Schikorr reaction
The Schikorr reaction formally describes the conversion of the iron(II) hydroxide (Fe(OH)2) into iron(II,III) oxide (Fe3O4).
The basis of this transformation reaction was first studied by Gerhard Schikorr, a German specialist of iron corrosion, in his early works (~1928-1933) on iron(II) and iron(III) hydroxides. The global reaction that Schikorr proposed to explain his observations onto the iron hydroxides conversion, and which later received his name, can be written as follows:
The Schikorr reaction involves two distinct processes:
The global reaction can thus be decomposed in half redox reactions as follows:
to give:
Adding to this reaction one intact iron(II) ion for each two oxidized iron(II) ions leads to:
Electroneutrality requires the iron cations on both sides of the equation to be counterbalanced by 6 hydroxyl anions (OH−):
For completing the main reaction, two companion reactions have still to be taken into account:
The autoprotolysis of the hydroxyl anions; a proton exchange between two OH−, like in a classical acid-base reaction:
it is then possible to reorganize the global reaction as:
Considering then the formation reaction of iron(II,III) oxide:
it is possible to write the balanced global reaction:
in its final form, known as the Schikorr reaction:
The Schikorr reaction can occur in the process of anaerobic corrosion of iron and carbon steel in various conditions.
Anaerobic corrosion of metallic iron to give iron(II) hydroxide and hydrogen:
followed by the Schikorr reaction:
give the following global reaction:
At low temperature, the anaerobic corrosion of iron can give rise to the formation of "green rust" (fougerite) an unstable layered double hydroxide (LDH). In function of the geochemical conditions prevailing in the environment of the corroding steel, iron(II) hydroxide and green rust can progressively transform in iron(II,III) oxide, or if bicarbonate ions are present in solution, they can also evolve towards more stable carbonate phases such as iron carbonate (FeCO3), or iron(II) hydroxycarbonate (Fe2(OH)2(CO3), chukanovite) isomorphic to copper(II) hydroxycarbonate (Cu2(OH)2(CO3), malachite) in the copper system.
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