What is the Principle of Cathodic Protection for Steel in Buried or Immersed Concrete Structures?

What is the Principle of Cathodic Protection for Steel in Buried or Immersed Concrete Structures?

Cathodic protection for steel in buried or immersed concrete follows the same basic principles as for atmospherically exposed concrete. The principle difference is that the concrete is likely to become water saturated, which will result in reduction in oxygen content at the steel surface under normal exposure conditions, which will be accelerated under the application of cathodic protection. Where oxygen depletion occurs, the potential will become very negative and the current required for cathodic protection will be reduced.

Hence, the current density required for steel in concrete that is buried or immersed for protection may be, if the concrete is fully water saturated, considerably less than that required for atmospherically exposed concrete. Typical current densities range from 0.2 mA/m2 to 2.0 mA/m2 for new structures (before corrosion initiation) in water saturated conditions. For structures that are not fully water saturated and are corroding before the application of cathodic protection, current densities may be as high as those for atmospherically exposed concrete, up to 20 mA/m2.

The current density is also dependent on whether the concrete is fully immersed or whether one face is exposed to air (e.g. as for tunnels, portions of diaphragm walls, underground storage tanks and where the thickness of the concrete structure is typically less than 0,5 m to 1 m). If this is the case, then a differential concentration (differential oxygen) cell can be created between the fully immersed face and the air exposed face. Where such conditions occur, a higher current density will be required on the immersed portion.

 Cathodic protection zones
Plan the anode zone size and layout and calculate the feeder spacing to ensure that local cathode current density requirements are met and to minimize the voltage and anode current density differentials within zones due to anode and cable resistances. Select the primary anode material and cross-section, its distribution and primary anode/“positive cable” connections to provide the required redundancy and to minimize voltage drops.

Different exposure conditions may dictate the use of different zones in the cathodic protection system of a single structure, for example, a hollow, floating reinforced concrete structure may be separated into the immersed zone utilizing immersed anodes in sea water.

Different elements of structures may require being combined into single zones: Typical zones for impressed current systems will have current ratings of 0.5 A to 2 A or possibly as high as 5 A if the steel/reinforcement distribution within the zone is uniform. With some anode systems, e.g. mixed metal-oxide-coated titanium (MMO/Ti), it is possible to vary the distribution and grade of anode within a zone in order to match the calculated local current demand and provide uniform cathode (steel) current density within a single complex zone for cathodic protection systems on atmospherically exposed structures or where a distributed anode is required to cope with a high electrolyte resistivity.