Cathodic corrosion protection of steel in concrete structures
The cathodic corrosion protection (CP) by protection 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 or prevent the loss of concrete reinforcement due to corrosion. Several million m² of cathodically protected concrete surface worldwide prove the success of this process. Protective current CP has evolved and improved over the years, both in terms of materials and systems available, and in terms of system design, operation and monitoring. The standard for the application of cathodic corrosion protection to reinforced concrete structures is DIN ISO EN 12696: 2012.
The conventional way of repairing concrete structures with reinforcement corrosion is the removal of cracked or chipped concrete areas and the replacement of the rusted reinforcement. In cases where the damage is limited and thus the problem of a local nature, such an approach may be sufficient. However, this procedure often requires the removal of large parts of otherwise healthy concrete. Another problem is the durability of such repairs.
In a CP application, the extent of concrete removal is significantly lower and the quality / resp. 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 virtually frozen. The remaining life of the structure is thus extended as long as the protection current is maintained.
First signs of corrosion at the reinforcement in concrete structures are usually slight cracks in the concrete cover and / or partial signs of rust color.
This is followed by more pronounced cracks and possibly spalling, as the corrosion products have a 5-7 times larger volume than steel.
The more the concrete is weakened and the cracks also make it more open, the more the penetration of 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 pollutants will not corrode due to its passivity. An atomic thin iron oxide layer on the steel surface, which is kept stable by the pH of the concrete (about 13), causes the steel to be in a state of negligible corrosion.
The passivation can be broken by two mechanisms (promoted by sufficient moisture and available oxygen):
• Carbon dioxide action, which reduces the pH and causes a uniform, areal 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, for example, salt salts, accelerators, etc.)