Principles of Corrosion Engineering and Corrosion Control

Höfundar: Ahmad, Zaki (Útgáfa: 0)
Principles of Corrosion Engineering and Corrosion Control

Kaup valmöguleikar

Corrosion is a huge issue for materials, mechanical, civil and petrochemical engineers. With comprehensive coverage of the principles of corrosion engineering, this book is a one-stop text and reference for students and practicing corrosion engineers. Highly illustrated, with worked examples and definitions, it covers basic corrosion principles, and more advanced information for postgraduate students and professionals.

Basic principles of electrochemistry and chemical thermodynamics are incorporated to make the book accessible for students and engineers who do not have prior knowledge of this area. Each form of corrosion covered in the book has a definition, description, mechanism, examples and preventative methods. Case histories of failure are cited for each form. End of chapter questions are accompanied by an online solutions manual.

* Comprehensively covers the principles of corrosion engineering, methods of corrosion protection and corrosion processes and control in selected engineering environments* Structured for corrosion science and engineering classes at senior undergraduate and graduate level, and is an ideal reference that readers will want to use in their professional work* Worked examples, extensive end of chapter exercises and accompanying online solutions and written by an expert from a key pretochemical university.

Nánar um bókina

Útgefandi
Elsevier S & T
ISBN
9780750659246
Print ISBN
9780750659246
Format
Page Fidelity (PDF)
Útgáfa
0
Höfundar
Ahmad, Zaki
Tungumál
English
Útgefið
09/2006
Prent takmörkun á líftíma
10

Kaflar

  • Principles of Corrosion Engineering and Corrosion Control
  • Front Cover
  • Front Cover
  • Title Page
  • Title Page
  • Copyright Page
  • Copyright Page
  • Table of Contents
  • Table of Contents
  • Preface
  • Preface
  • Acknowledgments
  • Acknowledgments
  • Chapter 1 - Introduction to Corrosion
  • Chapter 1 - Introduction to Corrosion
  • 1.1 Historical Background
  • 1.1 Historical Background
  • 1.2 Definitions
  • 1.2 Definitions
  • 1.3 Corrosive Environment
  • 1.3 Corrosive Environment
  • 1.4 Consequences of Corrosion
  • 1.4 Consequences of Corrosion
  • 1.5 Cost of Corrosion
  • 1.5 Cost of Corrosion
  • 1.6 Breakdown of Spending on Corrosion
  • 1.6 Breakdown of Spending on Corrosion
  • 1.7 Corrosion Science and Corrosion Engineering
  • 1.7 Corrosion Science and Corrosion Engineering
  • 1.8 Inter-disciplinary Nature of Corrosion
  • 1.8 Inter-disciplinary Nature of Corrosion
  • 1.9 Corrosion Education
  • 1.9 Corrosion Education
  • 1.10 Functional Aspects of Corrosion
  • 1.10 Functional Aspects of Corrosion
  • 1.11 Five Good Reasons to Study Corrosion
  • 1.11 Five Good Reasons to Study Corrosion
  • Questions
  • Questions
  • References
  • References
  • General References
  • General References
  • Websites
  • Websites
  • Software
  • Software
  • Chapter 2 - Basic Concepts in Corrosion
  • Chapter 2 - Basic Concepts in Corrosion
  • 2.1 Anodic and Cathodic Reactions
  • 2.1 Anodic and Cathodic Reactions
  • 2.2 Anodic Reactions Characteristics
  • 2.2 Anodic Reactions Characteristics
  • 2.3 Cathodic Reactions Characteristics
  • 2.3 Cathodic Reactions Characteristics
  • 2.4 Types of Corrosion Cells
  • 2.4 Types of Corrosion Cells
  • 2.5 Mechanism of Corrosion of Iron
  • 2.5 Mechanism of Corrosion of Iron
  • 2.6 Concept of Free Energy
  • 2.6 Concept of Free Energy
  • 2.7 Reversible Electrode Potential
  • 2.7 Reversible Electrode Potential
  • 2.8 Concentration Cell
  • 2.8 Concentration Cell
  • 2.9 Liquid Junction Potential
  • 2.9 Liquid Junction Potential
  • 2.10 Application of Free Energy to Corrosion Cells
  • 2.10 Application of Free Energy to Corrosion Cells
  • 2.11 Nernst Equation
  • 2.11 Nernst Equation
  • 2.12 Sign Convention
  • 2.12 Sign Convention
  • 2.13 Reference Electrodes
  • 2.13 Reference Electrodes
  • 2.14 Pourbaix Diagrams (Stability Diagrams)
  • 2.14 Pourbaix Diagrams (Stability Diagrams)
  • Questions
  • Questions
  • Suggested Books For Reading
  • Suggested Books For Reading
  • Keywords
  • Keywords
  • Chapter 3 - Corrosion Kinetics
  • Chapter 3 - Corrosion Kinetics
  • Faraday’s Laws of Electrolysis and its Application in Determining the Corrosion Rate
  • Faraday’s Laws of Electrolysis and its Application in Determining the Corrosion Rate
  • 3.1 The Laws
  • 3.1 The Laws
  • 3.2 Corrosion Kinetics
  • 3.2 Corrosion Kinetics
  • 3.3 Helmholtz Double Layer
  • 3.3 Helmholtz Double Layer
  • 3.4 Reverse Reaction (Cathodic Reaction)
  • 3.4 Reverse Reaction (Cathodic Reaction)
  • 3.5 Departure from Equilibrium [Activation Over-potential (n)]
  • 3.5 Departure from Equilibrium [Activation Over-potential (n)]
  • 3.6 Tafel Equation
  • 3.6 Tafel Equation
  • 3.7 Mixed Potential Theory and its Application
  • 3.7 Mixed Potential Theory and its Application
  • 3.8 Evans Diagrams
  • 3.8 Evans Diagrams
  • 3.9 Prediction of Corrosion Tendency on the Basis of Mixed Potential Theory
  • 3.9 Prediction of Corrosion Tendency on the Basis of Mixed Potential Theory
  • 3.10 Application of Mixed Potential Theory
  • 3.10 Application of Mixed Potential Theory
  • 3.11 Concentration Polarization
  • 3.11 Concentration Polarization
  • 3.12 Effect of Various Factors on Concentration Polarization
  • 3.12 Effect of Various Factors on Concentration Polarization
  • 3.13 Resistance Polarization (Ohmic Polarization)
  • 3.13 Resistance Polarization (Ohmic Polarization)
  • 3.14 Measurement of Corrosion
  • 3.14 Measurement of Corrosion
  • 3.15 Determination of Corrosion Rates by Electrochemical Measurements
  • 3.15 Determination of Corrosion Rates by Electrochemical Measurements
  • 3.16 Polarization Resistance (Linear Polarization)
  • 3.16 Polarization Resistance (Linear Polarization)
  • 3.17 Theoretical Background (Electrochemical Measurements)
  • 3.17 Theoretical Background (Electrochemical Measurements)
  • 3.18 Modern Developments
  • 3.18 Modern Developments
  • 3.19 Kinetics of Passivity
  • 3.19 Kinetics of Passivity
  • 3.20 Definition of Important Electrochemical Parameters for Active–Passive Metals
  • 3.20 Definition of Important Electrochemical Parameters for Active–Passive Metals
  • 3.21 Measured vs Actual Polarization Behavior of Active–Passive Metals
  • 3.21 Measured vs Actual Polarization Behavior of Active–Passive Metals
  • 3.22 Control of Passivity
  • 3.22 Control of Passivity
  • 3.23 Effect of Environment
  • 3.23 Effect of Environment
  • 3.24 Conversion Factors
  • 3.24 Conversion Factors
  • 3.25 Illustrative Problems
  • 3.25 Illustrative Problems
  • Questions
  • Questions
  • Suggested Reading
  • Suggested Reading
  • Keywords
  • Keywords
  • Chapter 4 - Types of Corrosion: Materials and Environments
  • Chapter 4 - Types of Corrosion: Materials and Environments
  • 4.1 Introduction
  • 4.1 Introduction
  • 4.2 Uniform Corrosion
  • 4.2 Uniform Corrosion
  • 4.3 Galvanic Corrosion
  • 4.3 Galvanic Corrosion
  • 4.4 Dezincification
  • 4.4 Dezincification
  • 4.5 Crevice Corrosion
  • 4.5 Crevice Corrosion
  • 4.6 Pitting Corrosion
  • 4.6 Pitting Corrosion
  • 4.7 Intergranular Corrosion
  • 4.7 Intergranular Corrosion
  • 4.8 Stress Corrosion Cracking and Hydrogen Damage
  • 4.8 Stress Corrosion Cracking and Hydrogen Damage
  • 4.9 Corrosion Fatigue
  • 4.9 Corrosion Fatigue
  • 4.10 Fretting Corrosion
  • 4.10 Fretting Corrosion
  • 4.11 Erosion–Corrosion and Cavitation Damage
  • 4.11 Erosion–Corrosion and Cavitation Damage
  • Questions
  • Questions
  • References
  • References
  • Keywords
  • Keywords
  • Chapter 5 - Cathodic Protection
  • Chapter 5 - Cathodic Protection
  • 5.1 Introduction
  • 5.1 Introduction
  • 5.2 Basis of Cathodic Protection
  • 5.2 Basis of Cathodic Protection
  • 5.3 Working of Cathodic Protection System
  • 5.3 Working of Cathodic Protection System
  • 5.4 Factors Leading to Corrosion of Underground Metallic Structures
  • 5.4 Factors Leading to Corrosion of Underground Metallic Structures
  • 5.5 Electrical Basis of Cathodic Protection
  • 5.5 Electrical Basis of Cathodic Protection
  • 5.6 Electrochemical Theory of Cathodic Protection
  • 5.6 Electrochemical Theory of Cathodic Protection
  • 5.7 Anodic Polarization
  • 5.7 Anodic Polarization
  • 5.8 Cathodic Protection Systems
  • 5.8 Cathodic Protection Systems
  • 5.9 Components of Galvanic Systems
  • 5.9 Components of Galvanic Systems
  • 5.10 Impressed Current System
  • 5.10 Impressed Current System
  • 5.11 Components of Impressed Current Systems and AC/DC Systems
  • 5.11 Components of Impressed Current Systems and AC/DC Systems
  • 5.12 Types of Ground-beds
  • 5.12 Types of Ground-beds
  • 5.13 Major Impressed Current Anodes
  • 5.13 Major Impressed Current Anodes
  • 5.14 Protection of Submarine Pipelines
  • 5.14 Protection of Submarine Pipelines
  • 5.15 Design Parameters in Cathodic Protection
  • 5.15 Design Parameters in Cathodic Protection
  • 5.16 Current Requirements
  • 5.16 Current Requirements
  • 5.17 Backfill
  • 5.17 Backfill
  • 5.18 Measurements in Cathodic Protection
  • 5.18 Measurements in Cathodic Protection
  • 5.19 Pipe-to-Soil Potential
  • 5.19 Pipe-to-Soil Potential
  • 5.20 Factors Affecting Pipe-to-Soil Potential
  • 5.20 Factors Affecting Pipe-to-Soil Potential
  • 5.21 Potential Survey
  • 5.21 Potential Survey
  • 5.22 Measurement of Current Flow
  • 5.22 Measurement of Current Flow
  • 5.23 Reference Electrode
  • 5.23 Reference Electrode
  • 5.24 Coating Resistance Tests
  • 5.24 Coating Resistance Tests
  • 5.25 Current Requirement Tests
  • 5.25 Current Requirement Tests
  • 5.26 Current Density Requirement for a Bare Structure
  • 5.26 Current Density Requirement for a Bare Structure
  • 5.27 Stray Current Corrosion
  • 5.27 Stray Current Corrosion
  • 5.28 Cathodic Protection Interferences
  • 5.28 Cathodic Protection Interferences
  • 5.29 Design Charts
  • 5.29 Design Charts
  • 5.30 Ground-bed Design
  • 5.30 Ground-bed Design
  • 5.31 Soil Resistivity and Pipe-to-Soil Potential Survey
  • 5.31 Soil Resistivity and Pipe-to-Soil Potential Survey
  • 5.32 Calculations in Cathodic Protection Design
  • 5.32 Calculations in Cathodic Protection Design
  • 5.33 Important Formulae in Cathodic Protection Calculations
  • 5.33 Important Formulae in Cathodic Protection Calculations
  • Questions
  • Questions
  • Recommended Literature on Cathodic Protection
  • Recommended Literature on Cathodic Protection
  • Keywords
  • Keywords
  • Chapter 6 - Corrosion Control by Inhibition
  • Chapter 6 - Corrosion Control by Inhibition
  • 6.1 Introduction
  • 6.1 Introduction
  • 6.2 Scope of Inhibitor
  • 6.2 Scope of Inhibitor
  • 6.3 Examples of Application of Inhibitors
  • 6.3 Examples of Application of Inhibitors
  • 6.4 Important Consideration in Selection of Inhibitors
  • 6.4 Important Consideration in Selection of Inhibitors
  • 6.5 Important Terms Related to Inhibitors
  • 6.5 Important Terms Related to Inhibitors
  • 6.6 Classification of Inhibitors
  • 6.6 Classification of Inhibitors
  • 6.7 Description of Inhibitors
  • 6.7 Description of Inhibitors
  • 6.8 Inorganic Anodic Inhibitors
  • 6.8 Inorganic Anodic Inhibitors
  • 6.9 Organic Inhibitors
  • 6.9 Organic Inhibitors
  • 6.10 Synergistic Inhibitors
  • 6.10 Synergistic Inhibitors
  • 6.11 Scavengers
  • 6.11 Scavengers
  • 6.12 Neutralizers
  • 6.12 Neutralizers
  • 6.13 Scale Inhibitors
  • 6.13 Scale Inhibitors
  • 6.14 Rebar Inhibitors
  • 6.14 Rebar Inhibitors
  • 6.15 Biocides
  • 6.15 Biocides
  • 6.16 Inhibitor Application Techniques
  • 6.16 Inhibitor Application Techniques
  • 6.17 Vapor Phase Inhibitors
  • 6.17 Vapor Phase Inhibitors
  • 6.18 Inhibitor Efficiency and Inhibitor Concentration
  • 6.18 Inhibitor Efficiency and Inhibitor Concentration
  • Questions
  • Questions
  • Suggested Reading
  • Suggested Reading
  • Keywords
  • Keywords
  • Chapter 7 - Coatings
  • Chapter 7 - Coatings
  • 7.1 Introduction
  • 7.1 Introduction
  • 7.2 Objectives
  • 7.2 Objectives
  • 7.3 Classification of Coatings
  • 7.3 Classification of Coatings
  • 7.4 Scope of Coatings
  • 7.4 Scope of Coatings
  • 7.5 Painting, Coating and Lining
  • 7.5 Painting, Coating and Lining
  • 7.6 Paint Coating System
  • 7.6 Paint Coating System
  • 7.7 Paint Coating Components
  • 7.7 Paint Coating Components
  • 7.8 Composition and Functions of Paint Coating Components
  • 7.8 Composition and Functions of Paint Coating Components
  • 7.9 Classification of Binders
  • 7.9 Classification of Binders
  • 7.10 Pigments
  • 7.10 Pigments
  • 7.11 Solvent
  • 7.11 Solvent
  • 7.12 Thinners (Diluents)
  • 7.12 Thinners (Diluents)
  • 7.13 Additives
  • 7.13 Additives
  • 7.14 Metallic Coatings
  • 7.14 Metallic Coatings
  • 7.15 Aluminum Coatings
  • 7.15 Aluminum Coatings
  • 7.16 Tin Coatings
  • 7.16 Tin Coatings
  • 7.17 Nickel Coatings
  • 7.17 Nickel Coatings
  • 7.18 Cadmium Coating
  • 7.18 Cadmium Coating
  • 7.19 Conversion Coatings
  • 7.19 Conversion Coatings
  • 7.20 Anodized Coatings
  • 7.20 Anodized Coatings
  • 7.21 Glass Flake Coatings
  • 7.21 Glass Flake Coatings
  • 7.22 Failure of Paints and Coatings
  • 7.22 Failure of Paints and Coatings
  • 7.23 Selection of Coatings
  • 7.23 Selection of Coatings
  • Questions
  • Questions
  • Suggested Literature for Reading
  • Suggested Literature for Reading
  • Keywords
  • Keywords
  • Chaper 8 - Corrosion Prevention by Design
  • Chaper 8 - Corrosion Prevention by Design
  • 8.1 Introduction
  • 8.1 Introduction
  • 8.2 Service Life of Equipment
  • 8.2 Service Life of Equipment
  • 8.3 Causes of Failures in the Context of Design
  • 8.3 Causes of Failures in the Context of Design
  • 8.4 Corrosive Environment
  • 8.4 Corrosive Environment
  • 8.5 Stages in the Design Process
  • 8.5 Stages in the Design Process
  • 8.6 Consideration of Areas Requiring Attention at Design Stage
  • 8.6 Consideration of Areas Requiring Attention at Design Stage
  • 8.7 Soldering and Threading
  • 8.7 Soldering and Threading
  • 8.8 Crevices
  • 8.8 Crevices
  • 8.9 Design to Control Corrosion in Water Distribution, Oil and Gas Pipelines
  • 8.9 Design to Control Corrosion in Water Distribution, Oil and Gas Pipelines
  • 8.10 Flowing Water Systems
  • 8.10 Flowing Water Systems
  • 8.11 Accessibility for Maintenance
  • 8.11 Accessibility for Maintenance
  • 8.12 Designs for Liquid Containers
  • 8.12 Designs for Liquid Containers
  • 8.13 Design in Packaging
  • 8.13 Design in Packaging
  • 8.14 Coating and Design
  • 8.14 Coating and Design
  • 8.15 Storage of Combat Vehicles
  • 8.15 Storage of Combat Vehicles
  • Questions
  • Questions
  • Suggested Reading
  • Suggested Reading
  • Commercial Technical Information Sources:
  • Commercial Technical Information Sources:
  • Internet Sources
  • Internet Sources
  • Keywords
  • Keywords
  • Chapter 9 - Selection of Materials for Corrosive Environment
  • Chapter 9 - Selection of Materials for Corrosive Environment
  • 9.1 Introduction
  • 9.1 Introduction
  • 9.2 Factors Affecting the Performance of Materials
  • 9.2 Factors Affecting the Performance of Materials
  • 9.3 The Material Selection Process
  • 9.3 The Material Selection Process
  • 9.4 Materials Classification
  • 9.4 Materials Classification
  • 9.5 Materials and Fluid Corrosivity
  • 9.5 Materials and Fluid Corrosivity
  • 9.6 Ranking of Performance of Materials
  • 9.6 Ranking of Performance of Materials
  • 9.7 Electrolyte Factors
  • 9.7 Electrolyte Factors
  • 9.8 Corrosion Indices
  • 9.8 Corrosion Indices
  • 9.9 Iso-Corrosion Charts
  • 9.9 Iso-Corrosion Charts
  • 9.10 Cost Effectiveness
  • 9.10 Cost Effectiveness
  • 9.11 Selection of Materials for Corrosive Environments
  • 9.11 Selection of Materials for Corrosive Environments
  • 9.12 Corrosion Characteristics of Carbon and Low Alloy Steels
  • 9.12 Corrosion Characteristics of Carbon and Low Alloy Steels
  • 9.13 Corrosion Rate of Carbon Steel in Seawater
  • 9.13 Corrosion Rate of Carbon Steel in Seawater
  • 9.14 Cast Irons
  • 9.14 Cast Irons
  • 9.15 Ductile and Cast Iron Pipes
  • 9.15 Ductile and Cast Iron Pipes
  • 9.16 Stainless Steels
  • 9.16 Stainless Steels
  • 9.17 Corrosion Behavior of Copper and Copper Alloys
  • 9.17 Corrosion Behavior of Copper and Copper Alloys
  • 9.18 Corrosion Resistance of Copper-base Alloys in Seawater
  • 9.18 Corrosion Resistance of Copper-base Alloys in Seawater
  • 9.19 Aluminum and its Alloys
  • 9.19 Aluminum and its Alloys
  • 9.20 Nickel and its Alloys
  • 9.20 Nickel and its Alloys
  • 9.21 Titanium and its Alloys
  • 9.21 Titanium and its Alloys
  • Questions
  • Questions
  • References
  • References
  • Suggested Reading
  • Suggested Reading
  • Keywords
  • Keywords
  • Chapter 10 - Atmospheric Corrosion
  • Chapter 10 - Atmospheric Corrosion
  • 10.1 Introduction
  • 10.1 Introduction
  • 10.2 Classification of Atmospheric Corrosion
  • 10.2 Classification of Atmospheric Corrosion
  • 10.3 Weather and Atmosphere
  • 10.3 Weather and Atmosphere
  • 10.4 Major Factors Affecting Atmospheric Corrosion
  • 10.4 Major Factors Affecting Atmospheric Corrosion
  • 10.5 Effect of Atmospheric Factors on Atmospheric Corrosion
  • 10.5 Effect of Atmospheric Factors on Atmospheric Corrosion
  • 10.6 Pollutants Chemistry and their Impact on Atmospheric Corrosion
  • 10.6 Pollutants Chemistry and their Impact on Atmospheric Corrosion
  • 10.7 Mechanism of Atmospheric Corrosion (Electrochemical Cycle)
  • 10.7 Mechanism of Atmospheric Corrosion (Electrochemical Cycle)
  • 10.8 Potential–pH Diagram (Fe–H2O)
  • 10.8 Potential–pH Diagram (Fe–H2O)
  • 10.9 Atmospheric Corrosion of Copper and its Alloys
  • 10.9 Atmospheric Corrosion of Copper and its Alloys
  • 10.10 Cracking of Copper Alloys
  • 10.10 Cracking of Copper Alloys
  • 10.11 Aluminum and its Alloys
  • 10.11 Aluminum and its Alloys
  • 10.12 Potential–pH Diagram (Al–H2O)
  • 10.12 Potential–pH Diagram (Al–H2O)
  • 10.13 Atmospheric Corrosion of Zinc
  • 10.13 Atmospheric Corrosion of Zinc
  • Questions
  • Questions
  • References
  • References
  • Suggested Literature
  • Suggested Literature
  • Keywords
  • Keywords
  • Chapter 11 - Boiler Corrosion
  • Chapter 11 - Boiler Corrosion
  • 11.1 Definition
  • 11.1 Definition
  • 11.2 Introduction
  • 11.2 Introduction
  • 11.3 Requirements of an Efficient Boiler
  • 11.3 Requirements of an Efficient Boiler
  • 11.4 Classification of Boilers
  • 11.4 Classification of Boilers
  • 11.5 Major Design
  • 11.5 Major Design
  • 11.6 Circulation
  • 11.6 Circulation
  • 11.7 Water Tube Circuit
  • 11.7 Water Tube Circuit
  • 11.8 Water Conditioning
  • 11.8 Water Conditioning
  • 11.9 Major Corrosion Problems in Boilers
  • 11.9 Major Corrosion Problems in Boilers
  • Questions
  • Questions
  • Suggested Reading
  • Suggested Reading
  • Keywords
  • Keywords
  • Chapter 12 - Concrete Corrosion
  • Chapter 12 - Concrete Corrosion
  • 12.1 Introduction
  • 12.1 Introduction
  • 12.2 Technical Terms
  • 12.2 Technical Terms
  • 12.3 Production, Types and Curing of Cement
  • 12.3 Production, Types and Curing of Cement
  • 12.4 Engineering Properties of Concrete
  • 12.4 Engineering Properties of Concrete
  • 12.5 Chemistry of Portland Cement
  • 12.5 Chemistry of Portland Cement
  • 12.6 Concrete Chemistry, Hydration Reactions
  • 12.6 Concrete Chemistry, Hydration Reactions
  • 12.7 Durability and Deterioration of Cement
  • 12.7 Durability and Deterioration of Cement
  • 12.8 Corrosion of Reinforcement (Rebar Corrosion)
  • 12.8 Corrosion of Reinforcement (Rebar Corrosion)
  • 12.9 Factors Affecting Corrosion
  • 12.9 Factors Affecting Corrosion
  • 12.10 Construction Practice
  • 12.10 Construction Practice
  • 12.11 Summary of Principles Involved in Corrosion Reinforcement
  • 12.11 Summary of Principles Involved in Corrosion Reinforcement
  • 12.12 Methods of Corrosion Control of Reinforcing Steel
  • 12.12 Methods of Corrosion Control of Reinforcing Steel
  • 12.13 Corrosion Protection of Reinforcement
  • 12.13 Corrosion Protection of Reinforcement
  • 12.14 Concrete Repairs and Rehabilitation
  • 12.14 Concrete Repairs and Rehabilitation
  • 12.15 Electrochemical Methods of Evaluation of Reinforcement Corrosion
  • 12.15 Electrochemical Methods of Evaluation of Reinforcement Corrosion
  • Questions
  • Questions
  • Suggested Reading
  • Suggested Reading
  • Keywords
  • Keywords
  • Index
  • Index