Capítulo VI: El Proceso Estratégico
6.3. Matriz Interna Externa (MIE)
An impressed current cathodic protection system consists of an anode system in permanent contact with the concrete, an adjustable direct current power supply, reference electrodes embedded in the concrete near the steel and possibly other monitoring probes. There will also be a monitoring and control system to measure voltages and currents and to adjust the power supply. These components are all wired together. A structure will be broken down
into zones which are powered, monitored and controlled independently of each other. The system is illustrated in Figure 4.2.
Impressed current cathodic protection anodes
The choice of anode is critical in the design of any cathodic protection system. For concrete there is a wide range of systems, each with their own advantages and limitations. Anode types currently widely available are as follows:
1 Mixed metal oxide coated titanium
a. Expanded mesh (in a concrete overlay on the surface) b. Expanded mesh ribbon (in slots cut in the surface) c. Probe anodes (in holes drilled or cored into the concrete)
2 Conductive coatings
a. Organic coatings (paint systems, typically chlorinated rubber or acrylics)
b. Thermal sprayed metal (usually zinc but titanium has been used) 3 Conductive cementitious overlay (one proprietary system uses nickel
coated carbon fibres in a wet sprayed mortar overlay, for example) 4 Conductive ceramic (non-stoichiometric titanium tube type probe
anodes).
Table 4.1 Impressed current anode types Anode Relative life Relative cost Advantages Limitations Mixed metal oxide coated titanium mesh – embedded in a concrete overlay, usually sprayed
Very long High Durable Overlay can debond if preparation and application is not done with great care and expertise
Deadweight and thickness of overlay can affect structure
Mixed metal oxide coated titanium mesh ribbon
Very long High Durable No change in dead load or sections
Need adequate cover to avoid short circuit from anode to steel
Leaves stripes on surface Mixed metal
oxide coated titanium probe anodes
Very long High Durable No change in dead load or sections
Titanium link wire can pit if voltage exceeds 8–12V. Anodes must be positioned to avoid short circuit from anode to steel
Leaves stripes on surface Requires coring or percussive drilling to install Conductive
ceramic probe anodes
Very long High Durable No change in dead load or sections
Titanium link wire can pit if voltage exceeds 8–12V. Anodes must be positioned to avoid short circuit from anode to steel
Leaves stripes on surface Requires coring or percussive drilling to install Conductive
organic coatings
Short Low Easy
application and replacement No change in dead load or sections Hides repairs and givens choice of finish colour and texture
Needs dry conditions during application and will deteriorate rapidly in wet conditions
Low resistance to wear and abrasion
Thermal sprayed zinc
Medium Medium Easy
application and replacement No change in dead load or sections Moisture tolerant Complex application process
A brief description is given in Table 4.1. For further information on anode systems see Broomfield (2007).
Monitoring probes
The reference electrodes are generally silver/silver chloride/potassium chloride electrodes or proprietary manganese/manganese dioxide electrodes designed and constructed for permanent embedment in concrete. They should be installed with minimal disruption to the concrete around the steel for representative measurement.
For systems on structures with very long lives, simpler, pseudo-reference electrodes may be used along with ‘true’ reference electrodes. This is because true reference electrodes have a life of about 20 years while a pseudo- reference electrode of graphite or mixed metal oxide coated titanium will last indefinitely but is less accurate.
Power supplies
The transformer/rectifier (T/R or rectifier) is the DC power supply that transforms mains AC to a lower voltage and rectifies it to DC. The positive terminal is connected to the anode and the negative to the cathode. The level of the output is controlled as described below. T/Rs can be run at constant voltage, constant current or constant potential (against a half cell). They can be adjusted manually, automatically by circuitry or computer control, or remotely using a telephone line and modem link or similar remote connection as described later.
Transformer/rectifiers for conventional cathodic protection systems of steel piles in docks or on pipelines can be very large and powerful, capable of delivering hundreds of amps, with oil-cooled transformers. However, for steel in concrete, the requirements are far more modest. Most systems are designed for a current density of about 10 to 20 mA per square metre
of steel surface for actively corroding structures and for 0.2 to 2.0 mA/m2
for new structures where there is no pitting and so no need to passivate pits.
Most steel in concrete needs less than 10 mA/m2 to provide protection,
usually at less than 10 V. The power for a 100 watt light bulb will typically
protect 10,000 m2. This means that a single phase, air cooled T/R will
usually protect even the largest structure and power consumption is rarely an economic concern.
When calculating the current demand in a system, there may be a requirement for allowances to be made for the current flow to lower layers of steel as well as the outer, corroding layer. Calculations must also make allowances for the voltage drops down the connecting cables and anode strings.
The T/R must be rugged and reliable with minimal maintenance requirements. It should be easy to maintain with good instruction manuals,
circuit diagrams for maintenance and easy access to fuses and other consumable and replaceable components.
There are two opposing directions of T/R design at the moment. The first is to make a simple, rugged reliable design with high-quality components. This is checked manually every one to six months and an annual ‘service’ carried out. The other is to attach a microprocessor and logger system that can monitor and control the system. This means that data can be collected remotely and in some designs the system can be adjusted without regular site visits, requiring only an annual inspection as long as local personnel carry out quick checks on the condition of the systems (loose wires, etc.).
The number of systems an organisation has in operation is one factor in choosing remote control. It becomes more cost effective to collect and review data without site visits as the number of cathodically protected structures increases. The sophistication of the client and his consultant is another factor.
The reliability of the microprocessor system has not been reported in the technical press; the author’s first few remote control systems installed in 1986/7 worked until the structures were demolished, although most of those were installed inside buildings, in very benign environments. Some systems are comparatively simple and will only monitor on and off potentials, current and voltage. It is not possible to change the current or voltage settings remotely on some of these systems. With modern microprocessors and the internet it is possible to store data online, and send alarms by email or text message if the system malfunctions or exceeds defined limit values of current, voltage or reference electrode potential. Systems have been developed that will commission and operate a system according to BS EN12696, the British and European standard for cathodic protection of steel in concrete.
Design, installation, monitoring and maintenance
An impressed current cathodic protection system can be procured by a number of routes:
• Design, build and commission by an experienced contractor, usually with
an independent design check with all responsibility on the contractor. The client must provide as much information as possible about the structure and the contractor may have to carry out investigations as part of the works.
• Detailed performance specification and outline design by the client’s
engineer with the contractor carrying out a detailed design. The client’s engineer should have carried out most of the necessary investigations and testing prior to issuing the tender documents.
• Detailed design by the client’s engineer with the contractor selecting
often requiring approval from the client’s engineer who retains design responsibility.
An impressed current cathodic protection system should be designed by a competent person with correct training and experience. In Europe a new standard BS EN 15257: 2006 ‘Cathodic protection – competence levels and certification of cathodic protection personnel’ is now being implemented in the UK by courses run by the Institute of Corrosion and the Corrosion Prevention Association for levels 1 and 2. Certificates are awarded by the Institute of Corrosion with level 3 certificates awarded after assessment of experience.
The design of a cathodic protection system is described in BS EN 12696:2000. It requires following steps:
• The selection of anode type or types (more than one may be needed) • Anode layout and the breaking down of the structure into zones
• Determination of current requirements by selection of a suitable current
density on the steel and calculation of the steel surface area for the concrete components of the structure
• Design of reinforcement connections
• Design the cable layout (numbers, sizes, junction boxes, conduits, cable trays etc.)
• Location and number of embedded reference electrodes and other monitoring probes
• Power supply location, type, capacity, a.c. power provision and remote monitoring telephone line or other telecoms options
• Documentation – drawings, specifications, method statements, quality plan, operations and maintenance manual.
Once a contractor is approved with necessary Heath & Safety, Quality Plan and other necessary systems in place, the usual installation process is as follows:
1 Conduct concrete repairs while checking electrical continuity of the steel and making negative connections to the reinforcement.
2 Install reference electrodes and other monitoring probes.
3 Install the anode system or systems.
4 Install wiring system.
5 Install control and monitoring system and wire up the system.
6 Carry out pre-commissioning checks.
7 Energise the system.
8 Commission the system.
9 Provide operation and maintenance manual to client containing all
Concrete Patch Concrete Anode Cathode Anode Anode Cathode Cathode BEFORE AFTER
Figure 4.3 Incipient anode schematic showing how the anode is displaced to the edge of the repair by the formation of a new cathode in the patch repair (from Broomfield 2007).
10 Operate the system for 12 months and supply a 12-month monitoring report.
11 Hand over system to client who appoints a suitably qualified and trained engineer to continue monitoring the system (this may be the client’s own engineer, the contractor’s engineer or an independent cathodic protection engineer).
Advantages and limitations of impressed current cathodic protection
The main reason for choosing to use an impressed current cathodic protection system as part of the repair and rehabilitation strategy for a structure is that it controls corrosion across the whole area where anodes are installed.
When carrying out repairs to corrosion-damaged reinforced concrete it is important to be aware of the ‘ring anode’ or ‘incipient anode’ effect. This is illustrated in Figure 4.3, 4.4a and 4.4b. The problem is that by repairing the corroding anode, we generate new anodes around the repair. This is especially prevalent in chloride-contaminated concrete where the higher moisture levels and the chloride content lower the electrical resistance of the corrosion cell and allow greater separation between anode and cathode.
Figure 4.4a Patch repair with surrounding spalling due to incipient anodes on a building (from Broomfield 2007).
Figure 4.4b Incipient anode showing repair (left side) and corrosion in original concrete (from Broomfield 2007).
Impressed current cathodic protection is frequently found to be the most cost-effective solution on reinforced concrete structures with chloride- induced corrosion with a long required life (over 10 years). However it is also widely used in other situations including carbonated structures and where any corrosion damage is unacceptable.
Another advantage of applying impressed current cathodic protection is that patch repairing can be made easier. As the corrosion protection is provided by the cathodic protection, cutting out and patching is easier as all chloride-contaminated (or carbonated) concrete does not have to be removed from behind the corroding reinforcement, as shown in Figure 4.5.
There are now well-proven national and international standards for supply, installation, monitoring and control of impressed current cathodic protection. These include the European Standard BS EN 12696 (2000), NACE standard RP 0290 (2000), and Australian/New Zealand standard AS 2832.5 (2002).
The key advantages of impressed current cathodic protection are: • It controls corrosion in all areas where anodes are applied. • It stops the ‘ring anode’ or ‘incipient anode’ effect.
• It simplifies repairs as there is no need to remove chloride-contaminated
concrete. This can avoid requirements to prop the structure during repairs.
• There are good, well-recognised international standards for applying the technique.
Feathered edges and poor preparation allow breakaway at edges and poor keying
Squared edges cutting behind the bar and removal beyond the corroded area restores passivity and removes contamination
Concrete removal beyond the corroded area is not required for electrochemical treatment
Figure 4.5 Patch repairs: bad, good and compatible with electrochemical treatments such as impressed current cathodic protection.
• There are good trade associations and learned societies with members skilled in the design, installation and operation of systems.
• There are certification schemes for engineers who design, install and operate the systems.
• It has a proven track record of over 20 years standing.
• Systems should last 20 to 50 years and can be designed for longer durability of the major components.
The key limitations of impressed current cathodic protection are: • They require specialist knowledge.
• Extreme caution and specialist advice is required to apply impressed current cathodic protection to:
• Structures containing prestressing steel
• Structures with coated steel (epoxy or galvanised)
• Structures that have been epoxy injected or have any other impediment to current reaching the steel.
• It requires a permanent power supply (and telephone line if it is remotely
monitored).
• It requires ongoing monitoring and maintenance.
• Initial cost is high and requires 10 to 20 years remaining life to justify the life-cycle cost.