Copyright 2008 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International, Copyright Division, 1440 South creek Drive, Houston, Texas 777084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

ROTATING CYLINDER ELECTRODE (RCE) SIMULATION OF CORROSION IN SWEET PRODUCTION

E. J. Wright ExxonMobil Development Company

12450 Greenspoint Drive Houston, Texas 77060

K. D. Efird

Efird Corrosion International, Inc. The Woodlands, Texas 77380

J. A. Boros

ExxonMobil Upstream Research Company Friendswood, Texas 77034

T. G. Hailey

ExxonMobil Upstream Research Company Houston, Texas 77098

ABSTRACT The major application of the rotating cylinder electrode (RCE) in the oil and gas industry is as a fluid flow corrosion test device for inhibitors. However, results reported previously by the authors demonstrated that while steel corrosion rates measured in CO2-containing brine using a parallel pipe flow apparatus and a jet impingement apparatus correlate with each other based on calculated wall shear stress, the RCE rotating cylinder corrosion rates do not correlate with either. This lack of correlation has implications for the use of the RCE to simulate flow accelerated corrosion in sweet production. This paper presents the results of an investigation to determine first, why the RCE results do not correlate to parallel pipe flow based on the calculated wall shear stress, and second, whether the conventional RCE can be modified to achieve correlation. Electrochemical corrosion testing of American Iron and Steel Institute (AISI) 1018 carbon steel in CO2-containing brines was carried out using direct current (DC) linear polarization and alternating current (AC) impedance techniques. This was coupled with examinations of surface films using scanning electron microscopy. Potential implications of these results for the use of the RCE as a screening device for corrosion inhibitor selection for upstream projects are discussed. Key terms: carbon dioxide, flow accelerated corrosion, rotating cylinder electrode, corrosion inhibitors, sweet corrosion, wall shear stress

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Paper No.

08629

INTRODUCTION There is a need for a realistic laboratory corrosion fluid flow test facility to investigate the corrosion behavior of carbon steel in CO2-brine environments. However, it is difficult to simulate effectively both the chemistry and the flow components of the sweet production environment in the laboratory. The traditional approach used for such investigations is a flow loop that is generally large and expensive both in terms of capital and operational costs. There is, therefore, an incentive to investigate the use of other techniques, such as the rotating cylinder electrode (RCE) and jet impingement, that potentially offer fluid flow corrosion testing in a more compact and inexpensive form. However, it is essential to correlate corrosion data obtained from these apparatus with pipe flow, so that the data can be effectively scaled up to service. Therefore, corrosion data obtained using the RCE must be correlated with corresponding tests carried out using pipe flow. The objective of this work was to develop a valid, reliable operational envelope for the use of carbon and low alloy steels in sweet production based on laboratory fluid flow corrosion test data. The work includes the design and construction of a flow loop facility for the simultaneous determination of carbon steel corrosion data in three types of flow apparatus: parallel pipe flow, jet impingement and the RCE. This eliminates any experimental artifacts caused by differences in the chemical composition of the fluid in each apparatus. Wall shear stress is a fundamental fluid flow parameter in the definition of flow accelerated corrosion, and can be readily calculated for most field situations. For these reasons, wall shear stress is referenced, instead of mass transfer coefficient to evaluate flow accelerated corrosion. However, the use of this fluid flow parameter does not imply the assumption by the authors of a shear stress mechanism for flow accelerated corrosion in this test environment. Conversely, any demonstration of an experimental correlation between the wall shear stress and the measured corrosion rate cannot be, and has not been, used to derive conclusions regarding the mechanism of corrosion. Results reported previously1 demonstrated that steel corrosion rates measured in a parallel pipe flow apparatus, and in a jet impingement apparatus correlate with each other based on calculated wall shear stress, while the RCE corrosion rates do not correlate with either. This paper presents the results of an investigation to determine first, why the RCE results do not correlate to parallel pipe flow based on the calculated wall shear stress, and second, whether the RCE can be modified to achieve a correlation. Further details of the experimental procedures used in the study have been presented earlier.2

BACKGROUND Hydrodynamics of the RCE The RCE consists of a cylindrical metallic electrode mounted on a corrosion resistant alloy shaft that is enclosed in an electrically insulating, non-metallic material which is flush with the electrode surface (see Figure 1). Electrical connection for electrochemical corrosion testing is made by silver-graphite brushes that are pressed against the metallic shaft. The electrode is immersed in the corrosion test environment and is then rotated at a defined, stable rate. This arrangement provides controlled hydrodynamic conditions at the electrode surface and can be used to investigate the effects of fluid flow on the corrosion and electrochemical processes occurring on that surface. Corrosion weight-loss measurements can also be made on the rotating cylinder either independently or together with electrochemical tests. The RCE and the accompanying electrochemical cell form a compact, relatively inexpensive system that uses small volumes of test fluids compared to alternative fluid flow corrosion testing apparatus such as flow loops.

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The hydrodynamic mass transfer characteristics and the applications of the RCE in the fields of electrochemistry and corrosion have been comprehensively reviewed by Gabe et al.3, 4 Two of the major influences on corrosion processes occurring on the surface of an electrode rotating in a corrosive environment are wall shear stress and mass transfer. The magnitude of these effects on the corrosion processes is a function of the rotational rate of the electrode, generally both effects increasing with rotational rate. It should be noted that these two effects are not independent; i.e., changes in the wall shear stress will also affect the rate of mass transfer to the electrode surface. The wall shear stress () is defined by the generally accepted equation: (Please refer to Table 1 for detailed symbol nomenclature.)

= 0.0791 Re-0.30 r2

2 [1]

where Re is the Reynolds number. This relationship was proposed in the corrosion literature by Silverman5 and its derivation is based on the empirically determined drag measurements of Theodoresen and Regier6 on rotating cylinders in various gases and liquids. The first dimensionless correlation of the mass transfer to hydrodynamic conditions of the RCE was achieved by Eisenberg et al.7 from an investigation of a nickel electrode-alkaline ferricyanide/ferrocyanide system. An empirical relationship between the mass transfer (Sh), and the Reynolds number (Re) and the Schmidt number (Sc) was derived from the data. This relationship is represented by the equation:

Sh = 0.0791 Re0.7 Sc0.356 [2]

that has been independently confirmed by Gabe et al.8 for copper plating solutions, Comet et al.9 and also Silverman11 for oxygen reduction on Monel Alloy 400. This relationship can also be expressed more directly in terms of the limiting current density measured for a mass-transfer controlled reaction at a RCE as a function of electrode rotational rate. Essentially the limiting current (ilim) for a mass transfer controlled reaction has been related to the electrode rotational rate () by the following equation:

ilim = 0.0791nFCb0.7d-0.3D0.644-0.344 [3]

where d is the characteristic dimension of the RCE equal to the diameter of the electrode. The wall shear stress and mass transfer equations presented above are based on predictions and measurements, respectively, on hydraulically smooth cylinders. The effects of surface roughness on the mass transfer characteristics have been investigated by Kappesser et al.10 who developed a modification of the correlation developed by Eisenberg that includes a surface roughness factor. This is important when examining the behavior of corroding surfaces that may show localized corrosion and pitting attack, thereby altering the wall shear stress and mass transfer on the metal surface from those theoretically predicted. Corrosion applications of the RCE Silverman5 has pointed out that most pipe and tubular components operate under turbulent conditions. He notes that the turbulent conditions predicted for the RCE can be used to investigate the effect of fluid flow on the corrosion behavior of engineering components under realistic flow conditions. Potentially the RCE has significant advantages for the investigation of the effects of flow on the corrosion of engineering materials. Silverman11,12 has presented a theoretical derivation to show hydrodynamic "equivalences" involving mass transfer and wall shear stress between jet impingement, parallel pipe flow and the RCE.

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Equivalences based on mass transfer assume that the corrosion rate is under mass transfer control. Furthermore, equations 2 and 3 define overall mass transfer from the RCE to the bulk environment. However, MacDonald and McKubre13 have described a rotating-cylinder collector electrode which consisted of a nickel strip insulated from and embedded in a copper cylinder. Species produced on the nickel could be reduced on the copper and vice versa. In the first case, a collection efficiency of 67 % was measured and in the second, 7.2 %. Reller et al.14 have shown that the limiting current density for the oxidation of ferricyanide on strip microelectrodes is significantly larger than that obtainable on a conventional RCE. The thinnest microelectrode used in this work (20 microns) showed an enhancement of a factor of 30 compared to the conventional RCE. The above suggests that mass transfer across the RCE surface influences the mass transfer characteristics of the RCE.

In summary, there are factors which are unique to the RCE flow test apparatus that may complicate correlation of corrosion rate data obtained with corresponding parallel pipe flow and jet impingement tests. Therefore, an empirical correlation of RCE corrosion rates with those measured in parallel pipe flow is required for a given corrosion test system. Two major studies have examined the behavior of alloys using different fluid flow corrosion test apparatus to establish such correlations. MacDonald et al.15 have examined pipe flow, annular flow and the RCE for 90:10 cupronickel in an aerated sodium chloride solution at room temperature and reported a correlation between the techniques based on mass transfer comparisons. However, these comparisons were complicated by the formation of surface films on the electrodes. Dawson et al.16 examined the corrosion behavior of carbon steel in sweet environments using jet impingement, RCE and a flow loop and reported a correlation between the techniques based on calculated wall shear stress for each of the test geometries but without the presentation of detailed data to support this conclusion. The main application of the RCE in the oil and gas Industry is as a screening test for corrosion inhibitors. 18 - 21 Several test approaches have been used, including prefilming of the electrodes and the observation of hysteresis effects between high and low rotational rates to evaluate inhibitor effectiveness. Generally, these tests examine the variation of inhibited corrosion rate versus the calculated wall shear stress created at the electrode surface. Inhibitor efficiency tests are frequently carried out to a defined maximum wall shear stress as calculated from equation 1. There has been some work on the fundamental investigation of CO2 corrosion of carbon steel using rotating cylinder weight-loss specimens,22, 23 but only limited work with the RCE. 24

EXPERIMENTAL Rotating Cylinder Electrodes The rotating cylinder electrode is mounted on an AISI 304 stainless steel shaft enclosed in Torlon washers. Figure 1 shows the electrode configuration. The specimen surface is finished to a 600 grade silicon carbide paper. Electrical connection is made by silver-graphite brushes pressed against the stainless steel shaft. The corrosion behavior of AISI 1018 carbon steel electrodes of two types was examined. First, conventional cylinder electrodes of three sizes: 1.27, 2.54 and 3.79 cms in diameter respectively. Second, a rotating cylinder strip electrode consisting of a modified 1.27 cm diameter cylinder electrode that had been machined down to 0.8 cm diameter, except for a strip 0.1 mm in width. The electrode outer diameter was then mounted in an epoxy resin and then finish-machined to obtain an exposed, flush-mounted strip of steel of 0.1 mm thickness in a dimensionally accurate cylinder. This functioned as the rotating strip electrode for corrosion tests and permitted a comparison of the corrosion data obtained from a conventional RCE.

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RCE - Apparatus and Electrochemical Test Cell The RCE used is an EG&G Princeton Applied Research Model 636 Electrode rotator. The assembled cell with the electrode in place is shown schematically in Figure 2. The cover is fabricated from polypropylene and has threaded holes in it for the Luggin capillary to the external reference electrode, solution inlet and outlet, a combined gas inlet and outlet, two platinum auxiliary electrodes and a thermocouple pocket. The lower surface of the cover also has six polypropylene baffles to minimize vortex formation and axial flow in the solution produced by the rotation of the electrode. The electrode rotator fits with a small clearance (~0.5 mm) through a neoprene stopper connected to a nitrogen purge (to prevent oxygen contamination) and is mounted centrally in the cell cover. The auxiliary electrodes are two platinum foils, each 100 mm X 50 mm, curved symmetrically around the specimen electrode. Potential measurements are made using a Luggin capillary probe. Once-through cell test fluid is supplied from an external flow loop. The cell temperature is monitored by an immersed thermocouple and controlled to 1C. Mass Transfer Characterization of the RCE It is important to characterize the mass transfer behavior of the RCE within this test cell to ensure that the RCE system performs as predicted from the technical literature. Therefore, the oxygen reduction reaction was used as a model system that, on a film-free electrode, is defined as being under mass transfer control in aqueous solutions at room temperature. Therefore, the limiting current densities for the oxygen reduction on a platinized Monel Alloy 400 surface were measured at a range of electrode rotational rates and the results compared to those predicted from the empirical equations presented by Eisenberg et al.7 The test electrolyte for these tests was an air-saturated (~ 6.8 ppm oxygen), 0.5 M anhydrous sodium sulfate solution. The specimen was polarized from the stable, free corrosion potential to - 0.75 vs. S.C.E. The limiting current density was measured across the entire range of electrode rotation rates. Triplicate experiments were conducted at 25C. Procedure for the RCE Corrosion Tests The test solution used for the carbon dioxide experiments was a 3.0 % NaCl solution containing a controlled concentration of initial bicarbonate ion (as sodium bicarbonate) to provide system pH buffering. The solution was prepared in the reservoir tank of the flow loop. This solution was circulated through the loop and deaerated with oxygen-free nitrogen for 2 hours, followed by sparging with the appropriate carbon dioxide/nitrogen gas mixture for 10 hours, to achieve saturation under the particular experimental conditions. The assembled RCE test cell was flushed with nitrogen gas for one hour. Solution at the required test temperature was then pumped under positive pressure from the loop to the RCE cell. The cell replenishment rate was ~ 3 cm3/sec to minimize any effects on the measured corrosion rates due either to pH changes or dissolved ferrous ion accumulation. Corrosion rates of AISI 1018 carbon steel were determined as a function of electrode rotational rate which can be theoretically related to wall shear stress by equation 1. After potential stabilization, the corrosion rates were measured by the linear polarization technique (LPR) which involved polarization from -15 mV below the open circuit potential to + 15 mV above at a sweep rate of 0.17 mV/s. Impedance spectra were determined using an EG&G Princeton Applied Research Model 273 Potentiostat with a Solartron Model 1250 Frequency Response Analyzer. The AC sinusoidal signal voltage applied to the electrode was 5 mV. The frequency range investigated was 0.005 Hz - 10 kHz. Weight-loss determinations were made to validate LPR corrosion rate estimates. Selected electrodes were examined by scanning electron microscopy. The influence of two solution chemistry parameters on the corrosion rate of carbon steel in this test environment was determined: (a) the effect of CO2 partial pressures of 2.5 and 14.7 psia and (b) the effect of an initial bicarbonate ion concentration of 1000 ppm HCO3

-.

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RESULTS AND DISCUSSION Electrical and Mechanical Characterization of the RCE The total contact resistance across the silver-graphite brushes used to connect the cylinder electrode for electrochemical polarization was measured as a function of rotational rate. These measurements showed that the low contact resistance (~ 0.15 ohms total) was unaffected by the rotational rate of the electrode. Oscilloscope measurements across the brushes confirmed that the system also exhibited very low electrical noise levels. Therefore no experimental artifacts, due either to varying resistance or electrical noise of the contacts, should occur for this system. Tachometer measurements confirmed that the unit can maintain accurate and stable rotational rates over extended time intervals. Both the accuracy and the stability of the setting were within 2 - 3 RPM. These observations confirm that the drive system can maintain the cylinder at the specified rotational rate required for corrosion investigations. Mass Transfer Characterization of the RCE The equation for mass transfer at cylindrical electrodes rotating inside concentric cells is well established as discussed above. Essentially the limiting current density (ilim) for a mass transfer controlled reaction is related to the electrode rotational rate () by equation 3. For an experiment under a given set of conditions, i.e., one electrode and constant oxygen concentration, temperature and solution, all of the variables are constant except the electrode rotational rate. Therefore the equation can be simplified to:

ilim = K0.7 [4]

where K is a constant for a given set of experimental conditions and a plot of ilim, versus

0.7 should be linear if the hydrodynamics are as predicted. This is the basis for the mass transfer characterization of the RCE. The results of mass transfer characterization of the system using the reduction of oxygen are presented in Figure 3 which shows a plot of the measured limiting current densities versus the corresponding electrode rotational rates raised to the power 0.7. Triplicate results are presented and show good reproducibility. These results demonstrate that a satisfactory agreement was obtained with equation 4. Therefore, within the limits of experimental error, the mass transfer/hydrodynamic relationship measured in the RCE cell is consistent with that predicted from the literature. Corrosion/Calculated Wall Shear Stress Data for AISI 1018 Carbon Steel RCE in CO2-Brines Figure 4 shows a plot of the corrosion rate of AISI 1018 carbon steel in the basic test solution (3% NaCI + 1000 ppm HCO3

- at 50C) determined using the LPR technique versus the calculated wall shear stress for the 1.27 cm diameter RCE electrode. This figure shows that the corrosion rate increases as the wall shear stress is increased. A limited number of weight-loss determinations at a constant wall shear stress were made to confirm the validity of the LPR estimates of corrosion rate. These are shown in Table 2 and show acceptable agreement between the two techniques. Comparison curves reported previously1 for the wall shear stress effect on carbon steel corrosion rate for the 1.27 cm diameter rotating cylinder electrode and pipe flow are shown in Figure 5, which shows that the corrosion rates for the RCE are significantly lower than for pipe flow. The power function regression analyses for the respective data are presented in Table 3. These analyses imply that the equation for the effect of wall shear stress on the carbon steel corrosion rate determined for the rotating cylinder electrode in this environment is expressed in the form:

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RCOR = 2.8 0.10 [5]

where RCOR is the corrosion rate in mm/y and is the calculated wall shear stress. The corresponding equation for pipe flow is:

RCOR = 7.7 0.10 [6]

Comparison of these equations shows, therefore, that the corrosion rate measured on the RCE is significantly less than that measured in pipe flow at an equivalent wall shear stress and does not directly correlate with pipe flow corrosion rate as a function of wall shear stress, using the generally accepted RCE equation for this parameter. An alternate approach is to attempt to correlate the corrosion rates measured on the RCE and pipe flow using calculated mass transfer coefficients, instead of the calculated wall shear stress. Figure 6 shows a plot of corrosion rate versus calculated mass transfer coefficient11 for the RCE and pipe flow. This figure demonstrates that the RCE corrosion rate does not correlate with pipe flow based on calculated mass transfer coefficient either. Figure 7 shows a plot of the corrosion rates of AISI 1018 carbon steel in the same 3.0 % NaCI + 1,000 ppm HCO3

- solution for the RCE determined for two CO2 partial pressures, i.e., 2.5 and 14.7 psia respectively at 50C. The corrosion rate again increases as the calculated wall shear stress increases. As expected, the lower CO2 partial pressure results in lower corrosion rates. However, as the wall shear stress increases the CO2 partial pressure exerts less influence on the measured corrosion rate. Figure 8 shows the corresponding plot determined for pipe flow. This figure shows a significantly different corrosion rate at the respective CO2 partial pressures across the range of wall shear stress. Figure 9 shows a plot of the corrosion rates of AISI 1018 carbon steel versus calculated wall shear stress in the 3.0% NaCI solution for the RCE at a CO2 partial pressure of 14.7 psia at 50C

with and without the addition of 1,000 ppm HCO3-. The absence of initial bicarbonate was

expected to result in a significant increase in corrosion rate. However, the corrosion rates measured are similar in each case at equivalent wall shear stresses. Figure 10 shows the corresponding data obtained for pipe flow, which are completely different from the RCE and exhibit the anticipated influence of bicarbonate ion on the corrosion rate. Specifically, this figure shows that the absence of bicarbonate results in a significant increase in the steel corrosion rate. These results demonstrate that the RCE responds very differently from pipe flow to changes in solution chemistry. Electrochemical Impedance and Scanning Electron Microscopy (SEM) Electrochemical impedance measurements were conducted for the RCE and the pipe flow. The objective was to determine if there are any significant differences between these two electrode surfaces which may explain the observed discrepancy in corrosion rates at equivalent calculated shear stress. The description of detailed equivalent circuit models is outside the scope of this paper. Figure 11 shows the Nyquist plot for AISI 1018 carbon steel in the RCE at a calculated shear stress of 10 N/m2. The RCE spectrum shows a large, depressed semicircle related to charge transfer and also an inductive loop at low frequencies. Figure 12 shows the corresponding plot for the pipe flow which has been rescaled for clarity. The figure shows a similar semicircle attributed to charge transfer resistance. However, there is no evidence of the inductive loop seen in the case of the RCE. In the case of pipe flow, a second semicircle at low frequencies may be related to a slow step in the steel corrosion process.25

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Representative electrochemical data for the RCE and the pipe are presented in Table 2. Caution should be used in comparing electrochemical data between these two cell geometries which are inevitably very different. Nevertheless, there are significant, reproducible and stable differences between the two sets of data that are difficult to rationalize in terms of cell artifacts. The major difference is the higher value of the estimated charge transfer resistance measured on the RCE, 121 ohms-cm2, compared to the pipe at 12.8 ohms-cm2. This indicates a greater degree of polarization on the RCE, possibly resulting from film formation. The higher apparent solution resistance measured on the RCE, compared to the pipe, also indicates the presence of a film on the RCE as does the inductive loop.26,27 Finally, the greater degree of depression observed in the RCE semicircle suggests a heterogeneous electrode surface, e.g., partially filmed.28 The DC corrosion potential on the RCE is also consistently lower by ~ 30 mV, compared to the pipe flow. Visual observations of the exposed RCE and pipe electrodes showed that after testing the RCE was covered with a very thin dark gray film, unlike the corresponding pipe electrode. Therefore, a RCE exposed surface and a metallographic cross section prepared from the same electrode were examined using a scanning electron microscope. Figure 13 is a scanning electron micrograph of the RCE surface after four hours' exposure to the test solution at 10 N/m2 and shows that corrosion attack on the surface is non-uniform, certain areas still show the original grinding marks, while others are attacked. Figure 14 is a corresponding micrograph of the metallographic cross-section and shows an unattacked area protected by a thin (~ 2 microns thick) film, probably of iron carbonate. These observations are consistent with the interpretation of the AC impedance data, i.e., that the RCE has a heterogeneous, partially-filmed surface. Corrosion/Calculated Wall Shear Stress Data for AISI 1018 Carbon Steel RCEs of Various Diameters and for the Rotating strip Electrode in CO2-Brines Figure 15 shows the corrosion rate versus calculated wall shear stress results for the three diameters of RCE examined, i.e., 1.27, 2.54 and 3.79 cms in the basic test environment. These experiments were designed to examine the effect of RCE diameter on the observed corrosion rates. Reference to equation 3 shows that each of the electrodes has different mass transfer characteristics. Nevertheless, this figure shows essentially the same corrosion rate versus calculated wall shear stress for the three electrodes. Therefore the effect of electrode diameter on the RCE calculated shear stress is consistent with equation 1. Figure 16 shows a plot of the corrosion rate of AISI 1018 carbon steel determined using the LPR technique versus the calculated wall shear stress on the surface of the 1.27 cm diameter rotating strip cylinder electrode in the basic environment. The corresponding plot for the conventional RCE is shown for comparison. This shows that the corrosion rates measured on the strip are similar to those for the conventional cylinder electrode under equivalent environmental and wall shear stress conditions. Therefore the use of a rotating strip electrode does not overcome the basic problem of lack of correlation with pipe flow. Why does the RCE Corrosion Rate not Correlate with Pipe Flow? The AC impedance data and SEM examinations demonstrate that there are significant differences between the RCE and pipe flow electrodes. Specifically, there is increased polarization on the RCE, resulting from the presence of a film. These results suggest that a localized environment exists around the RCE that is more conducive to film formation and therefore less corrosive than in pipe flow. This is proposed to be the principal reason the RCE corrosion rate does not correlate with pipe flow and also why the RCE does not respond to the effects of solution chemistry in the same manner as pipe flow. Two interrelated factors may be responsible for the creation of this localized environment. First, the generally accepted equation for calculating the wall shear stress on a rotating cylinder electrode may overestimate the actual

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wall shear stress because the original equation was derived from drag measurements of rotating cylinders in various liquids. Drag measurements inevitably include stress contributions from within the liquid layers around the rotating cylinder. Therefore, only a portion of the measured wall shear stress may act on the surface, producing a lower corrosion rate than expected. A second potential factor is that there are unique features of the fluid flow around rotating cylinders, e.g., at low rotational rates, the formation of Taylor Vortices that contain significant radial components of fluid velocity. At higher rotational rates, these are considered to break up to produce fully turbulent flow.29 Therefore, the effect of fluid turbulence on the RCE corrosion rate of steel may be intrinsically different to that in pipe flow. Specifically, the fluid flow may be less effective in removing corrosion products from the surface which would produce a localized environment, different from and less corrosive than that produced in pipe flow. The corrosion results from the rotating strip electrode were similar to those obtained from the conventional RCE, and this agreement shows that even a narrow strip appears to create a localized environment similar to that of a conventional RCE. This observation is consistent with the major flow component from the cylinder being in a radial (perpendicular to the surface), rather than in a parallel direction to the surface as in pipe flow. The detailed nature of turbulent flow and its effect on corrosion has been discussed in a separate paper by the authors30. The above results and discussion may have implications for the evaluation of corrosion inhibitors using the RCE that are not addressed in the applicable ASTM Standard Practice31. First, the baseline, uninhibited corrosion rate measured in the RCE may be lower than that measured in pipe flow and therefore not representative of field corrosion rates. Second, if a localized chemical environment exists around the RCE (e.g., higher pH), then a specific corrosion inhibitor may exhibit a performance that is not representative of the field. Finally, the presence of surface films, such as iron carbonate, creates a very different interface for electrode-inhibitor interactions in terms of electrochemistry and adsorption characteristics. As noted, the RCE has significant advantages for the screening of corrosion inhibitors under flow conditions. However, caution is needed in interpreting these data and independent confirmation of performance should be conducted for final inhibitor selection. Studies by Papavinasam et al.32 and Mora-Mendoza et al.33 have also highlighted specific cautions in the use of the RCE related to simulation of localized corrosion processes and oxygen contamination respectively.

CONCLUSIONS 1. The RCE corrosion rate of carbon steel does not directly correlate with pipe flow corrosion

rate as a function of calculated wall shear stress, using the generally accepted equation for RCE wall shear stress.

2. The RCE corrosion rate of carbon steel does not directly correlate with pipe flow corrosion

rate as a function of calculated mass transfer coefficients. 3. The RCE corrosion rate responds very differently from pipe flow to changes in solution

chemistry. Based on these results, corrosion rates generated by the RCE cannot be used to scale-up flow-accelerated corrosion of carbon steel in sweet production.

4. The AC impedance data and scanning electron microscope (SEM) examinations show that

there are significant differences between the RCE and pipe flow electrodes. Specifically, there is increased polarization on the RCE, resulting from the presence of a film, confirmed by SEM examinations and probably consisting of iron carbonate. These results suggest that a localized environment exists around the RCE that is more conducive to film formation, and therefore, less corrosive than in pipe flow.

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in Electrochemical Corrosion Testing, May 1979, Ed. F. Mansfeld and U. Bertocci, pp. 150 166, ASTM, 1981.

28. W. J. Lorenz and F. Mansfeld, " Determination of Corrosion Rates by Electrochemical DC

and AC Methods, Corrosion Science, Vol. 21, No. 9, pp. 647 672, (1981). 29. Electrochemical Systems, J. Newman, Prentice-hall, New York, 1973. 30. K. D. Efird, E.J. Wright, J. A. Boros and T.G. Hailey, Wall Shear Stress and Flow

Accelerated Corrosion of Carbon Steel in Sweet Production., Technical Paper TS 14 194, 12th International Corrosion Congress, Houston, Texas, U.S.A., September 1993.

31. ASTM G 185-06: "Standard Practice for Evaluating and Qualifying Oil Field and Refinery

Corrosion Inhibitors Using the Rotating Cylinder Electrode (2006).

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32. S. Papavinasam, R. W. Revie, M. Attard, A. Demoz, and K. Michaelian, "Comparison of

laboratory methodologies to evaluate corrosion inhibitors for oil and gas pipelines", Corrosion Vol. 57, No. 10, pp: 897-912.

33. J. L. Mora-Menoza, J. G. Chacon-Nava, G. Zavala-Olivares, M. A. Gonzalez-Nunuez and

S. Turgoose, "Influence of turbulent flow on the localized corrosion processes of mild steel with inhibited aqueous carbon dioxide systems", Corrosion Vol. 58, No. 7, pp: 608-619.

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TABLE 1 NOMENCLATURE

Symbols R resistance ( m2) kinematic viscosity (m

2/s)

c concentration (mol/l) RCOR corrosion rate (mm/y, m/y) shear stress

d diameter r radius or radical distance (m) rotation rate (rad/s)

F Faradays constant (96487 T temperature (C) Subscripts

c/mol) U mean velocity (m/s) b mass average or bulk

i current density z Charge number of a species cor corrosion

k mass transfer coefficient (m/s) Greek Symbols d diffusion

L characteristic length (m) Density (kg/m3) lim limiting

P pressure (pa, kg/m s2) Dynamic viscosity (kg/m s) w wall or electrode surface

TABLE 2 ELECTROCHEMICAL DATA FOR AISI 1018 CARBON STEEL IN THE RCE AND PIPE

FLOW AT A CALCULATED WALL SHEAR STRESSES OF 10 N/m2.

Test Apparatus

Corrosion Potential vs. S.C.E (mV)

Solution Resistance (ohms-cm2)

Charge Transfer

Resistance1 (ohms-cm2)

Corrosion Rate from LPR2 (mpy)

Corrosion Rate from weight-loss

(mpy)

RCE (1.27 cm dia.)

-742 6.6 121 2.3 (93) 1.7 (69)

Pipe Flow (1.27 cm dia.)

-717 0.7 12.8 16 (641) 11.5 (460)

1. Charge transfer resistance estimated after the method of Epelboin et al. in reference 27. 2. Corrosion rate estimates derived from DC polarization resistance.

TABLE 3 POWER FUNCTION REGRESSION RESULTS FOR CARBON STEEL CORROSION

RATE VARIATION WITH SHEAR STRESS

FLOW APPARATUS R2 b a, mm/y (mpy)

1.27 cm Rotating Cylinder Electrode 0.77 0.101 2.8 (111)

1.27 cm Diameter Pipe 0.85 0.103 7.7 (304)

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Figure 1. Rotating Cylinder Electrode.

Figure 2. Schematic of Rotating Cylinder Electrochemical Cell.

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Figure 3. Limiting current density measured for the oxygen reduction reachion versus RCE rotational rate (rads/s)0.7 showing a linear relationship in accordance with Eisenberg et al 7.

Figure 4. The effect of wall shear stress on the flow accelerated corrosion of carbon steel for the RCE of 1.27 cm diameter.

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Figure 5. Comparison of the effect of wall shear stress on the flow accelerated corrosion of carbon steel for the RCE (1.27 cm) and pipe flow.

Figure 6. Comparison of the flow accelerated corrosion of carbon steel as a function of calculated mass transfer coefficient for the RCE and pipe flow.

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Figure 7. The effect of CO2 partial pressures on the flow accelerated corrosion of carbon steel as a function of wall shear stress for the RCE.

Figure 8. The effect of CO2 partial pressure of the flow accelerated corrosion of carbon steel as a function of wall shear stress for pipe flow.

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Figure 9. The effect of initial bicarbonate ion concentration on the flow accelerated corrosion of carbon steel as a function of wall shear stress for the RCE.

Figure 10. The effect of initial bicarbonate ion concentration on the flow accelerated corrosion of carbon steel as a function of wall shear stress for pipe flow.

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Figure 11. AC Impedance Nyquist plot for the flow accelerated corrosion of carbon steel in the RCE at a constant wall shear stress of 10 N/m2.

Figure 12. AC Impedance Nyquist plot for the corrosion of carbon steel in the pipe flow at constant wall shear stress of 10 N/m2.

Z' ohms-cm2

Z'' o

hms-cm

2

Z' ohms-cm2

Z'' o

hms-cm

2

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Figure 13. Scanning electron micrograph of RCE surface after four hours exposure at a shear stress of 10 N/m2 (magnification = 1K X).

Figure 14. Scanning electron micrograph of metallographic cross-section of RCE surface After four hours exposure at a shear stress of 10 N/m2 (magnification =10K X).

FeCO3 film

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Figure 15. The affect of electrode diameter on the flow accelerated corrosion of carbon steel for the RCE as a function of wall shear stress.

Figure 16. Comparison of the effect of wall shear stress on the flow accelerated corrosion of carbon steel for the conventional RCE and the rotating strip electrode.

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