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Cathodic corrosion protection

Cathodic corrosion protection of steel in concrete structures

Cathodic corrosion protection (CP) using protective current is a modern, gentle and intelligent way of treating concrete structures with corrosion problems.

CP has proven to be an effective method in many countries to prevent the loss of reinforcement in concrete due to corrosion. Several million m² of cathodically protected concrete surface worldwide prove the success of this process. CP by means of protective current has been continuously developed and significantly improved over the years, both in terms of the materials and available systems, as well as in terms of the design, operation and monitoring of systems. The set of rules for the use of cathodic corrosion protection on reinforced concrete structures is DIN ISO EN 12696: 2012.

The traditional way of repairing concrete structures with reinforcement corrosion is to remove the cracked or chipped concrete areas and replace the rusted-away reinforcement. In those cases where the damage is limited and thus the problem is local, such an approach may be sufficient. In many cases, however, this procedure requires the removal of large parts of otherwise healthy concrete. Another problem is the durability of such repairs.

With a CP application, the extent of the concrete removal is significantly less and the quality or the effect can be permanently detected and quantified by built-in sensors. For the duration of the CP operation, the current state of the building is more or less "frozen". The remaining service life of the structure is thus extended as long as the protective current is maintained.


Corrosion

Corrosion with distributor iron that has already rusted away

The first signs of corrosion on the reinforcement in concrete structures are usually slight cracks in the concrete cover and / or partial traces of rust paint. This is followed by clearer cracks and, under certain circumstances, flaking, as the corrosion products have a volume that is 5-7 times larger than that of steel.

The more the concrete is weakened and the cracks open it even wider, the more the penetration by pollutants is promoted. In extreme cases, the loss in the steel cross-section leads to a reduction in the tensile strength of the reinforcement and ultimately to a disruption of the system.

Steel in normal concrete, without pollution, will not corrode due to its passivity. Due to an atomically thin iron oxide layer on the steel surface, which is kept stable by the pH value of the concrete (approx. 13), the steel is in a state of negligible corrosion.

Passivation can be broken by two mechanisms (promoted by sufficient moisture and available oxygen):

  • The effect of carbon dioxide, which reduces the pH value and causes a uniform, extensive loss of passivation (= corrosion induced by carbonation)
  • Presence of chloride ions, which locally break through the passive film and cause pitting corrosion (= chloride-induced corrosion by e.g. de-icing salts, accelerators, etc.)

Every type of corrosion always takes place in connection with a charge transport from the metal in the direction of the electrolyte, that is, a current flows in a positive direction from the metal structure into the electrolyte. Due to the charge transport, metal ions are released from the metal structure and corrosion occurs.

Cathodic corrosion protection

Cathodic protection is an electrochemical protection process in which an electrical direct current flows through the electrolyte (pore water solution in the concrete) to the reinforcement which has to be protected. The main effect of this protective current is cathodic polarization on the surface of the reinforcement, i.e. there is a shift in the electrochemical potential "E" of the steel towards more negative values. This prevents metal ions from being released from the metal surface (see corrosion). The anodic sub-process (iron dissolution) of corrosion is suppressed or prevented (metal removal rate <0.01 mm / A). The result is a change in potential to values at which the entire steel becomes cathodic compared to the anode, hence the term “cathodic protection”.

The operation of a CP system creates an electric field between the reinforcement and the anode, which in addition to the change in potential leads to two other positive side effects in the immediate vicinity of the steel. Under the influence of the field, all negatively charged ions (OH-, CL-, SO42-) migrate away from the reinforcement, towards the anode. This electromigration is limited in normal CP operation, but there is a certain distance between the chloride ions and the steel surface, which significantly reduces the further risk of corrosion. As a second side effect, it turned out that with the current densities that are usually used for CP, a thin layer of the (carbonated) concrete is created around the steel (pH value increase). The steel is brought into a more passive state, which means that the corrosion can no longer be triggered or continue to progress. This effect could also be observed at low chloride concentrations.

 

With the CP-B, an anode is attached, either drilled, slotted or mounted on the surface of the concrete. It is connected to the positive pole and the reinforcement to the negative pole of a voltage source. Various checks must be carried out before the anode is attached, such as measuring the continuity of the reinforcement.

In order to assess the quality of the protection achieved by the applied current, sensors are installed near the steel. There are different types of sensors. The most common are reference electrodes (RE). These electrodes measure the electrochemical potential in relation to a known potential. The most widely used and accepted criterion for verifying the effectiveness of CP is the 100 mV depolarization (switch-off) criterion. It indicates that the steel is adequately protected if a potential drop of at least 100 mV is recorded and that over a period of 24 hours after the power has been switched off. It should be noted here that each reading only reflects the location where the reference electrode is placed. It is important that the RE's are arranged in representative places and in sufficient numbers (> 1 RE per 100 m2).

Anode systems

Depending on the location and the associated situation of the reinforcement to be protected, the following anode types / systems can be used:

Advantages

  • High economic efficiency because of
  • no removal of chloride contaminated concrete necessary
  • low energy expenditure
  • less noise pollution
  • less vibrations
  • little interference with the structure and the environment
  • Repair possible during ongoing operation
  • low disturbance of the environment / surroundings
  • less construction time

Especially for multi-storey car parks / underground garages:

  • Little loss of rent
  • No full closure of a parking level

High sustainability / verifiability through:

  • continuous operation
  • continuous monitoring through built-in sensors
  • regular maintenance

Our range of services:

  • Advice during the planning of a repair
  • Preliminary examinations
  • System design and detailed planning of a CP plant
  • Execution CP-B
  • Operations
  • Maintenance (inexpensive with the help of remote data transmission)