Modeling of Corrosion Mechanisms in the Presence of Quaternary Ammonium Chloride and Imidazoline Corrosion Inhibitors ABSTRACT INTRODUCTION

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1 Pper No. 746 Modeling of Corrosion Mechnisms in the Presence of Quternry Ammonium Chloride nd Imidzoline Corrosion Inhibitors J.M. Domínguez Olivo, B. Brown, S. Nesic Institute for Corrosion nd Multiphse Technology. Ohio University Athens, OH 4571 ABSTRACT The mitigtion of corrosion in crbon steel pipelines due to the ddition of corrosion inhibitors hs trditionlly been described by using dsorption isotherms. Consequently, current models of corrosion mitigtion by inhibitors re bsed on the use of dsorption isotherms to predict surfce coverge, inhibitor efficiency nd ultimtely the corrosion rte s function of inhibitor concentrtion. However, coverge does not properly describe the underlying electrochemicl mechnisms, nor cn it predict the resulting chnge in corrosion potentil. The gol of this reserch is to nlyze nd explin how the underlying electrochemicl rections re ffected by the presence of dsorbed corrosion inhibitor nd the shift in corrosion potentil tht occurs. Two CO 2 corrosion inhibitors re studied here: til oil ftty cid / diehylenetrimine imidzoline nd quternry lkyl benzyl dimethyl mmonium chloride. A mechnistic model ws developed bsed on electrochemicl kinetics nd by using mitigtion fctor, θ, which ccounts for the overll retrdtion in the nodic nd cthodic rections. It ws found tht the retrdtion of the electrochemicl rections ffected by these inhibitor cn be modeled by using single prmeter: surfce coverge fctor. Key words: corrosion inhibitors, mechnistic model, mitigtion. INTRODUCTION The oil nd gs industry hs long considered the use of corrosion inhibitors s n effective nd ffordble method to mitigte corrosion. 1 Most literture chrcterize inhibitor effectiveness through the use of surfce coverge fctor. 2 4 A corrosion inhibitor is chemicl substnce tht significntly reduces the corrosion rte in certin environments when it is dded in smll concentrtions 2. Much of the knowledge bout corrosion inhibition for specific environments hs been obtined by simulting the operting field conditions in the lbortory nd mesuring the inhibition efficiency of different corrosion inhibitors 2 4. Engineers hve used inhibition efficiency to develop mthemticl models bsed upon dsorption isotherms by ssuming tht the inhibitors cover the metl surfce s uniform thin film nd this coverge is directly proportionl to the inhibition efficiency 3,5 8 This pproch to describing nd modeling inhibitor effectiveness is helpful, but there is need to develop more mechnistic pproch to modeling tht requires more detiled understnding of the dsorption nd electrochemicl mechnisms underlying corrosion inhibition. 1

2 The most widely ccepted clssifiction of corrosion inhibition mechnisms ws proposed by Lorenz 9 nd corroborted by the methodology proposed by Co 1. In this clssifiction, the open circuit potentil is the min prmeter. There re three possible effects of the corrosion inhibitor: Blockge effect: A surfce blocking effect is present when n inert dsorbed species ( species which does not reduce or oxidize) covers significnt frction of the corroding surfce nd thereby retrds both cthodic nd nodic kinetics eqully, so the open circuit potentil (corrosion potentil, OCP) does not chnge significntly. Preferentil retrdtion of nodic or cthodic rection: Preferentil dsorption occurs when the corrosion inhibitor fvorbly dsorbs onto either the cthodic or the nodic sites ffecting kinetics differently nd this results in chnge of the corrosion potentil. The corrosion potentil shifts into the positive direction when the inhibitor ffect the nodic rection more nd shifts in the negtive when the inhibitor ffect the cthodic rection preferentilly. Electroctlytic effect: The corrosion inhibitor interferes with the electrochemicl mechnism of the corrosion process nd the Tfel slope of ny given rection my chnge. The open circuit potentil my or my not chnge when n electroctlytic effect is present. However, this reltively esy to understnd clssifiction, bsed upon the chnge of the corrosion potentil, is not helpful when the corrosion process is under mixed or mss trnsfer control. 11 This is becuse the dsorption of the corrosion inhibitor redily ffects the chrge trnsfer rections (which re surfce phenomen) but does not ffect the diffusion of species (which is bulk electrolyte phenomenon). The sme holds true when corrosion is controlled by slow homogenous chemicl rection rte, such s for exmple CO 2 hydrtion. Consequently, the chnge of the open circuit potentil in CO 2 corrosion is not directly relted to the retrdtion of the individul electrochemicl rections, s illustrted in the schemtic in Figure 1. In tht prticle scenrio, positive shift on the corrosion potentil is obtined even if it is ssumed tht the blockge effect produces equl retrdtion of the nodic nd cthodic chrge trnsfer rection, wht is t odds with the Lorenz/Co clssifiction, which would suggest tht this is inhibitor retrds the nodic rection preferentilly. Mny subsequent studies hve used this methodology to interpret the effect of corrosion inhibitors on the chnge of the nodic nd cthodic corrosion mechnisms 9,1,12,13. However, this ws not done rigorously nd in most cses the overll effect of the corrosion inhibitor ws loosely qulified s being nodic or cthodic, bsed upon the chnge of the corrosion potentil 3, However, s rgued bove, this my led to incorrect conclusions bout the inhibition mechnisms. In order to void such errors in the interprettion of results, the work presented below proposes somewht different methodology to qulify nd quntify the retrdtion of corrosion due to the presence of n dsorbed corrosion inhibitor 17,18. 2

3 Potentil + (E corr ) ΔE E corr - (i corr ) i corr Current density Figure 1: A hypotheticl corrosion kinetics digrm illustrting the surfce blockge effect when equl retrdtion of chrge trnsfer processes is seen (both nodic nd cthodic), but one tht produces net positive chnge of potentil (ΔE) becuse the uninhibited cthodic rection is under mss trnsfer control. Solid lines: uninhibited rections. Dshed lines: inhibited rections. Ecorr: corrosion potentil of uninhibited rections, (Ecorr ) : corrosion potentil of inhibited rections, icorr : corrosion current density of uninhibited rections, (icorr ) : corrosion current density of inhibited rections. EXPERIMENTAL PROCEDURE A three electrode electrochemicl system ws set up in 2-liter glss cell nd used to perform series of experiments t 1 br, ph 4 nd 3 C. In 1 wt % NCl solution. An API 5L X65 steel rotting cylinder electrode (RCE) t 1 rpm ws used s the working electrode, s shown in Figure 2. The composition of the steel is shown in Tble 1. A pltinum covered titnium mesh ws used s the counter electrode nd n Ag/AgCl reference electrode ws used s the reference electrode connected with the cell using Luggin cpillry. Two different gses (N 2 or CO 2) were used for purging the system to remove dissolved oxygen nd sturte the solution. The solution ph ws mesured using glss electrode immersed directly in solution nd mintined t ph 4.±.1 during ech experiment. Liner polriztion resistnce (LPR) ws used to obtin corrosion rtes by polrizing the working electrode ±5 mv from the corrosion potentil (using B = 26 mv). Electrochemicl impednce spectroscopy (EIS) ws used for mesuring solution resistnce by employing n oscillting potentil ±5 mv with respect to the corrosion potentil nd using frequency rnge between 5mHz to 5KHz. Potentiodynmic polriztion ws pplied t the end of ech experiment t rte of 1.25 mv/s in order to obtin informtion bout the electrochemicl mechnisms underlying corrosion (cthodic nd nodic rections). The cthodic curve ws obtined by polrizing from the open circuit potentil to pproximtely -.5 V below. After witing for the corrosion potentil to return to the originl open circuit potentil, nother potentiodynmic polriztion ws performed in the positive direction for +.3 V, in order to obtin the nodic curve. Ech potentiodynmic curve ws corrected with the ohmic drop cused by the solution resistnce to show the relevnt current vs. voltge reltionship. Two different corrosion inhibitor pckges were tested nd nlyzed: quternry lkyl benzyl dimethyl mmonium chloride bsed corrosion inhibitor (referred to s qut in the text below) nd til oil ftty cid / diethylenetrimine (TOFA/DETA) imidzoline bsed corrosion inhibitor (referred to s + 3

4 imidzoline ). Concentrtions were selected bsed on their respective criticl micelle concentrtions in 1 wt% NCl solution: 11 ppm v/v for the qut t 36 ppm v/v for the imidzoline. The formultion of the corrosion inhibitor pckges is shown in Tble 2 nd the structure of the ctive components in the inhibitor pckges re shown in Figure 3 nd Figure 4. The experimentl conditions re shown in Tble Reference electrode (RE Sturted Ag / AgCl) 2. Rotting Cylinder Electrode (WE) 3. Counter electrode (CE) 4. Luggin Cpillry 5. ph meter 6. Thermocouple 7. Gs in 8. Gs out 9. Mgnetic Stirrer Figure 2: Three electrode set up used to perform experiments. Tble 1: Chemicl Composition of the X65 Steel Used s Working Electrode Elements Composition Cr Mo S V Si C Ni Mn P Fe Weight % Blnce Tble 2: Chemicl Composition of the Corrosion Inhibitor Pckges Description Active ingredient Components Generic inhibitor Generic inhibitor quternry lkylbenzyl dimethyl mmonium chloride til oil ftty cid / diethylenetrimine (TOFA/DETA) imidzoline 24% Alkylbenzyl dimethyl mmonium chloride Blnce wter 1% CH3COOH 13% C4H9OCH2CH2OH 24% TOFA/DETA imidzoline Blnce wter 4

5 Figure 3: Structure of quternry lkylbenzyl dimethyl mmonium chloride Figure 4: Structure of til oil ftty cid / diethylenetrimine TOFA/DETA imidzoline. Corrosion mitigtion efficiency Tble 3: Experimentl Conditions Description Prmeters Test mteril API 5L X65 Working solution 1 wt% NCl Purged gs N 2,.96 br CO 2 Temperture 3 C ph 4 Corrosion inhibitors none (bseline), qut, imidzoline Test durtion 2-12 hours (stbiliztion of corrosion rte) Mesurement methods LPR, EIS, potentiodynmic polriztion RESULTS AND DISCUSSION LPR corrosion rtes over 7-hour experimentl time period for the environmentl conditions defined in Tble 3 re shown in Figure 5 nd Figure 6. When the corrosion rte did not significntly chnge over time (less thn ±.1 mm/yer between mesurements), the corrosion mitigtion efficiency ws clculted. In order to clculte the corrosion mitigtion efficiency (ε), Eqution (1) ws used: CR 1 (1) CR Where (CR)θ is the corrosion rte of the system with corrosion inhibitor nd CR represents the corrosion rte of the sme system without the inhibitor present. The finl corrosion rtes nd the corrosion mitigtion efficiency re presented in Tble 4. As it cn be seen, the corrosion mitigtion efficiency for the imidzoline corrosion inhibitor is very high in ech condition tested (more thn 97% in ll cses) while the qut exhibits low corrosion mitigtion efficiency in either N 2 or CO 2 purged systems (75 nd 86% respectively). 5

6 Corrosion rte / (mm/y) Corrosion rte / mm/y Corrosion rte / (mm/y) Corrosion rte / mm/y 1 1 N 2 purged solution N 2 + qut.1.1 N 2 + imidzoline Time / (hours) / Figure 5: LPR mesured corrosion rtes over time for N 2 purged system t 3 C, ph 4, RCE t 1 rpm (B = 26 mv). 1 1 CO 2 purged solution CO 2 + qut.1 CO 2 + imidzoline Time / (hours) Figure 6: LPR mesured corrosion rtes over time for.96 br sturted system t 3 C, ph 4, RCE t 1 rpm nd. (B = 26 mv) 6

7 Potentil / E vs SHE / V Tble 4 : Finl Corrosion Rte (mm/yer) nd Corrosion Mitigtion Efficiency (ε) for Different Tests Purge Gs Blnk (mm/yer) Corrosion Rtes (mm/yer) Corrosion mitigtion efficiency (%) imidzoline qut imidzoline qut N ±.1.5 ±.2.4 ± ± ±.5 CO ±.2.5 ±.1.3 ± ± ±.5 Potentiodynmic polriztion curves The potentiodynmic sweeps for the N 2 purged solution re shown in Figure 7. The qut inhibitor does not led to significnt chnge in the corrosion potentil nor does it seem to chnge the corrosion mechnism, judging by the ppernce of the potentiodynmic sweeps before nd fter ddition of the inhibitor. The imidzoline corrosion inhibitor lso does not significntly modify the corrosion potentil, however, the mechnisms of the cthodic rection seem to chnge from diffusion limiting current to pure chrge trnsfer control. When the sme corrosion inhibitors re dded to CO 2 sturted solution (Figure 8), the sitution chnges. In the presence of qut corrosion inhibitor the open circuit potentil chnged significntly - bout 5 mv (considerbly more thn the experimentl error). From the potentiodynmic sweeps, this ppers to be becuse the iron dissolution (nodic rection) seems to be ffected more, while the cthodic rection is ffected in similr wy s in the N 2 purged solution. On the other hnd, the imidzoline-bsed corrosion inhibitor does not pper to significntly chnge the corrosion potentil, while it seems to ffect the both the cthodic nd the nodic rection proportionlly, nd in both cses much more tht it did for N 2 purged system N 2 + imidzoline N 2 + qut N 2 purged solution Current Density / i / (A/m 2 ) Figure 7: Potentiodynmic sweeps for N 2 purged system t 3 C, ph 4, RCE t 1 rpm. Solid blue line: bseline condition with no inhibitor; dshed ornge line: fter qut inhibitor dded; dotted red line: fter imidzoline inhibitor dded. 7

8 Potentil / E vs SHE / V Figure 8: Potentiodynmic sweeps for.96 br CO 2 sturted system t 3 C, ph 4, RCE t 1 rpm, nd. Solid green line: bseline condition with no inhibitor; dshed ornge line: fter qut inhibitor dded; dotted red line: fter imidzoline inhibitor dded. Mechnistic model In order to develop n nlyticl tool tht cn be used to investigte corrosion mechnisms, it is necessry to mke some ssumptions. In some previous corrosion inhibition studies 3,8 1, reserchers hve relted the retrdtion of the corrosion current density (icorr) with corrosion surfce coverge frction by the inhibitor (θ) by ssuming tht the re covered by the inhibitor does not corrode t ll while the surfce which is not covered corrodes s if the inhibitor is not present 8. This leds to simple reltionship between the inhibition efficiency nd surfce coverge 9,1 : CR 1 CR This llows substitution of the surfce coverge (θ) for efficiency (ε) in Eqution 2, long with substitution of corrosion current icorr for CR, to develop Eqution (3): where: corr CO 2 + imidzoline corr i corr i (3) CO 2 purged solution Current Density / i / (A/m 2 ) i is the corrosion current with surfce coverge by n inhibitor (A/m 2 ). i corr is the corrosion current in the bsence of inhibitor (A/m 2 ). θ is the surfce coverge frction (with vlues from to 1). CO 2 + qut If we further ssume tht this surfce coverge effect is not selective, we cn postulte tht the effective exchnge current density for both the nodic nd cthodic rections re ffected in the sme wy, leding to: (2) 8

9 where: 1 i (4) i i is the effective exchnge current density ffected by coverge of the inhibitor (A/m 2 ). i is the exchnge current density in the bsence of inhibitor (A/m 2 ). This hs been implemented in the electrochemicl CO 2 corrosion model for both cthodic nd nodic rections previously proposed by Nesic nd Zheng 17,18, including the buffering effect recently proposed by Remit 19. For the iron dissolution rection, tht is under chrge trnsfer control, this leds to simple correction of the nodic current density, to include the effect of surfce coverge, θ: ct 1 1 b b i i 1 i (5) where: ct i is the nodic chrge trnsfer current density ffected by coverge of the inhibitor (A/m 2 ). i is the effective nodic exchnge current density ffected by coverge of the inhibitor (A/m 2 ). i is the nodic exchnge current density in the bsence of inhibitor (A/m 2 ). is the nodic overpotentil: Epplied Erev Epp Erev b is the pplied potentil (V). is the reversible potentil (V). is the nodic Tfel slope (V/decde). For the cthodic rection tht hs limiting current density, only the chrge trnsfer portion of the current is ffected by the inhibitor coverge: where: c c i i ct ilim (6) c i is the cthodic current density ffected by coverge of the inhibitor (A/m 2 ). (ilim) is the limiting current density (A/m 2 ). c ct i is the cthodic chrge trnsfer current density (A/m 2 ) given by: c c c c bc c bc i i 1 i 1 ct 1 (7) 9

10 Potentil / E vs SHE / V where: c i is the cthodic exchnge current density in the bsence of inhibitor (A/m 2 ). is the cthodic overpotentil: Erev Epplied c bc is the cthodic Tfel slope (V/decde). By compring this model of corrosion kinetics with experimentl dt, it is possible to determine whether the ssumption tht ws built in is ctully correct, i.e. whether the corrosion inhibitor uniformly ffects the kinetics of both the nodic or cthodic rection or not. Evlution of the model The forementioned electrochemicl model ws compred to experimentl dt. Results indicte tht the in the presence of the qut inhibitor one cn use the sme surfce coverge fctor θ =.75 for both rections (nodic nd cthodic) nd for both the N 2 purged nd CO 2 sturted solution nd get very good fit, s shown in Figure 9 nd Figure 1. The success with use of the sme coverge fctor indictes tht both cthodic nd nodic rections re eqully ffected; consequently there is no preferentil dsorption on nodic sites, s it hd been previously suggested by other reserchers 3,15, even if the corrosion potentil incresed. The increse in potentil cn be ttributed to the chnge of the cthodic process control from limiting current to chrge-trnsfer control, fter the inhibitor ws dded, similrly s ws hypothesized in Figure 1. Furthermore, the fct tht the sme coverge fctor ws used for both the N 2 purged nd CO 2 sturted systems suggests tht the dsorption nd inhibition by the qut corrosion inhibitor ws not ffected by the presence of CO N 2 + Qut qut Current Density / i / (A/m 2 ) Figure 9: Comprison of predicted potentiodynmic sweeps including surfce coverge fctor θ =.75 for N 2 purged system t 3 C, ph 4, RCE t 1 rpm. Dshed ornge line: experimentl results; solid ornge line: model prediction. 1

11 Potentil / E vs SHE / V Potentil / E vs SHE / V CO 2 + qut Current Density / i / (A/m 2 ) Figure 1: Comprison of predicted potentiodynmic sweeps including surfce coverge fctor θ =.75 for.96 br CO 2 sturted system t 3 C, ph 4, RCE t 1 rpm. Dshed ornge line: experimentl results. Solid ornge line: model prediction. Applying the sme methodology for the imidzoline bsed corrosion inhibitor, nd by using θ =.97, the results of compring the clculted nd mesured potentiodynmic sweeps re shown in Figure 11 nd Figure 12. Agin, it cn be concluded tht the corrosion potentil nd corrosion mechnisms were predicted resonbly well by single surfce coverge fctor, suggesting tht this corrosion inhibitor lso does not exhibit ny preferentil effect. However, there seem to be devition of the cthodic rection from the expected behvior, prticulrly t higher overpotentils N 2 + Imidzoline imidzoline Current Density / i / (A/m 2 ) Figure 11: Comprison of predicted potentiodynmic sweeps including surfce coverge fctor θ =.97 for N 2 purged system t 3 C, ph 4, RCE t 1 rpm. Dshed red line: experimentl results; solid red line: model prediction. 11

12 Potentil / E vs SHE / V Figure 12: Comprison of predicted potentiodynmic sweeps including surfce coverge fctor θ =.97 for.96 br CO 2 sturted system t 3 C, ph 4, RCE t 1 rpm. Dshed red line: experimentl results; solid red line: model prediction. Finlly, in terms of corrosion rte nd open circuit potentil, this nlyticl tool cn predict the corrosion rte nd the open circuit potentil of CO 2 nd N 2 purged inhibited systems t ph 4 with resonble ccurcy, s shown in Tble 5 nd Tble 6. Tble 5: Comprison of Prediction of Corrosion Rte by the Mechnistic Model System imidzoline (experimentl) mm/yr imidzoline (model) mm/yr qut (experimentl) mm/yr qut (model) mm/yr N 2.5 ± ±.5.5 CO 2.5 ± ±.5.5 Tble 6: Comprison of Prediction of Corrosion Potentil by the Mechnistic Model System imidzoline (experimentl) mv vs SHE CO 2 + Imidzoline imidzoline Current Density / i / (A/m 2 ) imidzoline (model) mv vs SHE qut (experimentl) mv vs SHE qut (model) mv vs SHE N ± ± CO ± ±

13 CONCLUSIONS An nlyticl tool ws developed tht cn be used to quntify the chnge in electrochemicl corrosion kinetics due to the presence of corrosion inhibitor. When pplied to systems involving quternry mmonium chloride inhibitor nd n imidzoline inhibitor, it ws found tht: 1. There ws no preferentil dsorption of either of the inhibitors on the electrode surfce nd the sme corrosion mitigtion fctor, θ, could be used to model the retrdtion effect on the nodic nd cthodic rection. 2. The chnge of the corrosion potentil following inhibition cnnot be used s meningful indictor of the mechnism of corrosion inhibition, nd its preferentil nture. 3. The model cn predict the corrosion rte nd the open circuit potentil in CO 2 nd N 2 purged inhibited systems t ph 4 with resonble ccurcy. 4. A devition of Tfel slopes is seen in potentiodynmic cthodic sweeps for the imidzoline-bsed inhibited t lrge overpotentils, which ws not ccounted for. REFERENCES 1. D.A. Jones, Principles nd prevention of corrosion, McMilln, New York, p S. Ppvinsm, Corrosion Inhibitors, in: R.W. Revie (Ed.), Uhlig s Corros. Hndb., Second Edi, Ottw, 2: pp S. Nešić, W. Wilhelmsen, S. Skjerve, S.. Hesjevik, "Testing of inhibitors for CO2 corrosion using the electrochemicl techniques," Proc. 8th Eur. Symp. Corros. Inhib. (8 SEIC). 1 (1995): pp V.S. Sstri, Green Corrosion Inhibitors, 2nd ed., John Wiley & Sons, Hoboken, NJ, USA, 211. p D.M. Bstids, P.P. Gómez, E. Cno, "The isotherm slope. A criterion for studying the dsorption mechnism of benzotrizole on copper in sulphuric cid," Rev. Metl. 1 (25): pp S. Pri, K.C. Khilr, "A review on experimentl studies of surfctnt dsorption t the hydrophilic solid-wter interfce," Adv. Colloid Interfce Sci. 11 (24): pp R. Atkin, V.S.J. Crig, E.J. Wnless, S. Biggs, "Mechnism of ctionic surfctnt dsorption t the solid-queous interfce," Adv. Colloid Interfce Sci. 13 (23): pp L.M. Vrcr, D. Drzic, "Adsorption nd corrosion inhibitive properties of some orgnic molecules on iron electrode in sulfuric cid," Corros. Sci. 44 (22): pp W.J. Lorenz, F. Mnsfeld, "Interfce nd Interphse Corrosion Inhibition," Electrochim. Act. 31 (1985): pp C. Co, "On Electrochemicl Techniques For Interfce Inhibitor Reserch," Corros. Sci. 38 (1996): pp S. Nešić, "Key issues relted to modelling of internl corrosion of oil nd gs pipelines A review," Corros. Sci. 49 (27): pp M.A. Veloz, I.G. Mrtínez, "Effect of Some Pyridine Derivtives on the Corrosion Behvior of Crbon Steel in n Environment Like NACE TM177," Corros. Sci. 62 (26): pp

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