HIG H -TEM PERA TU RE, HIGH-PRESSURE ROTATING ELECTRODE SYSTEM

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1 International Pipeline Conference Volume I ASME 1998 IPC HIG H -TEM PERA TU RE, HIGH-PRESSURE ROTATING ELECTRODE SYSTEM S.Papavinasam and R.W.Revie CANMET/Materials Technology Laboratory Ottawa, Ontario Canada K1A OG1 ABSTRACT Corrosion in high pressure vessels, such as pipelines, furnaces, and steam generators, is influenced by composition (material and atmosphere), pressure, temperature and flow. To simulate the corrosion conditions in high pressure vessels, a simple system is necessary to control the parameters and measure instantaneous corrosion rates. This paper describes a simple, compact, and relatively inexpensive hightemperature, high-pressure rotating electrode (HTHPRE) system that can be used to control simultaneously pressure, temperature, and flow, and to measure instantaneous corrosion rates using electrochemical techniques. It can be used with corrosive gases, such as HZS and COz. Nomenclature: rp - wall shear stress AP - pressure drop AL - length of pipe d - specific diameter a - coefficient b - exponent Rp - polarization resistance IOT - corrosion current P Pc - constants, 0.1V. Key words: rotating electrode, high-pressure, high-temperature, pipeline, HZS, COj, flow, wall shear stress 1. INTRODUCTION Corrosion and its inhibition in gas atmospheres are highly complex processes. As a result, increasingly sophisticated evaluation techniques are being used to test inhibitors. Improvements in corrosion test methodologies are aimed at simulating field corrosion problems in the laboratory in a compressed time-scale. In order for a laboratory methodology to be useful and valuable, it must be relevant and predictive. Various parameters that influence corrosion rates, and hence, inhibitor performance, in a given system are: composition of material, gas and liquid, temperature, flow and pressure. In order for a test method to be relevant to a particular system, it should be possible to control various parameters that influence corrosion in that system. A test method is considered to be predictive if it can generate information regarding type of corrosion, general and pitting corrosion rates, nature of inhibition and life of inhibitor film (or adsorbed layer). Minister of Natural Resources Canada, 1998.

2 In this paper various parameters that influence corrosion rate and inhibitor performance are analyzed. Merits of various test methods to evaluate the performance of inhibitors are reviewed. A new high-temperature, highpressure rotating electrode (HTHPRE) system to control simultaneously pressure, temperature, flow and composition (steel, solution, and gas phase) in the laboratory is described. 2. VARIABLES 2.1 Flow Many corrosion experiments in flowing media have demonstrated that there is no simple correlation between corrosion and flow rates. The reason for this is that dimensionless parameters that are used to describe flow regime describe only bulk or average properties of the system, which are not directly related to the local forces responsible for accelerated local attack. The fluid flow on a metal surface induces corrosion either by impinging the second phase particles contained in the fluid on the surface (erosion-corrosion) or by increasing turbulence and mass transfer (flow-accelerated corrosion). Wall shear stress is used as a measure of turbulence. Wall shear stress is found to be simply a geometry-independent indicator of the degree of turbulence in the flow that can be effectively used for correlation of flow-accelerated corrosion in laboratory tests to field applications. This correlation exists whether the corrosion is controlled by charge transfer or mass transfer. The pipe wall shear stress, tp, can be measured experimentally in terms of the pressure drop (AP) over a specific length of pipe (AL) with a specific diameter (d)1: APd xp= (1) 4 AL The wall shear stress is related to the corrosion rate, C.R = a Tpb. The values of the coefficient, a, and exponent, b, are functions of the specific environment and solution chemistry. rp includes fluid flow parameters, such as fluid density, viscosity, and surface roughness. 2.2 Temperature Higher temperature generally increases the corrosion rate because of the accelerated electrochemical and chemical reactions. However, rate of precipitation increases with temperature; hence, elevated temperature may reduce corrosion rate when protective films are formed. Influence of temperature on inhibitor performance is quite complex. For inhibitors that physically adsorb on the metal surface, increasing temperature increases corrosion rate, as elevated temperature facilitates desorption. On the other hand, for those inhibitors that chemisorb on the metal surface, the chemical bond strength increases with temperature, and hence, corrosion rate decreases with temperature up to a certain temperature Composition Compositions of corroding material, gas phase and liquid phase influence corrosion rate. Variations of metallurgy affect both corrosion rates and types of corrosion. Extensive studies have been conducted on the effect of dissolved salts and gases on corrosion rates3'*. The factors controlling corrosion include concentrations of chloride, bicarbonate, hydrogen sulfide, and carbon dioxide, as well as ph. It has also been reported that the type of crude oil may also influence the corrosion rate Pressure Higher pressure (or partial pressure of corrosive species), may induce greater corrosion by increasing the dissolution of corrosive species and protective film on the metal surface. In view of this complex scenario, any test that is carried out to evaluate the performance of a corrosion inhibitor for a particular system should be carried out under conditions that are representative of all conditions of the system. Based on this consideration, the merits of various tests that are used to evaluate the performance of inhibitors are reviewed in the following section. 3. METHODOLOGIES 3.1 Wheel Test The wheel test6 is a very widely-used test in the industry for comparing or screening inhibitors. The wheel test is considered versatile in that the procedure may be adjusted to test a variety of inhibitors and may be performed on various test specimens. During the wheel test, the only variables that can be controlled are temperature, and composition. There is no mathematical correlation between the corrosion rate in the wheel test and those of an operating pipeline. The wheel test can be regarded as a screening test in a preliminary stage of inhibitor evaluation.

3 3.2 Rotating Cylinder Electrode (RCE) The RCE test system is compact, relatively inexpensive, and easily controlled. It provides stable and reproducible flow in relatively small volumes of fluid. At very low electrode rotation speeds, the flow around the RCE is laminar and occurs in concentric circles around the cylinder7. As rotational speeds increase further, the flow becomes fully turbulent, and eddies increasingly break up the regular flow pattern. The transition to fully turbulent flow occurs at Re 200. In the turbulent flow region, the rotating cylinder electrode can be applied to simulate flow behaviour by hydrodynamic analysis. RCE experiments are widely used to screen inhibitors. In RCE experiments, the variables such as composition, temperature and flow can be controlled; however, these experiments cannot be performed at elevated pressure using currently available commercial system. As a result, the influence of pressure on corrosion rate cannot be assessed using RCE. 3.3 Rotating Disk Electrode (RDE) The rotating disk system is simple and provides information quickly and inexpensively. It has a uniformly accessible surface. It creates a threedimensional boundary layer flow in a simple system. Therefore, the rotating disk system can simulate real, three-dimensional flow. All the transport equations can be solved analytically for the laminar flow of a rotating disk system. Even if the rotating disk is under turbulent flow, the mass transfer and fluid flow relationships can be formulated as in pipe flow. The criticisms for extrapolating experimental data from the rotating disk system to pipe flow are: (i) the rotating disk is usually under laminar flow while the pipe flow is turbulent and (ii) non-similarity of the geometry of the two systems. One way to resolve these problems is to model the systems separately and then compare the two models so that their relationship can be established. Because the corrosion rate may be controlled by different factors, a model for different geometries must allow for the possibility of rate control. Mathematical correlations between ROE flow and pipe flow have been recently established*. Using RDE, the composition, flow and temperature can be controlled, but the effect of pressure on the corrosion rate cannot be evaluated using available commercial systems. In addition, the commercially available RDE is not recommended at temperatures above 70 C nor in the presence of H2S. RDE experiments are not extensively used to evaluate corrosion inhibitors. 3.4 Rotating Cage In a rotating cage, the coupons are held between two teflon disks backed by steel plates and mounted on the stirring shaft9. Grooves are cut into the teflon disks to prevent the coupons from slipping at high rotational speeds. These grooves contribute to different types of corrosion artifacts. Local high turbulence at the leading and trailing edges of the grooves increase localized corrosion rates. A decrease in corrosion rate is observed inside the groove where the coupon is protected from the turbulent flow. The crevice formed by the coupon and the groove in the teflon disk provides a site for a protective scale to build up. In addition, holes are drilled in both the top and bottom of the cage, positioned so as to force fluids to the inside of the cage, so that turbulence on the inside surface of the coupon can be increased. During a rotating cage experiment, flow can be varied by altering the speed of the stirrer. 3.5 Jet Impingement The jet impingement test can reproducibly simulate high turbulence conditions at high temperature and pressure for gas, liquid, and multiphase turbulent systems. It requires relatively small volumes of test fluids and is controlled easily. Jet impingement tests are amenable for the use of electrochemical methods in high-temperature and high-pressure environments and for use with gas, liquid, and mixed-phase fluids (gas/liquid and two or more immiscible liquids with gas). Using the jet impingement system, all the variables, composition, flow, pressure, and temperature, can be controlled. 3.6 Flow Loop The flow loop simulates the flow regime of tubular flow in producing wells; however, the apparatus is very large and the experiments are expensive and time-consuming. In addition, controlling various parameters inside a flow loop involves considerable sophistication with respect to experimental design and operation. The controllable variables in various laboratory methodologies are presented in Fig. 1. Jet impingement and rotating cage are the only methodologies that can be used to control all four variables. In this paper, we demonstrate another methodology, high-temperature, high-pressure, rotating electrode (HTHPRE), to control simultaneously the composition, temperature, pressure, and flow, and to determine instantaneous corrosion rates using electrochemical techniques.

4 3.7 High-Temperature High-Pressure Rotating Electrode Conventional rotating electrode systems can function up to only 70 C and atmospheric pressure. Great care must be taken when using these systems in corrosive environments, such as H2S. In addition, it is very difficult to isolate the solutions used in these experiments from atmospheric oxygen. In order to overcome these obstacles and also to perform continuous electrochemical experiments at elevated pressure and temperature using rotating cylinder and disk electrodes, CANMET has designed and constructed a high-temperaturejiigh-pressure rotating electrode (HTHPRE) system. Using this system, it is feasible to cany out experiments with rotating cylinder (or disk) electrodes at high pressure and high temperature. A HTHPRE system should possess an electrically isolated electrode system, an electrically isolated motor for rotating the electrode, and a vessel that can withstand high pressure without leakage. Design of the vessel that can be used in hydrogen sulfide atmosphere is shown in Fig. 2. A standard one-litre Panautoclave was modified by lining it on the inside with Teflon. The stainless steel stirring rod was modified by drilling a hole in the rod into which a Teflon insulator was inserted. In addition, three O-rings were used to prevent leakage. Inside the Teflon insulator a metal rod was introduced. One end of the metal rod was threaded so that cylindrical (RCE) or disk (RDE) specimens could be attached. The other end of the rod, projecting slightly above the motor unit, was attached directly to the rotating motor, through which the electrical connection was made. The stainless steel rod was rotated by a separate motor connected to the rod using a belt. The counter and reference electrodes were replaceable carbon steel rods inserted inside the autoclave in Teflon lined stainless steel tubes. Experiments can be performed using a rotating cylinder or disc electrode as the working electrode and separate carbon steel rods as the reference and counter electrodes. Linear polarization (LPR) and electrochemical impedance spectroscopy (EIS) can be carried out using standard electrochemical instruments. The system was tested under various environments pertaining to pipeline operating conditions. At this stage, experiments can be carried out at pressures up to 1000 psi, at rotation speeds up to 5000 rpm and at temperatures up to 90 C in the presence of H2S atmosphere. 4. RESULTS AND DISCUSSION Using the HTHPRE, instantaneous corrosion rates were measured using LPR and EIS techniques at various total pressures, partial pressures of H2S, temperatures and rotation speeds. All experiments were carried out in synthetic formation water. The composition is presented in Table 1. Table 1: Composition of Formation Water Salt Concentration (gm/l) CaCI2.2H NaCL, MgCl2.6H AH experiments were carried out at 50 C. Linear polarization (LPR) and electrochemical impedance spectroscopy (EIS) were carried out A Solarton Model SI 1287 Electrochemical Interface was used to obtain linear polarization data, and a Solarton 1250 frequency response analyzer was used for EIS experiments. The LPR experiments were carried out by scanning at 0.1 mv/second from the potential 20 mv negative to the corrosion potential to that 20 mv positive to the corrosion potential. From the slope of the linear plot of potential vs current, polarization resistance was calculated. From the polarization resistance (R,), the corrosion current (I^) was calculated using the Stem-Geary equation: assuming P, = Pc = 0.1V, IOT = /Rp. The factor used to convert from IOT in pa/cm2 to corrosion rate in mm/year was 8.8 pa/cm2 = 0.1 mm/y and 1 mm/y = mpy. The EIS experiments were carried out at the corrosion potential in the frequency range 0.01 to 20,000 Hz and with an AC amplitude of 10 mv. The polarization resistance, Rp, was calculated by analysing the impedance spectra by equivalent circuit method. From the Rp values, the corrosion rate was calculated by the same way as in the LPR method. Several experiments were carried out to validate the HTHPRE system for high temperature and high pressure operation in the presence of H2S. Unless or otherwise specified, all experiments reported in the paper were carried out at 300 psi pressure (50 psi H2S, Bal. argon),

5 and at 50 C. Experiments were carried out at 0,500, 1000, and 2000 RPM rotation speeds. Figure 3 shows representative LPR curves obtained at 300 psi, 50 C, and 500 RPM rotation speed using a rotating cylinder electrode. Figure 4 presents a typical EIS curve obtained under the same conditions using a disc electrode. These figures demonstrate clearly that instantaneous corrosion rates can be measured in the laboratory while controlling the pressure, temperature and flow velocity. Figure 5 presents the corrosion rates obtained at various rotation speeds using a rotating cylinder electrode. After establishing steady state conditions, typically 3 hours after experimental set-up, the corrosion rate was measured at 0 rpm. Immediately after these measurements the rotation speed was set at 500 RPM, and the system was allowed to attain steady-state again, typically for an hour. In this manner, the experiments were also performed at 1000 and 2000 RPM speeds. The experiments were repeated four times. 5. CONCLUSIONS: 1. To evaluate corrosion inhibitors in the laboratory, four variables, composition, flow, pressure and temperature, should be controlled. 2. Using the high-temperature, high-pressure rotating electrode (HTHPRE) described in this paper, all four variables can be controlled simultaneously and corrosion rates can be measured instantaneously using LPR and EIS techniques. In addition, HTHPRE can be operated in the presence of H2S. 6. ACKNOWLEDGEMENT: The authors would like to acknowledge the helpful discussions with the members of the CANMET/Industry Consortium on Development of Standardized Methodologies for Evaluation and Qualification of Inhibitors for Sour Service and financial support from the Federal Interdepartmental Program of Energy R&D (PERD). In addition, the authors would like to acknowledge the technical help from the Materials Technology Laboratories, Yves Lafreniere and the Engineering and Technical Services Division, Ken Grenzowski, and Philippe Gosselin. Correlation of Steel Corrosion in Pipe Flow with Jet Impingement and Rotating Cylinder Tests, C orrosion 49, 992 (1993). 2. Metallic Corrosion Inhibitors, Chapter: Influence of Temperature on the Action of Inhibitors, I.N.Putilova, S.A.Balezin, and V.P.Barannik, 26 (1960) Pergamon Press. 3. K.Denpo and H.Ogawa; Fluid Flow Effects on C02 Corrosion Resistance of Oil Well Materials, C orrosion 49,442 (1993). 4. S.Nesic; Comparison of the Rotating Cylinder and Pipe Flow Tests for Flow-Sensitive Carbon Dioxide Corrosion, C orrosion 51,773 (1995). 5. K.D.Efird and RJ.Jasinski; Effect of the Crude Oil on Corrosion of Steel in Crude Oil/Brine Production, C orrosion 45, 165 (1989). 6. Wheel Test Method Used for Evaluation of Film Persistent Inhibitors for Oilfield Applications, NACE Report, M aterials P erform ance 21, 12,45 (1982). 7. D.R.Gabe; The Rotating Cylinder Electrode, J. A p p lied E lectroch em istry 4,91 (1974). 8. G.Liu, D.A.Tree, and M.S.High; Relationships between Rotating Disk Corrosion Measurements and Corrosion in Pipe Flow, C orrosion 50, 8, 584 (1994). 9. G.Schmitt, W.Bruckhoff, K.Faessler, and G.BIummel, Flow Loop versus Rotating Probes - Correlations between Experimental Results and Service Applications, N A C E C O RRO SIO N /90 C onference Paper Number 23 ( 1990). 7. REFERENCES: 1. K.D.Efird, E.J. Wright, J.A.Boros, and T.G.Hailey;

6 Fig. 1 : Controllable Variables in Laboratory Methodologies n Temperature : ^ : ^ Pressure S llllllll Flow... ^ ? g! Composition \ HTHPRDE HTHPRCE HTHPJT Rotating Caga RDE RCE JT Static 346 o 3 «*c a > Static «C o Atmospheric Pressura Experiments Elevated Pressure Experiments

7 ASM E Electrical Contact Unit; 2. Techometer (Rotation Speed Display); 3. Rotation controller; 4. Electrochemical Instruments; S. Rotating electrode units (working electrode); 6. Reference Electrode; 7. Water Cooler Coil; 8. Inlet (Gases and Solution); 9. Thermocouple; 10. Outlet (Gases and Solutions); 11. Counter Electrode; 12. Autoclave Body; 13. Solution; and 14. Teflon Liner. Fig. 2: Schematic Diagram o f HTHPRE System

8 Current, A

9 s Real Axis (O hm s)

10 Fig. 5: Variation of Corrosion Rate with Rotating Speed of HTHPRCE at 300 psi (H2S partial pressure 50 psi) C oir. Rate, mpy