A New Aproach for Monitoring Corrosion Flow Characteristics in Oil Brine Mixtures 2005

Electrochimica Acta 50 (2005) 2081–2090

A new approach for monitoring corrosion and flowcharacteristics in oil/brine mixtures

H. Bouazaze, F. Huet∗, R.P. Nogueira1

UPR15 du CNRS “Interfaces et Syst`emes Electrochimiques”, Universit´e Pierre et Marie Curie, 4, Place Jussieu, 75252 Paris Cedex 5, France

Received 9 June 2004; received in revised form 4 August 2004; accepted 21 August 2004

Abstract

Monitoring of corrosion processes and hydrodynamic flow pattern in oil/brine mixtures of various compositions between 0 and 80% involume of oil has been investigated by means of electrochemical impedance and noise measurements. In such two-phase electrolytes theelectrode behaviour is mostly characterized by the fact that the state of the corroding metallic surface evolves in a highly dynamic way asa result of local heterogeneities yielded by the non-uniform phase distribution near the electrode. This is why conventional electrochemicalc k sensitivityt e shown tob tion. Suchm©

K

1

odccamwcdctti

ce ofristic.owsseenuseofbleand

entlyico-ents.siclativeftenons,encyraffinofoil

0d

urrent and/or potential noise, as well as electrochemical impedance measurements, showed very poor reproducibility and weao electrolyte composition and flow changes. In contrast to conventional techniques, the electrolyte resistance fluctuations have a highly sensitive and reliable tool conveying real-time information about the two-phase flow pattern and electrolyte composieasurements can be an indirect way to evaluate the environment aggressiveness in industrial plant and field applications.2004 Elsevier Ltd. All rights reserved.

eywords:Electrochemical noise; Electrolyte resistance; Conductivity; Oil/water; Two-phase flow; Monitoring

. Introduction

Monitoring of corrosion processes in industrial plants andther field applications is an important raising research topicue to its technical and economic impacts. This is clearly thease of the petroleum industry, in which the consequences oforrosion can be particularly severe. Production, transport,nd transformation of multiphase electrolytes (usuallyixtures of more or less aggressive aqueous and oily phasesith or without the presence of gaseous components) involveomplex environments in which corrosion monitoring is aifficult task. Several authors have attempted to investigateorrosion processes in multiphase electrolytes[1–7], buthe conventional electrochemical impedance and noiseechniques employed in these works have shown to be rathernadequate to take into account the combined effect of the

∗ Corresponding author. Tel.: +33 1 44274136; fax: +33 1 44274074.E-mail address:[email protected] (F. Huet).

1 ISE member.

electrolyte corrosiveness and the concomitant presenother phases, often presenting a protective characteIt must be recognized that corrosion in multiphase flis actually a complex process. The oily phase can beas forming a protective layer inhibiting corrosion becaof its very low conductivity and the very low solubilitycorrosion products in oil. However, oxygen is more soluin the oil phase, which favours the cathodic reaction,consequently the overall corrosion reaction[1].

An alternative approach of the problem has been recproposed[8], which takes advantage of the distinct physchemical characteristics of the multiphase constituMore precisely, in oil/water based solutions the intrinresistivity of each phase associated to the continuous removement of the phases in the hydrodynamic flow – oturbulent – entails electrolyte resistance (ER) fluctuati�Re, that can be investigated both in the time and frequdomains. These fluctuations have been measured for paoil/brine – 3 wt.% sodium chloride (NaCl) – solutionsdifferent compositions between 0 and 33% of volumetric

013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2004.08.051

2082 H. Bouazaze et al. / Electrochimica Acta 50 (2005) 2081–2090

content (VOC), defined as the ratioφ = oil volume/solutionvolume. The ER has been shown to be a highly sensitivequantity that can provide reliable information about theflow pattern and composition, two parameters that stronglyinfluence the corrosion intensity. Taken into account thefact that the oily phase is less aggressive than the 3% NaClsolution, the intrinsic average aggressiveness of the mixedfluid can be roughly but reasonably estimated at any time inturbulent flows from the electrolyte composition assessed bymeans of the ER fluctuations. In laminar flows, ER fluctua-tions are prone to vanish because of phase segregation; suchinformation is also interesting since phase segregation islikely to favour corrosion in the lower part of the pipeline thatis in contact with the aggressive phase. Hence, by providinginformation about both the flow pattern and composition ERmeasurements appear as a way of indirectly estimating thereal-time corrosiveness of the solution.

The ER measurements were revealed to be much moreinformative than the conventional electrochemical current orpotential noise measurements, widely used in corrosion mon-itoring applications, that were poorly sensitive to changes inthe electrolyte composition and in the flow hydrodynamics.The power spectral density (PSD) of the ER fluctuations,�Re(f ), presented a monotonic five-decade increase for aVOCφ ranging between 0 and 33% at low frequency, whereastψ (onet lowV

, re-s eane aul 0a enceo ase,t C atl nol t-t le oilm ales-c

to al undt entsc om-p to thee er-i lowV n-t r re-g cec iouss pro-v erep ts ofb ance

of the interface, the potentiostatic control permitted electro-chemical measurements for high values of VOC that couldnot be obtained galvanostatically. The use of both control-ling devices allowed the straightforward comparison of theER fluctuations, expected to remain unchanged for equiva-lent hydrodynamic conditions, regardless the type of interfacecontrol. Furthermore, measurement drawbacks related to thechoice of the controlling device are also addressed showingthat intrinsic instrumentation limitations are a central prob-lem in the treatment of these complex two-phase systems.

2. Experimental

The cross-section of a pure iron (99.99%) rod of 0.5 cm indiameter embedded in epoxy resin was used as rotating-diskelectrode (RDE). Its surface was polished with emery paperup to 1200 grit and its rotating speed was fixed at 1500 rpm,which was deemed to be fast enough to avoid the blockageof the metallic surface by the formation of a permanent oilyfilm. The potential,V, of the working electrode (WE) wasmeasured against a saturated calomel electrode (SCE). A Ptcylinder of 0.5 cm in diameter and 7 cm in length was usedas counter electrode because its mechanical resistance to theelectrolyte flow was much lower than the Pt grid used in thep hases

cello lec-t re-a aceda neitya dert rac-t seeR so-lvt mr werec

underpt inw eadys sedi mw ededf thez

, firstd -qpq d tot oten-

he PSDs of the potential and current fluctuations,ψV(f) andI(f), respectively, showed a much less marked variation

o two decades) with an initial decrease in amplitude atOC followed by an increase at higher VOC.

Concerning the identification of flow pattern changesults indicated the existence of a linear evolution of the mlectrolyte resistance,〈Re〉, and of the low-frequency plate

evel,ΨRe(0), of the PSDΨRe(f ) for φ ranging betweennd 3%[8]. This was interpreted as being due to the presf oil droplets homogeneously dispersed in the brine ph

he droplet concentration being proportional to the VOeast up to this apparent 3% limit. The behaviour wasonger linear for 5%≤φ≤ 33%, indicating that the flow paern changed to more complex structures like deformabacrostructures formed as the result of oil-droplet co

ence.This work has been extended in the present paper

arger range of VOC between 0 and 80%, which was foo be the upper limit in which electrochemical measuremould be performed, as shown below. This clearly encasses a wide collection of water-cut values associatedvolution of the flow structure from oil-in-water to wat

n-oil emulsions. Special attention has been paid to theOC range (φ < 7%) in order to verify the interpretation me

ioned before and precisely determine the VOC point (oion) of flow-pattern transition. Another important differenoncerns the interface control, galvanostatic in the prevtudy[8] and potentiostatic in the present paper. Besidesiding direct assessment of current fluctuations, which wreviously obtained in an indirect way from measuremenoth the potential noise and the electrochemical imped

revious work that broke down the emulsion and led to pegregation at VOC higher than 30%.

Measurements were carried out in a cylindrical glassf 8 cm in diameter and 10 cm in height for a constant e

rolyte volume of 400 cm3. Besides the forced convection cted by the rotating disk, a 500 rpm-rotating magnet plt the bottom of the cell was used to ensure the homogend the dynamic stability of the oil/brine emulsions un

urbulent flow conditions (for a discussion about the chaerization of the hydrodynamic flow in such conditions,ef.[8]). The electrolytes were mixtures of 3 wt.% NaCl

ution and paraffin oil (0.87 g cm−3 density and 2.4 cm2 s−1

iscosity). Electrolytes with a VOC, defined as the ratioφ ofhe paraffin oil volume to the electrolyte volume of 400 c3,anging between 0 and 80% were used. All experimentsarried out at room temperature.

The electrochemical measurements were performedotentiostatic control at the corrosion potential,Ecorr. Before

he potential application, a time delay of minimum 15 mas left for the free evolution of the system towards a sttate and a stationaryEcorr value. Nevertheless, as discus

n the next section, forφ > 40% the electrochemical systeas prone to show more or less significant drifts that ne

urther correction of the applied potential in order to keepero average current expected for measurements atEcorr.

The ER was measured by means of a specific deviceesigned to be used under galvanostatic control[9] and subseuently extended to potentiostatic measurements[10]. Underotential control a sinusoidal voltage signal,νhf, with a fre-uency,fhf, at which the electrode impedance is reduce

he ER, was superimposed to the dc voltage of the p

H. Bouazaze et al. / Electrochimica Acta 50 (2005) 2081–2090 2083

tiostat. The current,I, flowing through the working electrodehad three components: the dc current, the current fluctuations,�I, and the sine wave current response,νhf/Re, at frequencyfhf. In the ER channel of the specific device, the first twocomponents were eliminated with a high-pass filter whilethe third component,νhf/Re, was amplified, rectified with adiode, and low-pass filtered to give a voltage signal,vRe, theamplitude of which was inversely proportional to the instan-taneous value of the ER. The fluctuations of the ER inducedfluctuations,�vRe, of vRe that could be easily obtained fromthe expression:

�vRe(t) = b�

(1

Re(t)

)= −b�Re(t)

〈Re〉2(1)

whereb is a proportionality factor (in V�) obtained from acalibration procedure with a variable resistor in place of theelectrochemical cell. With this value and the mean value ofvRe, the mean value〈Re〉 of the ER was also obtained. In thecurrent channel of the specific device, the current response,νhf/Re, was eliminated with a low-pass filter and the currentfluctuations,�I, were amplified, so that the specific devicemeasured the ER and current fluctuations simultaneously.

The analog ER and current signals were digitized succes-sively at different sampling rates by a real-time computerRTU 54501 with proper anti-aliasing low-pass filters, afterd igi-t afterH rier-t y oft 12t elec-t witht s ex-c entsa ncew than5 ve off m-p

3

3

icali re ofa Ref.[ 0t usly,d teel,wf n ER

, FortL

Fig. 1. Electrochemical impedance of a 0.2 cm2 iron RDE measured underpotential control at the corrosion potential in 3 wt.% NaCl with paraffin oilat different VOC (frequencies in Hz): (a)φ = 0%; (b)φ = 15%; (c)φ = 50%;(d)φ = 60% after corrosion potential drift; (e)φ = 60% after drift correction.

values, can be seen to increase monotonically with the VOC,as expected from the very high resistivity of the paraffin oil.In the low-frequency range, the diagram for a VOCφ = 15%also shows the expected trend of increasingRp value, indi-cating lower global corrosion rates, which would be coherentwith the protective properties of the oily phase. Nevertheless,this trend was not observed for all VOC values. For example,the impedance measured atφ = 50%, although presenting a〈Re〉 value consistently higher than that measured atφ = 15%,shows a surprisingly lowRp value inFig. 1, below that mea-sured atφ = 15%. This impedance collapse originates in theuse of the potentiostatic control, especially at high VOC.Indeed, if the corrosion potential diminishes during the sub-sequent electrochemical noise (EN) and impedance measure-ments, the applied potential becomes more and more anodicand, instead of a zero net current, an overall corrosion cur-rent appears, yielding lowerRp values. Curves d and e inFig. 1 illustrate this behaviour forφ = 60%. The impedancemeasured about 1 h after the beginning of the noise measure-ments gave a smallRp value of 12� cm2 (curve d) with thepresence of an incipient inductive loop, known to be relatedto corrosion processes[13]. The outcome current was foundto be positive after the measurement, which clearly indicateda potential drift. Once the applied potential was correctedso as to have a zero net current, the impedance was mea-s size( rrentv en-s w anym evT e

c-value elimination and amplification. The PSD of the dized signals were calculated between 10 mHz and 1 kHzann windowing to avoid artefacts related to the fast-Fou

ransform algorithm that assumes an intrinsic periodicithe measured signal[11,12]. The PSDs were averagedimes for increasing the measurement accuracy. Therochemical impedance measurements were performedhe same experimental set-up by using white noise aitation signal. From preliminary impedance measuremt different VOC, it was found that the electrode impedaas effectively reduced to the ER at frequencies higher0 kHz. The ER was then measured by using a sine wa

requencyfhf = 100 kHz and 150 mV peak-to-peak (pp) alitude as excitation signal in the specific device.

. Results and discussion

.1. Impedance and current noise analysis

Fig. 1shows the Nyquist diagrams of the electrochemmpedance measured at different VOC. The global featu

single capacitive loop is the same as that measured in8], although the polarization resistance,Rp, values are 1imes smaller for pure iron than for the steel used previoue to an unexpected presence of Ni and Mo in the shich has been identified only recently. InFig. 1, the high-

requency limits of the curves, which represent the mea

1 RTU 5450 is a trade name of Concurrent Computer Corporationauderdale, FL, USA.

ured again and gave a capacitive loop of much largercurve e). These deviations from the expected zero cualue took place mainly for high values of VOC, but theemble of measured impedance diagrams does not shoonotonic evolution of theRp values (normalized from th

alueRp(0) obtained atφ = 0%) with φ as seen inFig. 2.he crosses indicate the change inRp after correction of th


applied potential subsequent to the drift ofEcorr. This lackof long-term steady-state behaviour stems from the complexnature of these two-phase electrolytes: the state of the metal-lic surface may slowly oscillate between more or less activeconditions, following any slow change of the flow pattern,or more probably the formation and degradation of an oilyfilm at the electrode surface. It is interesting to note that thisproblem is clearly accentuated at VOC values (φ > 50%) thatcharacterize the transition region between oil-in-water andwater-in-oil emulsions[7]. In these conditions, the brine be-comes the dispersed phase in an oily bulk so that the metal-lic surface is more exposed to potential variations due toa temporarily protective oily film, as pointed out by Qing-dong[14]. Under potentiostatic control, the drift ofEcorr gaverise to a net DC current supplied by the potentiostat from/tothe counter-electrode, which could entail rapid changes atthe interface and retroactively accelerate the potential drift.In contrast, under galvanostatic control the imposed cur-rent value was constantly equal to zero, hence slowingdown the evolution of the surface state. This probably ex-plains why the drift ofEcorrdid not have a significant influencein the impedance measurements under galvanostatic control.

In spite of this shortcoming in the potential control, thecorrosion rates were estimated from theRp values given inFig. 2. Results showed that these two-phase electrolytes re-m ignif-i ratesea and2 thee one,w uctso ce ofa lytec eterf thes e for-m ace.I owp hich

F ono rc

control the corrosion processes. This will be done below frommeasurements of the ER.

Despite the uncertainty in this quantitative analysis, it ap-pears fromFigs. 1 and 2that the electrochemical impedance isnot able to distinguish between different hydrodynamic con-ditions, since the diagrams always present a single capacitiveloop atEcorr, regardless the significant flow-pattern changesdue to very different electrolyte compositions in the bulk. Aspointed out in Ref.[8], the stochastic nature of the turbulentflow close to the electrode, which induces large fluctuationsin the electrochemical quantities, as shown below, is filteredout by the time-averaging of the current and potential sig-nals in the impedance measurements. As a consequence, theimpedance diagrams characterize only the interfacial kineticsaveraged on the electrode surface area.

Another measurement limitation is associated to the veryhigh resistivity of high-VOC electrolytes. In water-in-oilemulsions the electrical continuity is not automatically en-sured by the brine phase. Depending on the brine distributionin the emulsion (dispersed brine droplets, stretched chains,etc.) the electrical continuity can be intermittently, or defini-tively for too-high VOCs, interrupted so that the potentiostatcan no longer control the interface. For VOC above 60%,this caused fluctuations of large amplitude in the controlledpotential, up to 12 mVpp atφ = 80%. The amplitude of theseu Vb ce int ostat,w th,h ove8 in-s tricalc in-t tedi nc-t ceda Vb t tran-s nceso dedt ut forφ

ua-t en-c d oft alsot ncyp am-p cedb de-c al tottc pt forφ

t ue

ain considerably aggressive even in the presence of scant oil-phase contents. As an example, the corrosionstimated from the two limitingRp values atφ = 80% (1460nd 360�) are approximately 1.4 and 5.7 mm/year (5524 mpy), respectively. These high values, obtained fromlectrolyte supposed to be a priori the less aggressiveere confirmed by visual observation of corrosion prodn the WE. There was no clear evidence of the presenny protective oily phase, which indicates that the electroomposition in the bulk is not the only determining paramor corrosion protection. A probable explanation is thathear forces associated to the turbulent flow preclude thation of a permanent protective film on the metallic surf

t is then very important to gather information about the flattern and electrolyte composition close to the WE, w

ig. 2. Normalized polarization resistance,Rp/Rp(0) measured as a functif VOC at the corrosion potential. Crosses indicateRp/Rp(0) values afteorrection of the corrosion potential drift.

ndesired fluctuations could be reduced down to 0.1 mppy a proper choice of the current-measuring resistan

he counter-electrode branch of the home-made potentihich had the effect of limiting its frequency bandwidence reducing high-frequency oscillations. At VOC ab0%, even if most of time the overall electric circuitide the cell was closed, local breakdowns of the eleconductivity in the solution occurred, which caused theerface control to temporarily switch-off. This is illustran Fig. 3 for a 85% VOC. The applied potential was puually lost at time 11 s in the time record, which indu

large-amplitude potential transient around the 0.1 mppackground noise, and, consequently, an anodic currenient of large amplitude too. The occurrence of sequef such randomly distributed switch-off impulses preclu

he impedance and noise measurements to be carried o> 80%.The overall evolution of the PSD of the current fluct

ions is depicted inFig. 4. The peaks present at high frequies correspond to the rotation of the stirrer magnet anhe RDE (ca. 8 and 25 Hz + harmonics, respectively) ando the 50 Hz power supply. All PSDs show a low-frequelateau followed by decreases with various slopes. Thelitude of the high-frequency plateau above 200 Hz, induy the background noise of the potentiostatic control,reased at high VOC because it is inversely proportionhe squared modulus of the impedance[15]. Results confirmhe poor sensitivity of the PSDΨ I to identify differences inorrosiveness or changes in flow pattern. Indeed, exce= 80%, all PSD curves merged in one decade (5× 10−13

o 5× 10−12 A2/Hz) at low-frequency. In addition, the val


Fig. 3. Current and potential impulses due to intermittent switch-off of thepotential control in mixtures of 3 wt.% NaCl and high-content oil (φ = 85%);V0 =−0.66 V/SCE.

Ψ I(0) of the low-frequency plateau ofΨ I(f) did not changemonotonically with the VOC:Ψ I(0) progressively decreasedfor a VOC increasing up to 70% and strongly increased forhigher VOC. This complex signal evolution can also be seenin the time records sampled at 2 kHz (Fig. 5), which cor-respond to the higher frequency range in the PSDs. Timerecords sampled at lower frequency are not shown since theypresent a white-noise aspect, which is not informative. Theinfluence of the forced convection generated by the magneticagitation was clearly picked-up by the current signal (period-icity of 0.13 s) for VOC up to 50%, but no major differencewas found in the shape and amplitude of the current fluctu-ations for increasing VOC values, except forφ = 80% as inFig. 4.

It is important to recall that monitoring corrosion pro-cesses in field applications from EN measurements consists

F t thec rentV

Fig. 5. Time records of the current fluctuations measured for an iron RDE atthe corrosion potential in mixtures of 3 wt.% NaCl and paraffin oil at differentVOC indicated in the figure. Sampling frequency of 2 kHz. Arbitrary scaleorigins.

in measuring the current and potential noise with two iden-tical WEs connected by a zero-resistance ammeter (ZRA),and a third electrode, often identical to the WEs, used aspseudo-reference electrode[16–18]. The basic idea in thisapproach is to derive the noise resistance,Rn, defined as theratio of the standard deviations of the potential and currentsignals, which is expected to be equal to the polarization resis-tance,Rp, and therefore to give the corrosion rate. However,in complex two- or multiphase flows, even if both WEs are inthe same macroscopic electrochemical conditions due to thezero-potential difference imposed by the ZRA, the hydrody-namic effects may be completely different on each electrode.Therefore, the resulting signals must convey the combinedeffects of highly stochastic events taking place on each elec-trode with intrinsically different noise levels. When the po-tential is measured against a pseudo-reference electrode, ithas been demonstrated that theRn values deviate from the ex-pectedRp values when the three electrodes are not identicallynoisy[19–21], which compromises the reliability ofRn mea-surements in complex electrochemical systems as oil/brinemixtures.

Fig. 6, which exhibits two PSDsΨ I measured for the sameVOC (φ = 80%) but in conditions in which theRp values weredifferent (seeFig. 2), shows that the current noise measure-ments present the same uncertainty related to the lack of welld s, ast ,t ouslyw tw tectl itionc tions(e inter-f oten-t le n thec enai

ig. 4. PSDs of the current fluctuations measured for an iron RDE aorrosion potential in mixtures of 3 wt.% NaCl and paraffin oil at diffeOC indicated in the figure.

efined steady-states and to instrumentation limitationhe electrochemical impedance (Figs. 1 and 2). In contrasthe PSDs of the ER fluctuations, measured simultaneith the PSDsΨ I , are identical inFig. 6, which is consistenith the fact that the ER is a purely ohmic quantity that de

ocal (time and space) changes in the electrolyte composlose to the electrode. Once the hydrodynamic condielectrolyte composition, flow pattern) are stationary,ΨRe isxpected to remain constant regardless changes in theacial kinetics, and, therefore, regardless the corrosion pial drift, as it actually happens inFig. 6. This is additionavidence that the ER is a more reliable parameter thaurrent fluctuations to investigate such complex phenomn multiphase flows as discussed below.


Fig. 6. PSDs of the ER (top) and current (bottom) fluctuations measured foran iron RDE at the corrosion potential in a mixture of 3 wt.% NaCl and 80%VOC paraffin oil before (Rp = 360�) and after (Rp = 1460�) correction ofthe potential drift.

3.2. Electrolyte resistance analysis

The evolution of the mean value〈Re〉 of the ER fluc-tuations for increasing VOC was first investigated. Anobvious remark is that the value delivered by the specificER measurement device must match that obtained as thehigh-frequency limit of the electrochemical impedance. Thiswas found to be the case for all VOC values, regardless thepotential control problems described above.Fig. 7 showsthe 〈Re〉 values, normalized from the corresponding valuein the oil-free electrolyte〈Re(0)〉 = 25.5�, against the VOC.Values obtained in similar conditions in Ref.[8], exceptthe smaller sizes of the cell and rotating magnet, the metalnature, the counter-electrode shape, and the interface contro

F tt theVI

(galvanostat), are also reported in the figure. An exponential-like increase of〈Re〉 with increasing VOC is observed inboth cases and good agreement is found in the〈Re〉 valuesobtained forφ < 10%. At higher VOC, deviations of the〈Re〉values may be explained by dissimilar overall hydrodynamicconditions induced by changes in the cell and magnet sizes,but above all by the use of different counter-electrodes. InRef. [8], the large-surface Pt grid used was a mechanicalobstacle that broke down the dynamic emulsion as theVOC increased, so that the value of the VOC close to theelectrode was higher than in the electrolyte bulk. This led toearlier flow-pattern changes so that the〈Re〉 exponential-likeincrease took place for lower VOC values than in the presentstudy, in which a cylindrical Pt tube offered a minor mechan-ical resistance. This means that a quantitative interpretationof results obtained from field applications must be precededby a specific calibration procedure so that a straightforwardand reliable correspondence between electrolyte-resistancedata (mean value and PSD) and flow composition couldbe established. The detailed view of the〈Re〉 evolution forsmall values of VOC in the inset ofFig. 7 shows a linearbehaviour for values of VOC below approximately 3%. Thislinear evolution indicates, as further discussed below, that inthese high water-cut oil-in-water emulsions, the increase inVOC had the simple effect of enhancing the concentrationo antfl

d oft vityo rries,e n-t ium,w ns.Am ets)i

w usm eoryi

T rgerr ford

ge-m〈 a-s s of〈 them be-t h the

ig. 7. Normalized mean value of the ER,〈Re〉/〈Re(0)〉, of an iron RDE ahe corrosion potential in 3 wt.% NaCl with paraffin oil as a function ofOC. 〈Re(0)〉 = 25.5� in the oil-free solution (�: values from Ref.[8]).

nset: detailed view for low VOC.

l

f the dispersed oil-droplets without inducing significow-pattern changes.

The overall evolution of the experimental〈Re〉 valuesepicted inFig. 7 may be compared to the predictions

heoretical models dealing with the electrical conductif heterogeneous systems as multiphase mixtures, slumulsions, etc.[22,23]. The Maxwell model concerns ide

ical spherical particles dispersed in a continuous medhich is the simplest kind of two-phase dispersioccording to this model, the electrical resistivity,ρ, of aixture consisting of non-conducting particles (oil dropl

n a conducting medium (brine), is expressed by

ρ

ρb= 1 + φ/2

1 − φ(2)

here ρb is the resistivity of the pure brine continuoedium. Later, Bruggeman proposed an asymmetric th

n which the resistivity of the mixture is

ρ

ρb= (1 − φ)−3/2 (3)

his expression was found to be applicable for a laange of VOC than Maxwell’s expression but is validispersions with a wide distribution of particle sizes.

In terms of electrolyte resistance, Maxwell’s and Brugan’s equations are obtained by replacingρ/ρb withRe〉/〈Re(0)〉 where 〈Re(0)〉 is the mean ER value meured in the oil-free electrolyte. The experimental valueRe〉/〈Re(0)〉 have been compared to those predicted byodels inFig. 8. There is a good qualitative agreement

ween the theoretical and experimental values althoug


Fig. 8. Comparison between experimental ER values fromFig. 7 (�) andpredictions (solid lines) from Maxwell model, Eq.(2)and Bruggeman model,Eq.(3). Inset: detailed view for low VOC.

latter were somewhat smaller, mainly for high VOC values.In a first glimpse this deviation could be associated to thefact that the validity of these models is limited to low VOCranges. Indeed, Maxwell’s model has been derived under theassumption that the potential fields surrounding each parti-cle do not perturb each other, which is only true in very di-lute solutions[22], whilst Bruggeman’s model fails beyondthe onset of direct interactions between dispersed particles,i.e. when flocculation or agglomeration takes place[23]. Itis worth noticing however, that the experimental values donot match the model predictions even at low VOC values, asseen in the inset ofFig. 8. This discrepancy could be relatedto the approximation done in considering the paraffin oil as astrictly non-conductive medium as for the dispersed phasesin Eqs.(2) and (3). Indeed, even if the paraffin oil resistivityis very much larger than that of the brine solution, the oildroplets could show a dielectric behaviour with ac-currentlines passing through the droplets, hence giving ER valueslower than expected. In other words, the perturbation signalfrequency of 100 kHz used for the ER measurements, whichis higher than those commonly used in solution resistivitymeasurements, could yield low ER values. However, no suchcapacitive effect could be detected between 1 Hz and 100 kHzin impedance measurements performed between two SCE ina four-electrode arrangement in oil–brine mixtures of vari-o inga

peci-fi eenc ag-n iledh heseu (sizea thev adyd n thet e-m ardcf ect

to the vortex induced by the magnet rotation, confirming thatthe concentration of oil droplets was not uniform in the cylin-drical electrochemical cell so that the theoretical models havenot a straightforward application in such experimental condi-tions. By comparing the slope of〈Re〉/〈Re(0)〉 versusφ at lowVOC inFig. 8to the value of 1.5 given by Eqs.(2) and (3), theapparent value of VOC measured close to the electrode canbe estimated to be close to 0.5φ in this experiment; however,different values were also found in further experiments de-pending on the position of the RDE with respect to the vortex.

It is important to notice that even if impedance measure-ments at high frequency may inform on the average ER evo-lution, the impedance technique cannot be used to monitorfast ER fluctuations, from which the flow pattern and com-position can be assessed, because only a few values ofRe canbe measured per second with a transfer function analyser.Fig. 9 presents typical time records of the ER fluctuationsmeasured with the specific device mentioned above for in-creasing VOC. It is important to stress that there is a 40-foldincrease in the�Re scale in the upper part of the figure. Thetime record measured forφ = 10% is reported in both parts ofthe figure as a guideline for emphasizing the extremely largevariations of the�Re amplitude when varying the VOC. Incontrast to current fluctuations (seeFig. 5), the�Re ampli-tude showed a highly sensitive and easily detectable mono-t thef -p thans t ex-h luesw (1 Hz

F at thec rentV caleo

us composition (φ < 70%), the solution impedance showpurely resistive behaviour in this frequency range.The discrepancy is probably related to experimental s

cities of the present study, mainly the fact that it has barried out under strongly turbulent flow conditions (met agitation combined to electrode rotation) that entaigh shear-stress fluctuations close to the electrode. Tncontrollable effects can yield emulsion parametersnd number of oil droplets) giving VOC values lower inicinity of the electrode than in the electrolyte bulk, as alreiscussed in the interpretation of the discordance betwee

wo curves presented inFig. 7. Solution resistivity measurents carried out in mixtures of low VOC with a stand

onductivity cell have actually shown variations inRe as aunction of the position of the conductivity cell with resp

onic increase with VOC. The corresponding analysis inrequency domain is given by the PSDs inFig. 10. The amlitude of the PSD increased monotonically over no lessix decades with increasing VOC while its shape did noibit any significant change, except at the highest VOC vahere the PSD began to decrease at lower frequencies

ig. 9. Time records of the ER fluctuations measured for an iron RDEorrosion potential in mixtures of 3 wt.% NaCl and paraffin oil at diffeOC indicated in the figure. Sampling frequency of 2 kHz. Arbitrary srigins.


Fig. 10. PSDs of the ER fluctuations measured for an iron RDE at the corro-sion potential in mixtures of 3 wt.% NaCl and paraffin oil at different VOCindicated in the figure.

at φ = 80%). As explained below, the shape of the PSD isgiven by the Fourier transform of the elementary ER tran-sient induced by the passage of an oil droplet close to theelectrode.Fig. 11shows the amplitude of the low-frequencyplateau (normalized from the valueΨRe,0%(0) measured inthe oil-free electrolyte) as a function of the VOC:ΨRe(0) in-creased progressively up to a VOC value of 70% and morerapidly above, as the mean ER value (Fig. 7). This change inbehaviour, obtained at lower values of VOC (about 20%) inthe previous study, is mainly attributed to the difference inthe counter-electrodes used, as explained above. It is inter-esting to notice that, in stationary hydrodynamic conditions,theΨRe(0) level allows the VOC to be directly assessed and,therefore, the intrinsic aggressiveness of the electrolyte to beindirectly estimated.

For low values of VOC,ΨRe(0) increased linearly with theVOC (Fig. 12). The linear evolution of〈Re〉andΨRe(0), whichcharacterizes the hydrodynamic steady-state behaviour of theinterface, indicates that for each VOC increment the systemresponse was limited to a correspondent increase in the am-plitude of both〈Re〉 andΨRe(0). In other words, up to thisapparent limiting 3% VOC value, there was a simple inten-sification of the events giving rise to ER fluctuations with-out any flow-pattern change. Hence, for these high water-cutemulsions, the electrolyte can be physically interpreted as be-

F en inF

Fig. 12. Normalized low-frequency plateau level of the ER PSDs given inFig. 10at low VOC (�: values from Ref.[8]).

ing a perfectly mixed dispersion of oil droplets. As the VOCincreased up to 3% the droplet concentration increased butthe emulsion remained homogeneous without interaction be-tween neighbouring droplets. The ER fluctuations were thenthe result of the continuous and independent passages of thesesmall droplets close to the electrode, which can be modelledby a Poisson process[8], so that〈Re〉 may be written as

〈Re〉 = λ

∫ ∞

0�Re(t) dt (4)

whereλ is the mean number of oil-droplet passages per unittime and�Re(t) is the elementary time transient at timetinduced by the passage at time 0 of a single oil droplet. ThePSD of the ER fluctuations can also be derived according tothe Poisson theory:

ΨRe(f ) = 2λ〈|�Re(f )|2〉 (5)

where�Re( f ) is the Fourier transform of�Re(t), expressedas

�Re(f ) =∫ ∞

0�Re(t) e−2iπftdt (6)

The magnet rotation created a non-uniform shear-stressfield throughout the bulk of the electrolyte, which means thatthe droplets could be deformed as they travelled across thec on-s sizec a lo-c nse,i es

〈Ψ

w

R

-t

ig. 11. Normalized low-frequency plateau level of the ER PSDs givig. 10(�: values from Ref.[8]).

ell. Nevertheless, it is likely that the droplets could be cidered as having approximately the same shape andlose to the electrode since the RDE tended to createally uniform and stationary shear-stress profile. In this set is reasonable to assume that allRe–time transients had thame shape and amplitude, so that Eqs.(4) and (5)give

Re〉 = λR (7)

Re(0) = 2λR2 (8)

hereR is the time integral of theRe transient:

=∫ ∞

0�Re(t) dt (9)

Eqs.(7) and (8)allow then the values ofλ andR to be deermined for each VOC. Results presented inFig. 13confirm


Fig. 13. Mean number of oil-droplet passages per unit time,λ (�), andRe–time transient integralR (�) calculated from data inFigs. 7 and 12withEqs.(7) and (8).

the validity of assuming a Poissonian behaviour of the oil-droplet passages for these high water-cut emulsions (φ up toabout 3%). In spite of some scattering in the calculated values,bothλ andR appeared as correctly confirming the Poissontheory. Indeed the proportionality ofλ with the droplet con-centration up to a 3% VOC indicates that there was a simpleintensification of the independent droplet passages close tothe electrode. The droplets were also considered in this ap-proach to be of approximately same shape and same size sothat the time integralR of theRe transients was expected tobe independent of the VOC, which is actually obtained inFig. 13: 〈R〉0–3%= 0.51 m� s (against 0.43 m� s in Ref.[8]).The values ofλ andRdeviate from the linear behaviour forVOC higher than 3%, which points out the change in flow pat-tern that can no longer be modelled by a simple Poisson pro-cess. This is certainly because the oil concentration reached athreshold value for which the distance between neighbouringdroplets was too low to assume their independence. Dropletcoalescence was probably a major component of the over-all phenomenon giving birth to more and more complex de-formable oil macrostructures passing by the metallic surface.Much work has to be done to understand how this transitionproceeds and affects the ER parameters, and why the shapeof the ER PSD remains unchanged when increasing the VOCup to 70%. Nevertheless, the ER analysis, mainly focusingo mon-i asefl

4

m-p ial bye sure-m ial inl oft lar-i es ina inu-o ded

on the time-varying flow pattern, was at the origin of someuncertainty in the measurement of the impedance and currentnoise PSD.

In contrast, electrolyte resistance measurements, per-formed with a home-made device delivering an analog signalallowing the mean value as well as the fluctuations of the ERto be measured in real time, were not sensitive to the corrosionpotential drift. Measurements could be carried out for volu-metric oil contents up to 80%, above which the potentiostatcould not control the electrode potential because of intermit-tent interruptions of electrical continuity in the solution. ThePSDs of the ER fluctuations were found to increase monoton-ically with the VOC on the whole frequency range, on morethan six amplitude decades. Thus, in steady-state hydrody-namic conditions the level of the PSD at a given frequencyallows the VOC to be assessed, and therefore the aggressive-ness of the oil/brine mixture to be estimated indirectly.

Analysis of the ER fluctuations gives information on thehydrodynamic flow pattern close to the electrode. For low val-ues of VOC (up to 3%), the ER fluctuations were shown to beinduced by continuous passages of independent oil-dropletsin front of the electrode, which could be modelled by a Pois-son process. At higher VOC, the flow pattern progressivelychanged to larger moving oil macrostructures, which couldnot be identified by naked eye. Even if further work is re-q netics ch-n msi

R

000)

OR-TX,

000,

RO-997.001)

58

991)

[ hem.

[ n 58

[ lec-

[ ook:SM

[

n the mean value and PSD, appears as promising fortoring the corrosion and flow-pattern in two- or multiphows.

. Conclusions

Corrosion of iron in oil/brine mixtures of various coositions has been investigated at the corrosion potentlectrochemical impedance and noise techniques. Meaents have shown that controlling the electrode potent

ow-conductivity solutions was a major difficulty becausehe drift of the corrosion potential that progressively pozed the electrode. This drift, which was related to changctivity of the metallic surface, probably due to the contus adsorption and desorption of an oily film that depen

uired these experiments on laboratory cells with magtirring show promising results on the ER fluctuation teique for corrosion monitoring in two- or multiphase syste

n the field.

eferences

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A New Aproach for Monitoring Corrosion Flow Characteristics in Oil Brine Mixtures 2005

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