Shamsuddeen A.Haladu ,Saviour A.Umoren *,Shaikh A.Ali ,Moses M.Solomon Abdul-Rashid I.Mohammed

1 Department Basic Sciencesand Humanities,College of Engineering,Imam Abdulrahman Bin Faisal University,Dammam 31451,Saudi Arabia

2 Centre of Research Excellence in Corrosion,Research Institute,King Fahd University of Petroleum and Minerals,Dhahran 31261,Saudi Arabia

3 Department of Chemistry,King Fahd University of Petroleum&Minerals,Dhahran 31261,Saudi Arabia

4 Centre for Engineering Research,Research Institute,King Fahd University of Petroleum and Minerals,Dhahran 31261,Saudi Arabia

Keywords:Tetrapolymer Metal Acid corrosion Corrosion inhibition Adsorption Synergism

ABSTRACT A novel tetrapolymer(TP)consisting of carboxylate,sulphonate,phosphonate and sulfur dioxide based comonomers was synthesized using Butler cyclopoymerization technique.The synthesized tetrapolymer w as characterized using FTIR,1H-NMR,13CNMRand elemental analysis.Theperformanceof thetetrapolymer as acorrosion inhibitor for St37 carbon steel in 15%HCl and 15%H2SO4 acid media was assessed using electrochemical impedance spectroscopy(EIS),linear polarization resistance(LPR),potentiodynamic polarization(PDP)and electrochemicalfrequency modulation(EFM)techniques.The in fl uence of addition of asmall amount of KIon thecorrosion inhibition ef fi ciency of TPw as also assessed.Results obtained showed that the tetrapolymer moderately inhibited the corrosion of St37 steel in the acid media with protection ef fi ciency of 79.5%and 61.1%at the optimum concentration of 1000 mg·L-1 studied in HCl and H2SO4 media respectively.On addition of 5 mmol·L-1 KIto the optimum tetrapolymer concentration,the protection ef fi ciency w as upgraded to 90.6%and 93.5%in HCl and H2SO4 environment,respectively.The enhanced performance of the polymer in the presence of KIis due to synergistic action deduced from synergism parameter(S1)which was found to be greater than unity.The tetrapolymer afforded the corrosion inhibition of St37 steel in the acid media by virtue of adsorption of the polymer moleculeson the steel surface which wascon fi rmed by ATR-FTIRanalysisof the adsorbed fi lm extracted from the steel surface.TP+KIformed complex with St37 steel surface in H2SO4 solution but not in HCl solution.

1.Introduction

In the process of acidizing stimulation or cleanup operations,metal tubulars,down hole tools/valves,surface lines,etc.mainly made of carbon steel are exposed to acidic fl uids[1].The exposure is detrimental to the carbon steel materials as they are prone to corrosion.In addition,metal substratesin high temperaturew ellsexperienceadrastic increase in corrosion rates.Therefore,controlling corrosion is critical and must be dealt w ith effectively.In addition,corrosion protection becomes inevitable in order to maintain the integrity and long life of dow n hole tools installed in an operating oil w ells.

Although there are many corrosion protection strategies adopted to control corrosion of metal substratesin diverse corrosive media,the use of corrosion inhibitors for internal pipeline corrosion protection offers the most practical and cost effective method.The key performance indicators for the selection of corrosion inhibitor include its economic availability,performance(effectiveness to inhibit the metal substrate)and its environmental side effects.Most of the inhibitors for corrosion of steel in acidic media are organic compoundscontaining nitrogen,oxygen and/or sulfur atoms.The inhibiting action of these compounds is often associated w ith their ability to adsorb on themetal/solution interface.Theuseof polymersand polymer compositesascorrosion inhibitor for steel in strong acid media can be found in the literature[2-7].In our laboratory,we have also reported on the use of cyclic cation polymer bearing bis-3-phosphorylpropyl pendants[8]and polypropylene glycol[9]as effective corrosion inhibitors for steel in 15%HCl medium.

Apart from corrosion,there are other problems in the oil and gas industry that required the use of specialty chemicals for treatment.For instance,antiscalantsor scaleinhibitorsarerequired for scaleinhibition,biocides to inhibit the grow th of microorganisms/biofouling and demulsi fi ers or emulsion breakersto separate emulsions,for example w ater in oil.The use of diverse chemicals simultaneously to treat different problems occurring in the oil fi eld sometimes results in the problem of incompatibility.It is highly desirable to have a single compound w ith dual or multi multi functions.Previous w ork has show n that the tetrapolymer acted as an effective antiscalant for CaSO4scale[10].In order to explorethepossibility of tetrapolymer serving a dual purpose as a scale inhibitor and as well as a corrosion inhibitor,thepresent w ork reportson thecorrosion inhibition performanceof tetrapolymer for carbon steel(St37 grade)in 15%HCl and 15%H2SO4environments using electrochemical techniques.The effect of the addition of KIon the corrosion inhibition performance of the investigated tetrapolymer was also assessed.Analysis of the adsorbed inhibitor fi lm on the steel surface in comparison w ith that of the bulk polymer using ATR-FTIRtechnique w as employed to elucidate the mechanism of corrosion inhibition by the tetrapolymer.

2.Experimental

2.1.Synthesis and characterization of tetrapolymer

The details of the synthesis of the tetrapolymer using Butler cyclopolymerization technique.The polymerization w as carried out in a solution of 1(2.75 g,15 mmol),2(3.83 g,15 mmol),and 3(3.29 g,15 mmol)SO2(2.88 g,45 mmol)in DMSO(12.5 g)using azobisobutyronitrile(AIBN)(225 mg)(Scheme 1).The mixture w as stirred under N2in a closed RB fl ask at 60°Cfor 48 h.Every 8 h,the fl ask w as opened to release N2generated from the decomposition of AIBN.The reaction mixture became immobile w ithin an hour;at the end of the time elapsed,the reaction mixture was soaked in water,fi ltered and w ashed w ith acetone.The w hite polymer 4 w as then grounded w ith a mortar and pestle and soaked in methanol,fi ltered,and dried under vacuum at 60°Cto a constant w eight(11.0 g,90%).The characterization of the obtained tetrapolymer w as done using FTIR,13CNMR,1H NMR,and elemental analysis.The results from FTIR,13CNMR,and elemental analysis can be found in reference[10].Only1H NMRspectrum is presented herein.

2.2.Corrosion inhibition studies

2.2.1.Materials and corrosive media

St37 steel with chemical composition as earlier reported[11]was used as the metal substrate.The corrosive media was 15%HCl and 15%H2SO4prepared by diluting analytical reagent grade 37%HCl(Sigma Aldrich)and 98%H2SO4(Sigma-Aldrich)w ith doubled distilled w ater.

Fig.1.1H NMRspectrum of tetrapolymer 4 in D2Oin the presence of NaCl.

The synthesized polymer was used as test corrosion inhibitor in the concentration range of 50-1000 mg·L-1.KI(Sigma Aldrich)concentration of 1 mmol·L-1and 5 mmol·L-1was added to 1000 mg·L-1of TP to evaluate the synergistic effect of KIadditive.

2.2.2.Electrochemical corrosion tests

Electrochemical corrosion tests w ere conducted using the Gamry Potentiostat/Galvanostat/ZRA Reference 600 together w ith Echem analyst softw are to analyze the experimental data.Conventional three electrode assembly w as used to accomplish the electrochemical measurements.The electrodes w ere made up of St37 steel as w orking,silver/silver chloride(Ag/AgCl)as reference and graphite rod as the counter electrodes.The w orking electrode w as in the form of a circular disc with an exposed surface area of 0.75 cm2.Prior to all electrochemical measurements,the w orking electrode was immersed for 1 h to ensure that the steady-state open circuit potential(OCP)w as established.Potentiodynamic polarization tests were performed at the potential of±150 m V from open circuit potential(OCP)at a scan rate of 0.5 m V·s-1.LPRexperiment w as carried out from-15 to+15 m V versus OCPat the scan rate of 0.125 m V·s-1.Electrochemical impedance spectroscopy(EIS)tests w ere monitored over the frequency range of 104to 10 m Hz,w ith acquirement of 10 points per decade and a signal amplitude of 10 m V(peak-to-peak)at Ecorr.EFM measurements w ere carried using potential perturbation signal w ith amplitudes of 10 m V.Measurements w ere conducted using tw o frequencies,2 and 5 Hz.The base frequency w as 1 Hz w ith 32 cycles so the w aveform repeats after 1 s.All measurements w ere repeated at least three timesand good reproducibility of the results w as observed.

Scheme 1.Synthesis of the tetrapolymer 4.

Fig.2.Variation of OCPw ith time for St37 steel in(a)15%H2SO4 and(b)15%HCl in the absence and presence of different concentrations of TPat 25°C.

2.2.3.Surface analysis

The morphology of the corroded steel surfaces immersed in the corrosive medium in absence and presence of TPand TPin combination of KI w ere determined using scanning electron microscope(SEM)(JEOLJSM 6610-LV)operated at an acceleration voltage of 20 kV and irradiation current of 10μA.

The UV-Visible spectra w ere recorded for solutions of 15%HCl and 15%H2SO4devoid of and containing TPand TP-KImixtures after 24 h immersion of the carbon steel using JASCO770-UV-Vis spectrophotometer from 250 to 450 nm using a dual beam operated at a resolution of 1 nm with a scan rate of 200 nm·min-1.

The ATR-FTIR analysis w as performed for pure TP and the fi lms extracted from the carbon steel surface immersed in 15%HCl and 15%H2SO4containing TPand TP-KImixtures for 24 h at 25°C.The ATRFTIRspectra w ere recorded in the range of 400-4000 cm-1using an IRspectrometry(TAinstrument with universal ATRattachment,Nicolet iS5,Thermo Scienti fi c).

3.Results and Discussion

3.1.Characterization of tetrapolymer(TP)

The1H NMRspectrum in D2O(Fig.1)revealed the formation of the polymer as indicated by the absence of alkene-proton signals of 1-3 which should appear between δvalue of 5.5-6.0 mg·L-1.

3.2.Corrosion inhibition by TP

The corrosion inhibition ability of TPfor St37 corrosion in strong acid environments w as assessed using electrochemical approaches.For results obtained from electrochemical experiments to be valid,the essential requirements of linearity,causality,and stability must be met[12].Fig.2 show s the variation of OCP of St37 steel w ith time in(a)15%HCl and(b)15%H2SO4solutions w ithout and w ith various concentrations of TP at ambient temperature(25°C).Although there is a slight difference in the length of time taken for a steady state condition to be attained in the various systems,it is obvious that a quasi-steady-state w as achieved in all cases w ithin the 3600 s w aiting time hence the electrochemical results presented herein should be valid.In the HCl(Fig.2(a))and H2SO4(Fig.2(b))solutions devoid of TP,the initial OCPvalues of the specimen are-456 m V/Ag/AgCl and-440 m V/Ag/AgCl respectively.The presence of TPin these media caused noticeable potential shift tow ards nobler OCPvalues indicating the appearance of TP adsorbed layer on the steel surface[13,14].

In Fig.3 is presented the potentiodynamic polarization curves obtained for St37 steel electrode in(a)15%HCl and(b)15%H2SO4solutions devoid of and containing various concentrations of TPat ordinary temperature.The relevant electrochemical polarization parameters namely,corrosion potential(Ecorr),corrosion current density(Icorr),and the anodic and cathodic slopes(βa,βc)deduced from the graphs are given in Table 1.Also displayed in Table 1 are the polarization resistance(Rp)and corrosion rate(CR)obtained from linear polarization graphs(not show n).The extent of corrosion inhibition(IE)by TPfrom potentiodynamic polarization and linear polarization techniques was computed making use of the Icorrand Rpvalues respectively according to the follow ing equations:

Fig.3.Potentiodynamic polarization curves for St37 steel in(a)15%H2SO4 and(b)15%HCl in the absence and presence of different concentrations TPat 25°C.

Table 1 Potentiodynamic polarization(PDP)and Linear polarization resistance(LPR)parameters for St37 steel in 15%HCl and 15%H2SO4 without and with different concentrationsof TPat 25 oC.

From Fig.3,it is observed that addition of TPto the corrosive media hasminimal in fl uence on Ecorr,i.e.the corrosion potential is only shifted slightly tow ards anodic direction.Both the oxidative dissolution of St37 steel electrode in the anodic region and the hydrogen reduction in the cathodic region are suppressed.These observations are common w ith mixed type corrosion inhibitors[15,16].The changes in bothβaandβcvalues relative to those of the blank(Table 1)also point to the mixed type behavior of TP in the studied environments[15,16].A quick comparison of Fig.3(a)to(b)revealsthat TPisa better corrosion inhibitor for St37 steel in HCl environment than in H2SO4medium.As could be seen in the fi gure and in Table 1,there is a remarkable reduction in Icorrby TPrelative to the blank in HCl solution than in H2SO4solution.For instance,in H2SO4solution,the addition of 1000 ppm TPresulted in a decline in Icorrvalue from 63.5 μA·cm-2to 35.2 μA·cm-2and the calculated IE is 44.6%.The addition of the same concentration of TPto 15%HCl solution caused a decrease in Icorrvalue from 649.0 μA·cm-2to 183.0 μA·cm-2w ith 71.8%as the IE.This seems to suggest that the anions(chlorides in HCl and sulphates in H2SO4)played a vital role in the adsorption process of TPonto the steel surface.It is also observed from the table that the Icorrand CR values decrease w hile Rpand IE valuesincrease with increasing TPconcentration.Thiscould mean availability of more inhibitor molecules for adsorption as the concentration increases.

Fig.4.EISplots for St37 steel in 15%H2SO4 and 15%HCl in the absence and presence of different concentrations of TPat 25°Cin(a)Nyquist and(b)Bode representations.

Fig.5.Equivalent circuit diagrams used to fi t impedance data in the(a)blank and(b)presence of TP.

Fig.4 show s the electrochemical impedance spectroscopy(EIS)results obtained for ST37 steel in free 15%HCl and 15%H2SO4solutions and in the acid solutions forti fi ed w ith various concentrations of TP in different formats namely(a,c)Nyquist and(b,d)Bode and Phase angle.In the Nyquist diagrams recorded in the free acid solutions,single capacitive loops often linked to charge transfer corrosion process are observed in the high frequencies and they correspond to one time constant in the Bode and Phase angle representations.In the case of TP inhibited systems,two capacitive loops corresponding to two time constants in Fig.4(b,d)at the high and medium frequencies can be identi fi ed.The two time constants could arise from the double layer structure of the TPadsorbed fi lm on the steel surface[17-19].To this end,a simple RQRequivalent circuit(Fig.5(a))w as used for the fi tting of the Nyquist curves recorded in the uninhibited systems w hile the RQ(RQR)equivalent circuit(Fig.5(b))w as deployed for the analysis of the Nyquist graphs obtained in the TP inhibited acid solutions.

The quite small chi-square values(in the range 19.4×10-5-82.4×10-5)(Table 2)and the low values of fi tting errors associated with the parameters(lessthan 5%in all cases)suggested the fi tted results were reliable.Accordingly,the physical meanings of the elements in Fig.5(b)are as follow;Rsrepresents the solution resistance,Rf1and CPEfstand for the resistance and constant phase element of theouter layer of theadsorbed TP fi lm respectively,Rf2and CPEdldenote the resistance and constant phase element of the inner layer of the adsorbed inhibitor fi lm respectively[18,19].The use of CPE was essential for good quality fi t ow ing to the imperfectness of the capacitive loops[19].The performance of adsorbed inhibitor fi lm can be judged from the polarization resistance(Rp)since inhibition ef fi ciency varies directly w ith fi lm corrosion resistance[19].The parameter Rpis de fi ned as Rp=(ZF)ω=0[18,19],where ZF=faradaic impedance of circuit and ω=angular frequency.According to Wang et al.[19],Rpcan be expressed as the summation of the resistances of the outer and inner fi lm layers(i.e Rp=Rf1+Rf2).For the uninhibited system,Rp=Rf2.Therefore,the corrosion protection performance of the TPadsorbed fi lm w as evaluated making use of Rpvalues according to the follow ing equation:

The values of all the parameters associated w ith the evaluated systems from this technique are displayed in Table 2.From the table,it can be deduced that charge transfer process w as dif fi cult in the inhibited systems compared to the uninhibited and this phenomenon may have been caused by the adsorption of TP molecules onto the St37 electrode surface in the inhibited systems.The in fl uence of increase in TPconcentration on the Rf2and Rpof the adsorbed fi lm is obvious in Table 2 and Fig.4.For instance,the diameter of the Nyquist graphs(Fig.4(a,c)),impedance and phase angle(Fig.4(b,d)),the Rf2and Rpvalues increased with increasing TPconcentration.Increase in inhibitor concentration may have resulted in larger surface coverage.Interestingly,the values of Rf1is found to increase w ith increase in TP concentration in H2SO4systems but decreases in HCl systems yet the inhibitor performed better in HCl medium than in H2SO4solution.This may re fl ect the dependency of corrosion protection ability of adsorbed fi lm on the resistive strength of the inner layer than the outer layer.Based on the values of n1and n2(Table 2)w hich are near unity,it is concluded that the interface behaved nearly capacitive since n=0 represents a pure resistor,n=-1 infers inductor,and n=+1 denotes a pure capacitor[7,11].

To further con fi rm the corrosion inhibition effect of TPon St37 steel in the considered acid solutions,electrochemical frequency modulation(EFM)w as undertaken.The beauty of EFM lies on the inherent data validation control through the use of causality factors(CF-2 and CF-3).As a rule,EFM results are adjudged valid only w hen the values of CF-2 and CF-3 are w ithin the range 0-2 and 0-3 respectively[4,20].The representative EFM spectra for ST37 electrodes in the studied media are show n in Fig.6.Tw o sets of peaks characterized the spectra—intense and clouded peaks.The intense peaks represent the harmonic and intermodulation signals w hile the clouded bands represent the background noise signals[4,21].Only the harmonic and intermodulation peaks were therefore selected for the computation of the corrosion parameterslisted in Table 3.Thevaluesof Icorrand CR in thetablevary in similar manner asthose in Table 1 with TPconcentration.Again,bothβaandβcvalues are affected with increasing TPconcentration supporting the mixed type behavior of TP earlier proposed.The CF-2 and CF-3 values are w ithin the acceptable limits and thus valid the obtained EFM results.There is good agreement betw een the IE values obtained from this technique with those from other electrochemical techniques(Tables 1&2).

Table 2 Impedance parameters for St37 steel in 15%HCl and 15%H2SO4 w ithout and w ith different concentrations of TPat 25°C

Fig.6.Intermodulation spectrum recorded for St37 steel in 15%HCl solution in absence and presence of different concentrations of TPat 25°C.

Table 3 Electrochemical frequency modulation parameters for St37 steel in 15%HCl and 15%H2SO4 in the absence and presence of different concentrations of TPat 25°C

Fig.7.Potentiodynamic polarization curvesfor St37 steelin(a)15%H2SO4 and(b)15%HCl in the absence and presence of TP(1000 mg·L-1)and TP+KImixtures at 25 °C.

3.3.Effect of KIon TPperformance

In a couple of our previous reports[22-24],w e had demonstrated the enhanced corrosion inhibition of polymers by iodide ions due to their strong chemisorption ability.In this present w ork,it was reasonable to attempt a strategy through w hich the corrosion inhibition strength of TPcan be improved ow ing to thefact that,theoptimum concentration of the polymer(1000 mg·L-1)could only afford 55.5%and 63.4%corrosion inhibition to St37 steel in 15%HCl and 15%H2SO4media respectively.In this regard,w e added small amounts(1 mmol·L-1and 5 mmol·L-1)of KIto the optimum concentration of TPand theeffect of thisaddition on theinhibition ef fi ciency w asstudied using PDP,LPR,EIS,and EFM.Fig.7 show s the comparative PDP graphsof St37 steel in(a)15%HCl and(b)15%H2SO4solutionswithout and containing 1000 mg·L-1TP,1000 mg·L-1TP+1 mmol·L-1KI,and 1000 mg·L-1TP+5 mmol·L-1KIrespectively.All the polarization parameters derived from the graphs are given in Table 4.Obviously,the addition of iodide ions to TP w as a bene fi cial strategy.In Fig.7 and Table 4,it is very clear that the addition of KIto TPresulted in signi fi cant reduction in the corrosion current densities than that of TP alone.This reduction manifested in higher corrosion protection.For instance,on addition of 1 mmol·L-1KI and 5 mmol·L-1KI to 1000 mg·L-1TP,the Icorrvalues of 35.2 μA·cm-2and 183.0 μA·cm-2for TP alone in H2SO4and HCl solutions respectively w ere drastically reduced to 4.1μA·cm-2and 2.9 μA·cm-2in H2SO4medium and 78.3 μA·cm-2and 76.4μA·cm-2in HCl solution.The inhibition effi ciency w as upgraded from 44.8%in H2SO4solution to 93.5%and 95.4%on addition of 1 mmol·L-1KIand 5 mmol·L-1KIrespectively.Similarly,the IE of TPin HCl w as raised from 71.8%to 87.9%and 88.2%by the addition of 1 mmol·L-1KIand 5 mmol·L-1KIrespectively.This is a clear indication that the TP+KIadsorbed protective fi lm covered greater surface area than TPadsorbed fi lm and probably w ere more stable.A close inspection of Fig.7(a)reveals that TP+KImixture suppressed cathodic reactions more than the anodic reactions.In the anodic branchesof the TP+KIgraphsin Fig.7(b),at corrosion potential nobler than-250 m V/(Ag/AgCl),the inhibition effect of the mixture seemsto declineand remain near constant for aw iderange of potential.For instance,the graphs appear somew hat parallel to the x-axis.Qian et al.[25]had reported similar observation and w as interpreted to mean desorption potential w here the desorption rate of the adsorbed fi lm is higher than the adsorption rate.By comparing the IE values of TP+KI in HCl systems to those in H2SO4solutions(Table 4),it is found that the values are higher in H2SO4than HCl which is a reverse of w hat w as observed in the case of TPalone(Tables1-3).The possible cause of this reversal is explained in Section 3.6.In Table 4 is also presented the Rp,CR,and IE values obtained from LPR measurements for the studied substrate and systems.In both acid media,the polarization and corrosion rate of the substrate are greatly reduced by TP+KImixture but the effect is more in H2SO4medium than HCl and thus agree w ith the PDP fi ndings.IE of 94.9%and 95.8%are obtained from this technique for TP+1 mmol·L-1KIand TP+5 mmol·L-1KIrespectively in H2SO4solution w hile the values are 88.7%and 89.3%in HCl environment.

Table 4 Potentiodynamic polarization(PDP)and Linear polarization resistance(LPR)parametersfor St37 steel in 15%HCl and 15%H2SO4 without,with 1000 mg/LTP,KIand TP+KImixturesat 25 oC.

The Nyquist,Bode,and Phase angle plots for St37 steel in(a,b)15%HCl and(c,d)15%H2SO4solutions w ithout and w ith additives are show n in Fig.8.The equivalent circuit in Fig.5(b)w as used for the analysis of the TP+KINyquist graphs.All the parameters pertaining to these systems are given in Table 5.As could be seen in Fig.8(a,c),the semicircles at high frequencies in the TP+KINyquist graphs are outstandingly larger than those of TPalone and the second loop in the medium frequencies becomes very pronounced.There is also a noticeable displacement in the impedance and phase angle in Fig.8(b,d)on the addition of KIto TP.All theseindicate the enhancement in corrosion inhibition ef fi ciency of TPby KI.The results in Table 5 reveals that the polarization resistance of St37 steel in the acid solutions was increased by a factor of more than 10 on the introduction of KIinto the corrosive systems containing 1000 mg·L-1TP.It can be concluded from the results in Table 5 that in 15%H2SO4environment,ST37 steel surface can be protected against corrosion by 93.4%and 93.5%while in 15%HCl,corrosion protection of 79.5%and 89.7%can be achieved on the addition of 1 m M KIand 5 m M KIrespectively to 1000 mg·L-1TP.

Fig.9 shows the EFM spectra recorded for St37 steel in strong acid media w ithout and w ith various additives as corrosion inhibitor and the parameters deduced from the analysis of the spectra are presented in Table 6.The obtained CF-2 and CF-3 values are w ithin the acceptable limit and the Icorrvalues are in perfect agreement w ith those derived from PDP experiments.There is also clear evidence of improved TP corrosion inhibition ef fi ciency by KIaddition.The IE values from all the electrochemical techniques are in good agreement and point to synergistic corrosion inhibition betw een TPand iodide ions.To determinewhether theobserved improvement in thecorrosion inhibition effi ciency of TPupon combination w ith iodide ions is due to synergistic effect,w e calculated the synergism parameter(S1).As it is know n,iodide ions and organic inhibitors can co-adsorb on a metal surface in tw o w ays;competitive or cooperative[26,27].In competitive coadsorption,iodide ions and organic cations adsorb on different sites in the metal surface[26]w hereasin cooperative co-adsorption,the iodide ions fi rst chemisorb on the metal surface and theorganic cationsadsorb on iodide ions adsorbed layer[26].Competitive co-adsorption leads to antagonistic effect w hile cooperative co-adsorption gives rise to synergistic effect[26,28].These tw o forms of co-adsorption can be differentiated using the synergism parameter.A value of S1＜1 signi fi es antagonistic effect w hile S1＞1 points to synergistic effect[26-28].The synergism parameter can be calculated using the follow ing equation[26,29]:

Fig.8.EISplots for St37 steel in 15%H2SO4 and 15%HCl in the absence and presence of TP(1000 mg·L-1)and TP+KImixtures at 25 °Cin(a)Nyquist and(b)Bode representations.

Table 5 Impedance parameters for St37 steel in 15%HCl and 15%H2SO4 without,with 1000 mg·L-1 TPand TP+KImixtures at 25 °C

Fig.9.Intermodulation spectrum recorded for St37 steel in 15%HCl solution in the absence and presence of TP(1000 mg·L-1)and TP+KImixtures at 25 °C.

Table 6 Electrochemical frequency modulation parameters for St37 steel in 15%HCl and 15%H2SO4 in the absence,presence of 1000 mg·L-1 TP,KIand TP+KImixtures at 25 °C

Table 7 Calculated values of the synergism parameter(S1)from the various methods used

Table 8 Langmuir adsorption parameters for St37 steel in 15%HCl and 15%H2SO4 containing TP from different experimental methods at 25°C

where IE1is the inhibition ef fi ciency of iodide ions,IE2is the inhibition ef fi ciency of the inhibitor,IE1+2is the inhibition ef fi ciency of inhibitor+iodide ions.In our case,the calculated S1values(Table 7)areall greater than unity hencew econclude that theobserved improvement on the corrosion inhibition ef fi ciency of TPby iodide ionsisdue to synergistic effect.

3.4.Adsorption isotherm consideration

The use of adsorption isotherm to probe the adsorption characteristics of organic inhibitors is still a valid model[30-32].The adsorption of organic inhibitor on metal surface depends on the degree of surface coverage(θ)[30].The degree of surface coverage(θ)at different concentrations of TPon the St37 steel surface w as calculated from the IE values(θ=IE/100)obtained from LPR,EIS,and EFM experiments(Tables 1-3).By virtue of the linear regression coef fi cient(R2)value w hich w as approximately 1 in all cases(Table 8),the Langmuir adsorption isotherm w as selected as the best fi t isotherm for the studied systems.It has the general form:

Fig.10.Langmuir adsorption isotherm for St37 steel in(a)15%H2SO4 and(b)15%HCl containing TPfrom different experimental techniques.

w here Cinhisthe inhibitor concentration and Kadsistheequilibrium constant for adsorption-desorption process[14].The linear relationship for TPadsorption onto St37 steel surface obtained by plotting Cinh/θversus Cinhis shown in Fig.10.

As should be expected,the slopes of thegraphs(Table 8)slightly deviated from unity required by an ideal Langmuir model indicating the existence of interaction in the adsorbed layer.Langmuir adsorption equation had been derived on the assumption that adsorbed molecules do not interact with each other[7].This is not true as authors[7,23,33]have demonstrated that macromolecules like TPin adsorbed layer are capable of interaction.The Kadsvalue wascomputed from the intercepts of the graphs in Fig.10 and as could be seen in Table 8,the value is low(i.e.between(1.00-4.18)× 10-2L·g-1)demonstrating low proportion of TPadsorbed on the St37 steel surface[14,34,35].It also suggests less adherence of the TPadsorbed fi lms on the metal surface.The higher the Kadsvalue,the tighter the adsorbed inhibitor fi lm on the metal surface and the better the inhibitive ability[31].To obtain information on the spontaneity of the adsorption process,the adsorption free energy(ΔG0ads)w as computed making use of the Kadsvalue according to the following equation[31,36]:

Fig.11.SEM micrographsfor St37 steel(a)in thepolished state(b)exposed to 15%HCl,(c)exposed to 15%H2SO4,(d)exposed to 15%HCl containing 1000 mg·L-1 TP,(e)exposed to 15%H2SO4 containing 1000 mg·L-1 TP,(f)exposed to 15%HClcontaining 1000 mg·L-1 TP+5 mmol·L-1 KIand(d)exposed to 15%H2SO4 containing1000 mg·L-1 TP+5 mmol·L-1 KIafter 24 h.

w here Cwateris the w ater concentration in the solution=1000 g·L-1,R is the molar gas constant,and T is the absolute temperature.The calculatedΔG0adsvalue from the various experimental methods are also given in Table 8.The value is negative and is indicative of spontaneous adsorption of TPmolecules and stability on the St37 steel surface in the studied corrosive media[13,37].

3.5.Surface analysis

Fig.11 show s the surface morphologies of St37 steel samples(a)before and after immersion in(b)15%HCl solution,(c)15%H2SO4solution,(d)15%HCl+1000 mg·L-1TP,(e)15%H2SO4+1000 mg·L-1TP,(f)15%HCl+1000 mg·L-1TP+5 mmol·L-1KI,and(g)15%H2SO4+1000 mg·L-1TP+5 mmol·L-1KIsolution for 24 h at 25 °C.Apparently,the smooth morphology of the specimens(Fig.11(a))is completely lost to corrosion upon exposure to the acid solutions.The surfaces thereafter exhibit rough and hilly-shaped morphologies(Fig.11(b&c)).Compared w ith the surface in Fig.11(c),the one in Fig.11(b)is rougher inferring that St37 steel sample deteriorated in 15%HCl solution than in 15%H2SO4solution.Undoubtedly,the metal samples w ere protected against corrosion in the acid solutionscontaining TPas evidenced in the relatively smoother surfaces in Fig.11(d&e)compared to the ones in Fig.11(b&c).It w as deduced from experimental results(Tables 1-3)that TPretarded St37 steel dissolution in HCl medium than in H2SO4environment.With asmoother surface morphology asobserved in Fig.11(d)than in Fig.11(e),the surface analysis resultsagree with the experimental results.Again,it is obviousthat addition of KIto TPenhanced the corrosion inhibition ef fi ciency of TPin the acid environments and the effect is higher in H2SO4environment than in HCl medium.Clearly,the image in Fig.11(g)presents smoother morphology than the one in Fig.11(f)but the surfaces in Fig.11(d&e)are rougher than the surfaces in Fig.11(f&g).Based on the experimental and surface analysis results,it is concluded that TP+KIcombination offers a good corrosion protection to St37 steel in strong acid environments.

3.6.Mechanism of corrosion inhibition

Inhibitorsin acid solution can interact w ith metalsand affect thecorrosion reaction in a number of w ays,some of w hich may occur simultaneously.It ispractically impossibleto assign asingle general mechanism of action to an inhibitor because the mechanism may change w ith experimental conditions.Thus,the predominant mechanism of action of an inhibitor may vary w ith factors such as its concentration,the p H of theacid,thenatureof theanion of theacid,thepresenceof other species in the solution,the extent of reaction to form secondary inhibitors,and the nature of the metal[30,38,39].Our experimental results reveal that TPinhibits St37 steel corrosion better in HCl solution than H2SO4solution but TP+KImixtureisabetter inhibitor in H2SO4than HCl solution.Generally,metal corrosion inhibition by adsorption mechanism is a quasi-substitution processand could be by physisorption or chemisorption[40,41].TP in the strong acids could be protonated and there is every tendency that St37 steel surface acquires net positive charge.Adsorption of protonated forms of TPonto charged St37 steel surface w ould be greatly in fl uenced by the anions present in the solution.As it is known,in HCl solution,chloride ions are speci fi cally adsorbed on metal surface w hile sulphate ions are adsorbed in H2SO4medium[26,28].The chloride ions have a higher shielding pow er than the sulphate ions therefore more of protonated TPshould be expected to adsorbed on the St37 steel surface recharged by chloride ions than one recharged by sulphate ions.Nevertheless,adsorbed protonated moleculescould be deprotonated and the freed heteroatomsgo into coordinate type of bond w ith metal surface[12,42].Such complex formation w ould lead to better corrosion protection[12,42].To gain insight into the corrosion inhibition of St37 steel in HCl and H2SO4media by TP and TP+KI and to explain w hy TP+KI better inhibits in H2SO4solution than HCl,FTIRand UV-vis experiments w ere undertaken.Fig.12 show s the FTIRspectra of pure TP,TP fi lm,and TP+KI fi lm extracted from St37 steel surface after immersion in(a)HCl solution and(b)H2SO4medium.The spectrum of the pure TPis characterized w ith peaks at 3500 cm-1,3100 cm-1,1850 cm-1,1613.2 cm-1,1320 cm-1,1282 cm-1,and 1000 cm-1.They correspond to O--H[7,11],N--H[43],C═O[15],S═O[44],C--O[45],C--N[46],and P--O[45]stretching vibrations respectively.Compared with the TP and TP+KI fi lm spectra in Fig.12(a),the P--Oand S═Opeaks shifted to 1100 cm-1and 1702 cm-1respectively and are more intense,the C--O,C═O,and N--H peaks diminish and are less intense.In Fig.12(b),the N--H peak almost completely disappear w hile the C--Opeak becomes more intense in the TPand TP+KIspectra compared to the pure TP.All these re fl ect the involvement of the heteroatoms in the adsorption process.How ever,the adsorption mechanism seems to be different for TPand TP+KIin HCl and H2SO4solutions.For instance,the N--H peak is almost absent in the TPand TP+KIspectra in Fig.12(b)but not so in Fig.12(a).Also,the C--O peak is stronger in the TPand TP+KIspectra in Fig.12(b)than in Fig.12(a)and new peak emerged in the TP+KIspectrum at 2500 cm-1in Fig.12(b).The UV-vis spectra presented in Fig.13 give a clearer picture to the adsorption processes in the studied systems.The solutions resultingfrom St37 steel immersion in theacid solutionsyielded spectra w ith tw o bands,oneat 250 nm and theother at 300 nm,w hilethespectrafor 15%HCl containingtheadditivesshow sno band.Thismeansthat,in thepresence of both TPand TP+KI,therew asno complex formation.The adsorbed inhibitor fi lms w ere held on the metal surface by electrostatic force.The non-formation or formation of unstable Fe-inhibitor complex even in the presence of iodide ions in the HCl solution may have been caused by competitive adsorption betw een Cl-and I-ions.As it is know n,in HCl environment,the speci fi c adsorption of chloride ions is on the anodic site while hydronium ions are adsorbed at the cathodic site[39,47,48].Our PDPresults(Fig.7(a))clearly show s that in HCl solution,addition of KIdid not affect the anodic reactions but affected only the cathodic reactions.It means that there w as some sort of opposition from chloride ions to iodide ions on the anodic site and this may have resulted in formation of unstable or non-formation of complex at all.In the Fe2++TP+KIspectrum of Fig.13(b),three clear bands at 250 nm,300 nm,and 350 nm can be identi fi ed.The appearance of the new peak at 350 nm justify the formation of Feinhibitor complex.Similar submission can be found in the corrosion literature[15].Thisnew band correspondsto the extrapeak at 2500 cm-1in Fig.12(b).The complex so formed in H2SO4solution ensured better corrosion mitigation than in HCl solution.It should also be mentioned that iodide ions suffered less opposition in H2SO4solution asevidenced in the PDPresult in Fig.7(b);both the anodic and cathodic reactionsare affected by KIaddition.

Fig.12.FTIRspectra of pure TP,and fi lm extracted from St37 surface immersed in(a)15%HCl and(b)15%H2SO4 containing TPand TP+KImixtures.

Fig.13.UV-Vis spectra of(a)15%HCl and(b)15%H2SO4 solutions w ithout and w ith 1000 mg·L-1 TPand 1000 mg·L-1 TP+KImixtures after St37 steel immersion for 24 h at 25 °C.

4.Conclusions

A novel tetrapolymer(TP)consisting of carboxylate,sulphonate,phosphonate and sulfur dioxide based co-monomers has been successfully synthesized using Butler cyclopolymerization technique and characterized using FTIR,1H-NMR,13CNMRand elemental analysis.The tetrapolymer is a moderate corrosion inhibitor for St37 steel in 15%HCl and 15%H2SO4acid media but better in HCl medium than in H2SO4solution.Addition of small amount of KIto thetetrapolymer synergistically enhanced TPcorrosion inhibition ef fi ciency.5 mmol·L-1KI upgraded TPcorrosion inhibition ef fi ciency from 61.1%and 79.5%in H2SO4and HCl solutions respectively to 93.5%and 90.6%according to the EISresults.TP and TP+KIafforded the corrosion inhibition of St37 steel in the acid media by virtue of adsorption mechanism according to ATR-FTIRanalysis results and the adsorption process follow ed Langmuir adsorption isotherm.TP+KIformed complex with St37 steel surface in H2SO4solution but not in HCl solution.

Acknowledgements

The authorsgratefully acknow ledged the fi nancial support provided by Imam Abdulrahman Bin Faisal University(IAU)through project number:2016-237-Eng.Some research facilities w ere provided by the Centre of Research Excellence in Corrosion;KFUPM is thankfully acknowledged.