Cathodic protection

The cathodic protection makes it possible to protect a Métal against the Corrosion. The basic principle is to carry the potential of a metal on a level known as of passivation. To modify the potential of metal to be protected cathodiquement, one uses an anode installed in same the electrolyte. The Anode S can be of two types is anodes having a potential more electronegative than metal to be protected (sacrificial anode), is anodes coupled to a generator of Tension continues imposing a potential difference between two metals (method with imposed current). Cathodic protection is a technique to control the corrosion of a metal surface by transforming this surface into the cathode of an electrochemical cell. Cathodic protection is used to protect the metal structures of corrosion, in particular steel, the pipelines water, the pipelines, the tanks, the metal pillars of the piers, the oil Navire S and platforms, the reinforced concrete structures.

Origins

The first use of cathodic protection goes up with 1824, when Sir Humphry Davy, of British Navy, outside fixed pieces of iron at the lower part of the water line on the copper coating of the hull of a ship. Iron corrodes more easily in the form of rust than copper when it is in contact with the hull so that the speed of corrosion of copper is some considerably reduced.

Galvanic cathodic protection

Today the reactive anodes or sacrificial can have various forms and are made by using Alliage S of Zinc, Magnésium and Aluminum. The Electrochemical potential , the capacity of current and the rate of consumption of these alloys are higher for cathodic protection than those of iron.

The sacrificial anodes are conceived and chosen to have an electrochemical potential more negative than the metal of the structure to be protected. So that cathodic protection is effective surface to be protected is polarized. The force which underlies cathodic protection is the potential difference electrochemical between the anode and cathode.

To determine the type and the quantity of anodes necessary for the protection of a given structure, the principal factors which intervene are the dimension of the surface exposed to the corrosive agent (sea water, for example), the nature of this surface (material, coating, roughness…) and lifespan of the structure or the interval of renewal of the anodes. The nature of the corrosive agent (the temperature and the salinity of sea water, for example) has also an influence to take into account in calculations. The sacrificial anodes are naturally self-regulated and answer at the request of corrosion of metal to protect, the currents follow the daily and seasonal thermal cycles then, as well as the ambient hygroscopy (rains, tides).

The structures can be various natures, steel, reinforced concrete or mixed. According to its form and the quantity of metal to protect a structure could be polarized more or less quickly. In the case of the reinforced concrete the sacrificial anodes are laid out regularly between the steel reinforcements. The characteristics of the anodes are generally given in capacitive load (kC: Coulomb kilo) and dimensions (Length and Diameter). The reactions of polarization which are relatively slow can be accelerated by a temporary load of a few days under imposed current, supporting with cathode the reduction of the oxidized medium and the creation of sites of oxidation on the surface of the anodes, it acts then of system says hybrid privileging the durability and the longevity of the anode in galvanic mode.

The mediums tending to balance, the currents are stabilized after a few months, the control of the potentials then becomes the means easiest to control the activity of the protective system. The alloys containing Zinc tend car-to be passivated, the contribution of alkaline coating maintaining the activity of the anode in the reinforced concrete structure is essential at the time of the installation of the anodes in the concrete structure.

Cathodic protection with imposed current

For installations larger or badly mechanically isolated by a coating from bad quality, the sacrificial anodes can not deliver sufficient current for an optimal protection. In the systems with current imposed of the anodes are then connected to a generator of permanent or cyclic D.C. current (the rectifier of current functions according to preset sequences of time). These anodes are in the shape of tube (to allow the hydrogen degasification) or of compact stem of various materials dedicated such as the Acier, of the cast iron, the Graphite, metallic oxides, covered wire of Platine and Niobium…

To test the installation

The Electrochemical potential is measured with electrodes of reference. Electrodes Copper-Copper II are used for structures in contact with the ground or of fresh water. Cerargyrite electrodes are used for the applications out of sea water. During repair of affected reinforced concrete by corrosion the use of electrodes of references Manganese/Manganèse Oxide makes it possible to have a stability of the measure to time without major drift. The electrodes with the cerargyrite release from the Chlore ions in the structure, thus disturbing true measurement.

Galvanized steel

The cars of today are made with galvanized steel parts (covered with Zinc). The steel not protected form a layer from iron oxide which is permeable with the air and water so that corrosion continues inside. On the other hand the Zinc oxide produced on the surface of the layer of Zinc is impermeable. As long as the zinc zinc and oxide are intact (i.e. as long as they are not exhausted or are not striped) steel that there is below will not corrode.

Galvanized steel has car-repairing properties, indeed small scratches which would expose steel will overlap with zinc. This occurs because zinc present around the scratch will dissolve and settle on steel by replacing that which disappeared during the scratch.

External bonds

the French Council of cathodic protection CFPC
Corrosion of the reinforced concrete

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