CATHODIC PROTECTION CATHODIC PROTECTION OF REINFORCED OF REINFORCED
CONCRETE STRUCTURESCONCRETE STRUCTURES
Center for Engineering ResearchResearch Institute, KFUPM
REINFORCEMENT CORROSION
Passivity• High pH leading to formation of passive layer• Chemical binding of chlorides• Dense and relatively impermeable structure of
concrete Depassivation
• Chloride ingress• Carbonation
Chloride-induced Reinforcement Corrosion
• Due to external chlorides
• Due to chloride contamination from the mix constituents
Chloride Limits
ACI 318 (0.1 – 0.15%; water soluble) ACI 224 (0.2%; acid soluble) BS 8110 (0.4%; total)
CARBONATION
• Ca(OH)2 + CO2 CaCO3 + H2O
• Reduction in pH (up to 8.5)
FACTORS AFFECTING CARBONATION
• Concrete quality• Concrete cover• Moisture condition• Temperature• Chloride-sulfate contamination
PREVENTION THROUGH DESIGN
Cover over reinforcing steel Water-cement ratio Cement content Cement type
PROTECTION OF STEEL
• Metallic coatings• Non-Metallic coatings• Inhibitors• Electrochemical protection
• Cathodic protection• Chloride removal• Realkalization
CATHODIC PROTECTION
Sacrificial anode Impressed current
Corrosion cell in concrete
Corrosion cell in a repaired concrete
Corrosion cell after CP
Comparison of CP systems (Merits)Impressed current system Sacrificial anode system
Longer anode life Inherently simple
Current can be controlled No monitoring and maintenanceNo requirement for electrical isolationRisk of hydrogen embrittlement on high strength steel is minimalSaw cutting and concrete encapsulation for anodes is not required
Comparison of CP systems (Demerits)
Impressed current system Sacrificial anode system
Requires monitoring and maintenance
Short anode life
Electrical isolation required between anode and steel
Anode current delivery is dependent on anode chemistry and surrounding environment
Conduit and wiring required Current cannot be adjusted or controlled
Sacrifical Anode System
Anode material• Alloys of Mg, Al, Zn
Anode shapes• Zinc foil (Zinc hydrogel)• Metallized zinc (thermal spray)• Jackets• Zinc capsules or discs encased in mortar
Corroded pile in sea
Pier with SACP
Discrete anode for repaired structure
Discrete anode
Placement of discrete anodes
Mesh anode
Placement of anodes
Specimens with discrete anodes
Specimens with jacket anodes
Specimens with mesh anodes
Specimens and measurement boxes
Sacrificial Anode
Impressed Current CP
ICCP Anodes Conductive coating
• Sparyed, rolled or brushed over a nibium wire (300 um thickness) Arc sprayed zinc
• Spraying metal on concrete with compressed air (300 -400 um) Titanium anode mesh Titanium anode mesh jackets Titanium ribbon Discrete anodes Thermally sprayed titanium
Current requirements
Atmospheric zones• 5 to 15 mA/m2
Under water• 0.2 to 2 mA/m2
Decreases with time
ICCP – Power System
Transformers/Rectifiers Constant voltage/current system Stable over long period of time Solar- or wind-powered units are also used
ICCP - Connections
Durable and corrosion resistant Low-voltage wires Diameter for minimum voltage drop Color coded Multiple connections and cables
recommended
ICCP Monitoring System
Potential measurements Embeddable reference electrodes
• Ag/AgCl• Mn/MnO2
Anode mesh fixed on concrete
Conductor bar welded to anode mesh
Negative connection to the steel bar
Connection of embedded items
Reference electrode
Installing overlay
Finishing operation
Anode and steel cage
Details of anode
Connection of anode ribbon to the titanium conductor bar
Steel connection
Placement of electrodes and connections to the steel bar
Pouring of concrete
Chloride Removal
CE - Details Anode
• Titanium or steel mesh Electrolyte
• Water or saturated calcium hydroxide solution Current
• 1 to 2 A/m2
Time • 6 to 10 weeks
Not advisable for prestressed concrete Impressed/sacrificial anode cathodic protection systems
may be used Side effects – Alkali-silica reaction
Re-Alkalization
Realkalization - details Anode
• Steel or titanium mesh Electrolyte
• Sodium carbonate solution (1 mole/L) Current
• 1 to 2 A/m2
Time• Days to weeks
Not advisable for prestressed concrete Impressed/sacrificial anode cathodic protection systems
may be used Side effects – Alkali-silica reaction
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