Cathodic Protection Of Reinforcement In Concrete – Overview And Experience Over 30+ Years

. This paper gives an overview of long term performance up to 30 years in the field of cathodic protection of steel reinforcement in concrete in The Netherlands, focusing on impressed current systems based on activated titanium anodes. Case studies are presented including applications to large numbers of precast elements corroding due to mixed-in chloride with drilled in titanium anodes, parking garages with titanium strips in cementitious overlays and industrial floors with titanium mesh in overlay. Based on observed failures and replacements, analysis of working life of various types of systems and components and end-of-life considerations are given. CP has become a fully accepted method of securing safety and serviceability of buildings and infrastructure that are prone to corrosion. Observations show that with activated titanium based systems, very long service lives can be obtained. The activated titanium itself keeps working well beyond the observed period of 30 years. Some other components have relatively higher failure rates. The repairs that were required in the cases discussed are dominated by failing anode-copper connections and to a minor extent power sources and potential sensors.


Introduction
Cathodic protection (CP) of steel reinforcement has become the major and most successful method to stop or prevent corrosion in concrete.Over time, many reinforced concrete structures develop corrosion of the embedded steel, due to long-term exposure to aggressive influences such as chlorides from sea water and de-icing salts or the effects of mixed-in chlorides; in some cases aggravated by carbonation [1,2].Steel corrosion causes cracking and spalling of concrete and steel cross section loss, compromising serviceability and eventually structural safety.Consequently, repair and protection of concrete structures has become a major industry in the past 40 years.However, in many cases conventional methods of concrete repair have been found to be ineffective or not durable [3,4].CP on the other hand, provides an effective and durable method for corrosion protection of steel in concrete.
CP of concrete structures was developed in the USA in the 1970s [5] and introduced in Europe in the 1980s [6].In the USA, a large number of concrete bridge decks was damaged by corrosion due to chloride ingress from de-icing salts.Early types of anodes were applied with varying results; many of these early anode types have been discontinued [7].Later, new anode materials became available, such as activated titanium and conductive coatings, variants of which are still widely used today; sacrificial (galvanic) anodes were introduced in the late 1990s.From about 1985 in the UK [6], Italy [8], Norway [9,10], Denmark, Switzerland [11,12] and The Netherlands [13] application of CP to concrete increased.A growing numbers of systems are being installed more recently in Germany, France, Belgium and Switzerland.Long term performance and an overview of interventions have been analysed [14,15].This paper focuses on impressed current CP (ICCP) with anodes based on activated titanium.Essential elements of every CP installation are the anode system, comprised of the anode material itself, current feeders, anode-cable connections, cathode-cable connections, anode and cathode cables, junction boxes, a power source, and the monitoring system.This paper reports on observations and analysis of the long term performance of activated titanium anode systems based on the authors' involvement in maintenance of a large number of CP systems in The Netherlands.Another paper reports on long term experience with monitoring systems [16].

Background
Reinforcing and prestressing steel in concrete are normally passivated due to the high alkalinity in the pore solution, caused by dissolved potassium and sodium hydroxides and buffered by solid calcium hydroxide, at pH values above 13.With passivation, corrosion is negligible and very low rates of oxidation and reduction occur.In aerated concrete steel potentials between about +100 and -100 mV versus a saturated calomel electrode (SCE) are present.Lowering of the pH due to carbonation, a reaction with carbon dioxide from the atmosphere, or the presence of chloride ions above a certain threshold concentration cause depassivation and initiation of corrosion.Subsequently anodic (oxidation) reactions and cathodic (reduction) reactions are strongly accelerated compared to the passive state, respectively described by the following equations: With chloride induced corrosion localised or pitting attack develops and anodic sites may be small; locally the potential drops several hundreds of millivolts and the pH drops to 2-3 [17,18].Cathodic potentials remain in the more positive ranges; strong potential gradients develop between anodic and cathodic regions that further accelerate corrosion.
The basic principle of cathodic protection is shifting the steel/concrete interface potential difference to more negative values by injecting a direct current, slowing down oxidation reactions (eq.1), accelerating reduction reactions (eq.2) and reducing potential gradients along the steel.In concrete, shifts of a few hundred millivolts are sufficient to depress corrosion rates to negligible levels [8].This is due to various favourable effects of current flow: increased cathodic reactions produce hydroxyl ions and increase the pH at the steel/concrete interface; negatively charged chloride ions migrate away from the steel/concrete interface.It follows that the potential difference between the steel and the concrete, the "steel potential", is a dynamic entity, indicating the corrosion state and varying with long term effects of current flow.Consequently, measuring the potential plays a major role in controlling CP.
In a CP system the current is injected into the concrete from an electrode installed on the concrete surface or in the cross section, called the anode, causing the interfacial potential to shift.At this anode, oxidation reactions take place that consume hydroxyl ions or oxidise materials at the anode/concrete interface described by the following equations: Reaction (3) may cause acidification and possibly loss of anode/concrete bond by dissolving the hardened cement paste; reaction (4) oxidises galvanic anode materials or carbon particles in conductive coatings.Possible side effects at the cathode (steel) are accumulation of alkali ions due to ionic migration; and hydrogen evolution according to an additional cathodic reaction that occurs when insufficient oxygen is available to sustain reaction (2): Hydrogen evolution at the surface of prestressing steel may cause its embrittlement, but it is harmless for reinforcing steel [1].Reaction (5) can be avoided by limiting the current density at the prestressing (or duct) surface.As this reaction only occurs at very negative potentials, typically -1100 mV versus SCE, monitoring the steel potential can be used as a safety measure.At more positive potentials than about -1000 mV, only reaction (2) is possible and hydrogen evolution according to eq. 5 does not occur.For more background information see [1].

Materials and Design
Two basic types of CP systems have been used on concrete structures: impressed current CP (ICCP) and galvanic (sacrificial) CP (GCP).The main components are illustrated in Figure 1.In ICCP systems a low voltage DC power source drives the current.The anode consists of a material that is not (or only slowly) consumed, such as activated titanium (titanium covered with noble metal oxides, Mixed Metal Oxide, MMO) [19] or a conductive coating.
Activated titanium has the shape of a diamond mesh embedded in a cementitious overlay on the concrete surface, typically 30 mm thick; or of fine mesh strips placed in boreholes in the concrete cross section or in slots cut in the concrete surface, forming a grid; holes and slots are filled with a cementitious grout.Titanium strip (also called ribbon) anodes have become quite popular.This is, among other factors, because they provide flexibility with regard to current output per square meter of concrete surface by varying the spacing between strips, which is typically 200-400 mm, or their width, typically 20-25 mm.
Conductive coatings are organic or inorganic polymers filled with carbon particles, applied to the concrete surface and usually covered with a (normal) top coat.
Primary anodes are metal wires or strips that feed the current into the actual anode material; they should be spaced closely enough to avoid large potential drops in the anode system.In titanium systems, current feeders are strips of plain titanium, that are spot welded to the anode material.
Cementitious overlays and conductive coatings should be durably bonded to the concrete surface for good long term electrical contact.Achieving good bond requires good surface preparation and proper application and curing are essential.Titanium mesh overlays are usually 25 -30 mm thick, applied by either spraying or casting.
In ICCP systems, the reinforcement (cathode) and the anodes are connected to the power source through isolated copper cables.Primary anodes are connected to the copper cables through anode-copper connections, either by splicing primary anodes and copper cables, isolating and embedding in the concrete or the overlay, or by connecting them in junction boxes.Reinforcement is connected by cathode connections, usually by some type of welding.Cable cross sections should be large enough for limited potential drop.Cables need to be redundant in order to avoid system failure due to cable interruptions, e.g. by vandalism; at least two cables should feed anodes as well as cathodes within a zone, with connections as far apart as possible.For practical reasons, cables can be collected and connected to main cables in junction boxes.Power sources are fed from the normal grid or by solar or wind energy, the latter two usually backed up by batteries.
In GCP systems the power originates from the electrochemical potential difference between the anode material and the steel.Corrosion of a sacrificial material, normally zinc, provides the current according to Zn → Zn 2+ + 2 e (6).
A GCP system can be made of zinc sheet with an appropriate activator and a conductive adhesive on the concrete surface; or by embedding zinc probes with activator inside cavities or boreholes in the cross section.Activators are needed to keep the zinc corroding; they are proprietary formulations based on alkalinity, complexing agents or halides.Zinc anodes are connected to the reinforcement using copper wires.
A CP system can be divided into separately controlled parts called zones, if current densities between different parts of a structure are anticipated to be different; or for practical reasons.Current densities are affected by differences in concrete electrical resistivity due to different exposure to precipitation or different cement types having been used [13,20].Practical reasons can be the desire to avoid large zones with poor controllability or a low capacity of available power units.Zones can be as large as many hundreds of square meters, or as small as 10 -20 m 2 or even smaller.
Basic design of CP is based on a current density of 20 mA/m 2 of steel surface, as a rule of thumb.It can be debated if this should concern all steel in the entire concrete cross section or only the steel closest to the surface.Experience shows that most ICCP systems, after some time, operate at much lower current densities, typically of 1 -5 mA/m 2 (steel surface).However, the long-term current density per unit of concrete surface area can be quite different.Buildings and precast beams or slabs usually contain less than 1 m 2 of steel surface, typically 0.5 m 2 per m 2 of concrete surface.The steel/concrete density of civil engineering structures can be much higher, in particular in areas with higher stresses, typically 1 or more.In practice, impressed CP current is kept as low as possible by applying a low voltage in order to limit system degradation and negative side effects, while maintaining sufficient protection.In galvanic systems, the current cannot be externally controlled and the zinc consumption rate is determined by concrete resistivity and steel density, which will determine anode working life.
The current density per unit surface of ICCP anode material is limited due to concerns with anode durability, in particular for conductive coatings, typically at 20 mA/m 2 , or regarding overlay durability related to acidification.Titanium anodes are thought to be durable up to 110 mA/m 2 of titanium surface.
Preventive application of CP for steel in concrete requires lower (design) current densities, in the range of 0.2 -2 mA/m 2 , because preventing corrosion is easier than stopping it [8,21].Compared to curative systems, less anode material is needed and the current distribution is more uniform, i.e. anodes have a larger throwing power.

Protection testing
As the current required to achieve protection varies from case to case and possibly with time (wetness, temperature), protection quality needs to be tested regularly.Various protection criteria have been developed.The International Standard ISO 12696 [21] provides the following rules or criteria of which at least one out of the last three (c, d and e) need to be met: a) An instant off potential more positive than -1100 mV versus Ag/AgCl/0.5MKCl for reinforcing steel b) An instant off potential more positive than -900 mV versus Ag/AgCl/0.5MKCl for prestressing steel c) An instant off potential more negative than -720 mV versus Ag/AgCl/0.5MKCl d) A potential decay over more than 24 hours of at least 150 mV from instant off e) A potential decay over a maximum of 24 hours of at least 100 mV from instant off.All of these criteria except e) need to be tested with a "true" reference electrode (RE) that has a good long term absolute potential stability.Criterion e) can be measured with a simple metallic "decay probe" (DP).True RE's most widely used are Ag/AgCl or Mn/MnO2 based, DPs can be based on activated titanium, carbon or other metals.Due to their simpler construction, DPs are less costly than REs.
In practice, 100 mV potential decay (depolarisation) up to 24 hours is the most widely used criterion for above ground structures.It is measured by interrupting the current for several hours to one day and monitoring the steel potential several times in the interval.In addition, criterion b) must always be used when prestressing steel is present.
ISO 12696 states that a minimum of two REs shall be installed in every zone of a CP system and allows for other sensors, e.g.DPs to be used, in addition to those two REs.This means that according to the Standard using only DPs is not allowed, even if only 100 mV depolarisation is tested for!In contrast, in The Netherlands DPs have been used without REs being present, in a large number of cases over several decades, with full satisfactory behaviour.This is allowed according to a National Technical Recommendation [22].This Recommendation [22] also states that in each zone at least one RE or DP should be placed per 100 m 2 with a minimum of two per zone.This makes sense for larger zones, as testing points can be spread out to better represent variation in aggressive conditions and distribution of protection current.For small zones the cost of two sensors must be compared to the need to get a good impression of protection quality.
Multiple REs or DPs should be embedded at representative locations in each zone.Regular monitoring consists of depolarisation testing at least twice a year and a visual inspection once a year.
Monitoring usually starts as part of a maintenance contract between the owner and the contractor who installed the system; such contracts are typically agreed for ten years.Increasing numbers of systems are remotely monitored and/or controlled using dataloggers and modern wireless connection technology.Nowadays more and more systems are being monitored by independent parties, after termination of the original monitoring contract.This implies that, at a minimum, proper documentation of the as built system must be available.

System working life
The working life of a CP system basically is the length of time over which sufficient current flows to all reinforcing steel that needs protection; the necessary controllability also requires the monitoring system to keep working properly.For ICCP systems this implies that the complete chain from power source to anode to steel and back needs to be conducting the current, as well as the monitoring circuit needs to transmit correct potential signals.Consequently, system failure can be caused by loss of anode material, loss of anode/concrete bond, anode-copper connection failure, cable failure and power source failure; or by failure of potential sensors and their cabling.
A previous study [15] has shown that failure of the following components occurred in a group of 105 ICCP systems: -Local attack of conductive coatings -Anode-copper connections -Power sources -Potential sensors In addition, in some cases loss of overlay/concrete bond was reported.Long term behaviour of conductive coatings was described by [23].Suppliers of activated titanium anodes claim very long life for their anodes, typically 50, 75 or 100 years.The cases described below report findings with titanium based systems.

Long term performance of Titanium based ICCP systems 6.1 General
The long term performance of titanium anode ICCP systems will be illustrated from several case studies of projects in The Netherlands and some additional observations.

Case Mozartlaan and Bachlaan, Tilburg
ICCP was installed in Tilburg on two identical 17-story apartment buildings in 1990 [24].These systems were the seventh and eighth concrete CP systems installed in The Netherlands, which made them pioneering in several respects.A substantial fraction of the cantilever beams showed corrosion and cracking due to mixed-in chlorides, apparent after about ten years.Subsequent repair and polyurethane coating did not stop corrosion and damage reappeared in few years.Replacement was highly impractical and consequently CP was chosen.
The concrete protected consisted of 1224 beams per building, with 18 beams of varying length per zone in 68 zones of approximately 25 m 2 of concrete surface each.The anodes were V-shaped activated titanium ribbon mesh strips (30 mm wide, Heraeus) in grouted boreholes along the length of the beams, as, illustrated in Figure 2. A reinforcement contact was made in every single beam by drilling a small hole adjacent to a bar in the beam head and hammering in a steel nail, because of poor continuity between beams.Anode and steel contact wires were connected to cables in junction boxes placed on the heads of the beams.For economic reasons, only half of the zones contained a single RE, i.e. a total of 34 per building, based on MnO2 (Force).As the other half of the zones did not contain an RE, these zones were operated at a predefined (fixed) current based on the performance of zones with an RE.This type of operation and control is now unusual and zones would now be larger and contain more RE's or DPs.
In 2005, so after 15 years, a large number of the junction boxes at the beam heads had been found to be leaking, which had resulted in corrosion of anode connections.All junction boxes were replaced and new anode connections were made.
At the time of a full system review in 2020, so after 30 years of operation, the strip anodes were judged to be still properly working.On the other hand, only about 35% of the REs were still fully functional.The performance criterion of 100 mV depolarisation in 24 hours, as per ISO 12696, was not met.This was identified as due to failure in the electro technical installation, cable ducts, conduits, connection boxes and such.Furthermore a large portion, approximately 50%, of steel contacts in the beams were failing and showed loss of continuity.It is likely that a large number of RE failures can be contributed to the failing connection to the steel in those particular beams.This was incidentally observed, but not verified in all cases.
One of these systems was renovated in 2021 by full replacement of the electro technical installation, while reusing the existing anodes, as the strip anodes themselves appeared to be in good condition.New anode connections were made and all were connected by new steel connections.The original REs were discarded and new electrodes were installed.In the new installation a total number of 34 zones has been realised, containing a total of 136 REs.The other is scheduled for renovation in 2022.

Case Saffierflat and Parelflat, Groningen
In Groningen an ICCP system was installed on two identical apartment buildings in 1996/7 and 1998, respectively.Corrosion and cracking in a significant number of cantilever beams was due to mixed-in chlorides at moderate levels, 0.1-0.6% by mass of cement and some carbonation.Each building comprised two zones of approximately 650 m 2 of protected surface for 559 beams per building; the zone division was based on façade orientation, South West versus North East related to the direction of prevailing winds and precipitation.The anodes were activated titanium ribbon mesh (type Lida Grid 20 mm, De Nora) placed in grouted longitudinal boreholes, generally similar to the Tilburg case.
Each zone contained 4 DPs based on an activated titanium wire precast in a mortar cylinder.During operation and at the time of a full system review in 2016, so after about 19 years, 9 out of 16 reference electrodes in the two systems were still fully functional, verified as described above.The performance criteria of ISO 12696 were generally met, but the electro technical installation, cable ducts, conduits, connection boxes and such, where starting to show signs of failure.The 7 DP failures were due to failing connections or failing cables and wiring in the electro technical installation.The strip anodes themselves were judged to be in good condition.In contrast, the beams on the top floor had been provided with a conductive coating anode system, which showed clear signs of failure, due to some debonding and failing primary anodes [25].
The original CP system in one building has been renovated by full replacement of the electro technical installation, while reusing the installed anodes.New anode connections were made and steel connections where checked and reused or replaced as needed.The second system is scheduled for renovation in the first half of 2022.

Case Parking garage UMC North, Utrecht
This parking garage is constructed of prestressed double-T-beams supported by prefabricated lightweight concrete support slabs placed in the perpendicular direction.The support slabs are typically 2 meters wide by an average length of 10 meters per element.The element height varies from the middle slab section with just 200 mm of thickness towards the integrated beams on both long sides of about 840 mm thickness.The two integrated beams are located above the columns and in turn support the double T-beams.
Large deformations were observed in the exposed top deck, mainly due to seasonal thermal cycles.Temperature differences in the Netherlands are typically from lowest temperatures in winter around -10°C to a maximum of 40°C during summer.Due to solar exposure of the upper mastic asphalt deck there was an additional temperature increase of the concrete on sunny days up to 10-20°C above the air temperature.The deformations caused cracking in the mastic asphalt leaving the concrete fully exposed.After cracking the concrete was no longer protected against ingress of chlorides from de-icing salts applied in winter.
The parking garage was refurbished in 2005.The exposed upper parking deck was equipped with CP based on activated titanium mesh (type DeNora Lida CN15) covered with mortar, in five separate zones located above the supports totalling 3.280 m 2 of protected concrete surface.The five zones are equipped with two reference electrodes per zone of type silversilver-chloride.The system is still fully functional in 2022 and complies to ISO 12696 after 17 years of operation.At 3 V of applied voltage a total of 11 A of protective current is supplied, corresponding to about 3.5 mA/m 2 of concrete surface.The depolarisation is well above 100 mV in 24 hours with values varying over time (seasonal, temperature and humidity related), ranging from 140 to 423 mV.

Case Parking garage Koperwiek, Eersel
In 2008 a newly built parking garage constructed in a basement under a residential building showed cracking in the floor.An investigation showed that the reinforcement of the basement was not properly designed with regard to the upward ground water pressure.The water pressure was reduced by installing a system of water level control using pumps in the direct perimeter of the building.The cracks in the concrete floor of the parking garage were deemed a durability risk with regard to de-icing salt that would be brought in by parked vehicles.The floor was upgraded by applying a CP system based on activated titanium ribbon mesh (type CPS TS5) with spacing 500 mm, for a capacity of 10 mA/m 2 .The mesh was embedded in a cementitious overlay and afterwards the floor was coated with an epoxy based coating suitable for parking decks.
The installation totals 650 m 2 of protected concrete surface in seven zones.Each zone is equipped with two silver-silver-chloride reference electrodes and two activated titanium depolarisation sensors.The system is still fully functional after 13 years of operation and complies to ISO 12696.At 4 V of applied potential a total protection current of 950 mA is supplied, corresponding to approximately 1,5 mA/m 2 (concrete surface).The depolarisations are well above 100 mV in 24 hours and show low fluctuations in time (indoor exposure) ranging from 230 to 475 mV.

Case Pickling line, IJmuiden
A number of reinforced concrete floors supporting various parts of an industrial pickling line were repeatedly and severely exposed to hydrochloric acid.Due to corrosion damage and high chloride levels in the concrete a total surface of 5.000 m 2 of concrete was protected with CP based on activated titanium ribbon mesh (CPS TS5) placed on the top side of the floors in slots with spacing 300 mm.After placing the anodes the slots were filled with cementitious grout.
The CP system was originally installed as 18 zones in 2003.A large fire in 2010 caused severe damage to some parts of the floor.The entire concrete construction in that section of approximately 900 m 2 was demolished and rebuilt.As part of the reconstruction a cathodic prevention system based on activated titanium ribbon mesh was incorporated.After the reconstruction the total protected area is unchanged, although 900 m 2 out of the 5.000 m 2 is cathodic prevention, but consists now of 16 zones.Each zone has four silver-silver-chloride reference electrodes and eight zones also have an additional two activated titanium depolarisation sensors.
After 19 years of service the system is still fully functional and complies to ISO 12696.At 4 V of applied potential a stable protection current is supplied (indoor exposure), but the current varies from zone to zone (mainly with their surface area) from 200 mA to 2 A per zone.On average a rather stable current density is found of approximately 2 mA/m 2 (concrete surface).The depolarisations are well above 100 mV in 24 hours and show low fluctuations in time ranging from 149 to 316 mV.

Other cases
A few other cases are mentioned without reference to their location for reasons of confidentiality.
Two cases concern disbonding of the shotcrete overlay of titanium mesh (walls of a building) or titanium strips (a ceiling in a tunnel).In both, large surfaces were found to sound hollow over patches between 0.3 and several square meters.In one case, this was just months after starting up the CP current, in the other case even before current was applied.Both were found to be caused by poor surface preparation, in particular insufficient roughening of the old concrete surface, or inadequate cleaning of exhaust fume deposits.Apparently, the disbonding had nothing to do with CP.
In a few cases, power units failed due to penetration of moisture into the cabinets, either due to insufficient sealing or poor detailing that allowed condensation from conduits leading into the power cabinet.Lightning strike has also caused power unit failures in a few older systems.
A previous analysis [15] mentioned three more cases of anode-copper connection failures.One, a bicycle deck of a bridge in Rotterdam called Stadionviaduct, was the first concrete CP system installed in The Netherlands, with a carbon filled polymer cable anode.Anode connections were made on site and embedded in the cementitious anode overlay.They failed after about ten years and were replaced two years later [26]; also power units and REs had to be replaced.The other two cases are a bridge and a building; the connections in the bridge, made on site, failed and were replaced after eight years.The connections in the building failed after five years, but no other information is available.

Discussion
As stated above, ICCP system life "ends" when any link in the chain in the protection current circuit fails, i.e. the anode itself, its bond to the concrete, its connections to copper cables, the cabling, the connections in junction boxes or the power units; or when any link in the monitoring chain fails, i.e. the potential sensors themselves, their cabling, connections or the measuring devices.
In the cases described the activated titanium anode material itself has not failed, in fact the material kept delivering current for durations of up to 30 years, and will most probably be able to do so for many more years.On the other hand, the electro technical installation showed various forms of failures.
Connection failures occurred in the fully discussed cases after 10, 15 and 19 years; and in the additional cases after 5, 8 and 10 years; this is out of about 100 documented titanium CP systems.Failing connections had been either embedded in the concrete/overlay/repair mortar or in junction boxes.Connection failure is due to accelerated corrosion as a result of anodic polarisation of the copper, which causes rapid dissolution and loss of continuity.This can only happen when the copper (or the connection clamp) is in electrolytic contact with the concrete.Small defects in the isolation of embedded connections may provide such electrolytic paths; moisture entering junction boxes may have a similar effect.Embedded connections should be of very high (isolation) quality, which may be difficult to achieve on site.A better option is to prefabricate copper wire connections to pieces of titanium wire under well controlled conditions in a workshop, and spot weld the titanium wire to the anode system on site.The best option would be to use all titanium connectors in the anode system all the way out of the concrete and make connections to copper wiring outside of the concrete.In this way there is no direct contact possible between copper and the electrolyte, even when the insulation would locally fail.Fig. 3. Prefabricated connection between isolated copper cable and plain titanium strip, spot welded on site to titanium conductor bars and mesh For connections in junction boxes to be durable, the junction boxes should have very good isolation properties and be located in sheltered places; moisture ingress from condensation in cable conduits should be avoided by careful design and placement.It should be noted that ICCP systems contain large numbers of anode-copper connections, whose long term performance may be strongly correlated.Consequently, failures in individual systems come in relatively large numbers and repair may be costly.
Power units have failed due to various causes, e.g.lightning strike and moisture ingress in cabinets.Proper isolation and detailing of cable inlets is necessary.However, replacing power units concerns a single or just a few units and hence may be relatively inexpensive.
Field experience shows that the general failure rate of potential sensors (REs or DPs) is low, up to at most a few percent of the installed numbers.In several documented cases all sensors kept working for more than 20 years, both REs and DPs.For more details see [16].
Returning to the matter of system working life, it appears that the activated titanium itself is not a limiting factor.As far as we can tell, manufacturers claims of working lives of 50 or 100 years may be correct.On the other hand, the working life of ICCP systems, or rather the time at which significant maintenance is needed, is much more strongly determined by various parts of the electro technical installation.The anode-copper connections are the most sensitive parts: either embedded or placed in junction boxes, they may fail at ages of ten to twenty years, depending on the execution quality of embedded connections or the weather protection and water tightness of junction boxes.It must be noted that all reported cases of connection failures concern early systems.It may very well be that contractors have improved their connection methods and components.Power unit failure has occurred in several cases, which makes the system stop working.This will be discovered during the next monitoring action, and their replacement is relatively simple.Reference electrodes and depolarisation sensor appear to have lives of twenty years and more.Failure has occurred, in low numbers.

Overview and conclusions
Cathodic protection reduces corrosion by injecting a current into the steel that depresses its potential.In concrete a negative shift of a few hundred millivolt is sufficient to reduce the corrosion rate to insignificant values, essentially independent of chloride levels or carbonation.This overview focused on long-term experience with ICCP systems based on activated titanium anodes and the following conclusions can be drawn.
A very long working life is possible with ICCP systems based on activated titanium, but it may be limited by failure of components of the electro technical installation.Manufacturers of activated titanium anodes claim lives for their material of 50 to 100 years, and no evidence was found of the contrary.However, several systems discussed showed evidence of some kind of failure in the current providing circuit or the monitoring system.
The parts that are most sensitive for degradation are the connections between (primary) anodes and copper cables.Accelerated corrosion of copper occurs when even the smallest electrolytic path to the concrete and the cathode (steel), arises, as the copper cannot resist anodic polarisation at typical system operating voltages.Moisture entering embedded connections or junction boxes is the culprit.Perfect isolation is necessary, which requires good execution of connections and proper sealing and/or sheltered positioning of junction boxes.Repair or replacement of failed connections usually concerns many items, and can be costly.
Failure of power units has occurred due to lightning strike, moisture entering the cabinets or simple ageing of the electronics.Such failures will be detected during monitoring sessions.Power unit replacement is relatively simple and generally not very costly.Anticipating replacement of power units around 15 years may be a good strategy [15].
Failure in the monitoring system may be due to potential sensors loosing contact, damaged cabling, connections or measuring devices.General failure rates are low [16], but with only two sensors in a zone, controllability is lost when a single electrode fails.Placing a few more sensors than strictly needed is a good strategy to maintain the ability of proper monitoring for a long time.

Fig. 2 .
Fig. 2. Cantilever beam with borehole for anode strip (left) and, after anode placement, with junction box opened for measurements (right)