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Corrosion behavior of galvanized steel reinforcements in MSE walls in the presence of soil organics Soriano Vazquez, Claudia Aide 2014

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CORROSION BEHAVIOR OF GALVANIZED STEEL REINFORCEMENTS IN MSE WALLS IN THE PRESENCE OF SOIL ORGANICS  by  Claudia Aide Soriano Vazquez  B. Sc., Universidad Nacional Autonoma de Mexico, 2009  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF APPLIED SCIENCES in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Materials Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  November 2014  © Claudia Aide Soriano Vazquez, 2014 ii  Abstract  Mechanically Stabilized Earth is a civil infrastructure technology that is widely used in retaining walls. Although the structures are designed for a service life of 75 years, early distress has been reported. Corrosion of the galvanized steel reinforcements has been pointed as one of the major causes that jeopardize their long-term performance.   The corrosion behavior of galvanized steel in the presence of organics found in soil is studied through electrochemical techniques as PDP, EIS and LPR in two types of solutions at various concentrations below 1 wt%. The first type of solutions aims to determine the corrosion behavior in the presence of individual organic reagents: humic acid, dextrose, citric acid and oxalic acid. The second type of solutions is prepared with a system that combines the organic reagents in proportions that simulate the typical composition of organic matter in soil and is called Simulated Soil Organic Matter (SSOM). Subsequently, the surface is analyzed using SEM and EDX.   The data shows that the corrosion effect of organic matter on galvanized steel depends on its composition. The comparison of the highest current density produced by the individual organics, allows ranging them in terms of their aggressiveness on galvanized steel in the following order: citric acid > oxalic acid > humic acid > dextrose. The Simulated Soil Organic Matter was able to corrode the zinc coating and the base steel.  iii  Preface  I was responsible for designing the experimental program presented in this thesis, conducting all electrochemical tests and surface characterization, and analyzing the results. Professor Akram Alfantazi supervised the progress of the research.  Results from this research work were presented in conferences as follow: 1. Soriano, C., Alfantazi, A. (2014). Individual effect of organics in soil on the corrosion of galvanized steel reinforcements in MSE walls. Student Poster at SAMPE1 Seattle Conference. 2. Soriano, C., Alfantazi, A. (2014). Effect of soil organic matter on the corrosion of galvanized steel reinforcements in MSE walls. Conference paper A20 and presentation at NACE2 International East Asia Pacific Rim Area Conference & Expo 2014.                                                  1 Society for the Advancement of Materials and Process Engineering, 1161 Park View Drive, Suite 200  Covina, CA 91724-3759, USA. 2 National Association of Corrosion Engineers, 15835 Park Ten Place, Houston, Texas, 77084, USA. iv  Table of Contents  Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iii Table of Contents ......................................................................................................................... iv List of Tables .............................................................................................................................. viii List of Figures .................................................................................................................................x List of Symbols .............................................................................................................................xv List of Abbreviations ................................................................................................................ xvii Glossary .................................................................................................................................... xviii Acknowledgements .................................................................................................................... xix Dedication .....................................................................................................................................xx Chapter 1: Introduction ................................................................................................................1 1.1 Mechanically Stabilized Earth walls ............................................................................... 1 1.2 MSE walls in British Columbia ...................................................................................... 4 Chapter 2: Literature review ........................................................................................................7 2.1 Corrosion......................................................................................................................... 7 2.1.1 Corrosion protection of steel by galvanizing .............................................................. 7 2.1.2 Corrosion process of galvanized steel ......................................................................... 8 2.1.3 Factors that influence the corrosion of galvanized steel in soil .................................. 9 2.1.4 Deterioration of MSE walls due to corrosion ........................................................... 11 2.1.5 The organic content in backfill soil .......................................................................... 13 2.2 Soil organic matter ........................................................................................................ 13 v  2.2.1 Definition and composition....................................................................................... 14 2.2.2 Characteristics of humic substances ......................................................................... 16 2.2.2.1 Oxidation-reduction reactions ........................................................................... 17 2.2.2.2 Surface adsorption ............................................................................................ 19 2.2.2.3 Chelation ........................................................................................................... 19 2.2.3 Carbohydrates in soil ................................................................................................ 20 2.2.4 Characteristics of organic acids ................................................................................ 21 Chapter 3: Objectives ..................................................................................................................23 3.1 Key technical objectives ............................................................................................... 23 Chapter 4: Approach and methodology ....................................................................................25 4.1 Materials selection ........................................................................................................ 25 4.1.1 Galvanized steel ........................................................................................................ 25 4.1.2 Organic reagents ....................................................................................................... 25 4.2 Preparation of solutions ................................................................................................ 27 4.2.1 Solutions with individual organics............................................................................ 27 4.2.2 Simulated soil organic solutions ............................................................................... 27 4.3 Electrochemical tests .................................................................................................... 28 4.3.1 Open Circuit Potential............................................................................................... 29 4.3.2 Potentiodynamic Polarization Resistance ................................................................. 29 4.3.3 Linear Polarization Resistance .................................................................................. 30 4.3.4 Electrochemical Impedance Spectroscopy ............................................................... 31 4.4 Equipment ..................................................................................................................... 32 4.5 Surface characterization ................................................................................................ 34 vi  4.5.1 Scanning Electron Microscope ................................................................................. 35 4.5.2 Energy Dispersive X-ray Spectroscopy .................................................................... 35 Chapter 5: Corrosion of galvanized steel due to individual organics .....................................36 5.1 Properties of organic solutions...................................................................................... 36 5.1.1 pH .............................................................................................................................. 36 5.1.2 Conductivity .............................................................................................................. 38 5.2 Control sample .............................................................................................................. 39 5.3 Humic acid .................................................................................................................... 40 5.3.1 Electrochemical tests in humic acid solutions .......................................................... 40 5.3.2 Surface characterization of galvanized steel in humic acid solutions....................... 44 5.4 Dextrose ........................................................................................................................ 47 5.4.1 Electrochemical tests in dextrose solutions .............................................................. 47 5.4.2 Surface characterization in dextrose solutions .......................................................... 50 5.5 Citric acid ...................................................................................................................... 53 5.5.1 Electrochemical tests in citric acid solutions ............................................................ 53 5.5.2 Surface characterization of galvanized steel in citric acid solutions ........................ 57 5.6 Oxalic acid .................................................................................................................... 60 5.6.1 Electrochemical tests in oxalic acid solutions .......................................................... 60 5.6.2 Surface characterization of galvanized steel in oxalic acid solutions ....................... 66 5.7 Chlorides and sulfates ................................................................................................... 70 5.8 Comparison of corrosion behavior of galvanized steel in four organic solutions ........ 71 5.9 Summary ....................................................................................................................... 78 vii  Chapter 6: Corrosion behavior of galvanized steel in solutions of simulated soil organic matter ............................................................................................................................................79 6.1 pH and conductivity ...................................................................................................... 79 6.2 Electrochemical measurements ..................................................................................... 80 6.3 Polarization resistance trend as a function of SSOM concentration in solution ........... 86 6.4 Surface characterization ................................................................................................ 87 6.5 Summary ....................................................................................................................... 93 Chapter 7: Conclusions ...............................................................................................................94 Chapter 8: Future work ..............................................................................................................95 Bibliography .................................................................................................................................96  viii  List of Tables  Table ‎2-1: AASHTO classification of aggressiveness of soils in terms of resistivity .................. 10 Table ‎2-2: Electrochemical characteristics of backfill soil ........................................................... 12 Table ‎2-3: AASHTO model of metal loss .................................................................................... 12 Table ‎4-1: Concentrations of solutions prepared for each organic ............................................... 27 Table ‎4-2: Composition of simulated soil organics by weight percent in 1L solution ................. 28 Table ‎5-1: pH of four organic solutions at four concentrations from 0.1 wt% to 1.0 wt% .......... 36 Table ‎5-2: Conductivity of solutions and kinetic parameters from PDP measurements for galvanized steel in humic acid solutions at concentrations ranging from 0.005 wt% to 1.0 wt% at pH 10 and 25°C ............................................................................................................................. 44 Table ‎5-3:  Conductivity of solutions and kinetic parameters from PDP measurements of galvanized steel in dextrose solutions at concentrations ranging from 0.10 wt% to 1.0 wt% at pH 7 and 25°C .................................................................................................................................... 49 Table ‎5-4: Conductivity of solutions and kinetic parameters from PDP measurements of galvanized steel in citric acid solutions at concentrations from 0.01 wt% to 1.0 wt% at pH 2 and 25°C .............................................................................................................................................. 55 Table ‎5-5: Conductivity of solutions and kinetic parameters from PDP measurements of galvanized steel in oxalic acid solutions at different concentrations at pH 1 and 25°C ............... 65 Table ‎5-6: Parameters of solutions with 100ppm chlorides and 200ppm sulfates and electrochemical measurements from PDP tests in 25°C ............................................................... 71 Table ‎6-1: Kinetic parameters obtained from PDP curves of SSOM solutions at 25°C and concentrations from 0.25 wt% to 1.0 wt% .................................................................................... 82 ix  Table ‎6-2: Polarization resistance at different concentrations of combined organics .................. 84 Table ‎6-3: Parameters of the components of the equivalent circuit that simulates the frequency response of galvanized steel in SSOM solutions at 25°C ............................................................. 85  x  List of Figures  Figure ‎1-1: Components of an MSE wall: 1) Level pad; 2) Vertical face element; 3) Backfill soil;......................................................................................................................................................... 2 Figure ‎1-2: Cross-sectional diagram of a MSE wall ....................................................................... 3 Figure ‎1-3: Growing tree on an MSE wall in B.C. ......................................................................... 6 Figure ‎1-4: Evidence of corrosion of steel and plant growth .......................................................... 6 Figure ‎2-1: Composition of organic matter in soil........................................................................ 14 Figure ‎2-2: Oxygen containing functional groups in humic acid: a) hydroxyl, b) carbonyl, c) carboxyl......................................................................................................................................... 17 Figure ‎4-1: Model of selected organics: a) oxalic acid, b) citric acid, and c) dextrose in open-chain form ..................................................................................................................................... 26 Figure ‎4-2: Three repeatable results after performing PDP test in galvanized steel in 1.00 wt% humic acid solution. ...................................................................................................................... 30 Figure ‎4-3: Three repeatable results of EIS test performed on galvanized steel in 0.05 wt% SSOM solution. ............................................................................................................................. 31 Figure ‎4-4: Experimental set-up of a standard electrochemical cell: 1. Reference electrode; 2. Sample holder; 3. Thermometer; 4. Counter electrode; 5. Corrosion cell; 6. Magnetic stirrer; 7. Water bath. .................................................................................................................................... 33 Figure ‎4-5: Components of the sample holder: 1. Body; 2. Flat O-ring; 3. Galvanized steel sample; 4. Metallic lid; 5. Polymer lid with O-ring ...................................................................... 34 Figure ‎5-1: E-pH diagram of the zinc-water system at 25 °C ...................................................... 37 Figure ‎5-2: Variation in conductivity of four organic solutions as a function of concentration .. 38 xi  Figure ‎5-3: SEM image of control sample of galvanized steel ..................................................... 39 Figure ‎5-4: EDX analysis of control sample of galvanized steel .................................................. 40 Figure ‎5-5: Measurement of open circuit potential of galvanized steel in humic acid solutions at different concentrations at pH 10 and 25°C .................................................................................. 41 Figure ‎5-6: Potentiodynamic curves of galvanized steel immersed in humic acid solutions at concentrations below 0.1 wt% at pH 10 and 25°C ....................................................................... 42 Figure ‎5-7: Potentiodynamic curves of galvanized steel immersed in humic acid solutions at concentrations between 0.1 wt% and 1.00 wt% pH 10 at 25°C ................................................... 43 Figure ‎5-8: SEM image of galvanized steel after PDP in 0.25wt% humic acid solution ............. 45 Figure ‎5-9: EDX analysis of region 1 of Figure ‎5-8 ..................................................................... 46 Figure ‎5-10: EDX analysis of region 2 of Figure ‎5-8 ................................................................... 47 Figure ‎5-11: Measurement of open circuit potential of galvanized steel in dextrose solutions at different concentrations at pH 7 and 25°C .................................................................................... 48 Figure ‎5-12: Potentiodynamic curves of galvanized steel immersed in dextrose solutions at different concentrations at pH 7 and 25°C .................................................................................... 49 Figure ‎5-13: SEM image of galvanized steel after PDP in 1wt% dextrose solution .................... 51 Figure ‎5-14: EDX analysis of region 2of Figure ‎5-13 .................................................................. 52 Figure ‎5-15: EDX analysis of region 1 of Figure ‎5-13 ................................................................. 52 Figure ‎5-16: Measurement of open circuit potential of galvanized steel in Citric Acid solutions at different concentrations at pH 3 and 25°C .................................................................................... 53 Figure ‎5-17: Potentiodynamic curves of galvanized steel immersed in citric acid solutions at different concentrations at 25°C ................................................................................................... 54 Figure ‎5-18: Predominance area diagram of zinc-citrate complexes in aqueous solutions. ......... 56 xii  Figure ‎5-19: Predominance area diagram of iron-citrate complexes in aqueous solutions. ......... 57 Figure ‎5-20: SEM image of galvanized steel after PDP in 0.25 wt% citric acid solution ............ 58 Figure ‎5-21: EDX analysis of region 1of Figure ‎5-20 .................................................................. 59 Figure ‎5-22: EDX analysis of region 2 of Figure ‎5-20 ................................................................. 59 Figure ‎5-23: Measurement of open circuit potential of galvanized steel in 0.10, 0.25 and 0.50 wt% oxalic acid solutions at pH 2 and 25°C ................................................................................ 60 Figure ‎5-24: Measurement of open circuit potential of galvanized steel in 1.0 wt% oxalic acid solution at pH 2 and 25°C ............................................................................................................. 61 Figure ‎5-25: Potentiodynamic curves of galvanized steel immersed in oxalic acid solutions at concentrations below 0.1 wt% at pH 1 and 25°C ......................................................................... 62 Figure ‎5-26: Potentiodynamic curves of galvanized steel immersed in oxalic acid solutions at concentrations between 0.1wt% and 1.0wt% at pH 1 and 25°C ................................................... 63 Figure ‎5-27: Passive-like behavior of galvanized steel immersed in 1.0wt% oxalic acid solution at pH 1 and 25°C ........................................................................................................................... 64 Figure ‎5-28: SEM image of galvanized steel after PDP in 0.25 wt% oxalic acid solution .......... 67 Figure ‎5-29: SEM image of crystals inside blisters ...................................................................... 67 Figure ‎5-30: EDX analysis of compact layer ................................................................................ 68 Figure ‎5-31: EDX of nests crystals ............................................................................................... 68 Figure ‎5-32: PDP curves of solutions with 100ppm chlorides and 200ppm sulfates at 25°C ...... 70 Figure ‎5-33: Comparison of potentiodynamic polarization resistance plots of galvanized steel immersed in 1 wt% organic solutions at 25°C .............................................................................. 72 xiii  Figure ‎5-34: Comparison of corrosion current densities of galvanized steel as a function of concentration of individual organics in solutions at 25°C: a) dextrose; b) humic acid; c) oxalic acid; d) citric acid.......................................................................................................................... 73 Figure ‎5-35: Conversion from 𝑖𝑐𝑜𝑟𝑟 values in 𝜇𝐴/𝑐𝑚2 to penetration rates (r) in 𝜇m/yr. ........... 75 Figure ‎5-36: Corrosion current densities at different pH values................................................... 76 Figure ‎5-37: Corrosion current densities and conductivity of organic solutions. ......................... 77 Figure ‎6-1: Parameters of the SSOM solutions as a function of concentration at 25°C: a) pH, b) conductivity................................................................................................................................... 80 Figure ‎6-2: Potentiodynamic curves of galvanized steel immersed in solutions with different concentrations of a combination of organics that simulate the typical composition of soil organic matter ............................................................................................................................................ 81 Figure ‎6-3: Graphic comparison of the current densities produces by individual organics and by the combination of them in SSOM solutions at 25°C ................................................................... 82 Figure ‎6-4: Nyquist plots of galvanized steel in SSOM solutions at different concentrations at 25°C. ............................................................................................................................................. 84 Figure ‎6-5: Equivalent circuit to EIS response ............................................................................. 85 Figure ‎6-6: Polarization Resistance (RP) obtained by three methods with error bar plots of standard deviation ......................................................................................................................... 86 Figure ‎6-7: SEM image of galvanized steel after PDP in 0.50 wt% SSOM solution at 25°C ...... 87 Figure ‎6-8: SEM image of GS sample after PDP immersed in solutions with 0.5 wt% SSOM... 88 Figure ‎6-9: EDX analysis of region 1 from Figure ‎6-8 ................................................................ 89 Figure ‎6-10: EDX analysis of region 2 from Figure ‎6-8 .............................................................. 89 Figure ‎6-11: SEM image of galvanized steel sample after PDP in 0.91 wt% SSOM solution .... 90 xiv  Figure ‎6-12: SEM image of region labeled as 1 in Figure ‎6-11 (0.91 wt% SSOM) .................... 91 Figure ‎6-13: EDX analysis of region 1 of Figure ‎6-11 ................................................................. 91 Figure ‎6-14: SEM image of region 2 of Figure ‎6-11 (0.91 wt% SSOM) ..................................... 92 Figure ‎6-15: EDX analysis of region labeled as 2 in Figure ‎6-11 ................................................ 93  xv  List of Symbols  a Atomic weight 𝛽𝑎 Anodic Tafel slope (V/dec) 𝛽𝑐 Cathodic Tafel slope (V/dec) c Proportionality constant D Density E Measured potential (V) 𝐸𝑏 Breaking potential (V) 𝐸𝑐𝑜𝑟𝑟 Corrosion potential (V) 𝐸𝑝𝑝 Primary passive potential (V) 𝐸𝑡𝑟𝑎𝑛𝑠 Transpassive potential 𝐸𝑍𝑛𝑜  Standard potential of zinc F Faraday’s‎constant‎(‎≈‎96500‎C/mole) i Current density (𝜇𝐴 𝑐𝑚2⁄ ) 𝑖𝑐𝑜𝑟𝑟 Corrosion current density (𝐴 𝑐𝑚2⁄ ) 𝑖𝑐𝑟𝑖𝑡 Critical current density (𝐴 𝑐𝑚2⁄ ) 𝑖𝑝𝑎𝑠𝑠 Passivation current density M Molar concentration Me Metal n Number of equivalents exchanges in oxidation-reduction reactions η Overpotential xvi  X Radical R Gas constant (8.314 J/(mole.K)) 𝑅𝑃 Polarization resistance 𝑅𝑠 Solution resistance Re Resistance of the electrolyte inside a pore Q1 Capacitance of the porous layer Q2 Capacitance of the double layer xvii  List of Abbreviations  AASHTO American Association of State Highway and Transportation Officials ASTM  American Society for Testing and Materials DOT  State Department of Transportation EDX  Energy Dispersive X-ray Spectroscopy EIS  Electrochemical Impedance Spectroscopy FHWA Federal Highway Administration GS  Galvanized steel HA  Humic acid LMWOA Low molecular weight organic acids LPR  Linear Polarization Resistance MIC  Microbially-Influenced Corrosion MSE  Mechanically Stabilized Earth NBS  National Bureau of Standards OCP  Open Circuit Potential PDP  Potentiodynamic Polarization Resistance SCE  Saturated Calomel Electrode SEM   Scanning Electron Microscope SOM  Soil Organic Matter SHE  Standard Hydrogen Electrode TAI  Terre Armée Internationale  xviii  Glossary  Abutment  A substructure that supports the end of a superstructure and retains some or all of the bridge approach fill Design life Period of time specified by the owner during which a structure is intended to remain in service Gabion  A wire mesh basket that is filled with stone or broken rock or concrete and forms part of a larger unit used for slope stability, erosion control, or related purposes Ligand Organic components capable of forming complexes with metals Retaining wall  Structure designed to hold back masses of earth. Retaining walls include footings and pilings supporting the walls   xix  Acknowledgements  I would like to express my sincere gratitude to my supervisor, Professor Akram Alfantazi, for introducing me to the world of knowledge of Corrosion Engineering but, most importantly, for guiding me and challenging me to improve my research, presentation, and instructional skills. I will take with me this invaluable learning.  Also, I would like to acknowledge the financial support provided by the British Columbia Ministry of Transportation and Infrastructure (BC MOTI), Atlantic Industries Limited (ALI) and Mitacs Accelerate. Special thanks are owed to Kevin Baskin, Ian Sturrock and Daryl Finlayson from the BC MOTI for sharing their experience and providing the questions and tools to apply my knowledge in field applications.   I owe thanks to my fellow students of the Corrosion Group for their continuous help and insight during my research. In particular, I offer my enduring gratitude to V. Padilla, whose answers always helped me to improve my work.   I also want to thank the staff of the Materials Engineering Department for their support during my work: Fiona Webster, Mary Jansepar, Norma, Michelle Tierney, Glenn Smith, and Jacob Kabel.  Finally, I want to express my recognition to my parents, who have supported me in countless ways throughout my lifetime and always encouraged and supported me in my years of education. xx  Dedication        To: My Mom, for her everlasting memory My Dad, for his unconditional support  1  Chapter 1: Introduction  1.1 Mechanically Stabilized Earth walls  Mechanically Stabilized Earth (MSE) is a construction technology of retaining walls that was originally developed in France in the 1960s by Terre Armée Internationale3. The technology was introduced in North America in the early 1970s with the support of the Federal Highway Administration (FHWA) of the US (RECo, 2011). Although the main use of MSE was for retaining walls, the applicability of the technology was extended to other heavily-loaded structures such as highway bridges, railway and industrial structures (Anderson & Brabant, 2010). At the beginning, only a few dozen MSE abutments were built each year. However, given the advantages that MSE walls offered in front of the traditional cast-in place concrete structures, such as more rapid construction, better aesthetics and cost effectiveness (Holtz et al., 1995), after 1990 the construction rate increased to approximately 600 abutments per year, i.e. 300 bridges (Anderson et al., 2012).   MSE walls have three main structural components (Chen, 2000): (i) the level pad, (ii) the vertical face element, and (iii) the soil reinforcement. The third component combines the tensile strength of horizontal reinforcements with the compressive and shear strengths of the backfill soil (Anderson et al., 2012). The reinforcements can be of two types depending on the material they                                                  3 Terre Armée Internationale (TAI) developed the Reinforced Earth® technology, which was the original denomination, but since it was a proprietary name of TAI, the generic name MSE was adopted. Later TAI changed its name to Reinforced Earth Company (RECo). 2  are made of: geosynthetic, called extensible; or metallic, called inextensible. In both cases, to build the MSE wall, the reinforcements are placed at specific heights in between layers of backfill soil according to the customary design. The inextensible reinforcements are made of hot-dip galvanized steel, which may be in the form of welded wire mesh or ribbed strips.    Figure ‎1-1: Components of an MSE wall: 1) Level pad; 2) Vertical face element; 3) Backfill soil; 4) Reinforcements   The components of an MSE wall can be observed in Figure ‎1-1 in two photographs that were taken at a site under construction located on Sneena Road, in West Kelowna, B.C. The picture on the left points the level pad and the concrete slabs that form the facing wall with labels 1 and 2, respectively, which will be the only components visible from outside upon completion of the abutment. The picture on the right illustrates how the galvanized steel mats (4) are placed on alternated layers of backfill soil (3) and anchored to the facing wall. These two components will be inaccessible once the construction is completed.  3  In addition, Figure ‎1-2 shows a diagram of the cross-section of an MSE wall, where it can be observed how the components will be integrated once the wall is completed. The vertical face element will appear as a wall that can be seen from the road. Behind the layers of backfill soil and galvanized steel reinforcements will lay horizontally, and the reinforcements will be anchored to the concrete slabs.   Figure ‎1-2: Cross-sectional diagram of a MSE wall  Furthermore, it is important to highlight that the soil used as backfill soil does not correspond to the foundation soil retrieved at the site of construction. Instead, the backfill soil is extracted from specific pits where it is known that the soil will meet the technical specifications required. Another remark about the materials used in this technology is that the steel in the reinforcements is of structural type. This implies that the steel will exhibit higher yield strength and tensile strength than carbon steel designed for other type of applications. The ASTM standards A36 and A709 provide the specifications for structural steel. The chemical composition will depend on the grade of the steel. For example, a structural steel of grade 36 will have a yield strength of 250 MPa, and for a rebar thickness over ¾ and below 1½ in (included), the maximum elemental composition is 0.27% C, 0.90% Mn, 0.04% P, 0.05% S, 0.40% Si, and 0.20% Cu. However, the 4  composition varies according to the grade, and the required grade for each wall will vary case by case according to the general design of the structure.   1.2 MSE walls in British Columbia  It is estimated that in British Columbia alone, there are over 1100 MSE walls (Padilla, 2013).  This is an important inventory of infrastructure assets that the British Columbia Ministry of Transportation and Infrastructure (BC-MOTI) is interested in managing to ensure that they perform as expected (CSA, 2006).  The several factors that influence the long term performance of MSE walls, closely associated with the metal loss of the galvanized steel rebars due to corrosion, have been considered in the design specifications that provide the electrochemical characteristics that should be met by the backfill soil, as will be described in chapter 2. However, those specifications were developed based on environmental conditions that are not necessarily the same as the ones in British Columbia. For this reason, there is an interest in understanding how the variables that are specific to this province may affect the corrosion behavior of the galvanized steel reinforcements. For instance, the increase of sulfate and chloride concentrations in the backfill soil due to the use of de-icing salts on the roads during the winter season has been addressed in a collaboration work between BC-MOTI and UBC (Padilla & Alfantazi, 2013; Padilla et al., 2013a). In 2012, several sites were visited by a PhD student from the University of British Columbia and personnel from the BC-MOTI to evaluate the current state of MSE walls. 5  Superficial soil samples were taken from 34 sites and sent to the laboratory to determine if the backfill soil characteristics were in compliance with the specifications after variable years in service. As a result, a numerical model was developed whose key innovating feature was the possibility to provide variable values of the electrochemical parameters of the soil as input to estimate the remaining service life of the reinforcements. Although various electrochemical parameters were incorporated, the organic content was not part of them. Therefore, a study that focused on this component was still required to complete the model (Padilla et al., 2013b).  From the information gathered on the field work performed in 2012, the soil analysis revealed that only in 8 sites, the organic content was within the established limit of 1% whereas, in the rest of the sites, the organic content was 2.22 ± 1.24%. Given the results, the immediate question was: Does the higher organic content in the backfill soil have an impact on the corrosion of the galvanized steel reinforcements? By visual inspection of the visited walls, it became evident that the low percentages of organic content in the backfill soil were sufficient to provide nutrients for vegetation in a number of sites. The most severe case is shown in Figure ‎1-3, where a growing tree is already damaging the concrete slabs on the facing wall. Similarly, in another wall (Figure ‎1-4), the organic content was enough to promote plant growth at the same time that the red stains on the concrete revealed that corrosion had reached the base steel of the reinforcements.  6   Figure ‎1-3: Growing tree on an MSE wall in B.C.  Figure ‎1-4: Evidence of corrosion of steel and plant growth   This research aims to determine the corrosion behavior of galvanized steel due, exclusively, to the presence of soil organics typically and to establish the path to find an equation that relates the content of organic matter with the polarization resistance that can be used to incorporate a new variable to the numerical model. 7  Chapter 2: Literature review  2.1  Corrosion  Corrosion is the degradation of a material due to its interaction with the environment. Four fundamental elements are required for corrosion to take place: (i) an anode, (ii) a cathode, (iii) an electronic path, and (iv) an ionic path. In the case of a civil infrastructure reinforced with steel, the reinforcements themselves provide the electronic path, while the medium into which they are embedded, either concrete or soil, provide the ionic path. The anode and the cathode are different regions of the steel reinforcements that act as one electrode or the other due to differences in the surrounding electrolyte that induce the formation of corrosion cells. At the anode, the metal dissolves and ions are released and the surface is negatively charged. At the cathode, ions are reduced and deposited on the surface. (Asgari et al., 2009)  2.1.1 Corrosion protection of steel by galvanizing  Galvanizing is the process through which steel is coated with zinc. During hot-dip galvanizing, zinc-iron alloy layers are formed with different compositions depending on their proximity to the base metal, with the highest content of pure zinc found on the surface. The zinc coating improves the corrosion resistance of the steel reinforcements in two ways: (i) acting as physical barrier and (ii) providing galvanic protection (Asgari et al., 2009). The physical barrier isolates the steel from the environment, first by the layer of zinc and, second, by the zinc corrosion products layer. 8  The galvanic protection means that even if the steel substrate gets exposed, the zinc layer will corrode preferentially (Yadav et al., 2004a).  2.1.2 Corrosion process of galvanized steel  The corrosion of galvanized steel can be described as a three-stage process (El-Mahdy et al., 2000; Padilla et al., 2013b; Yadav et al., 2004b). In the first stage, solid zinc corrodes near its standard potential (𝐸𝑍𝑛𝑜 ) releasing divalent ions, according to Eq. ‎2-1. 𝑍𝑛 → 𝑍𝑛2+ + 2𝑒− Eq. ‎2-1    The zinc corrosion products, composed mainly of zinc oxide (𝑍𝑛𝑂), have a very low solubility and precipitate forming a semi-compact, adherent, porous layer that has a characteristic white color (Thebault et al., 2008; Zhang, 1996). The galvanic protection that zinc provides to steel occurs in this stage within a limited distance from the coating (Thebault et al., 2008; Thebault et al., 2011).  In the second stage, the layer of corrosion products grows until zinc is depleted and a thick, porous, soluble and non-protective layer of corrosion products is formed. The main components of this layer are 𝑍𝑛(𝑂𝐻)2 and 𝑍𝑛𝐶𝑙2 ∙ 4𝑍𝑛(𝑂𝐻)2. It is in this stage, that the corrosion front reaches the Zn-Fe alloy layer and the galvanized steel experiences a shift of its corrosion potential to more noble values. In the third stage, corrosion of steel initiates and iron ions are released, according to Eq. ‎2-2. The corrosion products from steel become visible as red rust. 𝐹𝑒 → 𝐹𝑒2+ + 2𝑒− Eq. ‎2-2  9  2.1.3  Factors that influence the corrosion of galvanized steel in soil  Soil is the naturally-occurring, unconsolidated, mineral or organic material at the earth's surface (Soil Classification Working Group, 1998). Seen as a corrosive environment, soil is highly inhomogeneous and exhibits a wider range of chemical and physical properties compared to atmospheric and aquatic environments, due to the large number of chemical compounds that can be found in soils. Various properties have a synergistic influence on the corrosion rates of metals in soil, including pH, soil resistivity, moisture content, aeration, temperature, and soluble salt concentration.  For instance, the range of pH values that can be found in soils is wide, ranging from 2.6 to 10.2 (Soil Survey Division Staff, 1993). The soil resistivity itself depends on other factors such as particle size, porosity, and temperature. The size of the particles determines the specific surface area (i.e. total surface area per unit of mass). Soils with smaller particles have bigger specific surface areas and therefore exhibit higher conductivity at the surface than soils with coarse particles of the same mineralogy. For this reason, the fine fraction in backfill soil is limited to only 15% passing the 200 sieve. Porosity also affects resistivity because the distribution and geometry of the voids influences the proportion of air and water than can be retained in the soil. Finally, the day-night and seasonal temperature cycles may also have an effect con corrosion because an increase in temperature decreases the viscosity of water and increases the ion agitation resulting in a decrease of resistivity (Andrade et al., 2002). Therefore, comparison of soil resistivity measurements requires the value expressed at the standardized temperature of 25°C (Campbell et al., 1948; Samouelian et al., 2005). The American Association of State 10  Highway and Transportation Officials (AASHTO) of the US classifies the types of soil used in the construction of highways in different levels of corrosion aggressiveness in terms of their resistivity as follows: Table ‎2-1: AASHTO classification of aggressiveness of soils in terms of resistivity Aggressiveness of the soil Resistivity (𝜴 ∙ 𝒄𝒎) Very corrosive R < 700 Corrosive 700 < R < 2,000 Moderately corrosive 2000 < R < 5000 Mildly corrosive 5000 < R < 10000 Non-corrosive R > 10000  The moisture content also plays an important role in the corrosion of metals because higher moisture contents facilitate the transport of dissolved electrolytes in soil, whereas higher aeration levels increase the availability of oxygen that can participate in reduction reactions. The color of the soil provides an indication of the degree of aeration of the soil. Red, yellow and brown soils are typical of aerated soils, due to the presence of oxidized forms of iron; whereas gray soils are characteristic of poorly-aerated soils, where reduced forms of iron are predominant (Zhang, 1996). The electrode potential of zinc and steel has been measured in 12 different soils. The value for zinc varied from -0.8 V to -0.97 V, whereas for steel, the values ranged from -0.54 V to -0.66 V versus the Saturated Calomel Electrode (SCE) (Romanoff, 1957).    11  2.1.4 Deterioration of MSE walls due to corrosion  The design life of MSE walls is 75 years for general applications and 100 years in bridges (AASHTO, 2002; AASHTO, 2010; CSA, 2006). At forty years from the introduction of the MSE technology, information gathered from numerous state departments of transportation (DOTs), 39 from US and 5 from Canada, revealed that the DOTs consider corrosion of the reinforcements as one of the three major factors that compromise the long-term performance of MSE walls, along with drainage and global stability (Gerber, 2012). The concern about corrosion is that it reduces the cross sectional area of the reinforcements and, as a consequence, their tensile strength is also reduced, jeopardizing the internal stability of the structure.  Naturally, several efforts have been done to understand the variables involved in the corrosion of galvanized steel reinforcements in soil in order to predict accurately the loss of material. When the MSE technology was developed, the first predictions of the metal loss of the reinforcements were based on data from the US National Bureau of Standards (NBS) that conducted corrosion investigations over a period of 45 years, motivated by the economic losses in the pipeline industry (Romanoff, 1957). Subsequently, more specific research was conducted on the corrosion of galvanized steel reinforcements in MSE applications in periods that ranged from 8 to 10 years (Darbin et al., 1986). As a result, it was recommended to limit the content of chlorides in the backfill soil to 100 ppm and that of sulfates to 200 ppm (LCPC -SETRA, 1979). These recommendations were incorporated in the electrochemical parameters of the backfill soil given in the AASHTO specifications (AASHTO, 2002; AASHTO, 2010) and are shown in Table ‎2-2. 12  Table ‎2-2: Electrochemical characteristics of backfill soil Characteristic Accepted value pH 5 - 10 Resistivity R‎≥‎3000‎𝛺 ∙ 𝑐𝑚 Chloride concentration ≤‎100‎ppm Sulfate concentration ≤‎200‎ppm Organic content ≤‎1%  AASHTO developed its own model of metal loss (Table ‎2-3), according to which, provided the backfill soil specifications are met, it is estimated that the layer of zinc will corrode at a rate of 15‎μm/year‎ during‎ the‎ first‎ two‎ years‎ after‎ construction‎ and‎ then‎ corrosion‎will‎ continue‎ at‎ a‎lower‎rate‎of‎4‎μm/year until depletion of zinc, which is expected to occur after 16 years. After that,‎ steel‎will‎ corrode‎ at‎ a‎ rate‎ of‎ 12‎ μm/year‎ and‎ lose‎ thickness‎ during‎ 59‎ years‎ to‎ reach‎ a‎design life of 75 years in service. Using these predictions, sacrificial thickness of steel can be calculated and added to the dimensions of the rebars to guarantee that the tensile resistance of the reinforcements will be sufficient through the design life (Gerber & Billings, 2010).  Table ‎2-3: AASHTO model of metal loss Corroding material Period of time Rate of loss of material Zinc First 2 years 15‎μm/year Zinc From year 2 until depletion of zinc (approx. 16 years) 4‎μm/year Steel From initiation of corrosion of steel (approx. year 16) until end of service life (year 75) 12‎μm/year  Even though, the electrochemical parameters of the backfill soil and the metal loss model are based on extensive research, there is still a disagreement on the accuracy of the predictions of the 13  model. On one hand, (Gladstone et al., 2006; Gladstone et al., 2008) consider that the AASHTO model is too conservative because they claim that the zinc coating can protect the steel rebar for as long as 30 years, i.e. 14 more years than predicted by AASHTO. On the other hand, some cases have been reported where high levels of corrosion of the reinforcements were accidentally discovered (Thornley & Siddharthan, 2010; Thornley et al., 2010). The dissent may lay on the general assumption of the AASHTO model that the electrochemical conditions of the soil will remain constant through the entire service life of the structures or in the fact that some MSE walls were built according to early specifications that are no longer accepted by AASHTO.  2.1.5 The organic content in backfill soil  One specific electrochemical parameter of the backfill soil to which very little attention has been paid is the organic content in soil.  It has generally been assumed that the only mechanism through which it affects the corrosion of galvanized steel is by providing food to bacteria that participate in the Microbially-Influenced Corrosion (MIC) process (Elias et al., 1990; Uhlig & Revie, 2000). But the role of organics, themselves, on the corrosion of galvanized steel in soil has been neglected.  2.2 Soil organic matter  An extensive research of the type of organics that could be found in soil was essential in order to select the reagents that would be used in the electrochemical experiments to study the corrosion 14  behavior of galvanized steel in the presence of typical soil organics. This section summarizes the investigation.  2.2.1 Definition and composition  The presence of organics is what distinguishes soil from decomposed rock. Soil Organic Matter (SOM) is the whole of organic materials in soil, which includes both, living and non-living organisms. Figure ‎2-1 depicts the typical composition of organic matter in soils (Bohn et al., 2001; Schnitzer & Khan, 1978; Stevenson, 1994).   Figure ‎2-1: Composition of organic matter in soil 15  The living fraction represents less than 5% of the total SOM, while the non-living organic matter accounts for 95% or more of the total organic content in soils and is more important for the soil chemistry (Bohn et al., 2001). The living organic matter includes bacteria, fungi, algae, plant roots, soil fauna and un-decomposed debris derived from plants or animals. The non-living organic matter is an accumulation of dead plant and animal residues at different stages of decomposition. It can be divided into litter and humus. The former is partially-decayed products that still conserve a visible anatomy and are susceptible to further decomposition; whereas the latter is completely decomposed material that will not break down further and may remain as it is for centuries.  Humus is classified into humic compounds and non-humified material. Humic compounds are those that no longer exhibit specific chemical characteristics. They represent the bulk of humus, accounting for up to 70% of it and can be classified into humic acid, fulvic acid and humin according to their solubility characteristics: humic acid is the fraction soluble in alkali (pH > 2); fulvic acid is soluble in both alkali and acid solution; and humin is the insoluble fraction (Yasakau et al., 2008). Both, humic acid and humin, are considered passive humus because they are stable complexes that will not decompose further, whereas fulvic acid is the active fraction. The non-humified material is the remaining fraction of humus that still exhibits recognizable physical and chemical characteristics, e.g. melting point and elemental composition. Its major components are carbohydrates and lipids that represent, respectively, from 10 to 30% and from 2 to 6% of humus (Huang et al., 2009; Schnitzer & Khan, 1978). The rest of humus is a combination of various types of organics that are present in minute amounts such as organic acids, fatty acids, waxes, proteins, nucleic acids, and lignin. 16  2.2.2 Characteristics of humic substances  Humic compounds are acidic, dark-colored amorphous polymers that are responsible for the yellow to brown color of soil (Schulten & Schnitzer, 1993). These macromolecules have reached a point of stability through a process called humification where partially-decomposed plant and animal residues are synthetized by microbial activity and chemical reactions. The molecular weights of these structures vary from a few hundreds to 105 Daltons4 (Yasakau et al., 2008).   Humic compounds are composed of carbon and oxygen as major elements and smaller percentages of hydrogen, nitrogen, and sulfur. In humic acids, the proportions of carbon and oxygen are about 55% and 33%, respectively; whereas for fulvic acids, they are approximately 41% and 48%. Then nitrogen and hydrogen represent about 2-5% each, followed by less than 2% of sulfur.  Humic substances control various physical and chemical properties of soil. For example, they facilitate the warming of the soil and their hydrophilic nature is responsible for their capability to retain up to 20 times their weight in water. Also, they exhibit a buffer capacity. It has been reported that the pH range in which they act as a buffer goes from 5.5 to 8 (Pertusatti & Prado, 2007). Furthermore, humic substances contain a broad range of functional groups that allow them to participate as electron donors or acceptors. In particular, their high content of acidic functional groups facilitates metal binding and transport. The number of stable complexes that                                                  4 Dalton is the unified atomic mass unit that is usually used in molecular chemistry and corresponds to one twelfth of the mass of an unbound atom of carbon 12. It is approximately equal to 1.66 × 10−27 kg. 17  they can form with metal ions is almost infinite (Livens, 1991), but the interactions of humic substances with metal ions can be mainly of three types: oxidation-reduction reactions, surface adsorption, and chelation.   2.2.2.1 Oxidation-reduction reactions  The oxygen-containing functional groups, which retain about 80% of the total oxygen in humic compounds, are of particular interest to this research for their role in oxidation-reduction reactions (Struyk & Sposito, 2001). The functional groups of this type are hydroxyl (-OH), carbonyl (−𝐶 = 𝑂) and carboxyl (-COOH). They are shown in Figure ‎2-2, where the X represents a radical that may be any atom or group of atoms. The hydroxyl group (Figure ‎2-2a) is formed by an oxygen atom bonded to a hydrogen atom. The carbonyl group (Figure ‎2-2b) consists of a carbon atom double bonded to an oxygen atom. The carboxyl group (Figure ‎2-2c) is formed when the hydroxyl group is bonded to the carbon of the carbonyl group. This carboxyl group is the functional group that characterizes the carboxylic acids, also named organic acids.    a) Hydroxyl group   b) Carbonyl group  c) Carboxyl group Figure ‎2-2: Oxygen containing functional groups in humic acid: a) hydroxyl, b) carbonyl, c) carboxyl 18   The hydroxyl and carboxyl group provide an aprotic character to organics i.e. ability to readily provide protons because they self-dissociate as described in equation 2-3 where X represents any radical (Heitz, 1974).  𝐻𝑋 + 𝐻𝑋 ↔ 𝐻𝑋𝐻+ + 𝑋− Eq. ‎2-3  The negatively-charged sites provide humic substances the ability to form complexes with metal cations.  The role of the carboxyl group in the corrosion process of metals is explained using formic acid (HCOOH) for simplification purposes because it only contains one carboxyl group (El-Maksoud, 2008). At the cathode, the reactions are given by Eq. ‎2-4 and Eq. ‎2-5, where Me is any metal:  𝑀𝑒 + 𝐻𝐶𝑂𝑂𝐻 + 𝑒− → 𝑀𝑒𝐻𝑎𝑑𝑠 + 𝐻𝐶𝑂𝑂− Eq. ‎2-4 𝑀𝑒𝐻𝑎𝑑𝑠 +𝑀𝑒𝐻𝑎𝑑𝑠 → 𝐻2 + 2𝑀𝑒 Eq. ‎2-5   As a prerequisite for the anodic dissolution, the formate ions (𝐻𝐶𝑂𝑂−) need to be adsorbed on the surface. The reactions at the anode are given for the case of the corrosion of mild steel:  𝐹𝑒 + 𝐻𝐶𝑂𝑂− → [𝐹𝑒(𝐻𝐶𝑂𝑂)]𝑎𝑑𝑠 + 𝑒− Eq. ‎2-6 [𝐹𝑒(𝐻𝐶𝑂𝑂)]𝑎𝑑𝑠 → [𝐹𝑒(𝐻𝐶𝑂𝑂)]+ + 𝑒− Eq. ‎2-7 [𝐹𝑒(𝐻𝐶𝑂𝑂)]+ + 𝐻+ ↔ 𝐹𝑒2+ + 𝐻𝐶𝑂𝑂𝐻 Eq. ‎2-8 19  2.2.2.2 Surface adsorption  It is believed that adsorption of humic substances on metal surfaces is due to the interaction of the carboxylic functional group with hydroxylated oxide sites (Korshin et al., 1997). Additionally, organic compounds containing oxygen, sulfur, and nitrogen are well known as corrosion inhibitors of zinc because they can be adsorbed on the surface and block the active sites (El-Sherbini et al., 2005). Since these elements also are also components of humic substances, they may provide them adsorption capabilities. 2.2.2.3 Chelation  Chelation is the process of metal-organic complexation. The organic components capable of forming complexes with metals are referred to as ligands. Chelation occurs when two or more coordinate positions of a metal ion are occupied by groups of a single ligand and a ring structure is formed. If the chelating agent forms two bonds with the metal ion it is said to be bidentate. Similarly, terdentate, tetradentate, and pentadentate complexes are possible.   It has been determined that over a wide range of pH values, fulvic acids form complexes with Zn, according to the reaction described in equation 2-9, in which a moles of a cation 𝑀𝑒𝑥+ react with b moles of a ligand 𝐿𝑦 . 𝑎𝑀𝑒𝑥+ + 𝑏𝐿𝑦− ↔ 𝑀𝑒𝑎𝐿𝑏𝑎𝑥−𝑏𝑦 Eq. ‎2-9    The stability constant is given by: 𝐾 =(𝑀𝑒𝑎𝐿𝑏)𝑎𝑥−𝑏𝑦(𝑀𝑒𝑥+)𝑎(𝐿𝑦−)𝑏  Eq. ‎2-10 20   The value of the stability constant of zinc fulvates increases with increasing the pH values. Reported values are (log K) = 1.7 at pH 3.5 and 2.3 at pH 5, but also (log K) = 2.3 at pH 3 and 3.6 at pH 5 (Stevenson, 1994). Some authors proposed that the stability of the metal-organic complexes with humic acids is as follows: 𝑃𝑏2+ > 𝐶𝑢2+ > 𝑁𝑖2+ > 𝐶𝑜2+ > 𝑍𝑛2+ > 𝐶𝑑2+ > 𝐹𝑒2+ > 𝑀𝑛2+ > 𝑀𝑔2+. Others determined that the affinity of humic substances with metal ions decreases in this order: 𝐹𝑒3+ > 𝐴𝑙3+ > 𝐶𝑢2+ > 𝑁𝑖2+  > 𝑃𝑏2+  > 𝐶𝑜2+ > 𝑍𝑛2+ > 𝐹𝑒2+ > 𝑀𝑛2+ > 𝐶𝑎2+ > 𝑀𝑔2+ (Dick & Rodrigues, 2006).   2.2.3 Carbohydrates in soil  Carbohydrates are the second most significant component of SOM, accounting for up to 30% of it. The main source of soil carbohydrates is plant residues, given that they represent between 50 and 70% of the dry weight of plants. Carbohydrates are sugars with the general formula 𝐶𝑛(𝐻2𝑂)𝑚. They are classified into monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides are water-soluble, simple sugars that cannot be decomposed into smaller molecules. The most common monosaccharides in soil are fructose (𝐶6𝐻12𝑂6) and glucose (𝐶6𝐻12𝑂6), which are classified as hexoses because they contain 6 carbon atoms. Oligosaccharides are compound sugars that can yield from 2 to 6 molecules of monosaccharides when hydrolysed. Polysaccharides are water-insoluble, complex carbohydrates. Some examples of polysaccharides are hemicellulose, cellulose, and plant starches.  21  Carbohydrates in soil may occur in three forms: free sugars, polysaccharides, and polymeric molecules. The free sugars that have been identified in a variety of mineral and organic soils are glucose, galactose, mannose, fructose, arabinose, xylose, fucose, and ribose (McLaren & Peterson, 1967). Polysaccharides are the most common form of carbohydrates found in soil. They are insoluble in water and have molecular weights from 200,000 to 2 million. They may be decomposed into simpler sugars by microbial attack. Finally, the polymeric molecules in soil are strongly attached to clay colloids and are difficult to isolate.   2.2.4 Characteristics of organic acids  The most corrosive organic species are organic acids. They are ubiquitous in soil in concentrations that range from 10−2 to 5mM (Schwab et al., 2008; Stevenson, 1994). More than a dozen high molecular weight organic acids have been isolated from soil, but low molecular weight organic acids (LMWOA) are also normal constituents of mineral soils. LMWOA are exuded by plant roots, produced by microorganisms, or decomposed from plant and animal residues.‎They‎may‎also‎be‎incorporated‎into‎the‎soil‎by‎precipitations,‎which‎carry‎from‎0.6‎μM‎of‎them‎in‎remote‎areas‎to‎80‎μM‎in‎urban‎areas‎(Graedel et al., 1986). LMWOA are capable of forming complexes with metals due to their hydroxyl derivatives (Renella et al., 2004). The most common types of organic acids are carboxylic acids, which are classified according to the number of carboxyl groups that they contain into carboxylic, dicarboxylic, tricarboxylic, etc.   22  The feasibility of the complexation between organic acids and metal ions has widely been studied (Zaleckas et al., 2013). Although a dependence between the corrosion rates of SS41 steel and the concentration of formic and acetic acid has been demonstrated (Sekine & Senoo, 1984), it has been determined that they do not form as stable complexes with metals as citric and oxalic acids do (Kim et al., 2013). The capability of citric, acetic and oxalic acid to complex with Zn ions in soil was compared and it was found that citric acid exhibits the best binding capacity (Zaleckas et al., 2013).   23  Chapter 3: Objectives  The goal of this research is to quantify the corrosion behavior of hot-dip galvanized steel in the presence of selected organic reagents that represent the major organic components of soil.  3.1 Key technical objectives  1. Determine the electrochemical corrosion behavior of galvanized steel in solutions with additions of 0.01wt%, 0.02wt%, 0.1wt%, 0.25wt%, 0.5wt%, and 1.0wt% of three organic acids: a. Humic acid b. Oxalic acid c. Citric acid 2. Determine the electrochemical corrosion behavior of galvanized steel in solutions with additions of 0.1wt%, 0.25wt%, 0.5wt%, and 1.0wt% of glucose. 3. Characterize the surface of the galvanized steel after accelerated corrosion tests in order to determine the morphology of the corrosion products and the extent of corrosion due to each organic. 4. Determine the corrosion behavior of galvanized steel in solutions with additions of 0.1wt%, 0.25wt%, 0.5wt%, 0.75wt%, and 1.0wt% of simulated soil organics that combine the four reagents following typical compositions in soil using various electrochemical methods. 24  5. Characterize the surface of the galvanized steel after accelerated corrosion tests in simulated soil organic solutions. 25  Chapter 4: Approach and methodology  The purpose of conducting these experiments was to quantify the corrosion behavior of galvanized steel in the presence of organic reagents that represent the main components of SOM. In this chapter, the selection of materials, their preparation, and the electrochemical and surface characterization methods are discussed.  4.1 Materials selection  4.1.1 Galvanized steel The electrochemical tests were performed using galvanized steel samples with an exposed area of 1 𝑐𝑚2. The samples were cut from an as-received, low carbon, cold-rolled galvanized steel sheet that was fabricated according to ASTM A653 standard, with a bath composition for the hot-dip coating of 99.96wt% zinc and 0.04wt% Al, without a chromate finish. The sheet thickness was 0.03cm and the coating thickness approximately‎20‎μm‎±‎2‎μm.‎All‎samples‎were‎cleaned with ethyl alcohol and dried prior to immersion in the test solution.  4.1.2 Organic reagents  In literature review, previous corrosion experiments of the behavior of metals due to soil organics were found to be non-existent. Therefore, the selection of the organic reagents herein described is proposed as synthetic soil organics for electrochemical studies. Four organic 26  reagents were chosen to prepare the solutions for the electrochemical tests. The reasoning behind the selection was based on the information gathered in the literature review regarding the composition of soil organics that is applicable to all types of soils. Two of the organic reagents were chosen to represent the two major components of SOM: humic acid to represent the humic substances, and dextrose (D-glucose) to represent the carbohydrates. The other two organic reagents were oxalic acid (𝐶2𝐻2𝑂4) and citric acid (𝐶6𝐻8𝑂7). They were chosen to account for the effect of the most corrosive organic species that are ubiquitous in minute amounts (less than 5 mM): organic acids.     a)     b)  c) Figure ‎4-1: Model of selected organics: a) oxalic acid, b) citric acid, and c) dextrose in open-chain form  Oxalic acid (Figure ‎4-1a) is a dicarboxylic acid (it contains two carboxylic groups), citric acid (Figure ‎4-1b) is a tricarboxylic acid (it contains three carboxylic groups), and dextrose (Figure ‎4-1c) contains five hydroxyl groups.   27  4.2 Preparation of solutions  Two types of solutions were used. In the first type, solutions of each individual organic reagent were prepared separately. In the second type, the four organics were combined in solution in proportions that simulated the typical composition of soil organic matter. Such combination is referred to as Simulated Soil Organic Matter (SSOM) in this thesis.  4.2.1 Solutions with individual organics  Commercial humic acid sodium salt was purchased from Acros Organics. The other three reagents were ACS certified and supplied by Fisher Scientific as Dextrose Anhydrous, Citric Acid Anhydrous and Oxalic Acid Dihydrate. For each organic reagent, six solutions were prepared at the concentrations presented in Table 4-1. Table ‎4-1: Concentrations of solutions prepared for each organic Organic Concentration (wt%) Humic acid 0.01 0.05 0.10 0.25 0.50 1.00 Citri acid 0.01 0.02 0.10 0.25 0.50 1.00 Oxalic acid 0.01 0.07 0.10 0.25 0.50 1.00 Glucose     0.10 0.25 0.50 1.00  4.2.2 Simulated soil organic solutions  The second set of solutions was prepared with SSOM at four concentrations: 0.25 wt%, 0.50 wt%, 0.75 wt%, and 0.91 wt%. For each concentration, the percentage that was added to each individual organic reagent, mimicking the typical composition of SOM, is detailed in Table ‎4-2. 28  Table ‎4-2: Composition of simulated soil organics by weight percent in 1L solution Typical SOM composition 70% 30% ≤‎5‎mM Humic substances Carbohydrates Organic acids       Solution No. Total concentration of simulated soil organics Individual concentration of organics (wt%) Humic acid Dextrose Citric acid Oxalic acid 1 0.25 wt% 0.147 0.063 0.02 0.02 2 0.50 wt% 0.311 0.133 0.03 0.025 3 0.75 wt% 0.483 0.207 0.03 0.03 4 0.91 wt% 0.6 0.255 0.03 0.03   4.3 Electrochemical tests  To study the corrosion behavior of galvanized steel in the presence of soil organics, three accelerated corrosion tests were selected: Potentiodynamic Polarization Resistance (PDP), Linear Polarization Resistance (LPR), and Electrochemical Impedance Spectroscopy (EIS). To ensure that the samples had reached a stable state, the Open Circuit Potential (OCP) was measured prior to each test. To ensure repeatability of the results, each electrochemical test was performed three times for each solution described in section 4.3. For each run, a fresh solution and a fresh galvanized steel sample were used. The parameters applied for each electrochemical test are described here.     29  4.3.1 Open Circuit Potential  The Open Circuit Potential (OCP) was monitored through time. In most solutions, a stable value was obtained within 1 hour of immersion, but for some solutions it was necessary to wait longer before a steady state potential was reached.   4.3.2 Potentiodynamic Polarization Resistance  Potentiodynamic Polarization Resistance (PDP) tests were performed in all solutions from -0.250 V Ag/AgCl to +1.00 V Ag/AgCl with respect to the OCP, with a scan rate of 0.166 mV/s. The PDP curves presented in this thesis were selected from the three runs performed to simplify the visualization. With this test it was possible to obtain the corrosion potential (𝐸𝑐𝑜𝑟𝑟) and the corrosion current densities (𝑖𝑐𝑜𝑟𝑟), as well as to identify passive behaviors. The 𝑖𝑐𝑜𝑟𝑟 values were obtained by the intersection of the Tafel slopes from the anodic (𝛽𝑎) and cathodic (𝛽𝑏) branches, where the conditions of the Tafel behavior was met. In the cases where the Tafel criteria were not met, a least square regression analysis was used to fit the Butler-Volmer equation (Eq. ‎4-1) to the measured values on the cathodic and anodic branches at 100 mV from the 𝐸𝑐𝑜𝑟𝑟. The squared error between the measurements and the calculated values was minimized by modifying the Tafel slopes and the 𝑖𝑐𝑜𝑟𝑟 (V. Padilla et al., 2013a). 𝑖 = 𝑖𝑐𝑜𝑟𝑟 (exp (𝛼𝑎𝑛𝐹𝜂𝑅𝑇) − exp (−𝛼𝑐𝑛𝐹𝜂𝑅𝑇))  Eq. ‎4-1: Butler-Volmer equation  The method used to determine 𝑖𝑐𝑜𝑟𝑟 values from PDP curves is always a potential source of error. 30  An example of the repeatability of the three PDP test performed on galvanized steel in 1wt% humic acid solution is presented in Figure ‎4-2. A result is said to be repeatable when the shape of the curve and the Ecorr values are similar between the different runs of a test in identic experimental conditions.  Figure ‎4-2: Three repeatable results after performing PDP test in galvanized steel in 1.00 wt% humic acid solution.   4.3.3 Linear Polarization Resistance  Linear Polarization Resistance (LPR) was used for the samples in SSOM solutions. The potential was varied from -0.20 mV Ag/AgCl to +0.20 mV Ag/AgCl with respect to OCP with a potential 31  sweep rate of 0.166 mV/s. From the plots obtained of current versus potential, the polarization resistance (𝑅𝑃) was determined.  4.3.4 Electrochemical Impedance Spectroscopy  Electrochemical Impedance Spectroscopy (EIS) was performed for the samples in SSOM solutions. The measurements were taken in the frequency range of 0.01 to 10,000 Hz, with an amplitude of 10 mV and a sampling rate of 10 points per decade. The results were analyzed with ZSim software to find the equivalent electronic circuit that best represented the response of the system. The response of the circuit had a chi-square value in the order of 10−4 with respect to the experimental results.  Figure ‎4-3: Three repeatable results of EIS test performed on galvanized steel in 0.05 wt% SSOM solution. 32  Figure ‎4-3 presents an example of the repeatability of three EIS test performed on galvanized steel in 0.50 wt% SSOM solutions. In EIS, results are said to be repeatable, for example in the Nyquist plot, when the same number of time constants are observed as revealed by the number of semicircles and when the real and imaginary values of the total impedance are similar.   4.4 Equipment  The electrochemical corrosion tests were carried out in a conventional three-electrode corrosion cell, as shown in Figure ‎4-4. Each galvanized steel sample, the working electrode, was held in place by a sample holder, the counter electrode was a graphite rod and the reference electrode was Ag/AgCl ([𝐶𝑙−] = 4M). The cell was placed on a magnetic stirrer with a rotating speed of 300rpm to produce a vortex at the electrode. The temperature of the cell was maintained constant at 25°C using a VWR water bath. The measurements were taken with a Princeton Applied Research VersaStat 4 potentiostat.  33   Figure ‎4-4: Experimental set-up of a standard electrochemical cell: 1. Reference electrode; 2. Sample holder; 3. Thermometer; 4. Counter electrode; 5. Corrosion cell; 6. Magnetic stirrer; 7. Water bath.   A detail of the sample holder is shown in Figure ‎4-5. The body (1) of the sample holder is composed of a white polymer case with a cylindrical space to place the sample. The inner part of the case is of metallic material and is in contact with a rod that allows connecting electrically the potentiostat with the sample, while remaining isolated from the solution by a glass cylindrical shield.  The bottom side of the case has a circular orifice that has an area of 1 𝑐𝑚2. Before putting the sample in the body, a flat O-ring (2) is put in place to prevent the solution from filtering and getting in contact with a bigger are of the sample (3). A metallic lid (4) is screwed into the cylinder so that the sample is not loose. Finally, the ensemble is locked from the solution 34  on the open side of the case by a polymer lid that also has an O-ring to avoid filtrations of solution.   Figure ‎4-5: Components of the sample holder: 1. Body; 2. Flat O-ring; 3. Galvanized steel sample; 4. Metallic lid; 5. Polymer lid with O-ring    4.5 Surface characterization  The surface of the samples was characterized after PDP tests by means of two methods.  35  4.5.1 Scanning Electron Microscope After the electrochemical tests, the samples were extracted from the solution, rinsed with deionized water, and dried. The morphology of the surface and the corrosion products were observed with the Scanning Electron Microscope (SEM).   4.5.2 Energy Dispersive X-ray Spectroscopy The Energy Dispersive X-ray Spectroscopy (EDX) technique was used to determine the elemental composition of different areas of the surface. This method helped to confirm the extent of corrosion. 36  Chapter 5: Corrosion of galvanized steel due to individual organics  This chapter presents the corrosion performance of galvanized steel (GS) exposed to four different organic reagents at different concentrations below 1.00 wt%, a solution containing 100 ppm of chlorides, and a solution containing 200 ppm of sulfates. The kinetic parameters were obtained from PDP tests and the surface of the samples was characterized by SEM and EDX.  5.1 Properties of organic solutions  5.1.1 pH  The pH of the solutions of humic acid (HA), glucose, citric acid and oxalic acid were measured prior to the beginning of the electrochemical tests. The values are presented in Table ‎5-1. HA solutions were alkaline with pH values around 10. Glucose solutions were rather neutral with slight increases in pH as the concentration increased, from 6.2 to 7.4. As expected, solutions of citric and oxalic acid exhibited the lowest pH values, with oxalic acid solutions being the most acidic. Citric acid solutions had pH values between 2.9 and 2.6, while oxalic acid solutions had pH values between 2.4 and 2. Table ‎5-1: pH of four organic solutions at four concentrations from 0.1 wt% to 1.0 wt%  pH wt% HA Dextrose Citric acid Oxalic acid 0.10 10.1 ± 0.29 6.6 ± 0.98 2.9 ± 0.04 2.4 ± 0.14 0.25 10.1 ± 0.19 6.2 ± 0.58 2.9 ± 0.14 2.1 ± 0.42 0.50 10.2 ± 0.29 6.9 ± 0.58 2.7 ± 0.12 1.9 ± 0.21 1.00 10.3 ± 0.31 7.4 ± 0.76 2.6 ± 0.08 2.0 ± 0.03  37    Figure ‎5-1: E-pH diagram of the zinc-water system at 25 °C   The potential-pH diagram of zinc in water (Figure 5-1) provides an indication of the expected corrosion behavior of zinc. However, the diagram should be used with caution because it refers to a system that is not identical to the ones studied in this chapter. For humic acid and glucose solutions that exhibited pH values higher than 6, it was expected that they promoted the formation of zinc hydroxide (𝑍𝑛(𝑂𝐻)2) as the potential was increased in the PDP tests. In the case of citric and oxalic acid which produce lower pH values than 6,  it was expected that they promoted the formation of zinc cations (𝑍𝑛2+).   38  5.1.2 Conductivity  The influence of the concentration of the organic species on the conductivity of the solution is shown in Figure ‎5-2. The corresponding numeric values are given in the following sections of this chapter. The measurement of the conductivity shows that, while the conductivity of the glucose solutions remained constant at all concentrations (0.3 ± 0.2‎μS), it increased linearly with increasing concentrations of humic, citric, and oxalic acids. The conductivity in HA solutions increases from 0.8±0.1‎ μS‎ at‎ 0.01 wt% to‎ 65±2.3‎ μS at 1 wt%. Citric acid increased it from 3.5±0.5‎μS‎at‎0.1‎wt%‎to‎39.5±1.3‎μS‎at‎1‎wt%. The conductivity of humic acid solutions was higher than citric acid solutions at most concentrations. In oxalic acid solutions at 0.001 wt%, the conductivity was‎ 7.5±0.1‎ μS,‎ which was higher than the conductivity of 0.1 wt% citric acid solution. At 1.0 wt%, the conductivity of the oxalic acid solution‎was‎as‎high‎as‎530±7.2‎μS.  Figure ‎5-2: Variation in conductivity of four organic solutions as a function of concentration 39  5.2 Control sample  The surface of a galvanized steel control sample that was not exposed to any electrochemical test was characterized by SEM and EDX. The purpose of this characterization was to provide a reference of the morphology and composition to which the surface of samples exposed to organic solutions could be compared. The SEM image of the control sample (Figure ‎5-3) is mostly of one color and the characteristic grain boundaries formed during hot dip galvanizing steel can be observed.   Figure ‎5-3: SEM image of control sample of galvanized steel  The EDX performed on the surface (Figure ‎5-4) reveals that zinc is the main component on the surface. Oxygen and iron are also detected but they are present in insignificant amounts. 40   Figure ‎5-4: EDX analysis of control sample of galvanized steel   5.3 Humic acid  5.3.1 Electrochemical tests in humic acid solutions  The open circuit potential (OCP) was measured prior to each PDP test until a stable potential was reached, which for humic acid solutions, occurred within one hour after immersion (Figure ‎5-5). In all cases, the open circuit potential shifted towards more noble values as the immersion time increased from zero to one hour. Although no specific reference was found about the behavior of galvanized steel or zinc in the presence of humic acid, it has been proposed that zinc spontaneously develops an oxide/hydroxide film whose structure and protective properties 41  depend on the specific conditions of the corrosive environment (Sziraki, Szocs, Pilbath, Papp, & Kalman, 2001). This offers a plausible explanation for the increase of the potential.  Considering the final open circuit potential, no particular trend was observed on the measured value for concentrations between 0.005 wt% and 0.5 wt%. Only when the concentration was 1.0 wt% a significant shift towards more negative potentials was observed in comparison to all smaller concentrations. Moreover, the difference between the final open circuit potential and the potential measured immediately after immersion was smaller at 1.0 wt% concentration than the difference between the first and final OCP values at 0.005 wt%.   Figure ‎5-5: Measurement of open circuit potential of galvanized steel in humic acid solutions at different concentrations at pH 10 and 25°C   42  The PDP curves of galvanized steel in HA solutions at concentrations of 0.005, 0.01, and 0.05 wt% are presented in Figure ‎5-6. The shape of the cathodic branch confirms that the cathodic process was controlled by diffusion. The limiting current density (𝑖𝐿) increased slightly as the concentration was increased with values of 3.6, 4 and 7.5 𝜇𝐴/𝑐𝑚2 respectively. The shape of the anodic branches shows that the anodic process is controlled by activation. The 𝐸𝑐𝑜𝑟𝑟 of these solutions oscillated around -660 mV. For concentrations of 0.005 wt% and 0.01 wt%, the slope of the anodic branch in the vicinity of 𝐸𝑐𝑜𝑟𝑟 was approximately 20mV per decade, however at current density of 10−4 𝜇𝐴/𝑐𝑚2 the slope became bigger, with an increase in potential of more than 60mV is less than one decade. In the case of 0.05 wt%, the anodic slope was smaller and the sudden increase was not observed.    Figure ‎5-6: Potentiodynamic curves of galvanized steel immersed in humic acid solutions at concentrations below 0.1 wt% at pH 10 and 25°C 43  Figure 5-7 presents the PDP curves for GS in HA solutions at concentrations 0.10 wt%, 0.25 wt%, 0.50wt%, and 1.00 wt%. The diffusion process in the cathodic branch became less apparent. The slope in the anodic branch was similar than for smaller concentrations, i.e. approximately 20mV/dec. However, this slope was maintained until a current density of 10−3 𝜇𝐴/𝑐𝑚2, after which a high increase in the slope was observed. Furthermore, the 𝐸𝑐𝑜𝑟𝑟 measurements at concentrations 0.10 wt%, 0.25 wt%, and 0.50 wt% were close to each other, in the order of -781 mV. At the highest concentration of 1.0 wt%, the lowest 𝐸𝑐𝑜𝑟𝑟  was obtained with a value of -824 mV.    Figure ‎5-7: Potentiodynamic curves of galvanized steel immersed in humic acid solutions at concentrations between 0.1 wt% and 1.00 wt% pH 10 at 25°C   44  Table ‎5-2 summarizes the 𝐸𝑐𝑜𝑟𝑟 and 𝑖𝑐𝑜𝑟𝑟 values obtained from PDP measurements. In general, as the concentration of HA increased, the 𝐸𝑐𝑜𝑟𝑟 shifted towards more negative values. The same trend was not true for 𝑖𝑐𝑜𝑟𝑟. The maximum values of 𝑖𝑐𝑜𝑟𝑟 were reached at 0.01 wt%, however, the maximum most stable value was obtained at 0.05 wt%. After this concentration the 𝑖𝑐𝑜𝑟𝑟 decreased gradually until at 1.0 wt% the value was comparable to that obtained at 0.005 wt%.  Table ‎5-2: Conductivity of solutions and kinetic parameters from PDP measurements for galvanized steel in humic acid solutions at concentrations ranging from 0.005 wt% to 1.0 wt% at pH 10 and 25°C wt% σ‎(μS) 𝑬𝒄𝒐𝒓𝒓 (mV) 𝒊𝒄𝒐𝒓𝒓 (𝝁𝑨/𝒄𝒎𝟐) 0.005 0.9 ± 0.1 -681 ± 22 2.9 ± 1.3 0.01 1.9 ± 0.4 -674 ± 54 18.8 ± 14.9 0.05 6.6 ± 0.1 -638 ± 28 15.3 ± 0.3 0.10 10.7 ± 0.0 -767 ± 5 10.7 ± 3.2 0.25 21.3 ± 0.5 -793 ± 52 9.7 ± 4.6 0.50 36.9 ± 0.1 -784 ± 32 4.9 ± 4.8 1.00 65.4 ± 2.3 -824 ± 28 3.0 ± 1.4   5.3.2 Surface characterization of galvanized steel in humic acid solutions  It was observed that, independently of the concentration, when the galvanized steel samples were extracted from the solution immediately after the PDP tests, the exposed area appeared completely covered by a very thick layer (about 2mm) of dark deposits that could not be removed by rinsing the sample. However, those deposits were easier to remove using the same method after drying the layer at ambient temperature. The surface of the sample appeared to be non-uniformly covered by dark brown deposits in some regions (labeled with number 2 in Figure ‎5-8), while other areas seemed to be metallic (labeled with number 1 in Figure ‎5-8). 45   Figure ‎5-8: SEM image of galvanized steel after PDP in 0.25wt% humic acid solution  EDX confirmed that in region 1, humic acid at 0.25 wt% was able to corrode the zinc layer and reach the base steel (Figure ‎5-9). The feasibility of the interaction between humic acid and zinc has been studied in various contexts and the formation of humic acid-zinc complexes has been confirmed in various studies (Baker & Khalili, 2005; Gungor & Let, 2010; Li et al., 2010; Prado et al., 2006; Yang & Van, 2009; Yip et al., 2010).  The dark brown deposits had an elemental composition that contained carbon, oxygen, iron, zinc, and aluminum (Figure ‎5-10). The presence of aluminum can be explained because it is an alloying element that is typically incorporated in the zinc bath in hot-dip galvanizing to further improve the  corrosion resistance of the material by promoting the formation of a continuous layer of 𝐴𝑙2𝑂3 (Marder, 2000). The high counts of iron and oxygen may indicate that iron 46  corrosion products have been formed. It has been reported in the past that humic acid is capable of promoting corrosion of API 5LX65 steel in soil (Dick & Rodrigues, 2006) and carbon steel in natural freshwaters (Jiang et al., 2008). These corrosion products could be any of the ones that have been identified for galvanized steel: 𝐹𝑒𝑂, 𝐹𝑒3𝑂4, 𝐹𝑒2𝑂3, 𝐹𝑒(𝑂𝐻)2, and 𝐹𝑒(𝑂𝐻)3 (Pantazopoulou & Papoulia, 2001). The appearance of carbon in the EDX results is normal for two reasons: first, carbon is an alloying element of steel (Markham, 1913); second, humic acid contains up to 50% carbon (Schnitzer & Khan, 1978) and can be strongly complexed with iron (Szilagyi, 1971).    Figure ‎5-9: EDX analysis of region 1 of Figure ‎5-8   47   Figure ‎5-10: EDX analysis of region 2 of Figure ‎5-8   5.4 Dextrose  5.4.1 Electrochemical tests in dextrose solutions  The OCP was measured before performing the PDP test until a stable potential was reached (Figure ‎5-11). For concentrations of 0.10 wt% and 0.25 wt% the stability was reached in one hour. Nevertheless, solutions with higher concentrations of 0.50 and 1.0 wt% required an extra half an hour before stabilizing. At all concentrations, two changes in the potential were observed. First, the potential shifted towards more negative values during the first 1000 seconds and then the potential moved towards more noble values.  48    Figure ‎5-11: Measurement of open circuit potential of galvanized steel in dextrose solutions at different concentrations at pH 7 and 25°C    It was also observed that as the concentration increased, the difference between the stable potential and the potential immediately after immersion was higher than for lower concentrations. This difference was approximately of 60, 20, 20 and 10 mV for 1.0, 0.5, 0.25, and 0.1 wt% respectively. In Figure ‎5-12, the PDP curves of galvanized steel in 0.1, 0.25, 0.5, and 1.0 wt% dextrose solutions are presented. In the cathodic branch, the cathodic process was controlled by diffusion. The limiting current density (𝑖𝐿) for 0.1, 0.25, 0.5, and 1.0 wt% concentrations was, respectively, 4.7, 3.4, 2, and 0.7 𝜇𝐴/𝑐𝑚2, thus 𝑖𝐿 decreased as the concentration increased. In the anodic branch, the process was controlled by activation and the slope at all concentrations was small for potentials near 𝐸𝑐𝑜𝑟𝑟. This slope was smaller than 50 49  mV/dec for only one decade. After the current density of 10−5𝐴/𝑐𝑚2, the slope became sharply bigger, approaching 90 mV/dec.  Figure ‎5-12: Potentiodynamic curves of galvanized steel immersed in dextrose solutions at different concentrations at pH 7 and 25°C   Table 5-3 summarizes the conductivity, 𝐸𝑐𝑜𝑟𝑟 and 𝑖𝑐𝑜𝑟𝑟 values obtained experimentally for galvanized steel in dextrose solutions.  Table ‎5-3:  Conductivity of solutions and kinetic parameters from PDP measurements of galvanized steel in dextrose solutions at concentrations ranging from 0.10 wt% to 1.0 wt% at pH 7 and 25°C wt% σ‎(μS) 𝑬𝒄𝒐𝒓𝒓 (mV) 𝒊𝒄𝒐𝒓𝒓 (𝝁𝑨/𝒄𝒎𝟐) 0.10 0.5 ± 0.53 -740 ± 19 2.8 ± 0.3 0.25 0.5 ± 0.51 -793 ± 44 1.7 ± 0.8 0.50 0.2 ± 0.01 -743 ± 4 2.4 ± 1.7 1.00 0.2 ± 0.02 -748 ± 20 2.7 ± 2.1 50  Overall, the 𝐸𝑐𝑜𝑟𝑟 did not appear to be affected by the concentration as this potential oscillated around -756 mV with standard deviations of less than 44 mV at concentrations between 0.10 wt% and 1.0 wt%. Similarly, the concentration did not have a significant effect on  𝑖𝑐𝑜𝑟𝑟 values, which were all between 1.7 and 2.8 𝜇𝐴/𝑐𝑚2 in the same concentration range.   5.4.2 Surface characterization in dextrose solutions  Figure ‎5-13 shows a selected SEM image of a galvanized steel sample after PDP test in 1.0 wt% dextrose solution. A few randomly distributed black spots were observed and are pointed with number 2. The EDX analysis (Figure ‎5-14) performed on them indicates the presence of zinc and oxygen, which may correspond to zinc corrosion products. On the rest of the surface, in light gray shade in Figure ‎5-13, the grain boundaries of grains that measure about 0.7mm in length were observed. These large grains are formed during the hot-dip galvanizing process and are visible to the naked eye as spangles, that are a characteristic feature of galvanized steel (A. P. Yadav et al., 2004b). EDX performed on region 1 of Figure ‎5-15 only identified zinc (Figure ‎5-15).  This means that the zinc coating remained intact.   51   Figure ‎5-13: SEM image of galvanized steel after PDP in 1wt% dextrose solution   52   Figure ‎5-14: EDX analysis of region 2of Figure ‎5-13   Figure ‎5-15: EDX analysis of region 1 of Figure ‎5-13  53  5.5 Citric acid  5.5.1 Electrochemical tests in citric acid solutions  Previous to the PDP test, the OCP of solutions with 0.01, 0.02, 0.1, 0.25, 0.5, and 1.0 wt% concentrations of citric acid were recorded, as shown in Figure 5-14. The stable potential was reached for all solutions after 2000 seconds. Furthermore, the same trend of shifting gradually towards more negative potentials was exhibited at all concentrations, which is in agreement with the expected corrosion aggressiveness of citric acid. Although no apparent trend was observed in the final potential with regards to the concentration, it must be noted that the difference between all values was approximately 10 mV.   Figure ‎5-16: Measurement of open circuit potential of galvanized steel in Citric Acid solutions at different concentrations at pH 3 and 25°C 54  The PDP curves of the solutions of citric acid at six concentrations ranging from 0.01 to 1.0 wt% are presented in Figure ‎5-17. The shape of the PDP curves is similar at all concentrations. In particular, the PDP curves of 0.02 wt% and 0.1 wt% appeared overlapped. The 𝐸𝑐𝑜𝑟𝑟 was virtually the same at all concentrations of citric acid in the range 0.01 wt% to 1.00 wt% with a mean value of -982 mV. In the cathodic branch, a limiting current density was not easily identifiable within 25 mV below 𝐸𝑐𝑜𝑟𝑟. The only evident effect of the increase of citric acid concentration on the corrosion behavior of galvanized steel was the increase in current density, which is detected visually by the displacement towards the right of both, the anodic and cathodic branches. Furthermore the slopes of the anodic branches were large compared to the slopes of PDP curves in humic acid and dextrose solutions, since they were higher than 80mV/dec.  Figure ‎5-17: Potentiodynamic curves of galvanized steel immersed in citric acid solutions at different concentrations at 25°C 55  The conductivity and kinetic parameters at the different concentrations of citric acid are presented in Table ‎5-4.   Table ‎5-4: Conductivity of solutions and kinetic parameters from PDP measurements of galvanized steel in citric acid solutions at concentrations from 0.01 wt% to 1.0 wt% at pH 2 and 25°C wt% σ‎(μS) 𝑬𝒄𝒐𝒓𝒓 (mV) 𝒊𝒄𝒐𝒓𝒓 (𝝁𝑨/𝒄𝒎𝟐) 0.01 3.5 ± 0.5 -986 ± 13 55.0 ± 0.8 0.02 5.4 ± 0.3 -984 ± 6 53.2 ± 21.9 0.10 12.1 ± 0.4 -983 ± 4 72.4 ± 5.5 0.25 18.7 ± 0.1 -979 ± 16 108.0 ± 67.0 0.50 27.1 ± 7.2 -980 ± 3 186.5 ± 155.3 1.00 39.5 ± 1.3 -982 ± 3 153.2 ± 100.4   The values of 𝑖𝑐𝑜𝑟𝑟 in Table ‎5-4 confirmed that as the concentration increased, the current densities also increased. This same tendency in the current densities as the concentration of citric is increased has also been reported on steel and tin (Gouda et al., 1980; Gouda et al., 1981; Sekine et al., 1990). In general, the corrosive effect of citric acid has been recognized on various metals, including bronze, iron and tin (Abdel Rehim et al., 2003; Jafarian et al., 2008; Li et al., 2014; Ying et al., 2003; Zerfaoui et al., 2004). However, a maximum 𝑖𝑐𝑜𝑟𝑟 was obtained at 0.50 wt%, after which higher concentration produced a lower current density than this maximum value.  The electrochemical tests do not provide information about the complexes formed between zinc or iron with citric acid. However,  in order to determine the nature of the complexes that would be formed at a given potential and pH, a speciation diagram was created for each of these metals 56  with citrate using the MEDUSA5 software. The diagrams are presented in Figure ‎5-18 and Figure ‎5-19. For instance, considering the pH of the experimental conditions which is 3, zinc is expected to form stable complexes (Zn(Hcit)) at potentials above -1.00 V. However, zinc becomes leachable at neutral pH in a range that goes from 4 to 10, as indicated by the ionic form 𝑍𝑛(𝑐𝑖𝑡)−.  Figure ‎5-18: Predominance area diagram of zinc-citrate complexes in aqueous solutions.  Once the zinc layer is depleted and the base steel is exposed, at the same pH 3, the type of complexes that iron can form with citrate will depend on the potential. For potential values                                                  5 MEDUSA (Make Equilibrium Diagrams Using Sophisticated Algorithms), software developed by the School of Chemical Science and Engineering from the Royal Instute of Technology, 100 44 Stockholm, Sweden. 57  between -0.50 V and +1.0 V, the 𝐹𝑒2+ ions will be the predominant species, while at higher potentials, the predominant species will be stable complexes (Fe(cit)).  Figure ‎5-19: Predominance area diagram of iron-citrate complexes in aqueous solutions.   5.5.2 Surface characterization of galvanized steel in citric acid solutions  A representative SEM image of the galvanized steel samples surface after the PDP test is shown in Figure ‎5-20. The surface appeared to be covered in some areas by residual accumulations that seemed to be detached form the surface. By performing EDX analysis on the surface labeled as region 1 in Figure ‎5-20, it was determined that the galvanizing coating was completely corroded 58  by the citric acid solution at 0.25 wt%,  leaving the base steel surface exposed to the environment, which was confirmed by the high counts of iron in Figure ‎5-21. The elemental composition of the residues labeled with number 2 in Figure ‎5-20 was mainly Zn, Fe, and O, suggesting that most likely it was a form of zinc oxide as revealed by EDX results shown in Figure ‎5-22.   Figure ‎5-20: SEM image of galvanized steel after PDP in 0.25 wt% citric acid solution 59   Figure ‎5-21: EDX analysis of region 1of Figure ‎5-20   Figure ‎5-22: EDX analysis of region 2 of Figure ‎5-20 60  5.6 Oxalic acid  5.6.1 Electrochemical tests in oxalic acid solutions  The OCP of galvanized steel in solutions with different concentrations of oxalic acid was measured until a stable potential was reached. For solutions with concentrations of 0.10, 0.25, and 5.0 wt%, a stable potential was reached after 2000s (Figure ‎5-23). For these concentrations, the potential was between -0.94 V and -0.9 V.  Figure ‎5-23: Measurement of open circuit potential of galvanized steel in 0.10, 0.25 and 0.50 wt% oxalic acid solutions at pH 2 and 25°C  61  However, for the solution with 1.0 wt% of oxalic acid, it was necessary to continue recording the potential for more than 8 hours before a stable potential could be observed (Figure ‎5-24). At this concentration, the final OCP value was more noble than for the smaller concentrations.   Figure ‎5-24: Measurement of open circuit potential of galvanized steel in 1.0 wt% oxalic acid solution at pH 2 and 25°C   For concentrations of 0.10, 0.25, and 1.0 wt%, it was noticed that during the stabilization of the potential, the measurements tended to shift towards more noble values. This trend was not observed at 0.50 wt%, where each OCP measurement exhibited a different behavior.  62  The PDP curves at concentrations below 0.10 wt% did not exhibit a consistent shape (Figure ‎5-25). At the smallest concentration of 0.01 wt%, linearity within a decade could not be observed. Then, at 0.015 wt%, a passive-like behavior was exhibited at a current density of 160 𝜇𝐴 𝑐𝑚2⁄ . Finally, at concentration 0.07 wt%, passivation was observed by an inflection of the curve between potentials -1.0V and -0.7V. This could correspond to the formation of zinc corrosion products because the potential of the inflection corresponds to the potential of dissolution of zinc. Above this potential, active dissolution continued with a slope bigger than 80mV/dec in the anodic branch.     Figure ‎5-25: Potentiodynamic curves of galvanized steel immersed in oxalic acid solutions at concentrations below 0.1 wt% at pH 1 and 25°C  63  For concentrations of 0.1, 0.25, 0.5, and 1.0 wt%, the PDP curves did have a similar shape (Figure ‎5-26). In the anodic branches, slopes of about -10mV/dec were observed and a limiting current density could not be identified. In the anodic branch, a passive-like behavior became clearer with the vertical lines. At concentrations 0.25 wt% and 1.0 wt%, an inflection point to the right was observed that corresponded to the primary passive potentials (𝐸𝑝𝑝) and critical current (𝑖𝑐𝑟𝑖𝑡) densities after which the current decreased until the passivation current density (𝑖𝑝𝑎𝑠𝑠) was attained. For the 0.25 wt% solution, plotted in Figure ‎5-26, 𝐸𝑝𝑝 was about -0.650 V and 𝑖𝑐𝑟𝑖𝑡 150 𝜇𝐴 𝑐𝑚2⁄ . The corresponding values for 1.0 wt% were -0.360 V and 20 𝜇𝐴 𝑐𝑚2⁄ . Moreover, the measured value of 𝑖𝑝𝑎𝑠𝑠  was 120 𝜇𝐴 𝑐𝑚2⁄  for 0.1wt%; 40 𝜇𝐴 𝑐𝑚2⁄  for 0.25 and 0.5 wt%; and 9 𝜇𝐴 𝑐𝑚2⁄ for 1.0 wt%.  Figure ‎5-26: Potentiodynamic curves of galvanized steel immersed in oxalic acid solutions at concentrations between 0.1wt% and 1.0wt% at pH 1 and 25°C 64  The inhibition effect of oxalic acid has been reported on other metals. For example, oxalic acid has produced an inhibition of copper in dilute citric and concentrated propionic solutions at 30°C (Baah & Baah, 2000). In sulfuric acid solutions, additions of 10−7 to 10−3 M also had an inhibiting effect on carbon steel at room temperature (Giacomelli et al., 2004). An increase of corrosion rates as the concentration of oxalic acid increased was observed only at the boiling point on ferritic stainless steels (Sekine & Okano, 1989). icorr (A/cm2)1e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2E(V) vs Ag/AgCl-1.5-1.0-0.50.00.51.01.52.0 Figure ‎5-27: Passive-like behavior of galvanized steel immersed in 1.0wt% oxalic acid solution at pH 1 and 25°C  The passive-like behavior is desirable because it means that further corrosion of the material is being prevented. However, a risk associated with this condition is that if the passive region is trespassed and the trans-passive region is reached, pitting corrosion may occur. For this reason, additional experiments were carried out, aiming to determine the value of the breaking potential, 65  𝐸𝑏, at which a sharp increase in the current density would be observed. Figure ‎5-27 shows the PDP results of two identical experiments. The sharp increase in current density was found at 𝐸𝑏= +1.5 V.  At this potential, it is said that the material has reached the transpassive state. There are two possible explanations for the fast increase of current density. It is possible that the metal experienced severe dissolution or that another reaction took place, namely oxygen evolution (Zhang, 1996).   Table ‎5-5 summarizes the OCP measurements and kinetic parameters of GS in oxalic acid solutions. The increase of concentration between 0.01 and 1.0 wt% resulted in a shift of 𝐸𝑐𝑜𝑟𝑟 values towards more noble values. Three groups of corrosion potentials were observed. At concentrations below 0.1 wt%, the 𝐸𝑐𝑜𝑟𝑟values were close to -1 V. At concentrations of 0.1 and 0.25 wt%, an increase of 100 to 150 mV was detected. Finally, at concentrations of 0.5 and 1.0 wt%, the corrosion potential values were in the order of -767 mV. The corrosion behavior in terms of current density was clear. As the concentration increased, 𝑖𝑐𝑜𝑟𝑟 decreased.   Table ‎5-5: Conductivity of solutions and kinetic parameters from PDP measurements of galvanized steel in oxalic acid solutions at different concentrations at pH 1 and 25°C wt% σ‎(μS) 𝑬𝒄𝒐𝒓𝒓 (mV) 𝒊𝒄𝒐𝒓𝒓 (𝝁𝑨/𝒄𝒎𝟐) 0.01 7.4 ± 0.1 -990 ± 5 73.5 ± 5.1 0.015 11.7 ± 0.3 -986 ± 2 75.3 ± 8.6 0.07 45.9 ± 1.2 -1007 ± 2 49.9 ± 43.1 0.10 61.4 ± 0.9 -847 ± 64 19.5 ± 4.4 0.25 145.7 ± 2.1 -924 ± 70 12.3 ± 9.0 0.50 282.3 ± 7.8 -757 ± 7 6.4 ± 0.5 1.00 531.0 ± 7.2 -778 ± 33 2.9 ± 0.7   66  5.6.2 Surface characterization of galvanized steel in oxalic acid solutions  All galvanized samples appeared covered by white deposits that could not be removed by rinsing the samples with distilled water. By visual inspection of the samples, the amount of white deposits on the surface seemed to be related to the concentration of oxalic acid in solution. SEM imaging confirmed that the surface was completely covered by a compact layer of deposits whose homogeneity was interrupted by randomly-distributed nests (Figure ‎5-28), which were filled with crystals that appeared to be loose (Figure ‎5-29). At longer exposure time, the platelets would eventually spread over the surface (Asgari et al., 2009). Both the white compact layer and the nests, are characteristic of zinc corrosion products. It has been proposed that a membrane of colloidal oxyhydroxides is responsible for retaining the zinc corrosion products in the form of a compact layer and that diffusion of water into the layer, driven by high ionic strength of the solution, is responsible for bursting the membrane and forming the blisters (Weissenrieder et al., 2004).   The elemental composition of the compact layer (Figure ‎5-30) and the crystals inside the nests (Figure ‎5-31) were examined by EDX.  In both cases, the highest counts were found for zinc and oxygen. Therefore, the presence of zinc corrosion products was confirmed.  67   Figure ‎5-28: SEM image of galvanized steel after PDP in 0.25 wt% oxalic acid solution   Figure ‎5-29: SEM image of crystals inside blisters 68   Figure ‎5-30: EDX analysis of compact layer   Figure ‎5-31: EDX of nests crystals 69  The ability of oxalic acid to readily dissociate into oxalates (𝐶2𝑂42−) and act as an excellent ligand is known and has been studied for its applicability in soil remediation (Kim et al., 2013) and chemical cleaning of reactors (Borghi et al., 1996). The possible pathway for the formation of metal complexes due to the interaction of oxalic acid and metal ions (𝑀𝑒𝑎+) has been described (Sekine et al., 1990) as follows:  𝑀𝑒𝑎+ + 𝐻2𝐶2𝑂4 = 𝑀𝑒𝐶2𝑂4𝑎−2 + 2𝐻+ Eq. ‎5-1  𝑀𝑒𝐶2𝑂4𝑎−2 + 𝐻2𝐶2𝑂4 = [𝑀𝑒(𝐶2𝑂4)2]𝑎−4 + 2𝐻+ Eq. ‎5-2   [𝑀𝑒(𝐶2𝑂4)2]𝑎−4 + 𝐻2𝐶2𝑂4 = [𝑀𝑒(𝐶2𝑂4)3]𝑎−6 + 2𝐻+ Eq. ‎5-3  Given the surface characterization, it can be concluded that the passive-like behavior observed on PDP curves of galvanized steel in oxalic acid solutions is not a passivation effect, but rather, a passive-like behavior due to the formation of a layer of zinc corrosion products that provides a physical barrier that isolates the steel from the corrosive environment, blocking the anodic and cathodic reactions. At the breaking potential, 𝐸𝑏, the film dissolves and the corrosion process of the metal continues.     70  5.7 Chlorides and sulfates  In order to provide a parameter to compare the corrosion current densities produced by the individual organics, the PDP test was also performed in solutions containing 100 ppm chlorides (0.01 wt%) and 200 ppm sulfates (0.02 wt%). As for previous solutions, OCP was also measured until a stable potential was reached. However, only the final value is reported in the summary table (Table ‎5-6). The curves obtained exhibited a clear limiting current density in the cathodic branch in the order of 20 to 40 μ𝐴/𝑐𝑚2. In the anodic side, both had a slope of less than 50mV/dec with a sharp increase of about 600 mV in less than one decade when the current density values approached 1𝑚𝐴/𝑐𝑚2. An example of the PDP curves is presented in Figure ‎5-32.  Figure ‎5-32: PDP curves of solutions with 100ppm chlorides and 200ppm sulfates at 25°C 71  All the parameters that were reported for organic solutions are also presented for these solutions in the following table. However, for comparison purposes, the 𝑖𝑐𝑜𝑟𝑟 values are of interest.  Table ‎5-6: Parameters of solutions with 100ppm chlorides and 200ppm sulfates and electrochemical measurements from PDP tests in 25°C  pH σ‎(μS) 𝑬𝒄𝒐𝒓𝒓 (mV) 𝒊𝒄𝒐𝒓𝒓 (𝝁𝑨/𝒄𝒎𝟐) 100ppm chlorides 6.6 ± 1.46 8.5 ± 0.9 -789 ± 31 30.6 ± 4.8 200 ppm sulfates 8.1 ± 3.08 10.1 ± 0.2 -872 ± 20 23.4 ± 3.1    5.8 Comparison of corrosion behavior of galvanized steel in four organic solutions  The PDP curves of galvanized steel in the four organic solutions at 1.00 wt% are compared in Figure ‎5-33. The samples in dextrose and oxalic acid solutions exhibited the most noble corrosion potential values of the organics studied, with similar values of -749 mV and -778 mV, respectively. In citric acid solutions the most negative corrosion potential values were measured at -982 mV. In humic acid solutions, 𝐸𝑐𝑜𝑟𝑟 was found at an intermediate potential of -824 mV. From this PDP plots, it can also be observed that, at 1.00 wt% concentration, the aggressiveness of the organics decreased in the following order: citric acid > oxalic acid > humic acid and dextrose. Also, the passive-like behavior produced by oxalic acid that was explained in section 5.5.1 is observed.  72   Figure ‎5-33: Comparison of potentiodynamic polarization resistance plots of galvanized steel immersed in 1 wt% organic solutions at 25°C    Figure ‎5-34 presents together the plots of the corrosion current densities (𝑖𝑐𝑜𝑟𝑟) produced by each organic on galvanized steel as a function of concentration for comparison purposes. It is observed that the current densities produced by dextrose do not change significantly with the concentration, as all values were inferior to 3 𝜇𝐴 𝑐𝑚2⁄ . Humic acid produced higher current densities at low concentrations. The highest measured value with low variation was produced at 0.05 wt% with 15.3±0.3 (𝜇𝐴/𝑐𝑚2). However, the current density decreased as the concentration increased. At 1.00 wt%, the corrosion effect was identical to that produced by dextrose. Oxalic acid produced considerably higher current densities, also at the lowest concentrations. The maximum peak was 75 𝜇𝐴 𝑐𝑚2⁄  reached in 0.015 wt% oxalic acid solution. At concentration 73  0.07 wt%, the variation in the readings was big and for higher concentrations the value of the measurements dropped with small variation.    a) Dextrose  b) Humic acid  b) Oxalic acid  c) Citric acid  Figure ‎5-34: Comparison of corrosion current densities of galvanized steel as a function of concentration of individual organics in solutions at 25°C: a) dextrose; b) humic acid; c) oxalic acid; d) citric acid.  As discussed in section 5.5.2, the reason might be the formation of a compact layer of corrosion products. It is possible that the minimum concentration of oxalic acid that promotes the formation of zinc corrosion products on all the surfaces of the materials is between 0.015 wt% 74  and 0.10 wt%. Galvanized steel exhibited the highest current densities in citric acid solutions. Additions of citric acid resulted in an increase of the current density. The maximum value was observed at concentration 0.50 wt% that produced 187 𝜇𝐴 𝑐𝑚2⁄ . Furthermore, comparing the current densities of organic solutions with those produced by 100 ppm chlorides and 200 ppm sulfates, it can be concluded that dextrose and humic acid are less aggressive. Nevertheless, oxalic acid can be more aggressive at concentrations lower than 0.07 wt%, while citric acid is more aggressive at all concentrations.  Given the corrosion current density, it is possible to determine the mass (m) that will be lost due to‎corrosion‎by‎Faraday’s‎law‎(Eq. ‎5-4), where i is the current density, a is the atomic mass, n is the number of electrons transferred and F is‎Faraday’s‎constant.‎ 𝑚 =𝑖𝑎𝑛𝐹 Eq. ‎5-4:‎Faraday’s‎law From‎ Faraday’s‎ law‎ the‎ following‎ expression‎ is‎ obtained‎ that‎ directly‎ relates‎ the‎ corrosion‎current densities to penetration rates (r), in which c is a conversion constant equal to 3.27 to obtain‎r‎in‎micrometers‎per‎year‎(μm/yr)‎and‎D is the density of the material in 𝑔 𝑐𝑚3⁄ . 𝑟 = 𝑐𝑎 ∙ 𝑖𝑐𝑜𝑟𝑟𝑛𝐷 Eq. ‎5-5  For the samples used in the experiments, the 𝑖𝑐𝑜𝑟𝑟 values most likely correspond to the first stage of the corrosion of galvanized steel where only the zinc coating is corroding because the 𝑖𝑐𝑜𝑟𝑟 is obtained from PDP at potentials before an increase in the anodic reaction rates is induced. Under this consideration, the atomic weight of zinc (65.38 𝑔 𝑚𝑜𝑙⁄ ) and the density of zinc (7.14 𝑔 𝑐𝑚3⁄ ) can be replaced in Eq. ‎5-5 to produce the plot in Figure ‎5-35 that presents graphically 75  the conversion between 𝑖𝑐𝑜𝑟𝑟 in 𝜇𝐴/𝑐𝑚2to penetration rates (r) in 𝜇m/yr. In this figure, various conversion points are indicated, that correspond to the highest corrosion rates produced by each organic.    Figure ‎5-35: Conversion from 𝒊𝒄𝒐𝒓𝒓 values in 𝝁𝑨/𝒄𝒎𝟐to penetration rates (r) in 𝝁m/yr.  Dextrose produced similar 𝑖𝑐𝑜𝑟𝑟 independently of concentration, with values close to 3 𝜇𝐴/𝑐𝑚2, which‎corresponds‎to‎45‎μm/yr.‎The‎highest‎𝑖𝑐𝑜𝑟𝑟 due to humic acid was 15 𝜇𝐴/𝑐𝑚2, produced at‎concentration‎0.05‎wt%,‎is‎equivalent‎to‎225‎μm/yr.‎Similarly,‎𝑖𝑐𝑜𝑟𝑟 of 23 and 30 𝜇𝐴/𝑐𝑚2 due to‎ 100ppm‎ chlorides‎ and‎ 200ppm‎ sulfates‎ are‎ equal‎ to‎ 344‎ and‎ 449‎ μm/yr,‎ respectively.‎76  Likewise, the 73 𝜇𝐴/𝑐𝑚2 obtained‎in‎0.01‎wt%‎solutions‎of‎oxalic‎acid,‎are‎equal‎to‎1093‎μm/yr‎and the 186 𝜇𝐴/𝑐𝑚2 from‎0.50‎wt%‎citric‎acid‎solutions,‎to‎2785‎μm/yr.  A possible relationship between the corrosion current densities (𝑖𝑐𝑜𝑟𝑟) and the pH of the organic solutions was considered regardless of the concentration (Figure ‎5-36).   Figure ‎5-36: Corrosion current densities at different pH values.  The pH did not seem to have a significant impact on the 𝑖𝑐𝑜𝑟𝑟 values as is observed in Figure ‎5-36 by looking at the dots that represent the values obtained at concentration of 1.00 wt% for all organic species. Independently of the pH, the 𝑖𝑐𝑜𝑟𝑟 had values between 3 and 20 𝜇𝐴 𝑐𝑚2⁄ . Considering the 𝑖𝑐𝑜𝑟𝑟 at different pH of the organics at concentration 0.10 wt% the 77  range of values becomes larger. However, taking the citric acid as example, it appears that the concentration has a higher effect on the 𝑖𝑐𝑜𝑟𝑟 than the pH.  Similarly, the possible relationship between current densities and conductivity was considered. In Figure ‎5-37 each of the four dots of each organic represents a different concentration but the values are not detailed because the objective of this plot is to focus solely on 𝑖𝑐𝑜𝑟𝑟 and‎σ.  Figure ‎5-37: Corrosion current densities and conductivity of organic solutions.  It is observed that on one hand, while the conductivity of citric acid varies in a small range, from 12 to 39μS, the 𝑖𝑐𝑜𝑟𝑟 values increases from 72 to 186 𝜇𝐴 𝑐𝑚2⁄ . On the other hand, while the conductivity‎of‎oxalic‎acid‎ spans‎ in‎a‎ large‎ range,‎ from‎61‎ to‎531‎μS,‎ the‎variation‎of 𝑖𝑐𝑜𝑟𝑟 is relatively small, going from 19.5 to 3 𝜇𝐴 𝑐𝑚2⁄ . Therefore, the trend of the citric and oxalic acid 78  demonstrate that the relationship between 𝑖𝑐𝑜𝑟𝑟 and conductivity is non-existent. So, even though the conductivity measurements may provide an indication of the aggressiveness of the electrolyte, it does not provide sufficient information to predict the corrosion behavior of a material.    5.9 Summary  The corrosion potential (𝐸𝑐𝑜𝑟𝑟) and the current densities (𝑖𝑐𝑜𝑟𝑟) of galvanized steel in four organic solutions were obtained for concentrations ranging from 0.1 wt% to 1.0 wt%, from the potentiodynamic polarization curves. The increase of concentration of citric acid in solution was accompanied by an increase of current densities, while the opposite effect was observed for oxalic acid and humic acid, for which, as the concentration increased, the current densities decreased. This trend was not expected for oxalic acid because higher concentrations of it produced a higher conductivity. However, oxalic acid promoted the formation of a protective layer of zinc corrosion. In the case of glucose, the concentration did not seem to have an effect on the corrosion rates.  The comparison of the maximum current densities produced by each organic in the range 0.01 wt% to 1.0 wt% allows to classify their aggressiveness on galvanized steel in the following order: citric acid > oxalic acid > humic acid > glucose. 79  Chapter 6: Corrosion behavior of galvanized steel in solutions of simulated soil organic matter  In this chapter, the results from PDP, LPR and EIS electrochemical tests and surface analysis performed on galvanized steel in 0.25, 0.5, 0.75, and 0.91 wt% solutions of Simulated Soil Organic Matter (SSOM) are presented. The composition of SSOM is described in chapter 4. The focus of the results is to find the polarization resistance of galvanized steel by the different methods.    6.1 pH and conductivity  The pH and the conductivity of the solutions were measured prior to performing the electrochemical tests. Both parameters increased as the SSOM concentration was increased as shown in Figure ‎6-1. The pH moved towards from near neutral to more alkaline values. The pH values measured for 0.25, 0.5, 0.75, and 0.91 wt% concentrations were, 6±0.2, 7.2±0.1, 8.3±0.1, and 8.8±0.3. In the same range of concentration, the conductivity also increased following this order: 15.7‎±‎0.6‎μS at 0.25 wt%;  26.8‎±‎0.7‎μS‎at‎0.5‎wt%;‎‎37.3‎±‎43.1‎μS‎at‎0.5‎wt%;‎and‎43 ±‎0.5‎μS at 0.91 wt%.  80  Concentration (wt %)a)0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0pH5.56.06.57.07.58.08.59.09.5Concentration (wt %)b)0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0S1015202530354045 Figure ‎6-1: Parameters of the SSOM solutions as a function of concentration at 25°C: a) pH, b) conductivity   6.2 Electrochemical measurements  Figure ‎6-2 presents the PDP curves obtained. The cathodic process appeared controlled by diffusion at all concentrations with limiting current densities between 10−5 and 10−4 𝐴 𝑐𝑚2⁄ . The anodic branched of GS in solutions with 0.25, 0.5, and 0.75 wt% SSOM were very similar in shape among them, showing active dissolution of the material. However, at 0.91 wt%, the shape of the curve changed. In the anodic branch, two inflections were observed at potentials near -0.9 V and -0.3 V.  These potentials correspond approximately to the standard potentials of the dissolution of zinc and iron. Moreover, at this concentration the pH was close to 9. For both metals, at this pH value, the thermodynamics favor the formation of oxides. Thus it is possible 81  that the decrease in the current densities at this potential was due to the formation of zinc and iron corrosion products, respectively.   Figure ‎6-2: Potentiodynamic curves of galvanized steel immersed in solutions with different concentrations of a combination of organics that simulate the typical composition of soil organic matter   The corrosion potential values at each concentration are presented in Table ‎6-1. It can be considered that the corrosion potential was not affected by the additions of SSOM and remained close to -1015 mV. Comparing with the results presented in chapter 5, it appears that the minute additions of citric acid dragged the corrosion potential of the SSOM system to the same 𝐸𝑐𝑜𝑟𝑟 values of pure citric acid solutions.  82  Table ‎6-1: Kinetic parameters obtained from PDP curves of SSOM solutions at 25°C and concentrations from 0.25 wt% to 1.0 wt% wt% 𝑬𝒄𝒐𝒓𝒓 (mV) 𝒊𝒄𝒐𝒓𝒓 (𝝁𝑨/𝒄𝒎𝟐)  𝑹𝑷(𝒌𝜴) 0.25 -1011 ± 8 28 ± 23 2.11 ± 1.11 0.50 -998 ± 3 43 ± 10 1.59 ± 0.90 0.75 -1021 ± 13 84 ± 98 1.33 ± 0.92 0.91 -1041 ± 4 81 ± 95 1.18 ± 0.56   The calculated current densities increased as the concentration increased. To put these values in perspective, the mean 𝑖𝑐𝑜𝑟𝑟 values produced by the individual organics presented in the previous chapter are reproduced here in graphic form in the concentration range between 0.25 and 1.0 wt% together with the mean 𝑖𝑐𝑜𝑟𝑟 values of SSOM presented in the table above (Figure ‎6-3).  Concentration (wt%)0.4 0.6 0.8 1.0icorr (A/cm2)050100150200Humic acidDextrose Citric acidOxalic acidSSOM  Figure ‎6-3: Graphic comparison of the current densities produces by individual organics and by the combination of them in SSOM solutions at 25°C 83  It can be seen that the 𝑖𝑐𝑜𝑟𝑟 produced by SSOM were, at all concentrations, higher than those produced by individual solutions of dextrose, humic acid and oxalic acid, but lower than those produced by individual solutions of citric acid. Moreover, at concentrations of 0.25 and 0.5 wt%, the 𝑖𝑐𝑜𝑟𝑟 was higher than the values obtained in solutions with 100 ppm chlorides and 200 ppm sulfates, indicating that under the experimental conditions, SSOM can be more aggressive to galvanized steel than chlorides and sulfates.   The high standard deviation in the 𝑖𝑐𝑜𝑟𝑟 at concentrations of 0.75 wt% and 0.91 wt% puts in evidence that it became difficult to find reproducibility of the results.  At the time of performing the experiments no explanation could be found for the poor repeatability. However, it was later retrieved that the dissolution rates of humic acid particles can be increased up to 30 times in the presence of monocarboxylic acids (Brigante et al., 2008). Although in these experiments the organic acids used were di and tricarboxylic acids, a possibility exist that they exhibit a similar effect on humic acid and that an incomplete dissolution of humic acid particles could have affected the results at 0.75 wt% and 0.91 wt% concentrations.  The polarization resistance (𝑅𝑃) of galvanized steel in SSOM solutions was obtained from PDP results by means of Eq. ‎6-1: 𝑖𝑐𝑜𝑟𝑟 =𝐵𝑅𝑃  Eq. ‎6-1 Where 𝐵 =𝛽𝑎 ∙ 𝛽𝑐2.3(𝛽𝑎 + 𝛽𝑐) Eq. ‎6-2   84  The polarization resistance values were also obtained from LPR measurements by using the principle‎of‎Ohm’s‎law.   Table ‎6-2: Polarization resistance at different concentrations of combined organics wt% 𝑹𝑷 (kΩ) 0.25 9.0 ± 0.7 0.50 9.9 ± 0.5 0.75 7.6 ± 1.6 0.91 5.5 ± 1.2   The 𝑅𝑃 was also obtained from EIP results. Again, good repeatability was found at SSOM concentrations of 0.25 wt% and 0.5 wt%. The measurements correspond to samples in 0.5 wt% SSOM solutions. Similarly than in PDP results, repeatability was not observed at concentrations 0.75 wt% and 0.91 wt%. One representative Nyquist plot for each concentration of SSOM solution is shown in Figure ‎6-4.  Figure ‎6-4: Nyquist plots of galvanized steel in SSOM solutions at different concentrations at 25°C. 85  It was determined that at all concentrations the data was best fitted by the response of the equivalent electronic circuit shown in Figure ‎6-5, with which the chi square values were in the order of 10−4. This circuit represents the response of a porous layer, where Rs is the solution resistance, Re is the resistance of the electrolyte in the pore and Rp is the polarization resistance, Q1 is the capacitance due to the porous layer and Q2 is the capacitance of the double layer (ECS, 2008).  Figure ‎6-5: Equivalent circuit to EIS response  The parameters of the equivalent system are presented in the table below for the one test at each concentration.   Table ‎6-3: Parameters of the components of the equivalent circuit that simulates the frequency response of galvanized steel in SSOM solutions at 25°C  wt% Rs (Ω) Q1 (S.sec^n) f power, n Re (kΩ) Q2 (S.sec^n) f power, n Rp (kΩ) Chsq 0.25 631.000 7.365E-06 0.713 14.580 4.445E-05 1.000 2.351 5.47E-04 0.50 118.200 3.000E-06 0.800 8.190 2.366E-05 0.800 3.232 3.23E-04 0.75 252.500 4.973E-06 0.786 0.843 7.670E-06 0.517 7.699 1.22E-04 0.91 133.700 3.094E-06 0.822 8.270 1.061E-05 0.870 4.009 3.64E-04     86  6.3 Polarization resistance trend as a function of SSOM concentration in solution  The polarization resistance of galvanized steel obtained from three electrochemical methods (PDP, LPR and EIS) is presented graphically in Figure ‎6-6. The trend of the 𝑅𝑃 calculated from PDP and LPR measurements is similar, however the values of 𝑅𝑃 obtained from LPR are higher than the ones obtained by PDP. At concentrations of 0.25 wt% and 0.5 wt%, the values of 𝑅𝑃 calculated from EIS were similar to 𝑅𝑃 calculated by PDP results. At concentrations of 0.5 wt% and 0.91 wt%, the mean values of EIS were more similar to 𝑅𝑃 obtained by LPR.   Concentration (wt%)0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0RP (k)02468101214161820EISPDPLPR Figure ‎6-6: Polarization Resistance (RP) obtained by three methods with error bar plots of standard deviation   87  6.4 Surface characterization  By performing visual inspection of the galvanized steel samples after PDP tests at the different concentrations of SSOM, four distinct colors could be observed on the surface: dark gray, white, brown, and rusty red. These products were non-uniformly distributed on the surface of all samples exposed to SSOM concentrations of 0.25, 0.5, 0.75, and 0.91 wt% in solution. An example of the random distribution of deposits is given in Figure ‎6-7 where the different shades correspond to a different composition of the products formed in the presence of 0.5 wt% SSOM.    Figure ‎6-7: SEM image of galvanized steel after PDP in 0.50 wt% SSOM solution at 25°C  Further EDX analysis was required to identify the composition of the different types of products. For instance, in Figure ‎6-8, that shows another area of the surface of the sample immersed in 88  0.50 wt% SSOM, two different regions were identified and labeled with numbers 1 and 2.  As per EDX results (Figure ‎6-9), the higher counts in region 1 were of zinc, showing that, in this specific area, the sample was still protected by the zinc coating. On the other hand, in region 2, the zinc coating had been completely corroded and the base steel was exposed (Figure ‎6-10). The absence of oxygen indicated that corrosion products were not formed.    Figure ‎6-8: SEM image of GS sample after PDP immersed in solutions with 0.5 wt% SSOM  89   Figure ‎6-9: EDX analysis of region 1 from Figure ‎6-8   Figure ‎6-10: EDX analysis of region 2 from Figure ‎6-8 90  At 0.91 wt% SSOM concentration, two other regions were identified, labeled with numbers 1 and 2 in Figure ‎6-11. Figure ‎6-12 is a magnification of region 1 from Figure ‎6-11, where well-defined grains were observed. EDX results showed that they correspond to base steel that was exposed (Figure ‎6-13).     Figure ‎6-11: SEM image of galvanized steel sample after PDP in 0.91 wt% SSOM solution    91   Figure ‎6-12: SEM image of region labeled as 1 in Figure ‎6-11 (0.91 wt% SSOM)   Figure ‎6-13: EDX analysis of region 1 of Figure ‎6-11 92  In Figure ‎6-14, a magnification of region 2 from Figure ‎6-11 is observed, in which, deposits without defined shape were observed that formed a porous layer on the surface. EDX analysis of this area (Figure ‎6-15) revealed that the elemental composition was iron, oxygen, and carbon.    Figure ‎6-14: SEM image of region 2 of Figure ‎6-11 (0.91 wt% SSOM)  In this particular case, because the color of this area was dark brown rather than rusty red, it is speculated that the presence of iron and oxygen could correspond to the formation of metal organic complexes. For example, the formation of the metal oxalate complex [𝐹𝑒(𝐶2𝑂4)3]3− has been clearly identified for ferritic stainless steel in oxalic acid solutions (Sekine et al., 1990).  93   Figure ‎6-15: EDX analysis of region labeled as 2 in Figure ‎6-11   6.5 Summary  In summary, solutions with concentrations inferior to 1.0 wt% of the proposed system that simulates the typical composition of soil organic matter are capable to corrode the zinc coating and the base steel. The polarization resistance was calculated from electrochemical data obtained by PDP, LPR and EIS. A relationship between the polarization resistance of galvanized steel and the concentration of soil organics cannot be established with the given results. 94  Chapter 7: Conclusions  The results of the electrochemical test and the analysis presented in this thesis lead to the following conclusions:   The corrosion behavior of galvanized steel in organic solutions at 25°C in concentrations lower than 1.00 wt% depends on the composition of the organics.  The highest corrosion current density produced by each organic is reached at a concentration that is specific for each type of organic.  At concentrations below 1.00 wt% of solutions of individual organics, the comparison of the highest current density produced by each organic, allows to range their aggressiveness on galvanized steel in the following order: citric acid > oxalic acid > humic acid > dextrose.  Oxalic acid enhances the formation of a compact layer of zinc corrosion products that isolates galvanized steel from the environment and blocks the anodic reactions.  A system of organics (SSOM) that simulates the typical composition of soil organic matter is proposed to study the corrosion behavior of galvanized steel by electrochemical techniques  The system of combined organics produces higher corrosion rates than the individual organics at concentrations lower than 1.00 wt%  SEM analysis and EIS results confirm the formation of a non-protective porous layer in SSOM solutions at concentrations lower than 1.00 wt%. 95  Chapter 8: Future work   Perform further experiments in dextrose solutions, to confirm if the results found in this thesis correspond or not to the exchange current density of dextrose.  Increase the concentration range of SSOM solutions and improve the repeatability of the results in order to identify a trend in the polarization resistance as a function of concentration that can be used in the form of an equation that can be incorporated in the numerical model that has been previously developed to estimate the remaining service life of MSE walls.  Focus the next part of the investigation of the role of organics on the effect that they may produce on iron only to complement the results from this thesis, which focused mostly on the effect produced on zinc.  Analyze the organic composition of backfill soil used in the construction of MSE walls to validate that the proposed Simulated Soil Organic Matter is a suitable system to conduct experiments to characterize the corrosion behavior of galvanized steel in the presence of soil organics.  Conduct long-term corrosion tests of galvanized steel reinforcements in soil samples with controlled content of Simulated Soil Organic Matter.96  Bibliography   AASHTO (Ed.). 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