+and f（r→）-for optimized RTZ+using density function theory(DFT)method under theoretical level of GGA/BLYP/DND/COSMO

3.8.Molecular dynamics(MD)simulations

The adsorption mode of RTZ+on Fe(001)surface has been further investigated by MD simulations.Through analysis of temperature and energy,the adsorption system reaches equilibrium sate from 500 ps to 1000 ps.Fig.21 displays the favorable configuration of RTZ+on Fe(001)surface,and the calculated ΔEadsis-1210 kJ·mol-1.This large negative value of ΔEadsis apparently as a consequence of the strong interaction force of RTZ+with Fe(001)surface.Also,it can be observed from Fig.21 that RTZ+adsorbs on Fe(001)surface in a nearly parallel flat mode,which implies the simultaneous adsorption of tetrazole ring as well as three substituted phenyl groups on steel surface.Comparing with vertical and tilted orientations,the flat orientation is more stable and can cover more activated reactive sites on steel surface to form the barrier film,and slows down the corrosion rate of steel.

4.Conclusions

(1)The concentrated 7.0 mol·L-1H2SO4is an important transition concentration for the corrosion of CRS in sulfuric acid fromactive corrosion to passivated state.

(2)RTZ can be used as an efficient inhibitor for the corrosion of CRS in 7.0 mol·L-1H2SO4solution,and the maximum inhibition efficiency reaches higher than 95%with low inhibitor concentration of 2.0 mmol·L-1.The adsorption of RTZ on CRS surface completely follows Langmuir isotherm.

(3)RTZ acts as a mixed-type inhibitor,and the corrosion potential moves positive with the increase of RTZ concentration.EIS spectra exhibit two capacitive loops,both Rtand Rfsignificantly increases after adding RTZ to 7.0 mol·L-1H2SO4solution.

(4)SEM and AFM micrographs confirm that the addition of RTZ can remarkably retard the corrosion of CRS in 7.0 mol·L-1H2SO4.The corroded layer formed in inhibited solution is mainly composed of FeSO4·H2O,FeSO4·4H2O,Fe2(SO4)3,FeOOH,Fe2O3,Fe3O4and FeO,as well as RTZ.

Fig.19.The frontier molecule orbital density distributions of RTZ+using density function theory(DFT)method at theoretical level of GGA/BLYP/DND/COSMO:(a)HOMO;(b)LUMO.

Fig.20.Fukui function distributions of RTZ+using density function theory(DFT)method at theoretical level of GGA/BLYP/DND/COSMO:(a)f（r→）+and(b)f（r→）-.

(5)The adsorption center of RTZ+mainly relies on the tetrazole ring,while the substituted benzene rings can also adsorb on steel surface owing to the fact that HOMO and LUMO densities are partly located.RTZ+adsorbs on Fe(001)surface via the nearly flat orientation mode.

Fig.21.Equilibrium adsorption configuration ofRTZ+on Fe(001)surface obtained by MD simulations under 298 K using NVT ensemble with simulation time of1000 ps at1.0 fs per step.