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Cathodic Protection in Concrete and Reinforcing Steel

Feb. 14, 2022

Cathodic Protection in Concrete and Reinforcing Steel

Corrosion of reinforcing steel in concrete is a major cause of failure of reinforced concrete, concrete is a porous material formed by cement, sand and gravel and water mixed together after solidification, in the tiny pores of concrete contains a ph value of up to 12 or more concrete pore solution, in such conditions the pre-buried reinforcing steel in concrete will occur passivation and not rust, but with the passage of time or concrete components However, with the passage of time or cracks in the concrete, water, oxygen, carbon dioxide and chloride ions in the external environment will penetrate into the concrete, causing changes in the composition and nature of the pore solution around the reinforcement, which will lead to corrosion of the reinforcement once the passivation film on the surface of the reinforcement is destroyed. The structure loses its proper load-bearing capacity.

MMO Coated Titanium Anodes supplier DINOR shows you.

In coastal concrete buildings, the air contains salt, salt precipitation in the concrete structure, in the northern concrete structure, snowmelt in winter to remove snow. As the snowmelt contains chloride ions, chloride ions along the concrete gap or through the micro-pores in the concrete penetrate into the concrete structure, erode the passivation film of the reinforcing steel, and cause the corrosion potential of the local reinforcing steel to shift to the negative.

As the main component of the applied current cathodic protection auxiliary anode, the deep well anode has undergone various material improvements and developments. Traditional deep-well anodes mostly use silicon-iron anodes and precious metal oxide cylindrical anodes. However, the former is only protected on the side near the bottom bed of the anode and cannot effectively protect the internal pipeline. When used as deep well anodes, there are problems such as increased construction difficulties due to their heavy weight. Metal oxide MMO cylindrical anodes have the advantages of small footprint, large protection area, unrestricted point selection, long service life, and simple installation and maintenance.

Cathodic protection principle

The anode network is installed on the outside of the concrete structure and connected to the positive power supply, and the negative power supply is connected to the rebar. As the rebar potential is negatively shifted, a reduction reaction (O2+2H2O+4e-=4OH-) occurs near the rebar, producing hydroxide ions near the rebar, which increases the pH value near the rebar and enhances the alkalinity, which is conducive to the passivation of the rebar.

The cathodic protection current attracts chloride ions away from the rebar and tends to the anode, reducing the chloride ion content near the rebar and slowing down the corrosion.

The rebar potential is negatively shifted and cathodic polarization occurs.

Cathodic Protection in Concrete and Reinforcing Steel

The structure of concrete steel pipe

To prevent hydrogen embrittlement of the reinforcement caused by over-protection, zinc anodes are mostly used for protection. According to the difference of soil resistivity, different shapes and numbers of anodes are selected, filled with filling materials and then evenly arranged along the pipeline. In order to ensure the electrical continuity of the pipeline, a span connection is made at the interface of the concrete pipe.

In order to measure the disconnection potential of the reinforcement, a polarization probe is often used for disconnection potential measurement. The probe is made of reinforcement cast in concrete and buried near the concrete pipe, which is cathodically protected as well as the pipe and instantaneously disconnected during measurement. It is also possible to select some typical independent pipe sections where the sacrificial anode is connected to the pipe through a test pile, and the sacrificial anode is disconnected from the pipe during the measurement to measure the disconnection potential of the pipe.

Cathodic Protection in Concrete and Reinforcing Steel

MMO Titanium Anode Ribbon/titanium conductive tape

Design of cathodic protection

In the design process of concrete steel cathodic protection, the demand for anodes is first calculated according to the soil resistivity and the output current capacity of the anodes, then the service life of the anodes is accounted for, the amount of anodes is calculated, and a design safety factor of 1.5 times is selected. Place the anodes evenly along the water pipeline. As the prestressed concrete water pipeline is sensitive to the protection potential, in order to prevent the problem of hydrogen causing steel fracture brought by overprotection, sacrificial anode protection is mostly used, and zinc anodes are mainly used.

For more info and application, please contact DINORE today.

Cathodic Protection in Concrete and Reinforcing Steel