Effect of an Environmentally Friendly Diisooctyl Sebacate-based Mixed Corrosion Inhibitor on Reinforcing Steel

Ya Guo,Piao Jin,Minhua Shao,Shigang Dong,Ronggui Du,＊,Changjian Lin

1 Department of Chemistry,College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361005,Fujian Province,China.

2 Department of Metallurgy and Chemical Engineering,Gansu Vocational & Technical College of Nonferrous Metallurgy,Jinchang 737100,Gansu Province,China.

3 Department of Chemical and Biological Engineering,The Hong Kong University of Science and Technology,Klwloon,Hong Kong,China.

4 College of Energy,Xiamen University.Xiamen 361102,Fujian Province,China.

Abstract:Corrosion protection of reinforcing steel in concrete is an urgent task in modern society.Use of corrosion inhibitors in concrete is an effective,simple,and economical method for protecting reinforcing steel from corrosion.Mixed corrosion inhibitors usually perform better than a single inhibitor in actual reinforced concrete systems because of their synergistic inhibition effects.In recent years,environmentally friendly corrosion inhibitors have attracted increasing attention from corrosion researchers.Diisooctyl sebacate and sodium D-gluconate are environmentally friendly organic corrosion inhibitors,and ZnSO4 is an inorganic cathodic inhibitor,they may form an innovative,nontoxic,and pollution-free mixed corrosion inhibitor to control reinforcing steel corrosion.Additionally,diisooctyl sebacate and sodium D-gluconate serve as absorption-type inhibitors,and ZnSO4 acts as a precipitation-type inhibitor.We hypothesized that their combination might show a good synergistic corrosion inhibition effect on reinforcing steel.In this study,we developed a diisooctyl sebacate-based mixed corrosion inhibitor that includes D-gluconate and ZnSO4 and investigated its synergistic inhibition effects on reinforcing steel(Q235 steel)corrosion in a simulated polluted concrete pore solution.The reinforcing steel corrosion behavior and the properties of the mixed corrosion inhibitor were studied by polarization curve measurements,electrochemical impedance spectroscopy tests,and surface analysis methods(scanning electron microscopy,X-ray photoelectron spectroscopy,and Raman spectroscopy).The results indicated that the reinforcing steel in the simulated polluted concrete pore solution(pH 11.00,0.5 mol·L-1 NaCl)was in an active dissolving state and that localized corrosion took place.The mixed corrosion inhibitor,consisting of diisooctyl sebacate(59 mmol·L-1),sodium D-gluconate(0.5 mmol·L-1),and ZnSO4(1.5 mmol·L-1),had an obvious and powerful inhibition effect.Its inhibition efficiency reached 96.8% and 90.0% in the simulated polluted concrete pore solution and the cement mortar,respectively.The mixture of diisooctyl sebacate with sodium D-gluconate and ZnSO4 acted as a mixed-type inhibitor and effectively controlled both anodic and cathodic reactions of the steel corrosion.

Key Words:Reinforcing steel;Chloride;Localized corrosion;Corrosion inhibitor;Simulated concrete pore solution

1 Introduction

Reinforced concrete structures are widely used in various infrastructures and play a significant role in the economic development.However,the corrosion of reinforcing steel can cause severe durability issue and the premature degradation of reinforced concrete structures.Therefore,the study on the corrosion control of reinforcing steel in concrete is of great economic and social significance1-3.

In damage-free concrete,corrosion does not occur on the reinforcing steel resulting from a compact passive film on the steel surface due to the high alkalinity of the interstitial solution.However,the carbonation of concrete and the penetration of Clmay cause breakdown of the passive film easily,resulting in significant corrosion of the reinforcing steel1-7.In order to mitigate or ultimately eliminate the corrosion of reinforcing steel in concrete,many methods for corrosion protection have been used,including uses of protective coatings8,9,cathodic protection1,10and applications of corrosion inhibitors11-15.Among them,the use of corrosion inhibitors is a very promising method because it is effective,inexpensive and easy-to-operate.Mixed corrosion inhibitors are usually more effective than a single inhibitor due to the complexity of actual reinforced concrete systems1,15.Therefore,it is of practical significance to develop advanced mixed corrosion inhibitors.

Recently,the demand for eco-friendly corrosion inhibitors for reinforced concrete has increased with the increase of environmental awareness.Among them,mixed inhibitors containing transition metals have attracted considerable attention because of their super effects on corrosion inhibition.ZnSO4is often used as a precipitation inhibitor with a competitive advantage of non-toxicity and non-pollution16-19.It is an inorganic cathodic inhibitor and can combine with organic substances(such as sodium gluconate16,sodium dodecylsulphate17,and sodium octanoate19)into mixed inhibitors,and its mixed inhibitors showed synergistic inhibition effects on steel in aqueous solutions with NaCl.Gluconates(such as Sodium D-gluconate,SDG for short)as absorption-type organic corrosion inhibitors are also considered as environment-friendly organic inhibitors and have inhibition effects on steel12,20-23.For example,Otani et al.21investigated the influence of metal cations on the corrosion inhibition performance of gluconates for mild steel in fresh water and found that metal cations and gluconates conferred a synergistic inhibition effect on the corrosion of mild steel.Loto et al.22found that calcium gluconate exhibited mixed type inhibition property and suppressed the corrosion of mild steel in a 3.5%(mass fraction)NaCl solution with inhibition efficiency of over 80%.Therefore,gluconates are often used to prepare mixed inhibitors for corrosion control of steel in corrosive solutions12,21-23.Due to its low cost,abundant resources and lubricant property,diisooctyl sebacate(DOS for short)is increasingly valued24-26.It can be used as an absorption-type organic inhibitor and may have inhibition effect on steel corrosion.Based on the above discussions,a combination of these different types of inhibitors may show a good synergistic inhibition effect on reinforcing steel corrosion.So far,there is no report on this mixed inhibitor for corrosion protection.It is of interest to study corrosion inhibition behavior of the admixture containing of DOS,SDG and ZnSO4.

Therefore,in this work,we investigated the inhibition effect of a mixed corrosion inhibitor with DOS,SDG and ZnSO4on the corrosion of reinforcing steel in simulated concrete pore solutions with Cl-by the electrochemical techniques,together with surface sensitive techniques.The aim of this study was to assess whether this mixed corrosion inhibitor had an obvious synergistic effect and find the optimum concentration ratio of the mixed inhibitor.Furthermore,the likely inhibition mechanism of the mixed inhibitor for reinforcing steel was discussed.

2 Experimental

2.1 Steel specimens and solutions

The tested material was Q235 reinforcing steel bars with chemical composition as follows:C 0.390%,Si 0.083%,S 0.012%,P 0.015%,Mn 0.254%,Cu 0.101%,Ni 0.027%(mass ftraction)and balance Fe.The steel bar was cut into ? 1.12 cm ×0.40 cm cylindrical specimens for electrochemical measurements,the Raman spectroscopy investigation and the scanning electron microscope(SEM)analysis.The dimensions of the steel specimens for the X-ray photoelectron spectroscopy(XPS)analysis were ? 0.50 cm × 0.20 cm.Before use,all the steel surfaces for the tests were wet abraded with emery papers of grade from 400 to 1500,rinsed with deionized water,then ultrasonically cleaned in ethanol and finally dried in air.For the electrochemical tests,a Cu lead wire was soldered to the back of the steel specimen for electrical connection,and then the steel specimen(as a working electrode)was sealed with epoxy resin and mounted in a PVC holder only leaving a flat working surface of 1.0 cm2.

All the solutions were prepared with analytical grade reagents and deionized water.A pre-mixed solution with 0.6 mol·L-1KOH,0.2 mol·L-1NaOH and 0.001 mol·L-1Ca(OH)2was used as the simulated concrete pore solution27.The solution pH was adjusted to 11.00 by adding a 0.8 mol·L-1NaHCO3solution,NaCl was added resulting in a final concentration of 0.5 mol·L-1.Then,this solution was designated as the simulated polluted concrete pore(SPCP)solution or the test solution.Under normal conditions,reinforcing steel in concrete maintains its passivity in the concrete pore environment with high pH(12.5 to 13.5).The passivity may be lost when the pH is lower than about 11.3 due to concrete carbonation1,4,5,7.Therefore,the pH of the simulated concrete pore solution was adjusted to 11.00 for the purpose of investigating the inhibition effect of inhibitors on the steel.Chloride induced corrosion of reinforcing steel in concrete is an important cause of premature degradation of concrete structures1,14.Chloride ions may come from seawater or other environments.A test solution containing about 3.5%(mass fraction)or 0.5 mol·L-1NaCl is often used for studying corrosion behavior of reinforcing steel22,28,29.Therefore,0.5 mol·L-1NaCl was added to the SPCP solution in this work.

DOS,SDG and ZnSO4were used as corrosion inhibitors and combined into a mixed inhibitor.They were added to the SPCP solution with certain concentrations.If not otherwise specified,the mixed inhibitor refers to the inhibitor consisting of 59 mmol·L-1DOS,0.5 mmol·L-1SDG and 1.5 mmol·L-1ZnSO4in the following sections.

2.2 Electrochemical measurements

Electrochemical measurements were made by an Autolab Potentiostat Galvanostat,expanded with an FRA module.A three-electrode cell was used.The working electrode was the Q235 steel specimen with an exposed surface area of 1.0 cm2,a platinum sheet with a surface area of 1.5 cm × 1.5 cm served as the counter electrode and a saturated calomel electrode(SCE)was used as the reference electrode.All electrode potentials in this study were referred to SCE.The measurements were performed in air at ambient temperature(25 ± 3 °C).

The Tafel polarization curves were measured for the steel specimen in the SPCP solutions at a scan rate of 0.5 mV·s-1in the range from -120 mV to 120 mV versus corrosion potentials(Ecorr).The measurements of potentiodynamic anodic polarization curves were performed at a scan rate of 0.8 mV·s-1from Ecorrin the more positive direction.The electrochemical impedance spectroscopy tests were conducted in a frequency range of 100 kHz to 0.01 Hz with an AC signal of peak to zero amplitude 10 mV and data density of 5 points per decade.All impedance spectra were recorded at the steady open circuit potential,namely Ecorr,after exposure of the steel electrode to the solution for 40 min.The steady open circuit potentials of the steel specimens immersed to the test solutions with and without the mixed inhibitor were about -386 mV vs.SCE and -430 mV vs SCE,respectively.All potentials are reported vs SCE.The area of the steel electrode exposed to the solution was 1.0 cm2.All the fitting procedures and data processing were performed automatically by the software of Autolab to obtain electrochemical corrosion parameters of the steel specimen.

2.3 Surface analysis

After the steel specimens were exposed to the test solution for 1 h at ambient temperature,their surface morphologies were characterized by a field-emission SEM(Hitachi FE-SEM S4800).

Before the XPS analysis,the steel specimens were immersed in the test solution for 1 h,and then they were rinsed by deionized water,ultrasonically cleaned with ethanol for 10 min and dried in air.The surface compositions of the steel specimens were detected by XPS(PHI Quantum 2000 spectrometer).Al KαX-ray source(1486.6 eV)was set at 15 kV and 25 W.Binding energies were referenced to the C 1s peak at 284.8 eV.A 5 nm depth of the steel surface was obtained by Ar+sputtering with 4 keV for the compositional analysis of the surface layer.

Raman spectroscopy was examined for the steel specimen after 2 d immersion in the test solution.The specimen was pretreated by the same procedure as the XPS analysis.Raman measurements were conducted with an Xplora confocal Raman system(HORIBA Jobin Yvon)by using 532 nm laser light(7.95 mW).The Raman spectra in the range of 200-2800 cm-1were recorded by a CCD detector through a 100× objective lens(NA 0.9)with a spatial resolution of about 2 μm.

3 Results and discussion

3.1 Electrochemical results

3.1.1 Tafel polarization

Fig.1 depicts the Tafel polarization curves of the reinforcing steel in SPCP solutions with 0.5 mmol·L-1SDG,0.5 mmol·L-1ZnSO4and different DOS concentrations.The relevant electrochemical parameters derived from the curves are given in Table 1.Here,i0corrand icorrare corrosion current densities of the steel in the solutions without and with the inhibitors,respectively,βcand βaare cathodic and anodic Tafel slopes,respectively,and C represents the concentrations of the inhibitors.As shown in Fig.1 and Table 1,the Ecorrchange was not remarkable and within 40 mV after the addition of the inhibitors into the SPCP solutions,but the icorrvalue decreased significantly,showing that the mixed inhibitor could effectively inhibit the steel corrosion and mainly acted as a mixed type inhibitor.The inhibition efficiency η(%)from the Tafel polarization measurements was calculated from the following expression:

Fig.1 Tafel polarization curves of the reinforcing steel in the SPCP solution with 0.5 mmol·L-1 SDG,0.5 mmol·L-1 ZnSO4 and different DOS concentrations(mass fraction).

The corrosion current density of the reinforcing steel in the SPCP solution without inhibitors was 2.107 μA·cm-2.As shown in Table 1,for the solution with 0.5 mmol·L-1SDG and 0.5 mmol·L-1ZnSO4,the corrosion current density dropped to 0.561 μA·cm-2.Increasing the concentration(mass fraction)of the DOS from 1.0% to 2.5% resulted in a decrease in the corrosion current densities while an increase in the polarization resistances and inhibition efficiencies,which indicated that the steel was more effectively protected from corrosion by a protective layer with the addition of DOS to the SPCP solution.The presence of DOS caused a little change of the corrosion potential towards the positive direction.However,the corrosion potential decreased slightly and the corrosion current density increased with further increasing the DOS concentration higher than 2.5%(w).Thus,the mixed inhibitor exhibited the best inhibition effect when the concentration of DOS was 2.5%(w),namely 59 mmol·L-1.As shown in Table 1,the inhibition efficiency of the admixture consisting of SDG and ZnSO4was only 73.4%.After the 59 mmol·L-1DOS was added to the solution,the inhibition efficiency reached 94.2%,which might derive from the synergistic effect of the applied inhibitors(which will be further discussed in the following sections).

Fig.2 presents the Tafel polarization curves of the reinforcing steel in the SPCP solutions with 59 mmol·L-1DOS,0.5 mmol·L-1SDG and different ZnSO4concentrations.The electrochemical parameters resulted from the Tafel polarization measurements are shown in Table 2.For the solution with 1.5 mmol·L-1ZnSO4,the corrosion potential of the steel did not change markedly,but the current density decreased significantly,and the inhibition efficiency of the mixed inhibitor reached a high value(96.8%).However,when the ZnSO4concentration was increased to 2.0 mmol·L-1,the corrosion potential decreased,the corrosion current density increased,and theinhibition efficiency declined.It suggested that the inhibition effect of the mixed inhibitor on the steel corrosion in the alkaline chloride solution was dependent on the concentration of the individual inhibitors.The addition of 1.5 mmol·L-1ZnSO4to the solution made the inhibition efficiency of the mixed inhibitor reach 96.8%.This is due to the precipitation of Zn2+as Zn(OH)2on the cathodic sites of the steel surface and inhibition of its cathodic reaction,which showed the synergistic effect between Zn(OH)2and the other inhibitors16-19.

Fig.2 Tafel polarization curves of the reinforcing steel in the SPCP solution with 59 mmol·L-1 DOS,0.5 mmol·L-1 SDG and different ZnSO4 concentrations.

In general,the reinforcing steel is considered passive when the icorris lower than 0.1 μA·cm-230,31.The lowest corrosion current density(0.068 μA·cm-2)of the steel in the SPCP solution with the mixed inhibitor was observed when the concentrations of DOS,SDG and ZnSO4were 59,0.5 and 1.5 mmol·L-1,respectively.Under the above conditions,the molar ratio of DOS,SDG,ZnSO4,and Cl-was 11.8:0.1:0.3:100.This result indicated that the small amount of the mixed inhibitor could effectively control the corrosion of the reinforcing steel in the SPCP solution with Cl-.

In order to explain the observed phenomena,the inhibition mechanism of the mixed inhibitor was further discussed here.Adsorption mechanism could be used to understand the inhibition effects of DOS and SDG.The chemical formulas of DOS and SDG are(CH2)8[COOCH2CH(C2H5)C4H9]2and CH2OH(CHOH)4COONa,respectively.Both are composed of a polar group with oxygen atoms as the center which has a great electronegativity,and a nonpolar group with hydrocarbon as the center.The polarized group is adsorbed on the surface of the steel,while the nonpolar group stays away from the surface.Therefore,the inhibition effect can be attributed to the adsorption of the inhibitors on the steel surface and the assignment of nonpolar groups that form a hydrophobic film,which hinders the access of Cl-to the steel surface and subsequently prevents local corrosion.Hackerman proposed the concept that the organic inhibitor was Lewis base and the metal was Lewis acid32.Because the polar group of the inhibitor is electron donor,it is general base,whereas,the metal is electron acceptor,and is general acid.The hard acid and hard base can be stably combined.According to the polar group of organic inhibitors,RCOO-,ROCO-and ROH belong to the range of hard base,while the Fe3+coming from the oxide film on the steel surface belong to the range of hard acid.Therefore,the polar group of DOS and SDG can be strongly chemisorbed onto the oxidized steel surfaces by the formation of stable hard acid-hard base interaction with Fe3+on the surfaces of steel.The densely packed alkyl tails on the film can act as a barrier to Cl-and other aggressive anions diffusion and accumulation on the protective film of the steel surface,resulting in prevention of the film breakdown14,33-35.The counterpart of the metal corrosion is the oxygen reduction:

Zn2+from ZnSO4reacts with OH-in the solution to form Zn(OH)2,which may cover the reaction sites for the oxygen reduction reaction,i.e.,the cathodic reaction is greatly hindered16,17,19.Based on the above discussions,DOS and SDG served as absorption-type inhibitors with the dominant anodic inhibition effects.ZnSO4acted as a cathodic type inhibitor with the dominant cathodic effect.Therefore,the combination of these inhibitors might show a synergistic effect and provide high inhibition efficiency.Under our experimental conditions,one reason for the inhibition efficiency η exhibited the maximum value of 96.8% at the co-addition of 59 mmol·L-1DOS,0.5 mmol·L-1SDG and 1.5 mmol·L-1ZnSO4might be that the molecular layer formed on the surface of the reinforcing steel was relatively compact in this concentration ratio.The film was much thinner and could not well inhibit the corrosion of the steel at the lower concentration of inhibitors.Instead,with the increase of the concentration of inhibitors above the appropriate values,the corrosion inhibition efficiency decreased.Perhaps the incrassation of the precipitated film produced asymmetrical stress,which might destroy the film.Then the corrosion of the reinforcing steel would occur again and the corrosion inhibition efficiency decreased.

3.1.2 Potentiodynamic anodic polarization curves

Potentiodynamic anodic polarization curves of the reinforcing steel immersed in the SPCP solutions with and without the mixed inhibitor are shown in Fig.3.No stable passive region was observed without the mixed inhibitor as shown in the curve(a).Therefore,the steel under this condition was unstable.On the other hand,the steel immersed in the SPCP solution with the mixed inhibitor was in a passive state at the corrosion potential.Upon increasing the potential above the corrosion potential,a wide range of stable passive region was observed in the curve(b)until the potential reached about 500 mV vs SCE.This result indicated that the mixed inhibitor could effectively control the corrosion of the reinforcing steel and make it passive.

Fig.3 Potentiodynamic anodic polarization curves of the reinforcing steel in the SPCP solution(a)without and(b)with the mixed inhibitor.

3.1.3 Electrochemical impedance spectroscopy

Fig.4 shows the Nyquist plots for the reinforcing steel in the SPCP solutions without and with the mixed inhibitor,the solid fitting curves resulted from the experimental data represented with the points.The data were analyzed by the corresponding equivalent circuit presented in Fig.5,where Rsand Rctrepresent the solution resistance and the interfacial charge transfer resistance,respectively.The greater Rctvalue means a higher corrosion resistance of the steel,and the corrosion reaction is more difficult to occur.CPE is a constant phase element used to model the capacitance of the double-layer at the steel/solution interface,and its impedance value can be assessed by the following equation36:

Fig.4 Nyquist plots of the reinforcing steel in the SPCP solutions(a)without and(b)with the mixed inhibitor.

Fig.5 Equivalent circuit mode for the reinforcing steel in the SPCP solutions.

where Y0represents a modules with dimensions of Ω-1·cm-2·sn.In our case,n represents the departure of the capacitance from the ideal,and its value changes from 0 to 1.When n=1,the CPE serves as a capacitor,and when n=0,the CPE serves as a resistor37-40.Table 3 presents the parameters derived from the equivalent circuit,where the inhibition efficiency η(%)was calculated by the following expression:

Table 3 Impedance parameters and the corresponding inhibition efficiency for the reinforcing steel in the SPCP solutions without and with the mixed inhibitor at 25 °C.

Fig.4 clearly shows that the value of the impedance modulus in the SPCP solution without inhibitors was much smaller than that with the mixed inhibitor,which reflected an increase in the corrosion rate due to the presence of Cl-and the decrease of pH in the solution without inhibitors.As presented in Fig.4a,the steel electrode had a capacitive reactance arc at high frequencies and it shrink slightly at the low frequencies,which showed that the steel might be in the induction period of pitting41.However,after the addition of the mixed inhibitor,it was evident that the impedance modulus of the system increased greatly and the capacitive reactance arc did not shrink at the low frequencies as shown in Fig.4b.The above trend could be further supported by the quantified resistance values which can be seen from the fitting data of Rctin Table 3.

It is well known that in alkaline solutions,an electrical double layer is established rapidly at the steel/solution interface.A simple capacitor may be used as the model to describe this electrical double layer42.The parameters Y0and n can be used to evaluate the change of a steel electrode surface with a double layer.Bigger Y0values and smaller n values mean the electrode surfaces are more inhomogeneous,otherwise,smaller Y0values and bigger n values are characteristics of more homogeneous surfaces43-45.As presented in Table 3,after the addition of the mixed inhibitor,n increased and Y0decreased greatly,which indicated that the steel surface became smoother and more homogeneous due to the inhibition effect of the mixed inhibitor.

3.2 Surface analysis

3.2.1 SEM analysis

The surface morphologies of the reinforcing steel specimens immersed in the SPCP solutions with and without the mixed inhibitor for 1 h was examined by SEM(Fig.6).As shown in Fig 6a,the steel surface became rough and some obvious corrosion pits(the deep black points)appeared because of the occurrence of localized corrosion(pitting)on the steel resulting from the attack of Cl-.However,with the mixed inhibitor in the SPCP solution,the steel surface was smooth(Fig.6b),indicating that the mixed inhibitor had an obvious inhibition effect on the corrosion of the steel.

Fig.6 SEM images of the reinforcing steel surfaces after immersion in the SPCP solutions(a)without and(b)with the mixed inhibitor for 1 h.

Thus,we can conclude that the steel was unstable because of the attack of Cl-in the presence of 0.5 mol·L-1NaCl and with a lower pH(11.00)in the SPCP solution.The mixed inhibitor could effectively prevent the corrosion on the steel surface.

3.2.2 XPS analysis

The chemical compositions of the protective films formed on the reinforcing steel surfaces were further examined by XPS measurements.Fig.7 shows the spectra for Fe 2p3/2of steel surfaces after 1 h immersion in the SPCP solution without(Fig.7a)and with(Fig.7b)the mixed inhibitor.Fig.7c presents the relative atom fraction of Fe with different oxidation states identified by deconvolution for Fig.7a,b.To eliminate the effects of O2and CO2from the air on the composition of the surface films on the steel specimens,the deconvolution of Fe 2p3/2was conducted at 5 nm depth of the steel surface layers.In the SPCP solution without the mixed inhibitor(Fig.7a),the peaks at 707.0,708.6 and 711.6 eV were assigned to metallic Fe,the ferrous and ferric compounds,respectively46-49.The peak at 711.6 eV was characteristic for FeOOH50.In our work,the surface film of the steel specimen immersed in the test solution(pH 11.00,0.5 mol·L-1NaCl)without the inhibitor could be destroyed due to the high Cl-concentration and the lower pH.FeOOH could be in the form of β-FeOOH,γ-FeOOH and δ-FeOOH,etc.It has been known that FeOOH species were unstable and non-protective oxides48,49,51,and could not protect the steel from corrosion.This conclusion could also be obtained by the Raman spectroscopy analysis in the next section.For the test solution with the mixed inhibitor(Fig.7b),the wide peak of the ferrous compound appeared around at 707.8 eV(obtained from the result by deconvolution),the ferric compound at 710.9 eV and the corresponding satellite peak at 712.5 eV50,52,53.The peak at 710.9 eV was assigned to Fe3O4,which was known to be stable.The contents of Fe,Fe2+and Fe3+for the steel specimens tested in the solutions with and without the inhibitor are compared in Fig.7c.The molar percentage of metallic Fe,Fe2+and Fe3+was 45.5%,42.1% and 12.4%,respectively,for the specimen tested without the mixed inhibitor.Metallic Fe and Fe2+which have a low stability in the corrosive solution were the main species on the steel surface.For the specimen tested with the mixed inhibitor,the ratio of Fe2+and Fe3+was 10.6% and 89.4%,respectively.The significant increase of the ratio of Fe3+and the decrease of Fe2+and metallic Fe(actually did not present)suggested the passive film on the steel was more stable.It is obvious that the mixed inhibitor offered good protection to the steel surface from corrosion in the environment studied in this work.The mixed inhibitor not only inhibited the production of the ferrous compound,but also transformed the unstable ferrous compound to other stable ferric compounds.

Fig.7 XPS deconvoluted profiles of Fe 2p3/2 for the reinforcing steel surface film after 1 h immersion in the SPCP solutions(a)without and(b)with the mixed inhibitor at 25 °C,and(c)the relative atomic fraction of different valance states of Fe.

Fig.8 (A)Survey spectra and(B)high resolution spectra of C 1s for the reinforcing steel after 1 h immersion in the SPCP solutions(a)without and(b)with the mixed inhibitor.

Fig.8A shows the survey spectra measured from 0 to 1200 eV with the main peaks identified and labeled.The peak at 1024.1 eV in Fig.8A(b)for the specimen exposed to the test solution with the mixed inhibitor was Zn 2p3/254,from ZnO formed on the reinforcing steel surface,ZnO might originate from Zn(OH)2,which came from the reaction of Zn2+and OH-.The C 1s spectra were presented in Fig.8B.It can be seen from Fig.8B(b)that the content of C in the specimen for the solution with the mixed inhibitor was substantially higher than that without the mixed inhibitor as observed in Fig.8B(a).The strong peak centered around 285 eV was related to C-C and C-H bonds.For the specimen in the solution without the mixed inhibitor,C-C and C-H bonds originated from traces of dissolved C in the steel material.As for those with the mixed inhibitor,most of C-C and C-H bonds were mainly related to the adsorption of DOS and SDG on the steel surface.

The XPS results suggested that an organic layer was formed on the oxide film(FeO/Fe2O3).This layer might be composed of Fe3+-DOS and Fe3+-SDG complexes.The dense organic layer on the film acted as a barrier to Cl-attack and resulting in corrosion prevention of the steel.

3.2.3 Raman spectroscopy analysis

Fig.9 depicts the Raman spectra for the reinforcing steel after 2 d immersion in the SPCP solutions without and with the mixed inhibitor.Raman signals were very weak if the immersion time was too short.The Raman peak assignments were listed in the inserts.For the steel specimen tested without the mixed inhibitor,a strong peak at 503 cm-1and three weaker peaks at 245,349 and 1078 cm-1were observed(Fig.9a).The peak at 245 and 349 cm-1could be assigned to γ-FeOOH,the peak at 503 cm-1was attributed to δ-FeOOH and the peak at 1078 cm-1may be due to FeCO3.For the steel specimen tested with the mixed inhibitor,there was only one peak at 667 cm-1(Fig.9b),which could be attributed to Fe3O455-58.It has been known that the types of the structures of iron oxides affect their protective properties significantly.As mentioned above,β-FeOOH,γ-FeOOH and δ-FeOOH were unstable and non-protective oxides.Instead,some other phases of oxides such as Fe3O4were much more stable and bore compact morphologies.This indicated that the corrosion products,such as ferrous oxides and/or hydroxides formed on the steel surface due to the attack of Cl-and the decrease of the pH.These oxides were not stable and could not protect the steel from further corrosion.Raman spectroscopy provided more evidence that the mixed inhibitor could effectively restrict the formation of ferrous oxides and/or hydroxides and promote conversion of ferrous species to ferric ones.This is consistent with the results observed by the XPS analysis.

Fig.9 Raman spectra for the reinforcing steel after 2 d immersion in the SPCP solutions(a)without and(b)with the mixed inhibitor.

3.2.4 Inhibition effect of the mixed inhibitor for the reinforcing steel in cement mortar

As mentioned above,we have found that the mixed inhibitor consisting of DOS,SDG and ZnSO4had a good inhibition effect on the corrosion of the reinforcing steel in the SPCP solution.However,the further objective of our study is to verify whether the mixed inhibitor is suitable for practical applications.Therefore,we have conducted preliminary experiments to examine the inhibition effect of the mixed inhibitor on corrosion of the reinforcing steel in cement mortar for a longer immersion time.The sample preparation and electrochemical measurements were done as described in our previous study7.In brief,distilled water and an aqueous solution of DOS(59 mmol·L-1),SDG(0.5 mmol·L-1)and ZnSO4(1.5 mmol·L-1)were used to prepare cement mortars,respectively,and the reinforcing steel electrodes were embedded in the cement mortars(as a reinforced concrete specimens).No NaCl was added to the mortars.After the specimens were immersed in a 0.5 mol·L-1NaCl solution for 80 d,the Tafel polarization curves of the reinforcing steel were measured for obtaining its corrosion current densities.The electrochemical measurement results showed that the inhibition efficiency of the mixed inhibitor reached about 90.0%,suggesting that the mixed inhibitor had a good inhibition effect on the steel and had potential application in reinforced concrete.Further study is needed in our future work to evaluate compatibility of the mixed inhibitor with concrete properties such as concrete mechanical resistance,porosity and cement hydration.

4 Conclusions

Based on the results from the study on the corrosion inhibition of the reinforcing steel by the mixed inhibitor consisting of DOS,SDG and ZnSO4in the SPCP solution and cement mortar,the following conclusions can be drawn:

The mixed inhibitor exhibited excellent inhibition properties against reinforcing steel corrosion in the SPCP solution and cement mortar.The maximum inhibition efficiency(96.8%)was found for the SPCP solution with the admixture of 59 mmol·L-1DOS,0.5 mmol·L-1SDG and 1.5 mmol·L-1ZnSO4.The mixed inhibitor acted as a mixed type inhibitor and affected both anodic and cathodic reactions for the steel corrosion.These applied inhibitors had a synergistic inhibition effect on the corrosion of reinforcing steel.The anodic reaction was mainly controlled by the formation of Fe3+-DOS and Fe3+-SDG complexes on the steel surface.The cathodic reaction was mainly controlled by the formation of Zn(OH)2on the cathodic sites of the steel surface.SEM,XPS and Raman analyses showed that the protective film on the steel was not damaged due to the inhibitive effect of the mixed inhibitor in the solution containing Cl-.The XPS analysis indicated the presence of carbon species and the zinc oxide on the steel surface pertaining to the mixed inhibitor.