1 The marine corrosion of steel 1.1 The corrosion of steel in seawater 1.1.1 How much do we know? 1.1.2 Why does steel corrode? 1.1.3 How does corrosion happen? 1.1.3.1 A definition 1.1.3.2 A school corrosion experiment 1.1.3.3 Some electrochemistry 1.1.3.4 Aerated seawater 1.1.3.5 Deaerated seawater 1.1.4 What doesn’t the basic science tell us? 1.2 Corrosion rates 1.2.1 Laboratory tests 1.2.1.1 Weight loss tests 1.2.1.2 The importance of the “Blank” weight loss measurements 1.2.1.3 Limitations of laboratory testing 1.2.2 Seawater immersion tests 1.2.2.1 Effect of temperature 1.2.2.2 Effect of water depth 1.2.3 Information from existing structures 1.2.3.1 Shipwrecks 1.2.3.2 Harbour piling 1.2.3.3 Seabed burial 1.2.3.4 Intertidal and splash zones 1.2.4 How do we use corrosion rate information? 1.3 The microbiological dimension 1.3.1 Clean seawater 1.3.2 Slightly polluted seawater 1.3.3 Heavily polluted seawater and sediments 1.3.3.1 Sulfate reducing micro-organisms 1.3.3.2 MIC mechanisms 1.3.3.3 MIC morphology and rates 1.4 The forms of corrosion 1.4.1 General corrosion 1.4.2 Galvanic corrosion 1.4.2.1 Classic example 1.4.2.2 Why does it happen? 1.4.2.3 What are the risk factors? 1.4.3 Pitting 1.4.4 Crevice corrosion 1.4.5 Fatigue and corrosion fatigue 1.4.6 Other forms of corrosion References 2 Cathodic protection basics 2.1 A theoretical experiment 2.1.1 Removing electrons 2.1.2 Adding electrons 2.2 A simple model 2.2.1 How does cathodic protection work? 2.2.2 Implementation 2.3 The two views of current flow 2.4 Potential 2.4.1 What do we mean by “Potential”? 2.4.2 How do we measure the potential? 2.4.2.1 The problem 2.4.2.2 The solution 2.4.3 Potential measurement 2.4.4 What is the potential needed for protection? 2.5 Current 2.5.1 Bare steel 2.5.2 Coated steel 2.6 Power sources for CP 2.6.1 Davy’s work 2.6.2 Sacrificial anodes 2.6.3 Impressed current 2.6.4 Sacrificial anodes versus impressed current 2.6.4.1 Sacrificial anodes: advantages 2.6.4.2 Sacrificial anodes: disadvantages 2.6.4.3 Impressed current: advantages 2.6.4.4 Impressed current: disadvantages 2.6.4.5 Selecting between sacrificial anodes and ICCP 2.6.5 Hybrid systems 2.7 What does CP achieve? 2.8 Where do we go from here? References 3 Designing according to the codes 3.1 Do we need CP? 3.2 Who does the CP design? 3.2.1 In the land of the blind 3.2.2 Who is the “Expert”? 3.2.3 Certification of competence 3.2.3.1 Is it needed? 3.2.3.2 NACE 3.2.3.3 ISO 15257 3.3 The basis of design 3.3.1 System life 3.3.2 Environmental parameters 3.3.3 Coating 3.3.4 Sacrificial versus impressed current? 3.3.5 Which codes? 3.3.6 Cathode parameters 3.3.6.1 Protection potential 3.3.6.2 The protection current density 3.3.7 Anode parameters 3.3.7.1 General 3.3.7.2 Operating potential 3.3.7.3 Charge availability 3.3.7.4 Utilisation factor 3.4 The design process 3.4.1 Overview 3.4.2 Calculating the cathodic current demand 3.4.2.1 Interfaces 3.4.2.2 Uncoated zones 3.4.2.3 Coated zones 3.4.2.4 Additional current demands 3.4.3 Minimum anode mass 3.4.4 Anode output 3.4.4.1 General 3.4.4.2 Estimating anode resistance 3.4.5 Anode optimisation 3.5 Example calculations 3.5.1 Case 1 – uncoated structure 3.5.1.1 Life 3.5.1.2 Structure and area 3.5.1.3 Current densities 3.5.1.4 Current demand 3.5.1.5 Minimum anode weight 3.5.1.6 Anode selection 3.5.2 Case 2 – coated structure 3.5.2.1 Scope of CP design 3.5.2.2 Design parameters 3.5.2.3 Calculations 3.6 Anode locations 3.7 Anode manufacture and installation 3.8 Limitations of the codes 3.8.1 Can we use other codes? 3.8.2 What if the codes gets it wrong? 3.8.3 Where the codes are silent 4 Thermodynamics 4.1 Introduction 4.1.1 In the chemistry laboratory 4.1.2 In the real world 4.2 The science of thermodynamics 4.2.1 Background 4.2.2 Heat and mechanical energy 4.2.2.1 Parameters 4.2.2.2 The laws 4.2.3 Chemical thermodynamics 4.2.4 Application to corrosion 4.3 Electrode potential 4.3.1 The reversible electrode 4.3.2 The Nernst equation 4.4 E – pH diagrams 4.4.1 The hydrogen electrode 4.4.2 The oxygen electrode 4.4.3 The metal and its corrosion products 4.4.4 The metal-water system 4.4.4.1 Zinc 4.4.4.2 Copper 4.4.4.3 Gold 4.4.4.4 Iron 4.4.5 Limitations of E–pH diagrams 4.4.5.1 Pure metals 4.4.5.2 Pure water 4.4.5.3 Thermodynamic basis 4.5 CP and thermodynamics 4.5.1 Immunity 4.5.2 Passivity References 5 Electrode kinetics 5.1 Reversible electrodes 5.2 Electrochemical experiments 5.2.1 Some terminology 5.2.1.1 Electrodes, electrolytes, anodes, cathodes and half-cells 5.2.1.2 Potential 5.2.1.3 Polarisation 5.2.1.4 Overpotential and overvoltage 5.2.2 Galvanostatic polarisation 5.2.3 Potentiostatic polarisation 5.2.3.1 The potentiostat 5.2.3.2 Plotting polarisation curves 5.3 Obtaining polarisation curves 5.3.1 The experiments 5.3.2 Tafel behaviour 5.4 Analysing polarisation curves 5.4.1 Fitting theory to data 5.4.2 The concept of activation control 5.4.2.1 Activation energy 5.4.3 The Butler-Volmer equation 5.4.4 Tafel extrapolation 5.4.4.1 Exchange current density 5.4.5 Polarisation curves and polarisation diagrams 5.4.6 Departures from Tafel behaviour 5.5 Non-reversible electrodes 5.5.1 The mixed potential electrode 5.6 Corrosion in seawater 5.6.1 Oxygen-free seawater 5.6.2 Aerated seawater 5.7 Electrode kinetics and CP 5.7.1 General 5.7.2 The theory 5.7.3 Implications for CP References 6 Protection potential – carbon steel 6.1 Introduction 6.2 What does CP need to achieve? 6.3 What do the codes say? 6.3.1 Aerated seawater 6.3.2 Anaerobic environments 6.3.3 Elevated temperature 6.4 Aerobic environments: The –800 mV criterion 6.4.1 Theoretical considerations 6.4.1.1 Thermodynamics: immunity 6.4.1.2 Thermodynamics: passivity 6.4.1.3 Electrode kinetics 6.4.2 Laboratory testing 6.4.2.1 The predictions 6.4.2.2 The results 6.4.2.3 Evaluation of the evidence 6.4.3 Practical experience 6.4.4 Implications 6.5 Anaerobic environments: The –900 mV criterion 6.5.1 The codes 6.5.1.1 British standards institution 6.5.1.2 European and ISO standards 6.5.1.3 NACE 6.5.1.4 DNV 6.5.2 Theoretical considerations 6.5.2.1 Thermodynamics 6.5.2.2 Electrode kinetics 6.5.3 Laboratory investigations 6.5.4 Field test data 6.5.4.1 Onshore pipelines 6.5.4.2 Offshore pipelines 6.6 The effect of temperature 6.6.1 What the codes say 6.6.2 The theory 6.6.3 Laboratory testing 6.6.4 Field experience 6.7 Excessively negative potentials 6.8 Optimum potentials 6.9 Potential distribution References 7 Current and polarisation 7.1 What we need to know 7.2 What the codes advise 7.2.1 Current densities for seawater (offshore) 7.2.2 Current densities for seawater (near-shore) 7.2.3 Current densities for seabed burial 7.3 The problem with the codes 7.4 Laboratory testing: clean steel 7.5 Calcareous deposits 7.5.1 The chemistry 7.5.2 Importance 7.5.2.1 Benefits 7.5.2.2 Possible drawbacks 7.5.3 Laboratory investigations 7.5.3.1 Deposit growth 7.5.3.2 Deposit thickness 7.5.3.3 Factors affecting deposit growth 7.6 Site testing 7.6.1 The limitations of the laboratory 7.6.1.1 The microbiological dimension 7.6.1.2 Modes of polarisation 7.6.2 In-situ measurements 7.6.2.1 Monitoring of existing structures 7.7 Site experience 7.7.1 South China Sea 7.7.2 Middle East – operator 1 7.7.2.1 The requirement 7.7.2.2 Approach adopted 7.7.2.3 Example of analysis – structure A 7.7.2.4 Results of analyses – structures B - D 7.7.2.5 Application to other structures 7.7.3 Middle East – operator 2 7.8 Deeper waters 7.8.1 Codes 7.8.2 The theory 7.8.3 Laboratory testing 7.8.4 Site testing 7.8.5 The future 7.9 S-curves 7.10 The slope parameter 7.10.1 What is it? 7.10.2 Slope parameter versus “cookbook” 7.10.2.1 Two perspectives 7.10.2.2 SP0176 7.11 The rate of polarisation References 8 Corrosion resistant alloys 8.1 Why consider CRAs? 8.2 Passivity 8.2.1 What do we mean by passivity? 8.2.2 Thermodynamics 8.2.3 Electrode kinetics 8.2.4 Passivity breakdown 8.2.4.1 Nature of the passive film 8.2.4.2 Pitting 8.2.4.3 Crevice corrosion 8.2.4.4 Stress corrosion cracking 8.3 Stainless steels 8.3.1 Corrosion resistance 8.3.1.1 Passivity 8.3.1.2 Passivity breakdown 8.3.2 Designations 8.3.3 Grades used offshore 8.3.3.1 Allotropes 8.3.3.2 Ferritic stainless steels 8.3.3.3 Austenitic stainless steels 8.3.3.4 Duplex stainless steels 8.3.3.5 Superduplex 8.3.3.6 Martensitic and supermartensitic 8.3.3.7 Some stainless steels used subsea 8.4 High nickel alloys 8.5 Copper alloys 8.6 Aluminium alloys 8.6.1 Alloy types 8.6.2 Corrosion threats and mitigation 8.7 CP of corrosion resistant alloys 8.7.1 Protection potential 8.7.1.1 Theory and practice 8.7.1.2 The codes 8.7.2 Protection current densities 8.8 Summary References 9 Underwater coatings 9.1 Introduction 9.2 Some polymer basics 9.2.1 Polymerisation 9.2.2 Linear polymers 9.2.2.1 Polymers and copolymers 9.2.2.2 Flexibility 9.2.3 3-Dimensonal polymers 9.2.3.1 Cross-linking 9.2.3.2 Epoxies 9.2.4 Elastomers 9.3 Coatings and CP 9.3.1 Do coatings benefit CP? 9.3.2 Does CP benefit coatings? 9.3.3 Coating systems 9.4 Surface preparation 9.5 Coating system selection 9.5.1 Fixed steel structures 9.5.1.1 Early paints 9.5.1.2 Current systems 9.5.2 Ships and floating installations 9.5.2.1 External hulls 9.5.2.2 Ballast spaces 9.5.3 Submarine pipelines 9.5.3.1 Factory applied coating systems 9.5.3.2 Field joint coatings 9.6 Cathodic disbondment 9.6.1 Characteristics 9.6.2 Corrosion threats under disbonded coatings 9.6.2.1 Onshore pipelines 9.6.2.2 Submarine pipelines 9.6.3 Cathodic disbondment testing 9.7 Coating breakdown predictions 9.7.1 Coatings for fixed structures 9.7.2 Ships’ coatings 9.7.3 Pipeline coatings References 10 Sacrificial anodes 10.1 What properties do we need? 10.1.1 Potential 10.1.2 Current 10.1.2.1 Instantaneous output 10.1.2.2 Capacity, consumption rate and efficiency 10.2 Zinc alloys 10.2.1 Background 10.2.2 Present day alloys 10.2.3 Limitations 10.2.3.1 Elevated temperature 10.3 Magnesium alloys 10.4 Aluminium alloys 10.4.1 The benefits 10.4.2 Alloy research 10.4.3 Alloy development 10.4.4 Al-Zn-Sn and Al-Zn-Hg alloys 10.4.5 Indium-containing anodes 10.4.5.1 Al-Zn-In 10.4.5.2 Al-Zn-Mg-In 10.4.6 Al-Zn-Ga and Al-Ga 10.4.7 The future 10.4.7.1 The toxicity of indium? 10.4.7.2 Al-Zn and Al-Zn-Mg 10.5 Non-standard anodes 10.5.1 Limiting the polarisation of the cathode 10.5.1.1 The need 10.5.1.2 Alloy composition 10.5.1.3 Passive electronic components: resistors 10.5.1.4 Passive electronic components: diodes 10.5.2 Anodes for rapid polarisation 10.5.2.1 Motivation 10.5.2.2 Hybrid systems 10.5.2.3 Dual anodes 10.5.2.4 Shaped anodes 10.6 Future developments 10.7 Electrochemical testing 10.7.1 Parameters measured 10.7.1.1 Potential 10.7.1.2 Capacity 10.7.2 Testing modes and objectives 10.7.2.1 Screening tests 10.7.2.2 Performance tests 10.7.2.3 Deep water 10.7.2.4 Elevated temperature 10.7.2.5 Biofouling 10.7.2.6 Polluted environments 10.7.2.7 Seabed sediments 10.7.2.8 Estuarine waters 10.7.2.9 Pre-qualification and production testing 10.7.3 Testing configurations 10.7.3.1 Constant current tests 10.7.3.2 Constant potential tests 10.7.3.3 Free-running tests 10.8 Anode resistance 10.8.1 Relevance to design 10.8.2 How is R[sub(a)] calculated? 10.8.3 What the codes advise 10.8.3.1 Slender stand-off anodes 10.8.3.2 Shorter stand-off anodes 10.8.3.3 Long flush-mounted anodes 10.8.3.4 Short flush-mounted anodes and bracelets 10.8.4 Validation of resistance formulae 10.8.4.1 In-service testing 10.8.5 Anode clustering 10.8.5.1 The problem 10.8.5.2 The consequences 10.8.5.3 The solution 10.9 Anode design and manufacture 10.9.1 Who does the design? 10.9.2 The anode specification 10.9.3 Anode inserts 10.9.3.1 Insert configuration 10.9.3.2 Insert surface preparation 10.9.4 The casting process 10.10 Quality control 10.10.1 Sampling 10.10.2 Dimensional and weight tolerance 10.10.3 Casting quality 10.10.3.1 Non-destructive examination 10.10.3.2 Destructive examination 10.11 Anode installation References 11 Impressed current systems 11.1 The electrode reactions 11.1.1 Cathodic reactions 11.1.2 Anodic reactions 11.1.2.1 Consumable anodes 11.1.2.2 “Non-consumable” anodes 11.2 ICCP anodes 11.2.1 Requirements 11.2.2 Onshore origins 11.2.3 Anode development 11.2.3.1 Early ICCP anode alloys 11.2.3.2 Mixed metal oxide (“MMO”) anodes 11.2.4 Anode configuration and resistance 11.2.5 Anode shields 11.2.5.1 Why do we need anode shields? 11.2.5.2 Anode shield size 11.3 Basic design 11.3.1 Cathodic current demand 11.3.2 System output calculations 11.3.2.1 Current 11.3.2.2 Voltage 11.3.3 Design calculation process 11.3.4 Anode locations 11.4 Power supplies 11.4.1 What we need to know 11.4.2 The basics 11.4.3 Manual control 11.4.4 Automatic control 11.5 Control inputs 11.5.1 Ag|AgCl|seawater 11.5.2 Zinc reference electrodes 11.5.3 Dual references 11.6 Cables 11.6.1 Conductors 11.6.2 Insulation 11.6.2.1 What is required? 11.6.2.2 What do the codes say? 11.6.2.3 Candidate materials 11.6.2.4 Current practice 11.6.2.5 Mechanical protection of cables 11.6.2.6 Cables connections 11.7 Stray current interference 11.8 ICCP system safety 11.8.1 Transformer-rectifiers 11.8.2 Diver safety References 12 The effect of CP on mechanical properties 12.1 Introduction 12.1.1 Outline 12.1.2 Some basics 12.2 Materials of interest 12.2.1 Structures 12.2.1.1 Medium-strength steels (SMSY < 550 MPa) 12.2.1.2 Higher strength steels (>550 MPa) 12.2.2 Pipelines 12.2.3 Equipment 12.3 Fatigue 12.3.1 What is it? 12.3.2 S-N testing 12.3.2.1 Plain specimens 12.3.2.2 Notched specimens 12.3.3 Fracture mechanics 12.3.3.1 Basics 12.3.3.2 The Paris law 12.3.4 Reliability of testing 12.4 Corrosion fatigue 12.4.1 Discovery 12.4.2 Characterisation 12.4.3 Theories 12.4.4 Stress ratio (R-value) 12.5 The effect of CP 12.5.1 Information from S-N testing 12.5.2 The interaction with CP 12.5.3 The fracture mechanics perspective 12.5.4 S-N testing versus crack growth rate data 12.6 The codes 12.6.1 Code development 12.6.1.1 Laboratory testing 12.6.1.2 The Cognac fatigue “experiment” 12.6.1.3 Further testing 12.6.2 Using the codes 12.6.2.1 Overview 12.6.2.2 Elements and fatigue loadings 12.6.2.3 Select the S-N curve 12.6.2.4 Assessment 12.6.3 The role of the CP engineer 12.7 Hydrogen embrittlement 12.7.1 The problem 12.7.2 What is the source of atomic hydrogen? 12.7.3 What does the atomic hydrogen do? 12.7.3.1 A simplistic view 12.7.3.2 A less simplistic view 12.8 Low- and medium-strength carbon steels 12.9 High-strength low-alloy steels 12.9.1 General 12.9.2 Fasteners 12.10 Corrosion-resistant alloys 12.10.1 Stainless steels 12.10.1.1 Classes 12.10.1.2 Austenitic stainless steels 12.10.1.3 Ferritic stainless steels 12.10.1.4 Duplex stainless steels 12.10.1.5 Martensitic stainless steels 12.10.2 Nickel alloys 12.10.2.1 Solid solution alloys 12.10.2.2 Precipitation-hardened alloys 12.10.3 Copper alloys 12.10.4 Titanium References 13 Fixed steel structures 13.1 Structures for hydrocarbon production 13.1.1 Early sacrificial anode systems 13.1.2 Early ICCP systems 13.1.3 Deeper waters 13.1.4 To coat or not? 13.1.5 Weight saving 13.1.5.1 Same problem – different solutions 13.1.6 Into the sunset? 13.2 Offshore wind farms 13.2.1 Development 13.2.2 Foundation options 13.2.3 Monopiles 13.2.3.1 Is external CP needed? 13.2.3.2 CP design – codes 13.2.3.3 CP design – challenges 13.3 Harbour structures 13.3.1 Historical background 13.3.2 Current densities 13.3.3 Sacrificial anodes 13.3.4 ICCP systems 13.3.4.1 Seabed anodes 13.3.4.2 Pile-mounted anodes 13.3.4.3 Conventional “onshore” anodes 13.3.4.4 Harbour structures versus platforms 13.3.4.5 How not to do it 13.4 Allowances for current drainage 13.4.1 Simple rules 13.4.2 Well casings 13.4.3 Other buried steelwork 13.4.4 Concrete reinforcement 13.5 CP retrofits 13.5.1 What is a “retrofit”? 13.5.2 Information on retrofits 13.5.3 Do we need to retrofit? 13.5.3.1 Design mishaps 13.5.3.2 Life extension 13.5.3.3 Unnecessary retrofits 13.5.4 Retrofit requirements 13.5.5 Current demand 13.5.6 Retrofit strategies 13.5.6.1 ICCP vs sacrificial anodes 13.5.7 Retrofit implementation 13.5.7.1 Sacrificial anodes 13.5.7.2 Impressed current 13.5.7.3 Connections 13.6 The future References 14 Submarine pipelines 14.1 Early submarine pipelines 14.2 Pipeline types 14.2.1 Flowlines 14.2.1.1 General 14.2.1.2 Production flowlines 14.2.1.3 Injection and gas lift flowlines 14.2.2 Trunk and service lines 14.2.2.1 Export lines 14.2.2.2 Sea lines 14.2.2.3 Service pipelines 14.2.3 Risers 14.3 Code-based CP design 14.3.1 Methodology 14.3.2 Example calculation 14.3.2.1 Pipeline condition 14.3.2.2 Design factor 14.3.2.3 Current demand 14.3.2.4 Anode design – mean current 14.3.2.5 Anode design – final current 14.4 Anode spacing 14.4.1 Early practice 14.4.2 Extending the spacing 14.4.2.1 The crucial resistance 14.4.2.2 A worst-case approach 14.4.2.3 Norsok method 14.4.2.4 Potential attenuation 14.4.2.5 Recommendation 14.5 Electrical isolation: offshore 14.5.1 Early offshore practice 14.5.2 Recent codes 14.5.3 Current drain 14.5.4 Stray current interference 14.6 Pipeline landfalls 14.6.1 Some problems 14.6.2 Isolation 14.7 Hot pipelines and risers 14.7.1 Ekofisk alpha 14.7.2 CP criteria 14.7.2.1 Protection potential 14.7.2.2 Protection current density 14.7.2.3 Coating breakdown 14.7.2.4 Bracelet anode performance 14.7.3 Flow assurance 14.7.3.1 Keeping the product flowing 14.7.3.2 Insulation 14.7.3.3 Direct electrical heating 14.7.4 Seawater cooling 14.7.4.1 Pipelines 14.7.4.2 Subsea coolers 14.8 Pipeline retrofits 14.8.1 Why retrofit? 14.8.1.1 Something went wrong 14.8.1.2 Life extension 14.8.2 When to retrofit? 14.8.2.1 What you cannot see… 14.8.2.2 Lost in the Iron Mountain® 14.8.3 Retrofit strategies 14.8.3.1 Basic cases 14.8.3.2 Connecting anode sleds to pipelines 14.9 CRA and flexible pipelines References 15 Ships and floating structures 15.1 Ships’ hulls 15.1.1 Early days 15.1.2 CP design 15.1.2.1 Differences between fixed structures and ships 15.1.2.2 Current demand 15.1.3 Propellers and shafts 15.1.3.1 Materials 15.1.3.2 Bonding 15.1.3.3 Current demand 15.1.4 Rudders 15.1.4.1 Bonding 15.1.4.2 A cautionary tale 15.1.5 Sacrificial anode systems 15.1.5.1 Development 15.1.5.2 Design 15.1.6 Impressed current 15.1.6.1 Early days 15.1.6.2 Present day systems 15.1.6.3 Fitting-out 15.1.6.4 Laying-up 15.1.6.5 Alongside berths 15.1.7 Non-ferrous hulls 15.1.7.1 Aluminium 15.1.7.2 Copper and Cu-Ni hulls 15.1.7.3 Pleasure craft 15.2 Floating installations 15.2.1 Drill ships and semi-submersibles 15.2.1.1 Some history 15.2.1.2 A recent example 15.2.2 FPSOs 15.2.2.1 Hulls 15.2.3 Tension leg platforms 15.2.4 Moorings 15.2.4.1 Tethers and tendons 15.2.4.2 Chains 15.3 Jack-up rigs References 16 Internal CP 16.1 “Fully sealed” systems 16.1.1 Corrosion threats 16.1.1.1 Corrosion by dissolved oxygen 16.1.1.2 What really happens to the oxygen? 16.1.1.3 What happens then? 16.1.1.4 What about MIC? 16.1.2 Is CP needed? 16.2 Leaking systems 16.2.1 Monopile foundations 16.2.2 Corrosion implications 16.2.3 Internal CP of wind turbine foundations 16.2.3.1 A touch of schadenfreude? 16.2.3.2 Code guidance 16.2.3.3 Lessons learned 16.2.3.4 But is CP needed? 16.2.4 Water ballast tanks 16.2.4.1 Ballast water and its management 16.2.4.2 Corrosion management 16.3 Steel structures containing aerated seawater 16.3.1 Sea chests 16.3.2 Seawater intakes 16.3.2.1 Multi-metal systems 16.3.2.2 Shore-side seawater intakes 16.3.2.3 Seawater lift caissons 16.4 Seawater piping systems 16.4.1 Unlined carbon steel 16.4.2 Lined carbon steel 16.4.3 Corrosion resistant alloys 16.4.3.1 Materials for handling seawater 16.4.3.2 CRAs: selection and vulnerabilities 16.4.3.3 Internal CP of stainless steel pipework 16.4.3.4 Resistor-controlled cathodic protection 16.5 Heat exchangers 16.5.1 A corrosion machine 16.5.2 Early CP systems 16.5.3 Seawater exchangers References 17 Modelling 17.1 What is a model? 17.2 Physical modelling 17.2.1 Full scale 17.2.2 Reduced scale – reduced conductivity 17.2.3 Reduced scale – full conductivity 17.3 Early computer applications 17.4 Computer modelling – the basics 17.4.1 The convenience of the computer 17.4.2 Potential and current distribution 17.4.3 The Laplace equation 17.4.4 Solving Laplace 17.4.4.1 The need for a number-cruncher 17.4.4.2 Defining the space 17.4.4.3 Defining the boundaries 17.4.4.4 The finite element method (FEM) 17.4.4.5 The boundary element method (BEM) 17.4.4.6 FEM versus BEM 17.4.4.7 Other software approaches 17.5 Computer modelling – applications 17.5.1 Early days 17.5.2 Moore’s law 17.5.3 When modelling gets it wrong 17.5.4 The boundary conditions 17.5.4.1 The anodes 17.5.4.2 The cathode 17.5.5 What can computer modelling tell us? 17.5.5.1 Reasons to be careful 17.5.5.2 Uncoated steel structures 17.5.5.3 Coated structures 17.5.5.4 Sacrificial anodes 17.5.5.5 Internal spaces and complex geometries 17.6 Going forward References 18 CP system management 18.1 Surveying, inspection and monitoring 18.1.1 The need for measurement 18.2 Measuring the potential 18.2.1 The story so far 18.2.2 Alternative references 18.2.2.1 Standard hydrogen electrode 18.2.2.2 Saturated calomel half-cell 18.2.2.3 Silver chloride electrode 18.2.2.4 Silver chloride (0.5 M KCl) electrode 18.2.2.5 Copper sulfate electrode (CSE) 18.2.2.6 Zinc 18.2.3 Errors in potential measurement 18.2.3.1 Operatives 18.2.3.2 Equipment and operatives 18.2.3.3 Temperature 18.2.3.4 Liquid-junction or diffusion potential 18.2.3.5 The IR problem 18.2.3.6 IR-error mitigation 18.2.3.7 The effect of seawater flow 18.2.4 Potential surveys – structures 18.2.4.1 Dip-cell surveys 18.2.4.2 Diver and ROV surveys 18.2.4.3 Survey frequency – sacrificial systems 18.2.5 Potential surveys - pipelines 18.2.5.1 Background 18.2.5.2 Trailing wire surveys 18.2.5.3 ROV surveys 18.2.5.4 Beach crossings and shore approaches 18.2.5.5 Telluric currents 18.2.5.6 Survey frequency 18.2.6 Fixed potential monitoring 18.2.6.1 Structures 18.2.6.2 Pipelines 18.3 Current measurement 18.3.1 ICCP systems 18.3.2 Sacrificial systems 18.3.2.1 Current clamp meters 18.3.2.2 Monitored anodes 18.4 Current density measurement 18.4.1 Fixed monitoring 18.4.1.1 Current density plates and probes 18.4.1.2 Field gradients 18.4.2 Surveys 18.4.2.1 Pipelines 18.4.2.2 Structures 18.5 Interaction 18.5.1 Sacrificial systems 18.5.2 ICCP systems 18.5.2.1 General 18.5.2.2 Harbours 18.5.2.3 Ships and boats