What is cathodic protection?
Cathodic protection is an advanced method designed to prevent corrosion in the steel reinforcement of concrete structures.
It achieves this by electrically shifting the steel's potential to more negative values, effectively reducing the corrosion process by minimizing the potential differences between anodic and cathodic sites on the metal surface.
This method ensures the longevity of steel-reinforced concrete structures exposed to chloride contamination and carbonation.
What is cathodic protection used for?
Cathodic protection is primarily utilized to safeguard the steel reinforcement in concrete from corrosion, a common issue in structures prone to chloride contamination or carbonation.
By preventing or stopping the corrosion process, cathodic protection helps maintain the structural integrity of new and existing buildings, delaying or eliminating the need for extensive repairs that involve concrete removal and reinforcement replacement. It is a critical technique for extending the lifespan of infrastructure exposed to corrosive environments.
How does Cathodic Protection work?
Cathodic protection systems safeguard reinforced concrete structures from corrosion, by altering the environment around steel reinforcements to prevent corrosion processes.
There are two primary cathodic protection methods: Impressed Current Cathodic Protection (ICCP) and Sacrificial Anode Cathodic Protection (SACP).
ICCP uses an external power source to direct current from an external anode to the steel reinforcement, preventing corrosion by making the steel act solely as a cathode.
SACP employs a more reactive metal as a sacrificial anode, which corrodes in place of the steel reinforcement, naturally regulating protection based on environmental conditions.
Designing a cathodic protection system involves a thorough assessment of the structure's condition, including corrosion extent, chloride content, and concrete's electrical resistivity. Selection of appropriate materials and configurations for anodes and reference electrodes are vital for effective protection. It's also crucial to determine the right current density to ensure the steel reinforcement receives adequate protection without risking hydrogen embrittlement.
How do I carry out and control cathodic protection for concrete structures?
A procedure for effectively using cathodic protection to protect reinforced concrete structures is provided in BS EN ISO 12696:2022 and includes steps such as:
Conduct an assessment of the structures condition following BS EN 1504 to evaluate material condition, structural integrity, and repair requirements.
Review construction records, drawings, specifications, and notes to understand reinforcement details, concrete quality, and previous repairs.
Determine the optimal current density based the concrete structure's exposure environment (atmospheric, submerged, or buried), chloride ion concentration, and concrete resistivity, as these factors collectively influence the required current density for effective cathodic protection. For uncorroded structures this typically ranges from 0.2 – 2 mA/m2 but when corrosion is present higher charge densities are required (2- 20 mA/m2).
Calculate the total surface area of the steel reinforcement that requires protection.
Calculate the total current required to provide a high enough current density to protect the steel surface area using Equation 1.
Equation 1. Faraday’s law where I = Total current (Amperes), i = current density (Amperes/m2) and A = the surface area of steel requiring protection (m2)
Create detailed drawings showing the layout of anodes, reference electrodes, cabling, and power supply units. Ensure clear representation of anode types (impressed current or sacrificial), locations, and connections to the reinforcement.
Specify materials for anodes (e.g., titanium mesh for ICCP or zinc/magnesium for SACP), conductive coatings, reference electrodes, and power supply units. Equipment specifications should detail requirements for durability, electrical performance, and compatibility with concrete’s alkaline environment.
Document step-by-step procedures for installation, testing, energizing, commissioning, and operation. Include safety protocols, quality control measures, and guidelines for addressing unforeseen issues during installation.
Evaluate prestressed structures for susceptibility to hydrogen embrittlement, especially when using ICCP systems. Implement design considerations to minimize risks, such as limiting current density and ensuring uniform current distribution.
Analyse stray current risks by assessing the structure’s electrical environment and potential sources of stray currents. Design isolation measures or electrical shielding as needed to protect the integrity of the cathodic protection system and surrounding metallic structures.
Develop a testing plan to validate electrical continuity, anode functionality, and system integration before full-scale energizing. Include initial system energizing procedures, gradual ramp-up of current to desired levels, and stabilization periods to observe system behaviour.
Implement a pilot cathodic protection system in a section of the structure to measure actual performance. Adjust the current density based on potential measurements, corrosion rate assessments, and uniformity of protection achieved. Consider any specific conditions of the structure, such as the presence of cracks, repairs, or varying concrete quality, which might necessitate adjustments to the current density.
Establish monitoring routines for ongoing system evaluation.
Ensure continuity between reinforcing bars or steel elements using direct current reverse polarity resistance measurement technique to achieve a resistance of less than 10 Ω.
Install the anode system under controlled conditions, as demonstrated by trials or past projects, to prevent short-circuits between the anode system and reinforcement steel. The electrical resistance of all anode/cable connections should be tested and meet designed values.
Undertake all electrical installations in compliance with international or national electrical safety standards ensuring mains voltage cables are isolated from low voltage DC cables and ensuring cables are uniquely identified and protected from damage.
Perform polarity checks, continuity checks to confirm circuit resistance values, and insulation checks to ensure electrical isolation of DC positive and negative cables in impressed current systems.
Conduct a complete visual inspection, pre-energizing measurements, and initial energizing of impressed current systems according to the quality plan and design specifications.
Develop an operation and maintenance manual detailing the system, as-built drawings, routine maintenance, inspection intervals and procedures, future performance assessments, error-finding procedures, and maintenance/repair procedures for all components.
Follow the intervals and procedures recommended in the operation and maintenance manual, adjusting based on system performance. This may include function checks (e.g., measurement of output voltage and current) and performance assessments (e.g., measurement of "Instantaneous OFF" polarized potentials and potential decay).
What equipment and expertise are required for cathodic protection of concrete structures?
The equipment required for cathodic protection of concrete structures is outlined in BS EN ISO 12696:2022 and includes:
Anode systems able to provide a sufficient, continuous flow of direct current to the steel reinforcement or composed of sacrificial materials (zinc, magnesium, aluminium) which are more electrochemically active than steel and will preferentially corrode.
Reference electrodes, potential decay probes, and other sensors for monitoring cathodic protection system performance.
Digital meters and data loggers for collecting and analysing performance data.
Seven strand cables, with insulation and sheathing conforming to IEC 60502-1, suitable for long-term exposure to conditions ranging from acidic (pH = 2) to alkaline (pH = 13).
A transformer-rectifier for mains electrical power.
The installation and operation of cathodic protection systems requires significant, specialised expertise to ensure its success and, as such, the scope for many projects to implement these techniques may be limited. Designing successful cathodic protection systems is an extremely specialised task requiring significant expertise that may not be available to the majority of projects. Experts in the specific field are required to ensure the success of the operation.
What are the advantages of cathodic protection for concrete structures?
Cathodic protection effectively stops corrosion on steel reinforcement, prolonging the structure's service life.
By preventing corrosion, cathodic protection helps maintain the structural integrity and strength of concrete structures.
Despite initial costs, cathodic protection can be more economical over the lifespan of a structure due to reduced repair and maintenance needs. Once installed costs are low.
Cathodic protection systems can be tailored for different environmental conditions, making them versatile for various concrete structures such as those in marine environments or aggressive soils.
What are the disadvantages of cathodic protection for concrete structures?
Cathodic protection systems require ongoing maintenance and monitoring to ensure effectiveness, adding to the operational costs.
Proper installation requires all reinforcing bars to be electrically connected, which can be complex and labour-intensive.
There's a risk of hydrogen embrittlement for prestressed steel, which requires special caution in cathodic protection applications.
The set up and maintenance of these systems is extremely complex, time consuming and requires significant expertise.
What are the limitations of cathodic protection for concrete structures?
Cathodic protection cannot repair existing concrete damage or replace lost steel strength; it only prevents future corrosion.
For structures affected by alkali-aggregate reaction (AAR), cathodic protection can influence the progression of damage, necessitating evaluation of residual potential for expansion.
The effectiveness of cathodic protection relies on the electrical continuity of reinforcing bars, which may be challenging to achieve in complex structures or those with extensive damage.
Ancillary information
Service disruption: No
Preliminary works: Yes
Posterior works: Yes
Time consumption: Medium (one day)
Cost: Low