Corrosion protection

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The word corrosion was derived from latin word “corrodere” for zernagen. The DIN 50900 “corrosion of the metals” part of 1corrosion describes as “the reaction of a metallic material with its environment, which can lead a measurable change of the material effectuation and to an impairment of the function of a metallic construction unit or a whole system”. For example if iron or steel is exposed to environmental influences, then that sets in the best way admitted“rust”, thus that oxidize the iron to ferric oxide.

Bild:Korrosion.jpg

In order to prevent this chemical process, there are different procedures, which one generally calls corrosion protection. These chemical and physical techniques very different in the reason can be divided into few categories:

  • Protection by a barrier layer
oneBarrier layer is as impermeable and not electrically conductive a layer as possible like for example a lacquer or a conversion layer, i.e. a phosphatization, an anodized coat, a hard anodized coat, a chromate finishing or other transformation layers with a rather ceramic character. Also galvanotechnisch or chemically produced metallic surface layersfrom tin, gold, nickel, copper or alloy layers such as nickel phosphorus (chemical nickel) work as barrier layer. These layers are conductive, in addition, very close. Crucial it is here that these layers are nonporously separated. When galvanic nickel plating nickel works as cathodically more effective,passive corrosion protection. The nickel layer forms a surface layer, which prevents that corrosively active substances like e.g. Water and oxygen with the basic material (e.g. Iron) into contact come. Is nickel does not only corrosion, separates also heatproof, duktil and has decorative characteristics. At ambient temperature is nickelsteadily against air, water, not-oxidizing acids (like e.g. Hydrochloric acid), caustic solutions and most organic materials. The metal is however even not completely protected against corrosion. So diluted nitric acid can quite dissolve nickel. However a passivation layer made of nickeloxide under certain conditions, which those, formsStability of the surface continues to increase.
Unedlere layers protect the workpiece and function thereby than victim or protection anode - the layer dissolves preferentially and keeps the function of the construction unit thus as long as possible. A classical example is the galvanization of steel, in addition, the protection of hydraulic structures like e.g. Ships, air-locks, sheet pile walls, boat parts and rails by distant sacrificial anodes from zinc -, aluminum - or magnesium - alloys. In addition cathodic corrosion protection - short KKS - is possible also by means of foreign stream and foreign current anodes.
Bild:Zinkkorrosion.jpg
Iron stands inthe electro-chemical electromotive series more positively than zinc, i.e. Zinc is more unedler than iron and represents in the galvanic element the anode, and iron is the cathode. Iron as the nobler metal is so long cathodically protected therefore until the zinc away-corroded.
  • Protection of a piping system throughCorrosion inhibitors, those into the water circulation to be given
  • for the statement of the corrosion resistance of coatings are accomplished standardized environmental checks. Here e.g. become. Surfaces with cutting testing sets easily hurts, and then a salt spray suspended.




With the active corrosion protection one pursues the goal a metal that often in contact alsofor example water comes against rust to protect. For this one manages a unendlerem material, which is sacrificed quasi for the metal. The reaction runs off as follows: (by the example Fe/Mg) as soon as magnesium with the water comes into contact becomes it Mg^2+ and e^2 oxidizes.This reaction runs off only very slowly, can be accelerated however by changed conditions. Electrons the meeting the water split this up now into H2 and 2OH^. The iron does not change, since it can take up also the electrons delivered by the magnesium again. ThatMagnesium however dissolves you after a certain time and the rust protection for the iron must be renewed if necessary.


Attempt: [Example: Iron nail]

1. An unprotected iron nail is given to acidified seawater. To some time the iron goes into solution and corrodes, there no sacrificial anode availableis. The moreover hydrogen (H2) at the iron nail forms. Bild:eisennagel_neu.gifFig. 1
2. The iron nail is protected with the unedleren magnesium. It comes to the formation of a local cell, as magnesium works as anode and sacrifices themselves thus for the iron. It forms for hydrogen. Besides developsH2 at the Fe-cathode. The reason for this is to be looked for in the electron flow from the mg to the Fe, since mg is more unedler than iron and thus a larger reduction ability possesses.

Bild:eisennagel_anode_mg_neu.gif Fig. 2

3. If one takes for example nobler copper, then iron works as anode and thatCopper cannot protect the iron against the corrosion. It is remarkable that the iron oxidizes here faster. Also here an electron flow takes place, which runs from the Fe to the cu. Thus the corrosion is accelerated.

Bild:eisennagel_anode_cu_neu.gif Fig. With the fact

3 it is to be noted that depending upon application thoseAnode differently to select is. The moreover one the anode must be directly connected with the metal, so that a complete protective function is ensured.

see also

 

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