Operational Area

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Cathodic Corrosion Protection

Cathodic corrosion protection is an electrochemical protection method where an electric direct current is introduced into the protected object (metal) via the corrosive medium. Cathodic polarization of the system metal / plating solution takes place. The metallic potential shifts to higher negative values (protective potential). Cathodic corrosion protection is based on the corrosion rate as a function of the potential (see figure below).

The cathodic protection is achieved when the corrosion rate has fallen to a technically negligible smaller value.

Difference is made between the two types of cathodic corrosion protection: the full protection which can be achieved by all common carbon-steels and the partial protection which mainly compensates the cell currents of couplings of materials, e.g. carbon-steel / CrNi-steel, respectively placing the potentials of CrNi-steels out of the range of pitting corrosion.


Material loss in different electro-solutions - cathodic Corrosion Protection Material loss in different electro-solutions as a function of potential. The hatched areas show the data of atmospheric exposure corrosion. The corresponding potential is the free corrosion potential resp. the rest potential. With more negative potentials cathodic polarization is ensued i.e.cathodic corrosion protection.


Application - Examples:

 

 

  • For seawater and port facilities
  • For Deep-wells
  • For reinforcing steel used in concrete structures
  • For the inner surfaces of turbines in water power plants
  • For power plant condensers
  • For water tanks

Cathodic Corrosion Protection for Reinforcing Steel used in Concrete Structures

Impressed-current cathodic corrosion protection in reinforcing steel is a relatively new method of protection. Although it was already being applied on a trial basis at the end of the fifties, its use for rehabilitating corroded ferro-concrete structures was not persued because a lack of appropriate anode materials meant that cell-voltages of 15 to 200 volt were necessary.

As a result of previous experience there were fears that cathodic alkalization would lead to a breakdown in the structure of the steel-concrete compound, and therefore no further technical development occured.

Cathodic corrosion protection as a method of rehabilitation was taken up again in 1974 beacause of growing damage to traffic structures as a result of Cl-induced corrosion of the reinforcing steel, which was leading to high rehabilitation costs in the USA. This process was encouraged by the great deal of negative experience being made with other methodes of rehabilitation.

Reinforcing Steel for Concrete Piles

 

Cathodic Corrosion Protection for Reinforcing Steel used in Concrete Structures


Cathodic Protection for the Inner Surfaces of Turbines in Water Power Plants

Operators of water power plants are often facing corrosion problems with their water power turbines. In particular, these corrosion problems are occuring with high-chloride mediums, caused by couplings of materials. These corrosion problems can be prevented by installing a cathodic corrosion protection unit. However, an electrolytic compound between the surfaces to be protected and the anodes to be installed is required and all parts to be protected must guarantee for a problem-free electric connection. Depending on the turbine types as well as on engineering and flow conditions, anodes and reference alectrodes of various types are installed in the turbine and in the inlet and outlet chambers. Due to the geometric arrangement, special attention must be paid to the current distribution and to potential measurements in critical areas, e.g. near by the sites of contact of differentmaterials. Depending on the variability of the operating conditions, e.g. the alternating conductivity and flow rate of water, we are using potential controlled protective current units only. Units of this type ensure the constant flowing of the protective current for the cathodic protection and the reaching resp. holding of the present protective potential. The power consumption of these protective units is considered as very low.

Arrengements of Anodes and Reference Electrodes inside Water Turbines for Sea Water

 

diagram inside Water Turbines for Sea Water - cathodic Corrosion Protection

 

Inside Water Turbines for Sea Water - 1 - cathodic Corrosion Protection Inside Water Turbines for Sea Water - 2 - cathodic Corrosion Protection


Cathodic Corrosion Protection for Power Plant Condensers

Arrangement and design of anodes are of great influence for cathodic internal corrosion protection of power plant condensers, due to their relatively high electrolytic resistance. As material couplings of condenser tubes, tubesheets and water chamber cannot be avoided, it is important to arrange the reference electrodes in such a geometric order that all critical condenser areas are included. Therefore the following design and corrosion parameters must be taken into account when planning the cathodic corrosion protection unit:

 

 

  • As a direct metallic connection to the concerned tubesheet and the heat exchanger tubes exists via the flanges of the condenser lid and as the condenser tubes are usually made of uncoated nonferrous heavy metal, a voltaic cell builds up on the parts wetted by the elektrolyte. Due to its base potential in comparison with the nonferrous heavy metal, the carbon-steel builds the corroding anode in this cell. In order to protect the carbon-steel cathodically against corrosion, the control unit must compensate the cell current. The same applies for material couplings of CrNi-steel / carbon-steel.
  • The water conductivity, temperature and flow rate are of crucial importance for the design of the cathodic corrosion protection units.
  • When determining the surface to be protected, it must be distinguished between surface of coated carbon-steel, bright non-ferrous heavy metal and CrNi-Steel.


Arrengement of Anodes and Reference Electrodes in Power Plant Condensers

 

Power Plant Condensers - 1 - Cathodic Corrosion Protection Power Plant Condensers - 2 - Cathodic Corrosion Protection


Cathodic Protection for Water Tanks

The cathodic internal corrosion protection of water tanks is most economical when it has already been taken into account during planning. However, the equipment can also be installed later when rehabilitating older tanks in order to stop the progress of corrosion. The mounting parts for cathodic protected uncoated water tanks must have a faultless electrical connection to the tank in order to avoid damage caused by anodic influence in the cathodic voltage horn of the protected surfaces. The size of the water tank is of decisive importance when selecting the type and number of anodes and reference electrodes. If a tank has a small diameter, rod anodes of various types can be mounted into the tank shell by means of threaded pressure joints. In order to achieve an optimal current distribution, wire anodes are preferred for tanks with larger diameters. The number of rod and wire anodes is determined by the size of the structure to be protected, the type of medium and the quality of the coating. For protective potential measurements, the reference electrodes are to be mounted on geometrically suitable places by means of threaded pressure joints. Due to changing water levels and operation conditions, we only use potential controlling protective current units. All the materials which we install in tanks can also be used in drinking water tanks problemless.

Cathodic Protection for Water Tanks This figure shows a water tank with two wire anodes, braced vertically inside the tank an installed by means of R1 threaded pressure joints. Two Reference Electrodes of the Ag/AgCl type been intalled for the purpose of potential measurement.


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