Available online at http://ijcpe.uobaghdad.edu.iq and www.iasj.net Iraqi Journal of Chemical and Petroleum Engineering Vol.20 No.2 (June 2019) 41 – 50 EISSN: 2618-0707, PISSN: 1997-4884 Corresponding Authors: Name: Haider Hadi Jasim , Email: raidhani73@yahoo.com IJCPE is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Evaluation the Effect of Velocity and Temperature on the Corrosion Rate of Crude Oil Pipeline in the Presence of CO2/H2S Dissolved Gases Haider Hadi Jasim Chemical engineering department, College of engineering, Basrah University, Basrah, Iraq Abstract In this paper investigate the influences of dissolved CO2/H2S gases, crude oil velocity and temperature on the rate of corrosion of crude oil transmission pipelines of Maysan oil fields southern Iraq. The Potentiostatic corrosion test technique was conducted into two types of carbon steel pipeline (materials API 5L X60 and API 5L X80). The computer software ECE electronic corrosion engineer was used to predict the influences of CO2 partial pressure, the composition of crude oil, flow velocity of crude oil and percentage of material elements of carbon steel on the rate of corrosion. As a result, the carbon steel API 5L X80 indicates good and appropriate resistance to corrosion compared to carbon steel API 5L X60. The rate of corrosion acquired from the test in flow conditions is most significant than that in static conditions. The crude oil from Noor field has the largest value of corrosion rate, while the crude oil from Halfaya field has the lower value; other crude oils have moderate values. The dissolved CO2/H2S gases contribute by a low degree in internal pipeline corrosion because of the small concentrations. Keywords: CO2/H2S corrosion, crude oil velocity, chloride salts, Potentiostatic test, carbon steel Received on 02/12/2018, Accepted on 13/03/2019, published on 30/06/1029 https://doi.org/10.31699/IJCPE.2019.2.6 1- Introduction The pipelines are used to transport a crude oil or its products from one location to every other. Depending on the purpose, the oil pipelines can be labeled into three categories. First, gathering lines, this type has a small diameter and is used in short distances to handle crude oil. Second, transportation pipelines or feeders have a diameter greater than gathering pipelines and pass the oil product from storage or refinery units to distribution pipelines. Thirdly, a transmission pipeline, this type has the largest diameter and used to transport various grades of crude oil from the production area to the ultimate consumption area ‎[1]. Crude oil is a complex mixture contains different impurities which effect on the corrosion rate of metal such as sulfur, water, various dissolved ions (Na + , K + , Ca +2 , , Cl - …etc), organic acids, CO2 and H2S gases. Corrosion of carbon steel by CO2 gas is a significant problem in the crude oil transportation pipelines; CO2 gas dissolves in the water at different extents depending on temperature and pressure, which producing corrosive species. These species, besides the conventional salinity, producing the weak carbonic acid (H2CO3), bicarbonate ( ) and carbonate ( ) and becomes as driving the corrosion reactions depending on their concentrations and on pH levels ‎[2], ‎[3]. The electronic corrosion engineer (ECE) software is a good new tool that provides a useful approach for estimating the rate of corrosion of CO2 and H2S dissolved gases on different types of carbon steel material used in a pipeline and tubes for crude oil transportation. Various parameters that influence the corrosion in the presence of was taken by the ECE program include the CO2 partial pressure, temperature, and the crude oil velocity ‎[4]. The potentiostatic technique is one of the best electrochemical techniques used in corrosion testing for a quick time. This method includes the application of anodic discrete potential values in the scope of mV to the immersed specimen in a solution for a fixed time, and the potential-log current density curve is obtained. Analysis of the curve using Tafel technique can result in the corrosion current, which is used to calculate the value of corrosion rate ‎[5], ‎[6]. The problems of CO2 corrosion and its control of oil pipelines have been reported in many studies. Vuppu et al. ‎[7] studied the influence of temperature on the rate of corrosion and forms of protective carbonate layer on carbon steel pipelines at various crude oil velocities and oil-water mixtures. They show that both increases the temperatures and oil velocities have negative effect on protective carbonate layer formed on the steel surface. Xu et al. ‎[8] review the corrosion mechanism of CO2, H2S and SO2 of the inner side of oil pipelines. https://doi.org/10.31699/IJCPE.2019.2.6 H. H. Jasim / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 41 - 50 24 Their results show that the corrosion occurs as a result of the reactions between the crude oil compounds and the inner side of the pipelines through crude oil transport. Prakash et al. ‎[9] study the internal corrosion of the API 5L Grade-B carbons steel metal of Kuwait export crude oil pipeline network. Results of their study showed that in addition to the microbial effects, corrosion problems are caused by the effect of CO2 and H2S gases. Saleh et al. ‎[10] studied the corrosion rate of carbon steel pipe under various conditions including salinity, temperature velocity for both single and multiple flows of crude oil. Their result’s shows that for a single phase flow, the rate of metal corrosion in the NaCl brine solution is larger than in the multiphase flow (NaCl brine - CO2 - kerosene). Wang et al. ‎[11] reviewed and summarized the methods of measuring thecorrosion in oil-brine mixed conditions. They show that the effects of water and wetting behavior in corrosion process were very impressive in the case of flow conditions. Muhammed ‎[12] investigated the effects of soil properties contains by crude oil on the corrosion rate of API X70 steel pipeline used in oil transmissions from Iraq to Turkey. The results showed that the soil’s moisture content and pH had the most significant impact on the corrosion process. Mobbassar et al. ‎[13] demonstrated the factor influencing corrosion in inner oil pipeline made from low carbon alloy steel. They showed that analyzing the final corrosion product can help to select the most efficient and economical materials resistant to corrosion in the CO2 environment. Mursalov ‎[14] review the corrosion mechanism of H2S, the effect of pH value and temperature on the corrosion rate in the oil industry. The results of studies show that various oxides and chemical compounds are formed on the metal surface depending on the H2S concentration, pH value, and temperature and CO2 presence in the oil solutions. Cedeno et al. ‎[15] study by using the electrochemical measurements the effects of fluid speed, temperature and oil content on the corrosion rate of the AISI-SAE 1020 steel used in the petroleum transportation pipelines under a saturated CO2 gas. The results show that crude oil has the most significant effect of in the rate of corrosion of steel and increased as temperature and fluid velocity increased. Kolawole et al. ‎[16] give a comprehensive review of CO2 and H2S corrosion problems in API 5L steel pipeline and the proper methods to control the corrosion problems. They showed that the primary causes of corrosion in oil and gas pipelines are the presence of water together with acid gases (CO2 and H2S). This research aims to study the corrosion phenomenon of two types of carbon steel pipeline API 5L X60 and API 5L X80 used for the transport of crude oil through the use of Potentiostatic corrosion test method. Engineering corrosion software version 5.1 was used to calculate the rate of CO2/H2S corrosion of crude oil pipelines underneath different water fractions, temperature, velocity, and pressure of crude oil. 2- Experimental Methods 2.1. Pipeline Materials and Specimens Preparation In Iraq, there are two networks of pipelines extending over a distance of 300 km and transport crude oil from Maysan oil fields (Buzerkan, Halfaya and Faqa oil fields) to the port of Basrah southern Iraq. The networks pipeline materials are seamless carbon steels API 5L X80 and API 5L X60 and the chemical compositions are given in Table 1 and Table 2, respectively. The mechanical properties of these two materials are listed in Table 3. The API 5L X80 pipelines have a diameter of 122 cm, and the API 5L X60 have a diameter of 61 cm. Both of these pipelines have a thickness of 14 mm. Table 1. Chemical composition (wt. %) of API 5L X80 carbon steel ‎[17] Metals % Metals % C 0.08 Al 0.03 Si 0.26 Mo 0.27 Mn 1.75 Ni 0.22 P 0.011 Nb 0.07 S 0.0005 Fe Balance Table 2. Chemical composition (wt. %) of API 5L X60 carbon steel ‎[18] Metals % Metal % C 0.007 Al 0.008 Si 0.22 Cu 0.013 Mn 1.63 Ti 0.010 P 0.01 V 0.072 S 0.005 Nb 0.052 Cr 0.011 N 0.008 Ni 0.011 Mo 0.228 Table 3. Mechanical properties of API 5L X80 and API 5L X60 carbon steel ‎[17], ‎[18] Grade Yield strength MPa (Min.) Tensile strength MPa Elongation % (Min.) X80 555 Min. 625- 700 20 Min. X60 541 Min. 627 27 Min. The test specimens were prepared by cutting it in a cylindrical shape from a plate pipe material with a diameter of 1 cm and a thickness of 5 mm. According to ASTM G1-03 ‎[19] and ASTM G59- 97 ‎[20], the specimens were sequentially polished using silicon abrasive papers of 160, 180 and 200 grades, then washed with distilled water and dried with hot air in ambient conditions. The specimens are shown in Fig. 1. For each test, a new specimen was used for testing, a total of 50 specimens were used. H. H. Jasim / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 41 - 50 24 Fig. 1. Specimens for Potentiostatic test: A- X60 Carbon steel B- X80 Carbon steel 2.2. Crude Oil Samples Analysis The test oil samples used in this study were obtained from different oil fields in Maysan province (Noor, Buzerkan, Halfaya, Abu-Gharab, Faqa1 and Faqa2) southern Iraq. The material compositions of crude oils are analyzed in Nahr Umer laboratory of the south oil company and are listed in Table 4. The physical properties of crude oils obtained from field’s laboratory are summarized in Table 5. Table 4. Compositions of various crude oils Materials Fields Noor Bazerkan Faqa1 Faqa2 Halfaya Abu- Garab Iron ppm wt. 1.1 2.2 3.76 3.9 2.8 1.77 Copper ppm wt. 6.1 3 5.1 5.2 3.6 1.3 Vanadium ppm wt. 30 32 31 31 34 29 Nickel ppm wt. 8.7 10 11 11.6 7.1 11 Cadmium ppm 0.78 0.41 0.52 0.64 0.23 0.8 Aluminum ppm wt. 0.44 0.13 0.31 0.33 0.16 0.27 Manganese ppm wt. 0.30 0.17 0.22 0.22 0.27 0.10 Sulfur % 4.6 5.12 3.89 3.9 4.85 4.86 Water content % 2.4 1.85 1.96 2.05 2 2.3 NaCl ppm 44 31.5 24.4 22.3 14 11 H2S (g) ppm 0.32 0.21 0.42 0.40 0.49 0.45 CO2 (g) ppm 0.96 0.41 0.67 0.77 0.57 0.83 Table 5. Physical properties of crude oils Properties Fields Noor Bazergan Faqa1 Faqa2 Halfaya Abu -Gharab API 16 24.7 22.5 23.2 27 25 Asphaltenes % 13.8 13.2 10.5 12.6 15.1 14.8 Specific gravity at 20 o C 0.8792 0.8901 0.9048 0.912 8 0.8835 0.8584 Kinematics viscosity cst at 20 o C 33.5 45 40.1 33.6 44 39 Dielectric Constant 2.52 1.78 2.11 2.02 1.92 1.79 2.3. Potentiostatic Corrosion Test Techniques Fig. 2 demonstrates the collection of Potentiostatic corrosion test equipment includes (computerized M-Lap Potentiostatic, water bath and three electrodes corrosion cell. The three corrosion cell electrodes are a working electrode (the sample material), a counter electrode (Platonized Titanium (Pi-Ti)), and the Ag/AgCl reference electrode. Fig. 3 shows the electrochemical flow cell used in Potentiostatic test. Fig. 2. M-Lap Potentiostatic test collection Fig. 3. Electrochemical flow cell Due to the low conductivity of crude oil, the Potentiostatic corrosion test of the two types of pipelines material in crude oil is impossible. This test can increase the conductivity of crude oil through the use of the Potter method for conducting the Potentiostatic test ‎[21]. In the Potter method, a small quantity of toluene (methylbenzene C6H5CH3) was added to the crude oil solution. H. H. Jasim / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 41 - 50 22 The toluene is a solvent and selected for two reasons: first, the boiling point of the toluene must be higher than 60 o C and secondly, it must be causing the minimum interference. According to the Potter method, a 30 ml Toluene was added to 500 ml of crude oil and then added water to crude oil until the total water percentages reached to 5%. Then mix the results with a mixer to make sure the mixture is very well mixed and put it in the glass cell. A ligament is a round glass cell is used to ensure that the test electrolyte medium is isolated from the air environment. The glass cell is inserted into a temperature controller bath containing water solution and maintained constant (within ± 0.1 °C) to control the test solution temperature. For conducting Potentiostatic corrosion test, the specimen shall be a run under open circuit conditions for an exposure period of 60 minutes and the potential has been recorded at the end of the exposure period. The potential of ±250 mV is then applied to the measured open circuit potential of the working electrode at a scan rate of 0.5 mV/s. During the corrosion testing, a Potentiostatic test is carried out for a total time of 1 hour and new crude oil and new specimen are used for each test. The computer-operated and controlled M-Lap Potentiostatic device was used to record data (current, voltage and test time) during the test. The M-Lap science Bank-Electronics program version 5.1.exe is used. The output of the experiment test is a graph of the log current versus the potential plot. The curves result are analyzed using Tafel linearization method and final output can produce the corrosion current Icor.. Using the corrosion current density , the rate of corrosion (Cr) in mm/year was estimated by the following equation ‎[22]: (1) And (2) Where, As: Surface area of the specimen (cm 2 ). : Density of metal (g/cm 3 ). Icor : Corrosion current obtained from Tafel extrapolation method. The equivalent weight (Ew ) is given by the following relation ‎[23]: ∑ (3) Where, Ai : Atomic mass of the tested metal (g/mol). ni : Valiancy. Fi : Mass fraction of alloying elements. 3- Model of CO2 Corrosion Rate Calculation The corrosion model in the ECE program is based on the de Waard-Lotz corrosion model and two cathodic reactions are recognized ‎[24], ‎[25]: → (4) → (5) The metal anodic dissolution reaction balances the flow of electrons from these reactions, i.e. → (6) The basic rate of CO2 corrosion (Vcor.) is the combination of these two processes and given as: (7) Where: Vr and Vm represent the maximum kinetic corrosion rate of proton corrosion, and a mass transfer rate of the dissolved CO2. The equation for the proton reaction controlled part is ‎[26]: ( ) ( ) ( ) (8) Moreover, for the part controlled by mass transfer: (9) Where, T : Temperature ( o C), U : The velocity of crude oil (m/s). d: Pipe diameter in (m). : The pH of pure water saturated with CO2. : The fugacity of the CO2 (bar). pHactual: The pH resulting from the presence of dissolved salts. The result from the Eq.(7) is adjusted by the use of a number of multiplying factors for the presence of a protective scale ( ), H2S gas ( ) and crude oil ( ) on the base CO2 corrosion rate as followers: (10) Where: ( ) ( ) (11) (12) ( ) (13) And (14) H. H. Jasim / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 41 - 50 24 Where, : The angle of deviation from vertical. : Water friction in crude oil. : Partial pressure of gas in (bar). : Partial pressure of gas in (bar). API: The American Petroleum Institute gravity. The partial pressure of dissolved gas was calculated using Henry’s law as followers ‎[27]: (15) Where, : Henry’s constant 29.76 atm/ mol.lit. [CO2]: The concentration of dissolved CO2 gas in mol/lit. 3.2. Fugacity of CO2 Dissolved Gas ( ) The fugacity of CO2 ( ) is similar to its effective partial pressure and its expressed in Pa or atm., but corrected for non-ideality at high pressure and temperature. The basic equation for the correction of CO2 fugacity used in ECE software given as followers ‎[28]: ( ) ( ) ( ) (16) In ECE V.5.1, the maximum total pressure P is limited to 250 bar and the equation is adjusted slightly for temperatures above 80°C by scale factors to provide a better fit in the fugacity curves. 4- Results and Discussion The open circuit potentials or so called corrosion potential (Ecor.) of API 5L X60 and API 5L X80 carbon steel obtained from open circuit potential are illustrated in Table 6 at temperature 30 o C. The Potentiostatic test begins at a temperature of 30 o C due to the small values of corrosion potential obtained at 25 o C and it is not appropriate to conduct the tests. As shown in Table 6, the potential of the open circuit for different types of crude oil and steel has varied. Increasing the potential for corrosion shows an improvement in the performance of corrosion protection since the electrons transferred from the samples have decreased. The crude oil from Noor field has the lower potential, while crude oil from Halfaya field has the largest value; other crude oils types have moderate values. Fig. 4 shows polarization curves obtained from Potentiosatic test under static and flow condition of crude oil at 1 m/s and temperature 30 o C of the materials API 5L X60 and X80 carbon steel in the Noor crude oil. The slope of the polarization curves illustrated the velocity of reactions of the specimen’s surface with crude oil impurities. Both pipeline materials showed large oscillations of applied potential with measured current, and as the applied potential increased, the curve fluctuations increased. It also notes that a small change in the potential (ΔE) leads to a significant change in the current (ΔI), which means that the velocity of the corrosion reaction is high. The polarization curve obtained from testing of X60 steel in this type of crude oil showed a significantly large change in curves, which is attributed to the high reaction speed and then the transfer of electrons is larger than X80 steel. As indicates in the polarization curve the X60 steel shows high corrosion speed compared to the X80 steel. Table 6. Open circuit potential (corrosion potential (Ecor.) in mV) obtained from Potentiostatic test at T = 30 o C Static case Steel types Crude oil types Halfaya Faqa 1 Abu Gharab Faqa 2 Bazurkan Noor API 5L X80 -87 -104 -119 -133 -151 -160 API 5L X60 --130 -151 -163 -176 -198 -210 Flow case (1m/s) API 5L X80 -116 -136 -148 -161 -169 -180 API 5L X60 -160 -181 -201 -214 -227 -240 Fig. 4. Polarization curves under static and flow conditions in Noor crude oil at 30 o C By varying the temperature and types of crude oil and conduct the Potentiostatic corrosion analogue polarization curves of Fig.4 can be obtained. To obtain the corrosion current using the Tafel method, the polarization curves of Fig. 4 were analyzed. This was done using the software version 5.1.exe of the M-Lab science program to obtain the corrosion current. The equivalent weight of carbon steel X60 and X80 is calculated using Eq.(3) and the value of the corrosion rate is calculated using Eq.(1) at a constant temperature. The final results of the conclusions are illustrated in Fig. 5 A to F. Fig. 5 A to F show the rate of corrosion vs. temperature obtained from Potentiostatic test the pipeline materials in the six types of crude oils. Comparing the rates of corrosion results between these fields, it was found that the crude oil from Noor field shows the highest rate of corrosion, while crude oil from Halfaya field shows the least values; other crude oils show moderate values. H. H. Jasim / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 41 - 50 24 The rates of corrosion differ may be because the crude oil from Noor field has the highest water and impurities as given in Table 4 and Table 5. Both pipeline materials have a steady value of increases in the corrosion rate during the increases in temperature. The results of testing in the flow crude oil indicate that the corrosion current is significantly increased due to the effect of the crude oil velocity factor, which contributes to increasing the diffusion of different ions in steel surfaces during the test. Table 7 shows a comparison of the rate of corrosion between the two carbon steel materials API 5L X60 and API 5L X80 tested in the six types of crude oils at 30 o C. As shown in Table 7, when both materials tested in crude oil from Noor field, the carbon steel API 5L X60 has a corrosion rate of 0.190 mm/y and 0.295 mm/y in static and flow cases respectively, while the carbon steel API 5L X80 has a corrosion rate of 0.156 mm/y and 0.263 mm/y in static and flow cases respectively. The results of the test in crude oil from Halfaya field show that the carbon steel API 5L X60 has a corrosion rate of 0.081 mm/y and 0.092 mm/y in static and flow cases respectively, while API 5L X80 carbon steel has a corrosion rate of 0.042 mm/y and 0.053 mm/y in static and flow cases respectively. Other types of crude oils have moderates corrosion rate values. The crude oil flow is effects on the rate of corrosion by different methods. Compared to the static case, the corrosion ion and impurities transport in crude oil increases with crude oil flow. This accelerates the corrosion reaction within pipelines. Therefore, the rate of corrosion increased in case of flow compared to the static case. The flow effect on the inhibition element, in particular asphalt, which is collected at the bottom of the pipeline surface and thus weakens the inhibition element. On the other hand, higher crude oil flow leads to higher turbulence, which increases the wall shear stress and further increases the pipeline corrosion rate. The scouring effect of high-speed oil flows can quickly destroy films of corrosion products or prevent the formation of films due to corrosion, which reduces the effect of corrosion inhibitors. In static cases, the rate of corrosion is lower because the oil film protects the surface of the specimen from corrosion by reduces the contact of impurities and ions with the specimen surfaces. The conductivity of crude oils is one of the important factors affecting the corrosion rate values during the Potentiostatic test. It has depended on various factors including; water percent temperature, metal concentration, ion concentration, and presence of organic matter, dissolved gases and pH values. The current is relatively high at the start of the test and then started to decrease to lower steady values, this is due to ions and compounds that support corrosion reactions is removed and consumption. As the temperature of crude oil increases, the ions contained in crude oil contribute to increase the flow of electrons and move more freely through crude oil to the reference electrode and increase the oxidation and reduction processes, leading to faster rates of corrosion. Noor crude oil has the highest amount of metal compositions and water percent compared to other crude oils as given in Table 4. Chloride salts such as NaCl, CaCl2, MgCl2, it is found in crude oil with different concentrations. As the temperature or flow of crude oil increases, the mixture of this chloride salts with water in crude oil and hydrolysis increases. The hydrolysis of chloride salts in water leads to the initiation of a strong HCl acid, which increases the acidity of crude oil (pH value) and enhances the rate of corrosion. Chloride salt hydrolysis can be administered by the following reactions [25]: NaCl + H2O NaOH + HCl (17) MgCl2 + 2 H2O Mg(OH)2 + 2HCl (18) CaCl2 + 2 H2O Ca(OH)2 + 2HCl (19) The chloride salt NaCl and kinematics viscosity for Noor oilfields are 44 ppm and 35.5 cst, respectively, according to Table 4. Since the viscosity is low; this made the flow of crude oil is easily compared to other types. It also increases the contact of impurities contained in crude oil with the internal surface of pipelines leading to a faster rate of corrosion. Table 7. Corrosion rate values in (mm/year) obtained from test at T = 30 o C Static case Steel type Crude oil types Halfaya Faqa 1 Abu Gharab Faqa 2 Bazurkan Noor API 5L X80 0.042 0.059 0.094 0.116 0.132 0.156 API 5L X60 0.053 0.069 0.121 0.134 0.15 0.190 Flow case (1m/s) API 5L X80 0.081 0.130 0.152 0.163 0.204 0.263 API 5L X60 0.092 0.110 0.164 0.186 0.243 0.295 To illustrate the effect of concentration of CO2 and H2S dissolved gases in corrosion rate, engineering corrosion software (ECE) was used to calculate the corrosion rate at various temperatures for both types of pipeline material. The rate of corrosion for each type of steel tested in each type of crude oils is estimated at a constant temperature for a pipeline length of 100 m. For executing the ECE software program, the total pressure was taken as 50 bar and the pressure of dissolved CO2 gas was calculated according to the concentration of CO2 dissolved gas in each type of crude oils using Eq.(15). Then, the corrosion rate in the range of temperatures from 20 o C to 65 o C was obtained from ECE software program and drawn as a function of temperatures in Fig. 6. H. H. Jasim / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 41 - 50 24 Fig. 5. Corrosion rate vs. temperature for fields: A- Halfaya B-Faqa 1 C- Abu Gharab D-Faqa 2 E- Bazurkan F- Noor H. H. Jasim / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 41 - 50 24 As indicated in Fig. 6, due to the low concentration of dissolved gasses CO2 and H2S, which listed in Table 4, the corrosion rate is very small. Most corrosion rate occurs due to the effect of water and other impurities of crude oil in addition to the operating condition include temperature and pressure in pipes. Fig. 6. Effect of CO2 on corrosion rate at various temperatures for fields: A- Bazurkan B- Noor C- Faqa 2 D- Abu gharab E- Faqa 1 F- Halfaya The CO2 gas mixture with water forms weak carbonic acid (H2CO3) for both cases of static and flow, but the flow of crude oil has improved reactions, the transfer of ions and carried out more corrodes species to the exposure area of the steel specimen, this caused higher rates of corrosion. In general, both CO2 and H2S gasses in water were analyzed to form cations and anion compounds, and these increased reactions occurred in the anode and cathode parts of the Potentiostatic test. The following equations are illustrated the analyzed dissolved gasses CO2 and H2S in water: CO2 (g) + H2O (L) H + (aq) + HCO3 (aq) (20) H2S (g) + H2O (L) (aq) + (aq) (21) Fig. 7. Surfaces of specimens after test in the six different types of crude oils H. H. Jasim / Iraqi Journal of Chemical and Petroleum Engineering 20,2 (2019) 41 - 50 24 Fig. 7 illustrates the microscopic surface of specimens after conducted the Potentiostatic test in the six types of crude oil at ambient temperature. To remove crude oil deposits on the specimen's surface, kerosene was used to clean the specimens. There are two types of localized corrosion, pitting and general corrosion. The images of the surface of the specimens in Fig. 7 after the Potentiostatic test in static and flow conditions show that a rough and porous surface was produced, i.e. numerous cavities with small pits covering the surfaces. In the case of a flow condition, the number of cavities and pits are visually higher than that in a static condition due to the effect of flow. It is clear from the surface microstructures of Fig. 7, X60 shows more surface damage under flow conditions than in static conditions, and the pits were uniformly distributed over the surface, which was surrounded by a small area of corrosion. It also indicates the formation of oxides layer spread on the steel surface at various points obtained following a Potentiostatic test in crude oil solutions. A comparison of the specimen’s surfaces between static and flow conditions, it shows that the actual corrosion pits have more complex geometries in flow conditions with different pit depths and length than in static cases. 5- Conclusions Based on the results of this study, the following concluded are obtained: 1- The result of the Potentiostatic polarization tests showed that the polarization curves have a significant large oscillation associated with the initiation of unstable pits and high electrical resistance of crude oils. 2- The corrosion rate of CO2 and H2S dissolved gases obtained from ECE software program is minimal due to the small concentration of CO2 and H2S dissolved gases in the tested crude oil. 3- The X80 carbon steel has lower corrosion values, while the X60 carbon steel has largest values. 4- Noor crude oil has the highest corrosion rate value, while Halfaya crude oil has the lowest value; another crude oil has moderate values. 5- The corrosion rate in case of crude oil flow is larger than that obtained at static. 6- The surface of the samples tested showed pits and cavities covered the surfaces and both materials showed more damage in the surfaces under flow conditions than in a static case. 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