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According to newer investigations, good corrosion inhibiting layers or films contain a minimum of 25 % Fe2+ regarding the total iron - cations. A great part of this Fe2+ constitutes of FeCO3 (Siderite). Based on this, it is concluded that the structure and kind of corrosion products are responsible for the formation of good corrosion inhibiting layers. A further important part are the carbonic acid and their derivatives, since the OH-formed in the cathodic reaction react with bicarbonate according to the following equation:

OH + HCO3 CO3 + H2O

The secondary reactions can be summarized as follows:

I. Ca2+ + CO3 CaCO3 (Calcite)
II. Fe2+ + CO3 FeCO3 (Siderite)
III. 1/2O2 + H2O + 2FeCO3 FeOOH + 2CO2 (Goethite)
IV. 2Fe2+ + 1/2O2 + 3H2O FeOOH + 4H+ (Magnetite)
V. 3Fe2+ + 1/2O2 + 3H2O Fe3O4 + 6H+ (Magnetite)
VI. 1/2O2 + 3FeCO3 Fe3O4 + 3CO2 (Magnetite)

According to this newer theory the formation of siderite and not calcite is the key reaction in the corrosion process. Only if a considerable part of the secondary reaction leads according to equation II the formation of siderite, good corrosion inhibiting layers can be expected. The primary formation of insoluble Fe2+ components also assists in the formation of mixed Fe2+/Fe3+ oxides, which also show a good inhibiting effect. Reactions III and IV and thus do not negatively influence the formation of corrosion inhibiting layers.

Deposition of siderite which is by factor 100 less soluble than CaCO3, results by the actual corrosion process which yields Fe2+ and OH-. The pH-value at the metal surface thus increases, and CO3--ions for the formation of siderite are available.

Direct oxidation of Fe2+ according to equations IV and V will always be preferred to siderite formation, resulting in loose non-uniform layers giving no corrosion protection, if the pH on the metal surface is very high and the carbonate concentration very small.

This explains the influence of the buffer capacity of the water on the corrosion rate.

Based on the before-mentioned findings, the following conclusions can be made on the formation of corrosion protective layers (without the addition of treatment chemicals):

  1. In natural water high bicarbonate concentrations are always accompanied by high buffer capacities and relatively low pH-value. Due to the high buffer capacity the so- called "surface alkalinity" is low, resulting in a slow oxidation of the Fe2-formation of siderite, is preferred to calcite formation. In the presence of oxygen, the siderite can be partially or fully oxidized.

    The forming corrosion inhibiting layer decelerates the diffusion of oxygen more strongly than a layer of FeOOH x H2O developed by the direct oxidation of iron.
     
  2. Low bicarbonate concentrations and low buffer capacities result in unfavourable layers. The formation of Fe2+ is decelerated and its further oxidation to Fe3- accelerated because of a higher "surface alkalinity" and pH-value.

    Due to a further reaction of the Fe2+, mainly calcite is formed on the metal surface, and only loose, non-uniform layers giving no corrosion protection do result.
     
 
 
 
 
 
 

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