Cathodic corrosion protection (CCP)

Older building and civil engineering structures, such as bridges, tunnels and multi-storey car parks, are particularly affected. Unrecognised damage or underestimated damage patterns can result in cost-intensive renovation measures in the medium term and sometimes even the demolition of the property. This makes it even more important to recognise damage processes at an early stage and stop them before it is too late. Cathodic corrosion protection (CCP) is a fast, effective, time-saving and cost-effective method of repairing chloride-contaminated structures.

With conventional repair methods, the chloride-contaminated concrete often has to be removed very deeply, which represents a considerable intervention in the structure. In addition to the high costs for the intervention in the building fabric, the restriction of use during the sometimes lengthy repair work also speaks against these methods. Cathodic corrosion protection (CCP), on the other hand, is a largely non-destructive repair method. This globally unique and patented system solution, which MC-Bauchemie and Grillo-Werke AG have developed in co-operation, helps to preserve damaged but still functional reinforced concrete permanently and economically.

Danger for reinforced concrete: chloride ingress and carbonation

In principle, reinforced concrete itself is able to provide corrosion protection for the reinforcing steel used: due to the high pH value of over 13, a passive film forms on the steel surface that is only a few atomic layers thick and protects the underlying steel from corrosion. This passive protective function can be lost through carbonation and penetrating chlorides. The reinforced concrete then loses its alkalinity and begins to corrode. In the former process, carbon dioxide (CO2) diffuses from the ambient air into the concrete and reacts with the consumption of calcium hydroxide (Ca(OH)2) to form calcium carbonate (CaCO3). This reaction process dissolves the passive film, whereby the corrosion protection of the reinforcing steel is lost. Measures to prevent the passivation of concrete, such as the selection of a suitable concrete with a high alkalinity reserve and sufficient concrete cover or a surface protection system, are therefore advisable.

Pitting due to the penetration of chlorides

The far greater danger comes from penetrating chloride ions, which mainly reach the reinforcing steel through frost/de-icing salt ingress, as usually occurs in multi-storey car parks, and via moisture transport through capillaries in the concrete. There they interact with the passive film. If the critical quantity of chloride ions is exceeded, the passive film dissolves and the steel begins to corrode. This is also referred to as pitting, which occurs without being recognisable from the outside. Action should then be taken quickly, as the initiated corrosion process cannot be stopped by coating or concrete replacement measures alone and consequently the static function of the reinforced concrete and therefore the entire structure is at risk.

In the corrosion process of steel in concrete, the anodic partial reaction takes place with oxidation of the iron-to-iron ions (Fe2+), which dissolve in the moist concrete, the electrolyte. The cathodic partial process is the reduction of water and oxygen by the excess electrons with the formation of hydroxide ions (OH-), so that the charge balance in the electrolyte is maintained. The electrical circuit is closed by the electrical conductivity of the concrete pore solution, as a result of which the corrosion of steel in concrete takes place as an electrochemical process with the formation of a galvanic element.

The content of chloride ions at which the corrosion of the steel in the concrete begins is referred to as the ‘critical corrosion-inducing chloride content’. The content is decisive for the duration of the initiation phase and determines the initiation of corrosion. It is known that this content is not a concrete limit value but depends on factors such as porosity and the chemical composition of the concrete. In practice, different values are often found, which inevitably indicate depassivation of the concrete. It has been shown that the ‘critical’ chloride content must be determined for each structure with its specific boundary conditions.

Long-term protection through cathodic corrosion protection

Just like corrosion itself, cathodic corrosion protection (CCP) is based on electrochemical processes. Processes that intervene directly in the chemical or electrochemical reaction processes are referred to as active corrosion protection. The basic aim of CCP is to reduce the corrosion rate and polarise the reinforcement in the cathodic direction.

In corrosion protection using the MC-KKS/B system solution from MC-Bauchemie and Grillo-Werke AG, the less noble metal zinc is introduced into the electrochemical process as a sacrificial anode - comparable to a battery - and applied to the concrete as a layer around 150 µm thick. The zinc layer is connected to the reinforcing steel by means of a contact plate. The electrical circuit is closed by the concrete pore water, which serves as the electrolyte. The anodic partial reaction of iron dissolution is prevented by the protective current supplied by the zinc anode. This protective current flows to the reinforcement via the attached contacts. The negatively charged ions move towards the reinforcement and the positively charged ions move away from the reinforcement towards the anode.

The application

To prepare the substrate for cathodic corrosion protection, it is sufficient to remove the partially loose concrete to reprofile these areas, as well as cavities and breakouts, with a special repair mortar. All the chloride-contaminated concrete remains. At the same time as the concrete is being repaired, the bonding points that will later connect the zinc layer and the reinforcement in an electrically conductive manner are installed. Once the classic substrate preparation has been completed, the zinc coating is applied to the concrete surface by thermal arc spraying. Arc spraying describes the process in which an electrically conductive material is converted from a solid to a liquid state by means of an electrical discharge process. The melted zinc particles are then blasted onto the concrete surface with compressed air. There, the particles solidify and form a flat zinc spray layer. This process can be applied to floor, wall and ceiling surfaces. Application to floor surfaces can be carried out using an automated coating unit, which increases efficiency compared to manual application.

The zinc layer is then sealed on top and various standard structures can follow. Finally, an organic topcoat system is applied to the zinc layer, which ensures mechanical resistance and protects against the further penetration of chlorides and moisture.

Time and cost-saving repairs

The advantages of this system are, on the one hand, the high-cost savings due to the elimination of cost-intensive chiselling work. On the other hand, the system saves a great deal of time during installation, as there is no need for the curing times that would be required for a new structure with concrete and mortar. The system is quickly applied, and the renovated property is quickly ready for use again. In the event of a car park refurbishment, this means that downtimes are minimal. In addition, the headroom and load-bearing capacity are not reduced, as only an average 150 µm thin zinc coating is applied.

Cathodic corrosion protection using zinc as a sacrificial anode is based on the experience of almost 20 years of development and offers the possibility of repairing damaged but still functional reinforced concrete more cost-effectively and more quickly than with conventional methods and maintaining its condition permanently and safely.