Corrosion is caused when two dissimilar metals are submerged in an electrolytic substance such as water, soil, or concrete. This type of metal conducting path between the two dissimilar metals allows a pathway through which free electrons move from the more active metal (anode) to the less active metal (cathode). If free electrons from the anode do not reach active sites on the cathode before the arrival of oxygen, ions at the active sites can then recombine to produce rust. A major contributor to concrete deterioration is corrosion of embedded metals. In particular, corrosion of reinforcing steel, corrosion of epoxy coated steel, and corrosion of prestressed concrete when unchecked can eventually lead to major rehabilitation costs or structure replacement. Cathodic protection is used to mitigate corrosion damage to active metal surfaces. The anode is one of the most critical components for a cathodic protection system. It is used to distribute protective current to the reinforcing steel and provides locations for anodic reactions to take place in place of the reinforcing steel. By using relatively inert materials, such as catalyzed titanium, anode consumption is minimized.
Cathodic protection (CP) is a technique used to control (prevent or avoid) the corrosion of a metal by making it work as a cathode in an electrochemical cell. Application of CP on Reinforced Concrete (RC) structure is more recent than its use for marine steel structures or buried steel structures in soil. CP applied to RC structures is well described since decades both for existing RC structures (cathodic protection) or to new RC structures (cathodic prevention). It can be applied to any metallic structure in contact with a bulk electrolyte although in practice its main use is to protect steel structures buried in soil or immersed in water. Cathodic protection systems are used to protect a wide range of metallic structures in various environments. Its corrosion control method for protection of underground and undersea metallic structures, such as oil and gas pipelines, cables, utility lines and structural foundations. Cathodic protection is now widely applied in the protection of oil drilling platforms, dockyards, jetties, ships, submarines, condenser tubes in heat exchangers, bridges and decks etc. Cathodic protection connects the base metal at risk (steel) to a sacrificial metal that corrodes in lieu of the base metal. The technique of providing cathodic protection to steel preserves the metal by providing a highly active metal that can act as an anode and provide free electrons. By introducing these free electrons, the active metal sacrifices its ions and keeps the less active steel from corroding.
Cathodic protection works by placing an anode or anodes (external devices) in an electrolyte to create a circuit. Current flows from the anode through the electrolyte to the surface of the structure. Corrosion moves to the anode to stop further corrosion of the structure.Galvanic anodes are available in a variety of configurations, including:
- Bare metal anodes including magnesium, zinc, aluminum and other alloys
- Packaged in backfill for underground use
- Made with external steel straps for mounting to structures
- Ribbon types
- Rod and special shapes
Selection of cathodic Protection System
The following analysis is required for deciding a cathodic protection system
- Details of Structural Dimensions and drawings
- Surface Coating Scheme
- Details of Soil Strata / Terrain
- Presence of Foreign Metallic Structures.
- Details of cased crossings
- History of corrosive areas
- Stray current conditions
- Operating Temperature
- Availability of AC Power
- Corrosion history of structures in the area
- Electrolyte resistivity and pH survey report to understand corrosion rate
Types of cathodic protection
Galvanic cathodic protection
Galvanic cathodic protection is a corrosion prevention method that uses electrochemical means to protect a base material from corrosion. It does this through the use of a sacrificial anode that corrodes before the material being protected by the sacrificial anode. Galvanic cathodic protection is one of the most commonly employed forms of cathodic protection because of its ease of use. Zinc, Magnesium or Aluminum metal alloys are commonly used as the anode, as these metal alloys have a native potential (more negative) than the cathode structure metal. The asset metal is typically steel, but depending on the environment and corrosive nature of the electrolyte, the asset metal may consist of stainless steel, ductile iron, aluminum and or others. Sacrificial anode systems do not require an external power source, are self-controlling and require very little maintenance.
Impressed Current Cathodic Protection
Impressed current cathodic protection (ICCP) is a corrosion protection system consisting of sacrificial anodes connected to an external power source. The external power source, often a DC power supply, provides the current necessary to drive the electrochemical reaction required for cathodic protection to occur.In the absence of a DC power source, a transformer-rectifier connected to AC power can be used. Alternative sources may also be used to power the electrochemical cell, including wind, solar or gas-powered thermoelectric generators.
Impressed current cathodic protection systems are typically used in relatively large structures, where passive cathodic protection methods are ineffective or impractical. For example, passive cathodic protection is suitable for protecting individual structural members and appliances. However, this method generates minimal current, making it ineffective for protecting larger structures such as oil and gas pipelines and storage tanks.
Rather than depend on the natural potential difference between the anode and cathode material, ICCP systems use external sources to provide the necessary current. This increased current makes it possible for the cathodic protection to span a wider effective area.
Hybrid cathodic protection
Hybrid systems have been used for over a decade and incorporate the coordination, monitoring and high restorative current flow of ICCP systems with the reactive, lower cost and easier to maintain galvanic anodes.
The system is made up of wired galvanic anodes in arrays typically 400mm apart which are then initially powered for a short period to restore the concrete and power ionic migration. The power supply is then taken away and the anodes simply attached to the steel as a galvanic system. More powered phases can be administered if needed. Like Galvanic systems corrosion rate monitoring from polarisation tests and half-cell potential mapping can be used to measure corrosion. Polarization is not the goal for the life of the system.
Conclusion
Cathodic protection is a method to control the corrosion of steel in contaminated concrete that works by making the embedded reinforcing steel cathodic. When the steel becomes cathodic, hydroxyl ions are accumulated around it making it passive for a longer time. The reinforcing steel is electrically connected to another metal that becomes the anode with or without the application of an external power supply. Cathodic protection is used to protect almost any type of reinforced concrete structure, including horizontal slabs, walls, towers, beams, columns and foundations.