The Pennsylvania State University. The Graduate School. Engineering Science and Mechanics

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1 The Pennsylvania State University The Graduate School Engineering Science and Mechanics DEVELOPMENT OF NEW MULTIFUNCTIONAL COATINGS FOR PROTECTION AGAINST EROSION AND CORROSION IN QATAR OIL AND GAS PRODUCTION A Thesis in Engineering Science and Mechanics by Isin Dizvay 2014 Isin Dizvay Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science August 2014

2 The thesis of Isin Dizvay was reviewed and approved* by the following: Barbara Shaw Professor of Engineering Science and Mechanics Thesis Advisor Elzbieta Sikora Research Associate in Engineering Science and Mechanics Thesis Advisor Judith A. Todd Head of the Department of P. B. Breneman *Signatures are on file in the Graduate School ii

3 Abstract This thesis investigates some of the possible components of a multifunctional coating system to peotect against erosion and corrosion for the pipelines in Qatar oil and gas production. The coating system is divided in two parts according to the functions; the sacrificial and the barrier layer. The candidates were evaluated separately against only corrosion. Erosion studies are part of the overall program, but not part of this thesis. Zn, Al 1100 and Galvalume were chosen as the constituents of the sacrificial layer. A group of panels coated with organic coating were donated by NOV Tuboscope and were tested as the barrier layer. Also, three steel substrates (bare, burned-off and white-metal finish) were tested to observe the substrate behavior. The Zn, Al 1100, Galvalume coated steel samples, and the steel panels were tested for a week under constant immersion, while the coated panels were tested for weeks under constant immersion. The testing and constant immersion environments simulated to the temperature, chloride concentration, and ph of the environment in the oil and gas pipelines in Qatar. The solution used in these two environments was a-2000 ppm Cl - containing CO2 saturated NaCl-HCl mixture at a ph of 3.5 and at 60 o C. To investigate the corrosion behavior of the Zn, Al 1100, Galvalume, Open Circuit Potential, Polarization Resistance measurements and Electrochemical Impedance Spectroscopy scans were conducted. Polarization Resistance was not conducted for the coated panels. The data obtained from Open Circuit Potential and Polarization Resistance experiments were analyzed via Echem Analyst (from Gamry Software) and the data obtained from Electrochemical Impedance Spectroscopy was analyzed via Zview (version 3.2b) by fitting the data to electric circuit models. Even though the corrosion behavior of Zn, Al 1100 and Galvalume was similar, the corrosion rate of the samples showed that Al 1100 samples lasted longer. Therefore, they are more promising for as candidates for the sacrifical layer. As expected, substrated steel presented very low corrosion resistance for the entire time of testing. Almost all of the organic coatings showed no deterioration visually, though some of the coatings exhibited lower coating resistance values than their initial value. The parameters affecting the coating resistance and the coating capacitance are the filler amount, the coating thickness, and the coating type. In addition, one of the coatings on the panels was subjected to the scratch study. A handmade artifact was made on the coating to observe the healing properties of the coating after a defect was observed on the surface. iii

4 TABLE OF CONTENTS List of Figures List of Tables Acknowledgements vii xiii xv 1. Introduction 1 2. Literature Review Variables of corrosion mechanism ph Temperature Pressure Presence of Cl - /Salinity Multiphase environment CO 2 content H 2 S content Erosion Components of coating system Sacrificial layer Zn alloys Al alloys Al-Zn alloys Barrier layer Experimental Methods Specimens Panels Metals and alloys 45 iv

5 3.2 Equipment Heating baths Potentiostats Reference and counter electrodes Experimental environment Immersion environment Testing environment Characterization techniques and testing parameters Open Circuit Potential (OCP) Linear Polarization Resistance (LPR) Electrochemical Impedance Spectroscopy (EIS) Zview Results and Discussion Metals and Alloys Al Zn Galvalume Uncoated steel substrate panels Coated Panels TK 805 Powder Coatings with Low Amount of Filler TK 805 Powder Coatings with Standard Amount of Filler TK 805 Powder Coatings with High Amount of Filler TK 34XT Liquid Coatings with Low Amount of Filler Defected panels studies General Trends 96 v

6 5. Conclusion and Future Work 104 References 106 vi

7 LIST OF FIGURES Figure 2-1 LPR corrosion rates vs time for temperatures 80, 120, 150 and 200 o C, and at ph 4.0 and Figure 2-2 Polarization resistance curves of Zn at ph 1, 4, 7, 10, and 13 in 0.1 M NaCl solution 6 Figure 2-3 Solubility values of FeCO 3 from literature as a function of the reciprocal of temperature in 0.1 to 1.0 mol/kg NaClO 4 or 0.1 to 5.5 mol/kg NaCl solutions with CO 2 pressure from 0.01 to 0.93 bars 7 Figure 2 4 Specific mass loss as a function of time, tempering temperature and testing temperature in substitute ocean water slurry at ph Figure 2-5 Effect of temperature on corrosion rate of X70 steel in 3%NaCl stagnant soln saturated with CO 2 9 Figure 2-6 Corrosion rate vs total pressure in oil and gas pipelines 10 Figure 2-7 Nyquist plots of J55 as a function of NaCl concentration at 25 o C, ph of 4, 300 rpm stirring speed, and 0.97 bar of CO 2 partial pressure 12 Figure 2-8 Corrosion density and potential of API-X100 steel as a function of NaCl concentration at 20 o C and 90 o C 12 Figure 2-9 Phase inversions for different compositions of oil-water system 14 Figure 2-10 Nyquist plot of X65 in solution in different oil concentrated solutions at 60 o C and 3000 rpm electrode rotating speed 15 Figure 2-11 General corrosion rate as a function of exposure time at different CO 2 partial pressures in DI water, 70 o C, ph= Figure 2-12 Corrosion rate of tempered martensite steel as a function of temperature and P CO2 at ph 18 Figure 2-13 Total weight loss of API X65 pipeline steel as a function sand loading and flow rates at 20 o C 20 Figure 2-14 Open circuit potentials with respect to sand loading in 3.5% NaCl solution at 18 o C for a) UNS 31603, and b) UNS Figure 2-15 Photographs of cross-sectioned black pipe, galvanized pipe exposed to quiescent tap water for 12 months 23 Figure 2-16 Corrosion rates as a function of time of hot dip coatings in tidal zone 24 vii

8 Figure 2-17 Corrosion potentials of each hot dip coated sample vs SCE and galvanic corrosion potentials of hot dip coatings paired with steel vs SCE 25 Figure 2-18 Nyquist plots Al coated steel samples in 3.5 wt % NaCl for different exposure times 27 Figure 2-19 Images of the bare steels and coated specimens with different alumina percentages before and after the corrosion tests 28 Figure 2-20 Test rig at the coastal test site 30 Figure 2-21 Schematic of self-made impingement corrosion device 32 Figure 2-22 Change of weight of the samples during impingement test 33 Figure 2-23 Weight loss of HPOV and APS coatings in centrifugal erosion tester at 6000 rpm with 1 kg quartz sand 35 Figure 2-24 SEM images of a) HVOF_AT2 and b) APS_AT2 after erosion wear test at 90 o impact angle 36 Figure 2-25 SEM micrograph and EDS analysis of a) WC-10Co-4Cr powder, b) Al 2 O 3 +13TiO 2 powder 37 Figure 2-26 SEM micrograph of WC-10Co-4Cr (left), Al 2 O 3 +13TiO 2 on CF8M steel substrate 37 Figure 2-27 Erosion rate of uncoated, WC-10Co-4Cr-coated, and Al 2 O 3 +13TiO 2 -coated CF8M steel at 2250 and 4500 rpm 38 Figure 3-1 Tube-Kote Process- manufactured coatings on the panel 43 Figure 3-2 Connection area of the panel prior coating with epoxy 44 Figure 3-3 Water baths used in the experiments 46 Figure 3-4 Ag/AgCl reference electrode and the graphite counter electrodes used in the experiments 48 Figure 3-5 Immersion environment for the panels 49 Figure 3-6 Testing environment for all samples 50 Figure 3-7 OCP parameter entry screen for Reference 600 potentiostat, displaying the parameters selected for use during immersion testing of samples in CO 2 saturated NaCl+HCl solution, at 60 o C, ph of viii

9 Figure 3-8 LPR parameter entry screen for Reference 600, displaying the parameters selected for use during the immersion of metal and alloy samples in CO 2 saturated NaCL+HCL solution, at 60 o C, ph of Figure 3.9 EIS parameter entry screen for Reference 600, displaying the parameters selected for use during the immersion of metal and alloy samples in CO 2 saturated NaCL+HCL solution, at 60 o C, ph of Figure 4-1 Corrosion rates of the Al 1100 samples (#1, #2, and #3) via polarization resistance and electrochemical impedance spectroscopy in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion 58 Figure 4-2 Nyquist and Bode plots of the 2 nd Al 1100 sample from EIS in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion 59 Figure 4-3 Corrosion rates of the Al 1100 samples (#1, #2, and #3) via polarization resistance and electrochemical impedance spectroscopy in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion 61 Figure 4-4 Nyquist and Bode plots of the 2 nd Zn sample from EIS in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion 62 Figure 4-5 The corrosion rates of the Galvalume samples (#1, #2, and #3) via polarization resistance and electrochemical impedance spectroscopy in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph Figure 4-6 Nyquist and Bode plots of the 2 nd Galvalume sample from EIS in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion 65 Figure 4-7 The corrosion rates of the the bare (J1), burned off (J2) and white metal finish painted (J3) carbon steel panels via polarization resistance and electrochemical impedance spectroscopy in 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion 67 Figure 4-8 Electrical circuits for: an intact coating and a coating with a defect 68 Figure 4-9 Resistance of the coatings-estimated from EIS results- on 6B (P, low f, mils) and 6E(P, low f, mils) panels versus time during 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of ix

10 Figure 4-10 Capacitance of the coatings-estimated from EIS results- on 6B (P, low f, mils) and 6E(P, low f, mils) panels versus time during 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-11 Nyquist and Bode plots 6B (11-14 mils TK 805-powder coating with low amount of filler) after 6 hours, 5 days, and 14 weeks of immersion and 6E (4.5-6 mils TK 805-powder coating with low amount of filler) after 6 hours, 5 days, and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-12 Resistance of the coatings-estimated from EIS results- on A (P, std f, mils) and C (P, std f, mils) panels versus time during 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-13 Capacitance of the coatings-estimated from EIS results- on A (P, std f, mils) and C (P, std f, mils) panels versus time during 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-14 Nyquist and Bode plots 1A (4.5-6 mils TK 805-powder coating with standard amount of filler) after 5 days, 2, 10, and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-15 Nyquist and Bode plots 4A (4.5-6 mils TK 805-powder coating with standard amount of filler) after 5 days, 1 week, and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-16 Nyquist and Bode plots 2C and 4C (13-14 mils TK 805-powder coating with standard amount of filler) after 1 week, and 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-17 Resistance of the coatings-estimated from EIS results- on G (P, high f, mils) and D (P, std f, 8-12 mils) panels versus time during 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-18 Capacitance of the coatings-estimated from EIS results- on G (P, high f, mils) and D (P, std f, 8-12 mils) panels versus time during 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-19 Nyquist and Bode plots 1G (4.5-6 mils TK 805-powder coating with high amount of filler) after 24 hours, 3, 4, 6, 10, and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-20 Nyquist and Bode plots 5G (4.5-6 mils TK 805-powder coating with high amount of filler) after 24 hours, 10 days, 6, and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of x

11 Figure 4-21 Nyquist and Bode plots 1D and 6D (8-12 mils TK 805-powder coating with high amount of filler) after 1 week and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-22 Resistance of the coatings-estimated from EIS results- on H (L, std f, 3 mils) and I (L, std f, mils) panels versus time during 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-23 Capacitance of the coatings-estimated from EIS results- on H (L, std f, 3 mils) and I (L, std f, mils) panels versus time during 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-24 Nyquist and Bode plots 1H2 (3 mils TK 34XT-liquid coating with standard amount of filler) after 10, 11 days, 4, 6, 10, 12, and 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-25 Resistance of high filler TK 805 coating vs.time 86 Figure 4-26 Nyquist and Bode plots for 1I and 1I2 (6.5-7 mils TK 34XT-liquid coating with standard amount of filler) after 10 days, 3, 8, and 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-27 Resistance of the coatings-estimated from EIS results- on 1B (P, low f, mils) and 1E (P, low f, mils) panels versus time during 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-28 Capacitance of the coatings-estimated from EIS results- on 1B (P, low f, mils) and 1E (P, low f, mils) panels versus time during 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-29 Nyquist and Bode plots 1B (11-14 mils TK 805-powder coating with low amount of filler) after 6, 24 hours, 5 days and 1 week of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-30 Hand-made scratch on 1B at its 4 th week of immersion 91 Figure 4-31 Nyquist and Bode plots 1B (11-14 mils TK 805-powder coating with low amount of filler) after scratched ( after 4 weeks of immersion), and 3, 4 days of immersion after the scratch, and 6 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-32 Nyquist and Bode plots 1B (11-14 mils TK 805-powder coating with low amount of filler) 4 days after the scratch and after 8 and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of xi

12 Figure 4-33 Nyquist and Bode plots 1E (4.5-6 mils TK 805-powder coating with low amount of filler) after 1, 4, 8, 10 and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-34 Nyquist and Bode plots 4A (4.5-6 mils TK 805-powder coating with standard amount of filler) after 2 and 18 weeks of immersion, 5G (4.5-6 mils TK 805-powder coating with high amount of filler) after 2 and 14 weeks of immersion and 6E (4.5-6 mils TK 805-powder coating with low amount of filler) after 2 and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-35 Nyquist and Bode plots 4C (13-14 mils TK 805-powder coating with standard amount of filler) after 1 week and 18 weeks of immersion, 6B (11-14 mils TK 805-powder coating with low amount of filler) after 1 week and 14 weeks of immersion and 6D (8-12 mils TK 805- powder coating with high amount of filler) after 1 week and 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-36 Nyquist and Bode plots 4A (4.5-6 mils TK 805-powder coating with standard amount of filler) and 1H2 (3 mils TK 34 XT-liquid coating with standard amount of filler) after 2 and 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-37 Nyquist and Bode plots 2C (13-14 mils TK 805-powder coating with standard amount of filler) and 1I (6.5-7 mils TK 34 XT-liquid coating with standard amount of filler) after 2 and 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-38 Resistance of the coatings-estimated from EIS results- on A (P, std f, mils) and I (L, std f, mils) panels versus time during 18 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of Figure 4-39 Capacitance variation in an organic coating versus immersion time 102 xii

13 LIST OF TABLES Table 2-1 Electrochemical parameters (j corr, E corr, E pit ) for austenitic stainless steel samples in different NaCl solutions at 25 o C 11 Table 2-2 Anodic reactions of Iron in CO 2 containing media 16 Table 2-3 Corrosion rates based on weight loss measurements during salt fog exposure 23 Table 2-4 Coating specifications 29 Table 2-5 Evaluation of the coatings in 3 different zones after 7, 10, and 15 years 31 Table 2-6 Specifications of the coatings manufactured (F/C-Fused and crashed, A/S-Agglomerated and sintered) 35 Table 3-1 Chemical composition of 34J grade steel 39 Table 3-2 Coating Characteristics of the Panels tested in this thesis 41 Table 3-3 Chemistry and dimensions of metal/alloy samples 45 Table 3-4 Specifications of the water baths 46 Table 3-5 The specifications of Reference 600 potentiostat 47 Table 3-6 Electrochemical test protocols for coated panels and metal/alloy samples 50 Table 4-1 Polarization resistance and estimated corrosion current density and corrosion rates of the triplicate sample set of Al 1100 samples during a week of immersion in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and at 60 o C 57 Table 4-2 Polarization resistance and estimated corrosion current density and corrosion rates of the triplicate sample set of Zn samples during a week of immersion in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and at 60 o C 60 Table 4-3 Polarization resistance and estimated corrosion current density and corrosion rates of the triplicate sample set of Galvalume samples during a week of immersion in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and at 60 o C 63 Table 4-4 Polarization resistance and estimated corrosion current density and corrosion rates of the bare (J1), burned off (J2) and white metal finish painted (J3) carbon steel panels during a week of immersion in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and at 60 o C 66 Table 4-5 Coating system characteristics for panels E and B 69 Table 4-6 Coating system characteristics for panels A and C 72 Table 4-7 Coating system characteristics for panels D and G 77 xiii

14 Table 4.8 Coating system characteristics for panels H and I 83 Table 4-9 Coating system characteristics for panels 1B and 1E 88 xiv

15 ACKNOWLEDGEMENTS First of all, I want to thank my advisors Prof Barbara Shaw and Dr Elzbieta Sikora for their guidance, patience and support. I really appreciate that they have always been very helpful during my studies. I wish to express my gratitude to Qatar National Research Foundation for providing the financial support to this project. Also, I would like to thank to NOV Tuboscope for the samples used in this research. I also would like to express my appreciation to my company, Turkish Petroleum Corporation (TPAO), for giving me this opportunity to and providing me the financial support to study corrosion in Engineering Science and Mechanics at the Pennsylvania State University. I sincerely thank to all Corrosion lab members for sharing their experience, invaluable assistance, and keeping things fun. Last but not least, I am thankful to my family for their support. Also, I would like to thank to my friends in Turkey who give endless support and never leave me alone even in the US. I dedicate this thesis to my grandmother Ayse Saribay. Isin Dizvay August, 2014 xv

16 1. Introduction 1.1 Background of study Corrosion is a type of material loss or destruction that results from chemical reactions in the presence of an electrolyte. Corrosion happens in pipelines when the pipeline material such as iron, steel, etc. is in contact with an electrolyte i.e. water. The contact of water polarizes the pipeline material resulting in anodic and cathodic sites. Oxygen reduction occurs on cathodic sites, whereas on anodic sites, metal is oxidized from Fe to Fe 2+, causing metal dissolution and material loss. The most probable two reactions are given below. The metal dissolution usually continues with a scale formation on the surface, such as FeO, Fe(OH) 2, Fe(OH) 3, Fe 2 O 3, etc., which is a somewhat protective and rather stable surface film. Thus, for corrosive environments, self-protective metals such as Zn and Al that react with water and oxygen in the air and form stable, corrosion-resistant and adherent films, are preferred. The basic corrosion reaction occurs spontaneously, the presence of CO 2 or erosion at high temperature and pressures makes the electrolyte more aggressive and accelerates this reaction. CO 2 gas plays an important role in the formation of carbonic acid, H 2 CO 3. It has an impact on the ph of the solution; whereas, the presence of solid particles disturbs the protective layer. Increasing need for energy forces countries with large oil and gas reserves to produce more oil and gas. Thus, even the reserves once found to be infeasible become valuable. The main reason behind their infeasibility is low recovery from the oil wells rather than the conversion of crude oil. The recovery from the oil wells is possible with injection of fluids containing CO 2 and O 2 that initiate corrosion in the pipelines where high temperature and high pressure conditions dominate. The challenges among recovery of oil: the recovery from the oil reserves requires high strength and chemically more corrosionresistant and stable pipeline materials. Qatar being the 4 th largest dry gas producer in the world in 2012 according to US Energy Information Administration [1] suffers from quick degradation of pipeline materials under corrosive conditions. A 1

17 recent field survey shows that local territory fluid transported by pipelines contains the formation fluids, i.e. natural gas, hydrocarbons, brine, in addition to 2% CO 2, and 3-6% H 2 S. Pressure measured along the pipeline is bars at 70 o C. The fluid may be accompanied by liquid slugs and sand particles depending on the area. Although the constituents of the system are not constant, erosion and CO 2 are taken into account as landmarks of Qatar reserves. X65 pipeline steel has been a durable option for the transportation of the extracted fluid at high pressure because of the addition of vanadium and titanium to iron which increase its toughness and strength. In spite of the addition of V and Ti, the erosion activator sand particles reduce the life time of the pipeline. 1.2 Research objectives This research project, founded by Qatar National Research Foundation, investigates pipeline material degradation caused by the corrosive liquids, gases and sand particles. The objectives of this thesis are focused on the following below: To investigate candidate sacrificial layers that could protect the steel substrate from corrosion and last as long as possible To create a testing environment similar to that experienced in the field (without H 2 S because that work will be done in Qatar) in order to evaluate possible components of a multifunctional coating system To characterize the corrosion behavior of the possible sacrificial layer components and NOV Tuboscope coatings in the simulated environment To find a suitable model for analysis of the electrochemical impedance data to better understand the components of the system To analyze the data obtained from the electrochemical corrosion tests by fitting the previously found model to the data To evaluate the electrochemical behavior of the samples and to discuss the key findings 2

18 1.3 Thesis structure This thesis is divided into 5 chapters, starting with the introduction. The introduction chapter provides background information about the research area with the specific objectives of the study and the structure of the thesis. The second chapter consists of the literature review of variables of the corrosion mechanism and components of the multilayered coating system. The first section investigates the recent studies of the effects of ph, temperature, pressure, salinity, multiphase environment, CO 2 & H 2 S content, and erosion on corrosion rate. The second section explores the possible specific components of the multifunctional coating system and the specifics of sacrificial and barrier layers. The third chapter describes experimental methods including specimens, testing environment, equipment, and characterization methods used during the study. The specimens physical and chemical properties are given in the first section. Then, the testing and immersion media are described. Afterwards, all the equipment used during the experiments is specified. Finally, characterization methods for the specimens are theoretically explained, and testing parameters of the methods are given with respect to the type of specimen. The results and discussion chapter represents the data obtained from electrochemical experiments performed for metals and alloys, bare and coated panels. The data analyzed in this section is coating resistance, coating capacitance and the plots obtained from electrochemical impedance spectroscopy. In the first section, Al 1100, Zn, Galvalume and bare steel panels are investigated with respect to their corrosion resistance properties. The second section represents the evaluation of the coatings on the panels by discussing the important parameters that affect the coating performance. At the end of this chapter, a discussion section clarifies the irregularities in the data. The last chapter summarizes the conclusions from the experimental work and the key verdicts. Based on the verdicts in this research, further work is proposed at the end of this chapter. 3

19 2. Literature Review This literature review is divided into two main sections. The first section is focused on the parameters that are present in the oil and gas environment and how they affect the corrosion behavior of the material. The oil and gas pipelines transport oil and gas mixtures extracted from the wells located underground. Since the pipelines transport this multiphase mixture, they experience high temperature and pressure. The extracted fluid is rich in CO 2, and rarely H 2 S and O 2 gas content. Another component of the pipeline system are the sand particles present by the nature of the extraction process; they cause erosion. Also, the ph and the salinity of the multiphase mixture are important parameters that greatly effect on the corrosion behavior of the pipeline materials. Therefore, this section reviews the effects of the mentioned parameters on the corrosion rate of the materials. The second section categorizes the possible components of the proposed multifunctional coating components in the previous work under sacrificial or barrier layer components Variables of corrosion mechanism ph The ph of the electrolyte has a strong effect on the direction and the products of corrosion reactions. Because it represents the molar amount of H + ions in the electrolyte, change in the ph may cause a complete change of the reaction mechanism. H + ion concentration domination creates an acidic environment, while excess OH - ions correspond to a basic electrolyte. Since the environment contains, H 2 CO 3, a weak acid formed by the dissolution of CO 2 gas in water, the ph of the electrolyte and the components of the chemical equilibria, anodic and cathodic reactions would change. The chemical equilibria of a CO 2 containing system is composed of three main reactions: dissolution of CO 2 gas in the electrolyte, hydration of dissolved CO 2, and dissolution of aqueous H 2 CO 3 to its components [2]. The first and the second reactions are not ph-dependent; on the contrary, the dissolution of CO 2 gas in any aqueous environment strongly depends on the temperature and pressure. The 3 rd reaction being phdependent, forms weak acid equilibrium of two steps. 4

20 Since both reactions include common species, any change in one of the species concentration causes a shift of the reactions accordingly. At ph 3.9, the reaction favors bicarbonate ion, HCO 3 -, which leads a possible reaction for the formation of FeCO 3. The critical importance of ph in the corrosion reaction was confirmed in [3], in CO 2 saturated, 5% NaCl solution at 25 o C and 1 atm. Traces of FeCO 3 were found at a ph of 3.9, while it is the main corrosion product at a ph of 6.5 summarizes that lower ph destroys the main corrosion product, FeCO 3. To see the effect of ph at different temperatures on mild steel, 1% NaCl solution in an autoclave system was used in [4]. Dissolved CO 2 was kept constant at 0.03 M for every experiment to prevent the temperature variability of the CO 2 dissolution. Corrosion rates were calculated for 20 hour linear polarization resistance test shown in Figure 2.1. Figure 2.1 LPR corrosion rates vs time for temperatures 80, 120, 150 and 200 o C, and at ph 4.0 (on the left) and 6.0 (on the right) [4] As seen in Figure 2.1, a general decrease in the corrosion rate for each temperature is achieved by changing ph from 4.0 to 6.0. Especially at 200 o C, it can be said that corrosion product formed on steel sample had better corrosion protection than the corrosion products formed at 80, 120, 150 and 180 o C by judging the significant difference between initial and final corrosion rates at both phs [4]. Decreasing corrosion rate with a shift of ph to basic values was also seen in 99.99% pure Zn samples [5]. In a kinetics and thermodynamics study of Zn, the effect of ph on polarization was determined in 0.1 M NaCl solution where the ph was adjusted accordingly with NaOH and H 2 SO 4. Potential was applied in a 5

21 range of -0.2V sce to -0.7 V sce, with a sweep rate of 0.75 mv/s which provided the polarization curves given in Figure 2.2. Figure 2.2 Polarization curves of Zn at ph 1, 4, 7, 10, and 13 in 0.1 M NaCl solution [5] In Figure 2.2, the polarization curves presented a significant shift for ph 1. It is clearly seen that severe corrosion attack on Zn occured at ph 1 because of the high corrosion current density value. There was milder corrosion at ph 4, 7 and 10, concluding that changing the environment from acidic to a mild ph decelerates the corrosion rate [5] Temperature Temperature has an opposite effect on corrosion of steel and formation of a protective layer on steel. On one hand, higher temperature increase not only increases the rate of corrosion, but also facilitates the mass transfer. On the other hand, high temperature has an adverse effect on the passive film formation because of the inverse proportionality of the solubility constant of iron carbonate (FeCO 3 ) and temperature as seen in Figure 2.3 [6]. 6

22 Figure 2.3 Solubility values of FeCO 3 from literature as a function of the reciprocal of temperature in 0.1 to 1.0 mol/kg NaClO 4 or 0.1 to 5.5 mol/kg NaCl solutions with CO 2 pressure from 0.01 to 0.93 bars [6] A study [7] of AISI 420 and high nitrogen martensitic steels 410N and 410SN, at testing temperatures of 0, 25 and 70 o C, showed that mass loss due to erosion and corrosion is directly proportional to increasing temperature. This is shown in Figure 2.4. The samples tempered at 200 and 450 o C were placed in a slurry containing substitute ocean water with 20% quartz particles at ph

23 Figure 2.4 Specific mass loss as a function of time, tempering temperature and testing temperature in substitute ocean water slurry at ph 8.25 [7] A study [3] about corrosion in CO 2 containing brine solutions was conducted on X70 steel samples in 3% NaCl saturated with CO 2 at different temperatures ranging from 25 to 75 o C. Results show that increasing temperature up to almost 60 o C increases corrosion rate, and at higher temperatures, corrosion rate starts to decrease as can be seen in Figure

24 Figure 2.5 Effect of temperature on corrosion rate of X70 steel in 3%NaCl stagnant solution saturated with CO 2 [3] Pressure High pressure in oil and gas operations is an existing condition and a necessary characteristic for both the extraction and transportation processes of oil and gas. Even though the influence of high pressure on flow characteristics is hard to simulate in the laboratory environment because of the need of a long line; its direct effect on corrosion rate is known. Assuming CO 2 and water vapor are the only two gases in the environment, the solubility of CO 2 increases with the increasing total pressure [6]. Since CO 2 concentration in aqueous phase is directly proportional to the corrosion rate, a rise in total operational pressure causes higher corrosion rate as plotted in Figure

25 Figure 2.6 Corrosion rate vs total pressure in oil and gas pipelines [6] Presence of Cl - / Salinity Cl - concentration is another important factor in pipeline corrosion due to the high salinity (10, ,000 ppm) of geologic sequestration environments as well as high temperature and high pressure [8]. Presence of Cl - is commonly associated with localized attack and pitting corrosion in liquid phase systems. The correlation is a result of the cyclic formation and breakdown of protective passive films created by chloride ions [9]. The destructive behavior of chloride ions are a result of small size, highly penetrative structure and strong acid anion behavior. To keep the pits electrically neutral, Cl - moves through the passive film under the influence of the electric field, because of its small size, it reaches the pits and causes hydrolysis of corrosion products as well as acidification that inhibits the repassivation of the surface [10]. To see the effect of Cl - concentration on corrosion rate, 4 types of austenitic steel samples were tested via potentiodynamic polarization in variously concentrated and aerated NaCl solutions (0.5-3 M) at 25 o C in a study [10]. Electrochemical parameters collected from polarization curves are shown in Table

26 Table 2.1 Electrochemical parameters (j corr, E corr, E pit ) for austenitic stainless steel samples in different NaCl solutions at 25 o C [10] An increase in the Cl - concentration results in an increase in corrosion current density referred as jcorr in the study, and was proportional to corrosion rate. However, the increase in the corrosion density also depended on the chemical composition of the steel at a given NaCl concentration [10]. EIS measurements were carried out at a frequency range of 10 mhz to 200 khz with 20 mv peak amplitude at 300 rpm stirring speed in different NaCl solutions [8]. The Nyquist plots of EIS are shown in Figure

27 Figure 2.7 Nyquist plots of J55 as a function of NaCl concentration at 25 o C, ph of 4, 300 rpm stirring speed, and 0.97 bar of CO 2 partial pressure[8] Nyquist plots in Figure 2.7 showed improvement in the corrosion resistance of J55 samples with increasing Cl - concentration [8]. Another study [11] was conducted on API-X100 steel. Their study not only broadened the the range of NaCl concentration, but also achieved results at high temperature. CO 2 was bubbled into the unbuffered solutions ( ) to deoxygenate the solution. The corrosion density and corrosion potential of API- X100 steel versus NaCl concentration plots are given in Figure 2.8. Figure 2.8 Corrosion density and potential of API-X100 steel as a function of NaCl concentration at 20 o C (left) and 90 o C (right) [11] 12

28 In Figure 2.8, the highest values of corrosion density were at 15 g/l at 20 o C (left) and at 5 g/l at 90 o C (right). This study shows that there is a maximum value of corrosion density for a NaCl threshold value for each temperature beyond which the corrosion density decreases. The relationship between Ecorr and NaCl concentration was found to be inversely proportional at both temperatures [11]. 13

29 Multiphase environment Streams from wells may have multiple components such as water, hydrocarbons and gasses. Even though the fluid is composed of liquid and gas phases, because of the immiscibility of water and oil, there are several phase combinations [12], shown in Figure 2.9. Figure 2.9 Phase inversions for different compositions of oil-water system [12] It is important to know the main phase of the flow, because it determines not only the flow type, but also the main phase covering the surface. The importance of the phase coverage is derived from the likelihood of corrosion when the surface is covered with water and the low possibility of corrosion when surface is covered with oil [13]. A recent study [14] showed the effect of oil percentage on corrosion rate via electrochemical impedance spectroscopy; their results are shown in Figure The tests on X65 samples were run in 0.02M NaCl M NaHCO 3 with a surfactant agent for the addition of oily phase. ph is changed from 8.59 to 8.62 with the addition of oily phase. To create the turbulent flow effect, a rotating cylinder electrode was used. 14

30 Figure 2.10 Nyquist plot of X65 in solution in different oil concentrated solutions at 60 o C and 3000 rpm electrode rotating speed [14]. In Figure 2.10, at 0% oily phase, the corrosion reaction was controlled by diffusion process whereas at 10 or 30 % oil, reaction mechanism was taken over by the activation controlled mechanism implying that the oily surface film formation inhibits the charge transfer step [14] CO 2 content In the extraction process of oil and gas, CO 2, H 2 S, and water are involved in the geological system. While dry CO 2 has almost no contribution to the corrosion process of the system, wet or dissolved CO 2 has tremendous corrosive effect. In fact, it becomes the main reactant after being dissolved in brine or water. In order for CO 2 to affect the general corrosion mechanism, the following reactions must occur as shown below [3]: 15

31 However, the cathodic reaction depends mostly on ph. Thus, hydrogen evolution occurs for solutions at ph lower than 4, while at higher ph values, carbonic acid reduction prevails. There are many CO 2 mechanism proposed, but a most likely and ph dependent mechanism [15] is shown in Table 2.2. Table 2.2 Anodic reactions of Iron in CO 2 containing media [15] The FeCO 3 film is formed as a corrosion product via several reactions. The structure of the passive film depends on the environmental conditions such as ph, temperature, composition of the solution, etc. In a study [16], partial pressure of CO 2 was investigated as one of the parameters. Corrosion rates were calculated via the weight loss coupons on API X65 steel samples after 21 days of deionized water exposure, at 70 o C. The ph of the liquid phase was changed from 3.5 to 4.8. The results are shown in Figure

32 Figure 2.11 General corrosion rate as a function of exposure time at different CO 2 partial pressures in DI water, 70 o C, ph= [16] Uniform corrosion was observed as the partial pressure of CO 2 was increased. A stable FeCO 3 film was observed on the samples where P CO2 was at 2 and 7 bars and resulted in decrease in the corrosion rate after 14 th day. However, the film formed on the sample with the lowest CO 2 partial pressure was not as uniform and thick as the other samples. While high pressure of CO 2 corresponds to higher initial corrosion rates, it will eventually result in a lower corrosion rate because it keeps conditions suitable for the formation of FeCO 3 [16]. In another study [17], the effect of partial pressure of CO 2 on the corrosion rate during activation controlled state was investigated. A low alloy tempered martensite steel was used for the direct weight loss experiments after being exposed to deaerated DI water for 24 hours in a high temperature high pressure (HTHP) autoclave. After CO 2 saturation, the ph was measured as Figure 2.12 shows the corrosion rates with respect to temperature and CO 2 partial pressure. 17

33 Figure 2.12 Corrosion rate of tempered martensite steel as a function of temperature and P CO2 at ph [17] As anticipated from the Figure 2.12, the CO 2 partial pressure is directly proportional to the corrosion rate in the first 24 hours. Also, when P CO2 is kept constant, it can be clearly seen that a positive temperature gradient increases the corrosion rate [17]. 18

34 H 2 S H 2 S is extremely corrosive under HPHT (High pressure/high temperature) environments, such as oil and gas wells and pipelines. The protection against corrosion depends on the protective surface layer formed by the reaction of the surface metal and the active ions in the solution. However, in the case of H 2 S, more than one type of film forms on the surface instead of a single, stable film as in CO 2 corrosion. Even though mackinawite (Fe,Ni) 9 S 8 is expected to form as the dominant product of the reaction of H 2 S with Fe occurs, amorphous ferrous sulfide (FeS), cubic ferrous sulfide (FeS-cubic), smythite ((Fe, Ni) 3+x S 4, x=0-0.3), greigte, pyrrhotite ((Fe 1-x S, x=0-0.2), troilite (FeS) and pyrite (FeS 2 ) are other types of iron sulfide that may form depending on the conditions. But, the lesser known mechanism of H 2 S corrosion makes it difficult to evaluate the formation rate of the films [18] Erosion Erosion is an important component of a corrosive environment because it is responsible for the wear of not only the surface layer, but also the substrate. Erosion-corrosion is more destructive than pure erosion by degrading the material and causing it exposed to the corrosive media. As mentioned before, a protective film forms on the surface of the material when exposed to corrosive fluids that makes the material electrochemically passive. But with the addition of solid particles in the corrosive fluid, the protective, passive film is destructed unevenly. Also, the surface film is known for its preventing action to the corrosive fluid diffusion. With the removal of the surface layer, corrosion is under control of mass transfer of corrosive chemicals, and will be accelerated [19]. A study [20] was conducted for the degradation of pipeline steel API X65 in a simulated oil and gas pipeline environment. A jet impingement apparatus with dual nozzle was used. The impingement angle was 90 o and the 4 hour tests were run at different temperatures, flow rates and sand loadings. The total weight loss versus sand loading plots are given in Figure

35 Figure 2.13 Total weight loss of API X65 pipeline steel as a function sand loading and flow rates at 20 o C [20] The linear relationship between total weight loss and sand loading can be clearly seen in Figure 2.13 for moderate flow rates. However, at a flow rate of 7 m/s, sand loading increase has almost no effect on the weight loss. Also, increase in the flow rate causes higher material removal. In the same study [20] open circuit potentials of high alloy steel UNS and stainless steel UNS were measured while increasing silica sand load in 3.5% NaCl solution. The impingement angle is kept constant at 90 o. Open circuit potentials versus sand load are given in Figure

36 Figure 2.14 Open circuit potentials with respect to sand loading in 3.5% NaCl solution at 18 o C for a) UNS 31603, and b) UNS [20] As can be seen in Figure 2.14, the open circuit potentials for both steels become more negative and more active as the loading increases. 21

37 2.2. Components of coating system Sacrificial layer Application of the sacrificial layer in coating systems utilizes galvanic corrosion to protect the substrate metal from corrosion. According to the galvanic series, sacrificed metal or alloy should be active enough to corrode before the substrate. Hence, the more negative the metal s half cell potential, the better the protection. Accordingly, zinc, aluminum, magnesium and their alloys are usually preferred as sacrificial layer components depending on the environment. The protection mechanism of the sacrificial metals is the formation of an oxide film immediately in the presence of oxygen in the environment. The metal oxide is the most stable form of the metal in the nature, thus, with lack of disturbance, this passive film protects the underneath layer from the corrosive environment. In addition, sacrificial layers have to be applied directly on the substrate because of the electrical contact necessary between the substrate and sacrificial layer so that a galvanic couple is formed [21] Zn alloys Zn and its alloys have the advantage of providing a higher potential difference than most metals provide and thus a higher driving force towards the protection of the substrate. After zinc couples with the steel substrate, it becomes an anode in the galvanic couple. Upon corrosion of zinc, corrosion products are varied accordingly to the environment such as ph, temperature, alloy content, solution, and etc. Usually, formed surface oxides performed well enough to protect the underlying substrate unless there is a chemical or mechanical disturbance. In a recent study [5], it s been proven that the composition and type of the protective thin film upon the corrosion of Zn depends on the ph of the solution in 0.1M NaCl solution at room temperature. The experimental results show that the most stable form of Zn is Zn 2+ at ph 1 to 5, and can possibly undergo a reaction to form ZnCl + if there is a source of Cl -. Besides its active open circuit potential compared to steel, it s important for Zn to last as long as possible in order to serve as a sacrificial layer. 22

38 In a corrosion study [22] for sprinkler systems, a black pipe and a galvanized pipe were exposed to quiescent tap water for 12 months. The pictures of the cross-section area of the both pipes were taken and shown in Figure Figure 2.15 Photographs of cross-sectioned black pipe (left), galvanized pipe(right) exposed to quiescent tap water for 12 months [22] Even though, the black pipe s cross section showed a higher level of affection from the corrosion process in Figure 2.15, no further corrosion evidence was found under the microscope. But, this does not prove that corrosion rate was the same for both pipes. To see accelerated results, salt fog test was applied to the samples with 5% NaCl solution up to 12 months. Weight loss data taken during the experiment were used to calculate corrosion rates given in Table 2.3. Table 2.3 Corrosion rates based on weight loss measurements during salt fog exposure [22] Specimen Average CR (mpy) 6 months exposure CR (mpy) 12 months exposure CR (mpy) Black steel with weld 39.5 ± ± seam Galvanized steel with weld seam 7.7 ± 1.3 Weld intact-minimum corrosion 15.8 As seen in Table 2.3, the corrosion rates were much lower for the galvanized pipe under rough conditions simulated by salt fog exposure. As indicated before, Zn was sacrificed in the Zn-steel galvanic couple, which mitigates the corrosion rate of steel unlike those for black pipe [22]. A study [23] showed that Zn should be coupled with Al in order to provide a higher level of sacrificial protection. Galvanized, Zn-Al alloy, and Zn-55Al-Si (known as Galvalume coating) samples were exposed 23

39 to seawater of ph 8.14 and at 17 o C in tidal and immersion zones. The corrosion rate of the samples during 18 months of exposure are given in Figure Figure 2.16 Corrosion rates as a function of time of hot dip coatings in tidal zone [23] As seen in Figure 2.16, the initial corrosion rates of galvanized coatings were almost twice high as the Zn-25Al, and Galvalume coated samples. The initial higher corrosion rate of Zn-25Al in tidal zone can be explained with the Zn constituent corrosion in the alloy. In order to understand the level of cathodic protection of each coating, corrosion and galvanic couple potentials were examined and given in Figure

40 a) b) Figure 2.17 Corrosion potentials of each hot dip coated sample vs SCE (left) and galvanic corrosion potentials of hot dip coatings paired with steel vs SCE (right) [23] The corrosion potentials of Zn, Galvalume and Zn-25Al alloys were around mv, mv, and mv vs SCE respectively. The galvanic potentials of Zn and Zn-25Al when coupled with steel were and mv vs SCE respectively. However, the Galvalume sample was at its corrosion potential, mv vs SCE when coupled with steel, and then showed a noble shift after the 50 th hour. An explanation for this behavior is that a preferential corrosion occurs for the first 50 hours, then the alloy behaves like Al and shifts its potential to a higher value. Thus, Galvalume coatings behaved like Zn initially, supplying cathodic protection, and after the Zn-rich phase corroded off, it behaved like Al at a nobler potential. A result drawn from these studies is Zn is recommended to be alloyed with a nobler metal so that sacrificial corrosion of Zn lasts longer and the substrate is cathodically protected by Al [23]. 25

41 Al alloys Aside from having a low open circuit potential like Zn, the aluminum oxide formed upon corrosion is not only insoluble in water, but also it forms immediately upon mechanical damage and passivates the surface. The reactions for the protective oxide formation are given below [24].. or In an article [25], Al coated steel samples were evaluated via EIS in 3.5 wt % NaCl solution after being subjected to 1500-hr salt spray assay. Seen in Figure 2.18a, the resistance of the coating improved with the time of immersion proving a formed oxide layer improves the resistance. In Figure 2.18b, Nyquist plots measured after 28, 35 and 44 days showed one time constant in contrast to the plots in Figure 2.18a. This can be explained by Al corrosion products filling up the pores before corrosion of substrate takes place. 26

42 a) b) Figure 2.18 Nyquist plots Al coated steel samples in 3.5 wt % NaCl for different exposure times [25] A study [26] investigating the size of the powder and hard particle effects on a coating s protective features treated with a cold spraying coating technique, pure Al and Al-Al 2 O 3 coatings were deposited on Al7075 substrate. Specimens were exposed to a salt spray test using 3.5 % NaCl according to ASTM 85 standard for 1000 hours and a 30-minute immersion in 3.5 % NaCl and 30-minute dry cycle for 1000 hours. Different percentages of alumina were used in each coating to see the effect of hard particle. Bare steel specimens were tested for only 24 hours because of the immediate deterioration. The results of the both tests are given in Figure

43 Figure 2.19 Images of the bare steels and coated specimens with different alumina percentages before and after the corrosion tests [26] Aside from bare steel samples surfaces turning red, the coated specimens showed no significant difference with respect to the alumina percentages and the surfaces turned to white after the salt spray test. However, surface roughening was observed after the cyclic immersion test as a function of alumina percentage. The specimen surfaces were examined by backscattered electron imaging and no sign of localized corrosion was found besides a thin, less than 25 µm corrosion product film which explains the whitening of the coated surfaces [26]. 28

44 Al-Zn alloys Both Zn and Al are preferential metals due to their sacrificial properties in protection of steel. The Zn-Al coatings don t last long because of several reasons such as erosive fluid, solubility of the corrosion product at service conditions and other factors. Without disrupting Zn s protection of steel against corrosion, an enhancement in its sacrificial property can be done by alloying Zn with a nobler-yet notmore- electropositive-than-steel metal such as Al supplying the passivity [27]. In a 20-year study in Japan [28], various combinations of Al and Zn coatings were applied by arc and flame spraying onto steel pipes which were then partially immersed in the seawater at a coastal site. The evaluation criteria for the pipes were observation of swelling, exfoliation, cracking, white/red rust, color change, delamination, and other qualitative reactions. The severity of the pipe appearance was ranked from A to E, A being no change in the coating and E being general rusting due to corrosion on air, splash and tidal zones of the pipes. The specifications of each coating are given in Table 2.4 Table 2.4 Coating specifications [28] 29

45 As seen in Table 2.4, the coating systems were combinations of WP (wash primer), PE (epoxy paint), CP (epoxy primer with calcium plumbate) and PU (polyurehthene paint). In order to be more explanatory, a picture of the test rig at the test site is given in Figure Figure 2.20 Test rig at the coastal test site [28] The evaluation of the coatings based on their appearance and thickness measurements is given in Table

46 Table 2.5 Evaluation of the coatings in 3 different zones after 7, 10, and 15 years [28] The evaluation table shows the coating can be improved by alloying for both Al and Zn. An example for this phenomena was the evolution of Zn-alone (1) or Al-alone (3) flame sprayed coatings performance by alloying into Zn-Al flame spray (2). This assessment also valid for sealed coatings illustrated by #7 s better performance than #8 and especially #9 [28]. In an arc-sprayed comparative metal coatings study [29], of Al, Al top-coated-zn base coating, Zn-Al pseudo alloy and Al-Ti pseudo alloy coatings were prepared and then sealed with epoxy priming and aliphatic polyurethane topcoat to test erosion-corrosion in simulated splash zone by a water impingement corrosion device as shown Figure

47 Figure 2.21 Schematic of self-made impingement corrosion device [29] A 3.5 % NaCl solution with a ph of 8.0 at 55±5 o C was used for the salt spraying simulation. Flow velocity was adjusted to 5.4 m/s and the experiment lasted 370 hours with 24-hour intervals interrupted for cleaning and weighing the samples. The weight changes of the samples are given as a function of time in Figure

48 Figure 2.22 Change of weight of the samples during impingement test [29] The figure shows change in weight is directly proportional to the erosion-corrosion rate. Thus, Al/Zn pseudo alloy coating performed better under erosion-corrosion. This can be explained with the high plasticity of Zn or Al. In contrast, materials with high hardness such as Ti did not tolerate the damage from micro jet-cutting [29] Barrier layer A barrier component is added to the corrosion resistant coating system because the abrasive and corrosive solution causes local corrosion and the rupture of the passive protective film on the surface. Even though the passive film usually is protecting the substrate, it is not the most uniform layer regarding its thickness, thus the film can easily be wore off by the sand particles in the slurry. Another reason of the sacrificial layer rupture is corrosion-erosion where aggressive fluid dissolves the film at its weakest zone and causes exfoliation of the film. Many parameters contribute to the wear of surface layer, such as sand concentration, particle size, velocity of the particles, etc. A study [30] for observation of erosion and corrosion synergy, a slurry pot 33

49 erosion tester was used to determine the influence of sand concentration, particle size and velocity of the particles in 0.1 M NaOH with addition of silica sand on stainless steel (SS316L) and carbon steel (AISI 1020). As expected, increase in the sand concentration resulted in increase in the rate of erosion because of the increasing number of impacting particles on the surface. A similar effect was found for the sand velocity. However, for low velocities such as 3 and 5 m/s, the mass losses were almost the same which can be explained by a threshold velocity which is needed for suspension of particles in the fluid to make an impact on the surface. Similarly, to see the effect of the sand particle size on weight loss, the particle size of the sand should be increased upto a threshold value. Above that critical value, weight loss increases linearly with the increase in the particle size. As a less expensive option, organic coatings can be used for erosion protection except in the case of a high energy solid particle impingement [31]. Under low energy flows, organic coatings also provide reasonable protection from corrosion. For high level impingements, ceramic coatings are best suited but with high cost. For moderate conditions, a rather low cost type of coating is thermally sprayed metallic coatings with a common use of aluminum, zinc, copper, nickel, their alloys and composite materials based on them. Ni and Co based coatings are used for more severe environments. For highly erosive flows, cermets like WC-CoCr and WC-NiCr are preferred due to their mechanic properties. To be specific, for intense wear conditions at high temperature, alumina oxide and combinations of alumina/titania/silica are recommended by the producers. There are many mechanical properties of a coating that can be used to evaluate its wear performance [32]. Contrary to the general belief, no strong correlation is found between hardness and wear resistance of a coating. But, microstructure, such as the presence of voids, cracks, unmelted particles, oxide inclusions, and porosity, weakens the strength of the coating. The advantage of alumina (Al 2 O 3 ) comes from its good wear, corrosion and thermal resistance availability at low price. Its low fracture toughness can be improved with a small addition of titania (TiO 2 ). It is possible to lower the quality of wear resistance of Al 2 O 3 by the mechanical addition of TiO 2. The consequences of mechanical blending of two compounds can be fixed by blending fused or crashed powders of alumina and titania, and HVOF spraying a more evenly distributed coating. Another method developed besides HVOF spraying is using nanostructured powders for plasma spraying, such as agglomerated and sintered powders from nanosized particles. In a study [33], Al 2 O 3 - TiO 2 coatings were manufactured by HVOF and plasma spraying, and compared with pure Al 2 O 3. The coatings manufactured for this study are given in Table

50 Table 2.6 Specifications of the coatings manufactured (F/C-Fused and crashed, A/S-Agglomerated and sintered) [33] The coatings at 30 o and 90 o impact angle were tested in centrifugal erosion tester at 6000 rpm, and with 1 kg quartz sand after fine-grinded with 1200 SiC paper. Weight losses in erosion test are shown in Figure Figure 2.23 Weight loss of HPOV and APS coatings in centrifugal erosion tester at 6000 rpm with 1 kg quartz sand [33] As seen in weight loss measurements in Figure 2.23, HVOF coatings were slightly more resistant than APS coatings. Both types perform better at 30 o impact angle. The ranking of the coatings for erosion wear test were HVOF_A, HVOF_AT1, APS_AT2, HVOF_AT2, APS_A, and APS_AT1. 35

51 After the weight loss measurements, SEM images of HVOF_AT2 and APS_AT2 are taken and given in Figure Figure 2.24 SEM images of a) HVOF_AT2 and b) APS_AT2 after erosion wear test at 90 o impact angle [33] HVOF_AT2 sample showed one particle impact site with some brittle fracture on impact mark edges. Also, the coating was observed to be plastically deformed. In addition to the mentioned deformations, APS_AT2 showed debris from erosive particle and material loss because of the brittle fracture around the wear marks [33]. As stated before for applications in highly erosive environments such as turbines, WC-Co-Cr combinations are preferred for the surface coating. In the following study [34], HVOF WC-10Co-4Cr and Al 2 O 3 +13TiO 2 coatings were compared under highly erosive slurry conditions. The underlying substrate used was CF8M turbine steel. The structure of WC-10Co-4Cr was composed of agglomerated-sintered, spherical particles whereas Al 2 O 3 +13TiO 2 was composed of fused and crashed, ellipsoid and elongated particles as seen in Figure

52 Figure 2.25 SEM micrograph and EDS analysis of a) WC-10Co-4Cr powder, b) Al 2 O 3 +13TiO 2 powder [34] SEM images of the coatings taken after spraying process are shown in Figure Figure 2.26 SEM micrograph of WC-10Co-4Cr (left), Al 2 O 3 +13TiO 2 (right) on CF8M steel substrate [34] Even though the spraying techniques were the same, WC-10Co-4Cr showed a laminar, splat-like, uniform surface structure with a defect free interface, while the Al 2 O 3 +13TiO 2 coating had nodule-like unmelted particles unclearly surrounded by splat like fully melted splat lamellae, with many porosities. In a high speed erosion test rig, the coatings were compared with each other and CF8M steel, with respect to their corrosion rate under different particle velocities impinged at different rotational speeds. The erosion rate measurements are given in Figure

53 Figure 2.27 Erosion rate of uncoated, WC-10Co-4Cr-coated, and Al 2 O 3 +13TiO 2 -coated CF8M steel at 2250 and 4500 rpm [34] It is apparent that WC-10Co-4Cr coating performed better at each condition, but it should be noted that CF8M turbine steel is a very high performance steel and it performed outstandingly better than Al 2 O 3 +13TiO 2 at 4500 rpm. Also, the rotation speed increased erosion rate as expected [34]. In this chapter, the fundamentals of the corrosion process and its parameters were summarized. The main factors that increase the material degradation were reviewed with their role in the corrosion process. Also, the materials for sacrificial and barrier layer were briefly revised in the previous work. In summary, the constant immersion and testing environments should be kept at high temperature, acidic ph, and in CO 2 saturated solution to simulate the natural pipeline environment. It can also be concluded that Zn, Al and their alloys are promising constituents for sacrificial layer while organic coatings with ceramic particles have high ability to resist to erosion and corrosion. 38

54 3. Experimental Methods 3.1 Specimens Panels All of the steel panels and polymer-based coatings used in this thesis were donated by NOV Tubescope. The grade of the steel used as substrate for the panels was 34J. The chemical composition of 34J grade steel is given in Table 3.1. Table 3.1 Chemical composition of 34J grade steel Heating id/ E2C226 W2D789 Elements C Mn P S Si Total Al Cu Ni Cr Mo Cb V Ti Ca

55 The panels were four by six inches in size and had a coating system applied onto the steel substrate. The coating system on the panels was composed of a primer layer, one of two types of coatings and different amounts of filler. The primer was applied to improve adherence between the coating to the substrate and was present on all coated panels. Five of the panels were coated with only the primer to study the properties of this layer by itself. Thirty other panels were coated with TK 805-a powder-phenolic novolac coating with low, standard, and high amounts of filler at (two different thickness ranges). Another ten panels were coated with TK 34XTliquid-epoxy phenolic- coating with only a standard amount of filler at two different thickness ranges. Three other panels were bare steel substrates (bare carbon steel without any pretreatment on the surface), burned-off steel (carbon steel exposed to high temperature to form an oxide layer on the surface for cleaning the surface from dirt, grease, etc.) and a white-metal finish steel (carbon steel had an anchor-tooth profile on the surface created by grit blasting the surface with Al 2 O 3 ). The general schematic of the coatings on each panel and coating characteristics of the panels analyzed in this thesis are presented in Table 3.2. Images of the panels from each group are given in Figure 3.1. As mentioned above, the filler on the coatings was at three different levels, low, standard, and high. Filler was composed of conductive and non-conductive species. The low filler coatings had 0.17x the standard filler amount; whereas, the high filler coatings had 1.3x the standard filler amount. The low and standard amounts of filler had the same ratio between conductive to nonconductive parts; while, the high filler coatings contained more conductive of the part, (14.7x), higher than standard and low filler coatings. The panels were coated by Tube-Kote Process. The same process is used for the application of the brand s commercial coatings. The process starts with thermal baking at 750 o F for 4-8 hours (as part of the substrate cleaning process). Before the application of primer, the surface is blasted with aluminum oxide to a white metal finish to increase the adhesion between the substrate and primer. Then, the liquid primer coating is applied. The rest of the process continues depending on the coating. If the coating is liquid, TK 34-XT, it is applied as necessary depending on the number of layers required in the description of the coating with intervals of baking at o F between each layer application. A final baking stage occurs at o F for various times. For powder coatings, TK 805, a preheating stage is required at o F before the powder coating application at F. 40

56 An electrical connection to the steel substrate was necessary to provide a working electrode connection for the electrochemical experiments. A corner of each panel was kept bare to expose the steel substrate in this connection. The coated panels were used for long-term constant immersion testing. To maintain a permanent electrical connection, a hole was drilled on the bare area to bolt a copper wire for the connection and then the bare site with the wire connection was covered with epoxy paint to protect the connection from corrosion. Figure 3.2 displays the bare connection area. Table 3.2 Coating Characteristics of the Panels tested in this thesis Panel code Coating code Coating type Filler amount Coating Panel drawings thickness 1A, 3A, 4A, TK 805 Powder- standard mils 6A, 7A phenolic novolac 1B, 2B, 3B, TK 805 Powder- low mils 5B, 6B phenolic novolac 2C, 3C, 4C, TK 805 Powder- standard mil 6C, 7C phenolic novolac 1D,2D, 4D, TK 805 Powder- high 8-12 mils 5D, 6D phenolic novolac 1E,2E, 4E, 5E, TK 805 Powder- low mils 6E phenolic novolac 41

57 Panel code Coating code Coating type Filler amount Coating Panel drawings thickness 1F, 2F, 3F, Primer - - < 1 mil 4F, 5F G1, G2, G3, TK 805 Powder- high mil G4, G5 phenolic novolac 1H, 1H2, TK 34XT Liquid-epoxy standard 3 mils 1H3, 1H4, phenolic 1H5 1I, 1I2, 1I3, TK 34XT Liquid-epoxy standard mils 1I4, 1I5 phenolic J1 bare/unaltered steel J2 Burned off J3 White metal

58 Figure 3.1 Tube-Kote Process- manufactured coatings on the panels a)powder coating (on the A, B, C, D, E, G panels), b)liquid coatings (on the H and I panels), c)primer only (on the F panels), d) bare steel (J1) e)burned-off steel (J2), and f)white-metal finish steel (J3) 43

59 Figure 3.2 Connection area of the panel prior coating with epoxy 44

60 3.1.2 Metals and alloys Metals chosen for the sacrificial coating studies were 99.9% pure Zn, 1100 Al alloy and Galvalume (a Zn- Al combination). The chemical composition of the samples and their dimensions are given in Table 3.3. Table 3.3 Chemistry and dimensions of metal/alloy samples Zn 1100 Al Galvalume Purity/Chemistry 99.9 % Zn 99 % (min) Al, % 55 % Al, 43 % Zn, 1.6 % Si (max) Cu, 0.05 % (max) Mn, 0.95 % (max) Si+Fe, Zn 0.1 % (max) Dimensions 6 x 0.4 x 0.06 in 6 x 0.4 x 0.1 in 4 x 1.18x 0.02 in Epoxy paint was applied on Al 1100 and the Galvalume samples on the waterline area to provide an evenly exposed area (to mitigate the effect of waterline corrosion). The Zn samples were covered with 3M tape for the same reason. The samples described in this section were constantly immersed in the solution up to their half-length for a week. 3.2 Equipment Heating baths In order to simulate the high temperature environment in the oil and gas pipelines, two VWR brand water baths were used. Both have high grade stainless steel tanks and a robust outer case for durability. The smaller one was used only during conducting electrochemical experiments. Two samples can be tested at the same time in the smaller bath. The larger bath; on the other hand, was used for the constant immersion of samples, and can hold up to 16 samples at the same time. In order to keep the temperature constant in the larger bath, a polycarbonate lid was put on the top of the bath. This condition cannot be provided during the testing in the smaller bath because of the cables connecting electrodes to the potentiostat. Detailed information about the heat baths is given in Table 3.4 and pictures of both baths are given in Figure 3.3. Both baths were fitted with high temperature resistant plastic containers so that the aggressive electrolyte did not contact the stainless steel tanks. 45

61 Table 3.4 Specifications of the water baths Test bath Immersion bath Type Manual Analog Volume 5 L 26 L Dimensions 15.1 x 30 x 15 cm 50.5 x 30 x20 cm Max temperature 99 o C 99 o C Electrical 115 V, 50/60 Hz 115 V, 50/60 Hz Stability ±1 o C ±1 o C a) b) Figure 3.3 Water baths used in the experiments a) Test bath, b) Immersion bath Potentiostats Reference 600 Potentiostats, a product of Gamry Instruments, were used to run all corrosion related electrochemical tests, including: Open Circuit Potential, Linear Polarization Resistance and Electrochemical Impedance Spectroscopy. The potentiostat comes with Gamry Software which runs electrochemical corrosion experiments and analyzes data. The Reference 600 cable has 6 connections 46

62 consisting of working, working sense, counter, counter sense, reference, and ground. The hardware features of Reference 600 are listed in Table 3.5. Table 3.5 The specifications of Reference 600 potentiostat Max current Max Voltage Frequency Range Data Sampling Rate Input Impedance Current Detectable at Max ±600mA 11V 10μHz 1MHz 3μs Ohms 5pA Impedance Bandwidth Weight 15MHz 3000g Reference and counter electrodes The temperature of the simulated environment puts a limitation for the type of the reference electrode (RE). A commonly used reference electrode, saturated calomel electrode (SCE), becomes less stable as the temperature of the system is raised. Even though the SCE s limit is given as 80 o C, it s recommended to use Ag/AgCl reference electrodes at temperatures higher than 40 o C. A Gamry brand Ag/AgCl electrode consists of a AgCl dipped or AgCl electroplated Ag wire, immersed in saturated KCl solution. The solution and the wire are placed in a tube. The reference electrode is in contact with the testing electrolyte by its porous plug end. The potential of Ag/AgCl electrode is determined as V versus SHE at 60 o C. Ag/AgCl reference electrodes were used in this thesis research. A counter electrode in electrochemical measurements is required to complete the cell circuit so that electrons flow into and leave the solution through the counter electrode. The counter electrode is a conductor selected from electrochemically inert materials such as platinum, gold or carbon. In this 47

63 research, a graphite counter electrode was used for all experiments. Figure 3.4 represents a graphite counter electrode and a Gamry Ag/AgCl reference electrode used in the experiments in this thesis. a) b) Figure 3.4 a) Ag/AgCl reference electrode and b) graphite counter electrodes used in the experiments 3.3 Experimental Environment Immersion Environment All panels were kept immersed half way in an inert solution container that was submerged in a closed, constant temperature water bath. The solution was heated by DI water in the water bath, and the temperature of the solution was kept constant at 60 o C, under atmospheric pressure. The solution contained 2000 ppm [Cl - ] and the ph was 3.5. The solution was prepared by adding 3.27 g/l NaCl and 26.3 μl 37% HCl to 1 L of DI water. In addition, 99.5% pure CO 2 gas was fed into the solution from both ends of the container. The water bath keeping the solution container at 60 o C had a lid on the top, so there would be CO 2 gas-liquid equilibrium in the simulated pipeline environment. The immersion environment set up can be seen in Figure

64 CO 2 tanks Panels Water o C Figure 3.5 Immersion environment for the panels Testing Environment All specimens were tested under the same conditions. The panels were immersed in a water bath and removed for testing. The solution was the same NaCl-HCl-DI water mixture at ph 3.5 and was heated to 60 o C. Testing takes place in a smaller water bath where only two samples can be tested at the same time. Tests are run in two separate containers for each sample. Although CO 2 was bubbled in each test solution separately, there was no lid on the top of the smaller containers. A reference electrode, a counter electrode and a working electrode were placed in each test container during experiment. A panel, as a working electrode, and a Ag/AgCl reference electrode, were used for every test. A counter electrode was used for polarization resistance and electrochemical impedance spectroscopy. All electrodes in the electrolyte were connected to the potentiostat during tests. The environment described is shown in Figure

65 A panel Potentiostats Electrical conections Testing o C Figure 3.6 Testing environment for all samples 3.4 Characterization techniques and testing parameters There are two different protocols followed for the coated panels and the metals/alloys. The panels were only tested via EIS with an initial delay of OCP. The open circuit potential of the metal/alloy sample was measured for a half hour. Then, polarization resistance and electrochemical impedance spectroscopy with a half hour of open circuit potential measurement were run. The protocols are also given below in Table 3.6. Table 3.6 Electrochemical test protocols for coated panels and metal/alloy samples Tests Zn / 1100 Al / Galvalume/steel panels Coated panels Open circuit Potential 30 min 30 min Linear Polarization Resistance ±0.015 V vs E oc Open circuit Potential 30 min Electrochemical Impedance Spectroscopy 1 mhz-100 khz 1 mhz khz 50

66 3.4.1 Open Circuit Potential (OCP) Open circuit potential measurement is the monitoring of the corrosion potential of a specimen against a reference electrode in an electrolyte with respect to time. Based on their corrosion potentials, metals are rated accordingly for their tendency to corrode in a given electrolyte. OCP of a metal would be more positive if passivated by a protective film formed on the surface and would be more negative, if it is actively corroding. To make the measurement, the specimen is connected to the potentiostat against a suitable reference electrode and the potential difference between the reference electrode and the specimen is measured by a high impedance voltmeter. Getting a stable OCP value may take time. Thus, the OCP measurement is not just used for providing the state of the specimen, it also enables the specimen reach its steady state condition before other electrochemical tests. The OCP of the panels is measured prior to EIS measurements to allow the panels reach their steady state. However, for metal and alloy samples, OCP is also measured in between two other tests (LPR and EIS) just to stabilize the samples after being disturbed by LPR. In Figure 3.7, the testing parameters for OCP measurement are shown. Figure 3.7 OCP parameter entry screen for Reference 600 potentiostat, displaying the parameters selected for use during immersion testing of samples in CO 2 saturated NaCl+HCl solution, at 60 o C, ph of

67 3.4.2 Linear Polarization Resistance (LPR) LPR enables the determination of corrosion current density which then used to determine the corrosion rate. In this method, a small range of potential negative and positive vs OCP is applied to the specimen and the resulting current density is measured as a function of applied voltage. Polarization resistance (Rp) is estimated from the slope of potential-current density curve produced. Corrosion current is calculated by either using Tafel extrapolation or Stern-Geary equation, with βa and βc being anodic and cathodic Tafel constants, given below: After the estimation of Icorr and dividing by the area to get icorr, corrosion rate (CR) is calculated by the equation given below; a= atomic weight i= corrosion current density (µa/ cm 2 ) n= oxidation number D= density (g/cm 3 ) CR= corrosion rate (mpy) The applied, small-ranged potential producing a high current indicates a low polarization resistance and thus, a high corrosion rate; whereas, a low current indicates high polarization range and low corrosion rate. This test is run by connecting 3 electrodes, a working, a reference and a counter electrode, to the potentiostat in an electrolyte under desired conditions. The testing parameters are given in Figure 3.8 for metal and alloy specimens. 52

68 Figure 3.8 LPR parameter entry screen for Reference 600, displaying the parameters selected for use during the immersion of metal and alloy samples in CO 2 saturated NaCL+HCL solution, at 60 o C, ph of

69 3.4.3 Electrochemical Impedance Spectroscopy (EIS) Electrochemical impedance spectroscopy works by applying a range of frequencies to the sample and measuring the resulting amplitude and phase shift of the signals. Thus, the impedance is also a function of time and frequency, composed of a complex number with a real (Z (ω)) and an imaginary (Z (ω)) part. The measured current is a response signal with an amplitude ((Z (ω)) and a phase shift (Z (ω)). Ohm s law describes the impedance as; Ɵ represents the phase as the shift of the current signal with respect to the potential. Two common plots are produced form the application, Nyquist plot, Z (ω) versus Z (ω), and Bode plot, log Z and log Ɵ versus log f. Analysis can be done by fitting the response plots to an equivalent electrical circuits consisting resistors, capacitors, inductors which defines the features of the coating. EIS is run on a sample by connecting all 3 electrodes as in LPR procedure. The testing parameters for the EIS experiments are given in Figure

70 Figure 3.9 EIS parameter entry screen for Reference 600, displaying the parameters selected for use during the immersion of metal and alloy samples in CO 2 saturated NaCL+HCL solution, at 60 o C, ph of Zview Zview (version 3.2b) is the commercial software that was used for analysis of the data obtained by EIS. To be able to interpret the EIS measurements, the actual system is needed to be related with the elements in EIS, and then discussed quantitatively. At this point, Zview is used to find an appropriate model or make a new model of an electric circuit that reflects the real system parameters, and then the elements in the circuit are fitted to the model. The equivalent circuit is used for the estimation of the parameters by Zview. The elements in the equivalent circuit can be varied according to the actual system, i.e. the resistance of the coating is simulated with an appropriately placed resistor in the circuit. According to the change in the values of the circuit elements provided by the simulation, the actual system is analyzed. 55

71 4. Results and Discussion The experimental results are divided into two sections. The first section presents the electrochemical behavior of the metals and alloys (proposed as sacrificial layer) and of carbon steel used as a substrate. For the sacrificial layer tests, triplicate sample sets of Al 1100, pure Zn and Galvalume were tested. The experiments were conducted immediately after immersion and 1, 3, 5, and 7 days after immersion. The waterline area of the samples was covered because it was observed in the previous experiments that the corrosion products were concentrated around waterline. This phenomenon occurs because the high evaporation rate of the electrolyte causing Cl - ions to concentrate around the waterline area. In addition, the surface of the electrolyte is open to the air, thus high oxygen concentration at the waterline changes the kinetics of corrosion and increases corrosion in this area. Thus, by covering the area with an epoxy coating or tape, the corrosion rate calculations can be based on the area where the corrosion reactions occur evenly. The bare, burned-off and white-metal finish blasted steel substrates were tested immediately after immersion and 1, 3, 5, and 7 days after immersion. The second section presents the electrochemical behavior of coated panels. The coating system on the panels was comprised of a primer coat on the top of the prepared substrate, and one of the two types of coating (liquid or powder) with the addition of filler at various amounts. The panels have been immersed and tested regularly for almost 4 months as described in the Experimental Methods Metals and alloys Al 1100, pure Zn (99.9%), and Galvalume were chosen as probable candidates for the sacrificial layer because of their lower open-circuit potentials with respect to steel. Open circuit potential, polarization resistance and electrochemical impedance spectroscopy measurements were conducted on the triplicate samples of the Al 1100, Zn and Galvalume. In addition to these, bare, burned off and white metal finish steel substrate panels were tested under the same experimental conditions. The waterline area of the Galvalume and Al 1100 samples was painted with epoxy, and 3M tape was used for Zn samples. 56

72 4.1.1 Al 1100 A triplicate sample set of Al 1100 was tested as described in the Experimental Methods. The polarization resistance values of the Al 1100 samples and the corrosion rate calculations are given in Table 4.1, and corrosion rate of the samples versus immersion time plots are given in Figure 4.1. Table 4.1 Polarization resistance and estimated corrosion current density and corrosion rates of the triplicate sample set of Al 1100 samples during a week of immersion in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and at 60 o C Immersion Rp 1 Icorr 1 CR 1 Rp 2 Icorr 2 CR 2 Rp 3 Icorr 3 CR 3 time ohms.cm 2 µa/cm 2 mpy ohms.cm 2 µa/cm 2 mpy ohms.cm 2 µa/cm 2 mpy 0 hour 5.36E E E day 3.55E E E days 6.68E E E days 9.06E E E week 5.54E E E

73 CR (mpy) 3.0 CR vs immersion time for Al 1100 via PR and EIS #1- EIS #2- EIS #3- EIS #1- PR #2- PR #3- PR Time (hours) Figure 4.1 Corrosion rates of the Al 1100 samples (#1, #2, and #3) via polarization resistance and electrochemical impedance spectroscopy in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion According to Table 4.1, it is observed that the polarization resistance values were still fluctuating during the week of immersion. Thus, it is difficult to determine the effect of longer immersion time on the Al samples. The fluctuation rates can be clearly observed in the corrosion rate values (calculated from both LPR and EIS data) in Figure 4.1. It was shown that both methods provided the same results. Also, the 2 nd and 3 rd samples exhibited similar changes in the corrosion rate values showing that the fluctuations are present for all the samples. In addition to the corrosion rate changes versus time graphs, the Nyquist and Bode plots of the 2 nd Al 1100 sample obtained from EIS are given in Figure

74 Figure 4.2 Nyquist and Bode plots of the 2 nd Al 1100 sample from EIS in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion 59

75 As seen in Figure 4.2, the Nyquist plots show the 2 nd Al 1100 sample had initially two time constants, but the 2 nd semicircle disappeared after 7 days of immersion. This may be associated with the sample having a porous surface, and as can be seen in the 3 rd and 5 th days Nyquist plots in Figure 4.2, corrosion occurs in the pores. Corrosion products filling the pores on the surface might have resulted in an increase in the coating resistance value after the 5 th day Zn A triplicate sample set of Zn was tested as described in the Experimental Methods. The polarization resistance values of the Zn samples and the corrosion rate calculations are given in Table 4.2, and corrosion rate of the samples versus immersion time plots are given in Figure 4.3. Table 4.2 Polarization resistance and estimated corrosion current density and corrosion rates of the triplicate sample set of Zn samples during a week of immersion in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and at 60 o C Immersion Rp 1 Icorr 1 CR 1 Rp 2 Icorr 2 CR 2 mpy Rp 3 Icorr 3 CR 3 time ohms.cm 2 µa/cm 2 mpy ohms.cm 2 µa/cm 2 ohms.cm 2 µa/cm 2 mpy 0 hour 2.01E E E day 7.37E E E days E days 2.42E E E week 1.50E E E

76 CR (mpy) 60 CR of Zn vs immersion time via EIS & PR #1- EIS #2- EIS #3- EIS #1- PR #2- PR #3- PR Time (hours) Figure 4.3 Corrosion rates of the Al 1100 samples (#1, #2, and #3) via polarization resistance and electrochemical impedance spectroscopy in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion As can be seen in Table 4.2 and Figure 4.3, the 1 st and 2 nd samples display more reproducible data for EIS and PR. In contrast to the Al 1100 samples, the corrosion rates of the Zn samples decreased in time. Also, a gradual decrease in the corrosion rates of the Zn samples with time signifies less fluctuation in the corrosion rate values. The difference observed in using different techniques (EIS and PR) to estimate the corrosion rates of the same samples decreased after 120 hours of immersion which may signify that the corrosion products accumulated on the surface, held onto the surface and provided additional resistance against corrosion. In addition to the corrosion rate changes versus time graphs, the Nyquist and Bode plots of the 2 nd Zn sample obtained from EIS are given in Figure

77 Figure 4.4 Nyquist and Bode plots of the 2 nd Zn sample from EIS in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion 62

78 The Nyquist plots of the 2 nd Zn sample in Figure 4.4 show that the corrosion process was a diffusioncontrolled mechanism. Even though the corrosion rates of the 2 nd sample decreased compared to the rates at the beginning of immersion, the corrosion process started to occur on the surface after a week of exposure to the media. Thus, if the experiment were to be continued, the decrease in the corrosion rate may or may not happen Galvalume Galvalume is a Zn-Al (55% Al, 43% Zn, and 1.6% Si) alloy. The coating is formed by dipping the steel substrate into molten Galvalume. The results from polarization resistance experiments are given in Table 4.3, and corrosion rates of the samples versus immersion time plots are given in Figure 4.5. Table 4.3 Polarization resistance and estimated corrosion current density and corrosion rates of the triplicate sample set of Galvalume samples during a week of immersion in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and at 60 o C Immersion Rp 1 Icorr 1 CR 1 Rp 2 Icorr 2 CR 2 Rp 3 Icorr 3 CR 3 time ohms.cm 2 µa/cm 2 mpy ohms.cm 2 µa/cm 2 mpy ohms.cm 2 µa/cm 2 mpy 0 hour 2.24E E E day 3.07E E E days 4.19E E E days 3.99E E E week 2.70E E E

79 CR (mpy) 14 CR of Galvalume vs immersion time via EIS & PR #1- EIS #2- EIS #3- EIS #1- PR #2- PR #3- PR Time (hours) Figure 4.5 The corrosion rates of the Galvalume samples (#1, #2, and #3) via polarization resistance and electrochemical impedance spectroscopy in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion The corrosion rate values of the Galvalume samples show more stable behavior compared to those of the Al 1100 samples. Aside from the 3 rd day data of the 1 st sample and the initial corrosion rate value of the 3 rd sample, the rest of the values stay almost the same. Also, it is more obvious in Figure 4.5 that EIS measurements yield in a slightly higher corrosion rate than the PR method; however, at the end of the 7 days of immersion, it can be seen the corrosion rate values of all samples estimated by EIS and PR methods were close. This may be interpreted as a steady state for the corrosion process. Overall, Galvalume samples have the highest corrosion rates among the possible sacrificial layer constituents. The Nyquist and Bode plots of the 2 nd Zn sample obtained from EIS are given in Figure

80 Figure 4.6 Nyquist and Bode plots of the 2 nd Galvalume sample from EIS in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion 65

81 Figure 4.6 shows that for the 2 nd Galvalume replica longer immersion times not only decreased the coating resistance, but also caused an appearance of a second time constant. However, according to the EIS plots, the 2 nd semi-circle disappeared after 7 days of immersion Uncoated steel substrate panels The uncoated 34J- grade carbon steel substrate panels were tested without replicas, their surface preparation was varied. The J1 coded panel was the bare carbon steel with no pretreatment on the surface. J2, the burned-off carbon steel panel was exposed to high temperature to form an oxide layer on the surface for cleaning the surface from dirt, grease, etc. J3, the white-metal finish panel, has an anchor pattern to improve adhesion of the subsequent paint layer. The white-metal finish was accomplished by grit blasting the substrate with Al 2 O 3.The results from polarization resistance experiments on these panels are given in Table 4.4, and corrosion rate of the samples versus immersion time plots are given in Figure 4.7. Table 4.4 Polarization resistance and estimated corrosion current density and corrosion rates of the bare (J1), burned off (J2) and white metal finish painted (J3) carbon steel panels during a week of immersion in a 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and at 60 o C Immersion Rp J1 Icorr J1 CR J1 Rp J2 Icorr J2 CR J2 Rp J3 Icorr J3 CR J3 time ohms.cm 2 µa/cm 2 mpy ohms.cm 2 µa/cm 2 mpy ohms.cm 2 µa/cm 2 mpy 0 hour 7.60E E days 1.29E days E week

82 CR (mpy) 60 CR of steel panels vs Immersion time via PR & EIS J1- PR J2- PR J3- PR J1- EIS J2- EIS J3- EIS Time (hours) Figure 4.7 The corrosion rates of the the bare (J1), burned off (J2) and white metal finish painted (J3) carbon steel panels via polarization resistance and electrochemical impedance spectroscopy in 2000 ppm NaCl- HCl mixture saturated with CO 2 at ph 3.5 and 60 o C at the 0 th hour and on the 1 st, 3 rd, 5 th, 7 th days of the immersion As can be seen in the polarization resistance results in Table 4.4., the corrosion rates of the two steel substrates (J1 and J3) tended to decrease with increasing immersion time except for J2 (the burned-off) steel panel. The highest final (on the 7 th day of immersion) corrosion rate (26 mpy) was estimated for J3, the white-metal finish steel substrate. Despite the high corrosion rate values, the accumulation of the corrosion products causes the decrease in the corrosion rate by blocking the surface reactions. Also, the Nyquist and Bode plots obtained from EIS did not follow a trend, but displayed only fast dissolution of the steel resulting in uniform corrosion. The Nyquist and Bode plots are not given. 67

83 4.2. Coated Panels In this section, EIS data of the coated panels were analyzed by fitting into equivalent circuits using a commercial software package, Zview (version 3.2b). Assessing the performance of a coating system uses electrical circuits built from components such as resistors and capacitors to represent the electrochemical behavior of the coating and the metal substrate. Changes in the values for the individual components indicate their behavior and performance. The equivalent electrical circuit used for an intact coating and for the calculations are shown in Figure 4.8. Figure 4.8 Electrical circuits for: an intact coating (left) and a coating with a defect (right) Both the intact coating and the coating with a defect have a common subcircuit represented by (Rc, Cc). The (Rc, Cc) part of the circuit represents an intact coating. The resistance, Rc, and capacitance, Cc, values of the coatings are estimated by fitting the 1 st semicircle (at the higher frequencies) in the Nyquist and the lowest plateau (at the higher frequencies) of the Modulus curve in the Bode plot into the model. The (Rct, Cdl) parts represent the defect and the corrosion process occurring through the defect at the metal-coating interface. The charge transfer resistance, Rct, and the double layer formed due to the corrosion process, Cdl, are estimated by fitting the 2 nd semicircle (that is at the lower frequencies in the Nyquist plot) and the higher plateau (at the lower frequencies) of the modulus curve (in the Bode plot) into the model. The coatings are categorized according to their filler amount and discussed accordingly. 68

84 4.2.1 TK 805 Powder Coatings with Low Amount of Filler The coated panels discussed in this section, 6B and 6E, are not analyzed with their replicas, 1B and 1E. The coatings on 1B and 1E panels are discussed in the Scratch Studies section because a defect was detected on 1B. A sudden decrease in the resistance of 1E was observed which could also be associated with a defect, but no such defect has been visually observed. The characteristics of the TK 805 coating system on panels B and E are given in Table 4.5. The coating resistance and coating capacitance values with respect to time are plotted in Figures 4.9 and 4.10, and the Nyquist and Bode plots for the coatings are presented in Figure Table 4.5 Coating system characteristics for panels E and B Panel code Coating code Coating Filler amount Thickness 1B, 6B TK 805 Powder-phenolic novolac 1E, 6E TK 805 Powder-phenolic novolac low low mils mils 69

85 Coating capacitance (Farads/cm 2 ) Coating resistance (ohms.cm 2 ) 1.00E+13 Rc-6B & 6E 6B 6E 1.00E E Time (hours) Figure 4.9 Resistance of the coatings-estimated from EIS results- on 6B (P, low f, mils) and 6E(P, low f, mils) panels versus time during 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of E-09 6E Cc-6B & 6E 6B 1.00E E Time (hours) Figure 4.10 Capacitance of the coatings-estimated from EIS results- on 6B (P, low f, mils) and 6E(P, low f, mils) panels versus time during 14 weeks of immersion in a NaCl-HCl solution saturated with CO 2 at 60 o C and ph of