Schikorr reaction
Encyclopedia
The Schikorr reaction formally describes the conversion of the iron(II) hydroxide
(Fe(OH)2) into iron(II,III) oxide
(Fe3O4).
The bases of this transformation reaction were 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 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 counter-balanced 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:
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.
Iron(II) hydroxide
Iron hydroxide or ferrous hydroxide is a compound produced when iron ions, from a compound such as iron sulfate, react with hydroxide ions. Iron hydroxide itself is practically white, but even traces of oxygen impart it with a greenish tinge...
(Fe(OH)2) into iron(II,III) oxide
Iron(II,III) oxide
Iron oxide is the chemical compound with formula Fe3O4. It is one of a number of iron oxides, the others being iron oxide , which is rare, and iron oxide also known as hematite. It occurs in nature as the mineral magnetite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙...
(Fe3O4).
The bases of this transformation reaction were first studied by Gerhard Schikorr, a German specialist of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
, 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:
- 3 Fe(OH)2 → Fe3O4 + H2 + 2 H2O
Reaction mechanism
The Schikorr reaction involves two distinct processes:- the anaerobic oxidation of two Fe(II) (Fe2+) into Fe(III) (Fe3+) by the protons of water. The reductionReductionReduction, reduced, or reduce may refer to:- Chemistry :* Reduction, part of a reduction-oxidation reaction where oxygen is being removed from a compound.** Reduced gas, a gas with a low oxidation number...
of two water protonProtonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s is accompanied by the production of molecular hydrogen (H2), and; - the loss of two water molecules from the iron(II) and iron(III) hydroxides giving rise to its dehydration and to the formation of a thermodynamically more stable phase iron(II,III) oxide.
The global reaction can thus be decomposed in half redox reactions as follows:
- 2 (Fe2+ → Fe3+ + e–) (oxidation of 2 iron(II) ions)
- 2 (H2O + e– → ½ H2 + OH–) (reduction of 2 water protons)
to give:
- 2 Fe2+ + 2 H2O → 2 Fe3+ + H2 + 2 OH–
Adding to this reaction one intact iron(II) ion for each two oxidized iron(II) ions leads to:
- 3 Fe2+ + 2 H2O → Fe2+ + 2 Fe3+ + H2 + 2 OH–
Electroneutrality requires the iron cations on both sides of the equation to be counter-balanced by 6 hydroxyl anions (OH–):
- 3 Fe2+ + 6 OH– + 2 H2O → Fe2+ + 2 Fe3+ + H2 + 8 OH–
- 3 Fe(OH)2 + 2 H2O → Fe(OH)2 + 2 Fe(OH)3 + H2
For completing the main reaction, two companion reactions have still to be taken into account:
The autoprotolysis of the hydroxyl anions; a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
exchange
Exchange
-Finance and Commerce:* Exchange of goods and services, or trade* Exchange between a buyer and seller, a financial transaction* Exchange , where securities are sold and bought-Communications and Technology:...
between two OH–, like in a classical acid-base reaction:
- OH– + OH– → O2– + H2O
- acid 1 + base 2 → base 1 + acid 2, or also,
- 2 OH– → O2– + H2O
it is then possible to reorganize the global reaction as:
- 3 Fe(OH)2 + 2 H2O → (FeO + H2O) + (Fe2O3 + 3 H2O) + H2
- 3 Fe(OH)2 + 2 H2O → FeO + Fe2O3 + 4 H2O + H2
- 3 Fe(OH)2 → FeO + Fe2O3 + 2 H2O + H2
Considering then the formation reaction of iron(II,III) oxide:
- Fe(II)O + Fe(III)2O3 → Fe3O4
it is possible to write the balanced global reaction:
- 3 Fe(OH)2 → (FeO·Fe2O3) + 2 H2O + H2
in its final form, known as the Schikorr reaction:
- 3 Fe(OH)2 → Fe3O4 + 2 H2O + H2
Occurrences
The Schikorr reaction can occur in the process of anaerobic corrosion of ironIron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and carbon steel in various conditions.
Anaerobic corrosion of metallic iron to give iron(II) hydroxide and hydrogen:
- 3 (Fe + 2 H2O → Fe(OH)2 + H2)
followed by the Schikorr reaction:
- 3 Fe(OH)2 → Fe3O4 + 2 H2O + H2
give the following global reaction:
- 3 Fe + 6 H2O → Fe3O4 + 2 H2O + 4 H2
- 3 Fe + 4 H2O → Fe3O4 + 4 H2
At low temperature, the anaerobic corrosion of iron can give rise to the formation of "green rust" (fougerite
Fougerite
Fougerite is a recently discovered representative of clay minerals, chemically and structurally related to so-called green rust, hydrotalcite-like minerals and other layered double hydroxides. The formula of the mineral is complex due to structural variabilities, as is for green rust, thus...
) an unstable layered double hydroxide (LDH
LDH
LDH is an acronym which may refer to:* Lactate dehydrogenase, an enzyme in a wide range of plants, animals and other organisms; is measured as part of the complete blood test* Large diameter hose, a type of fire hose...
). 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
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
ions are present in solution, they can also evolve towards more stable carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
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
Malachite
Malachite is a copper carbonate mineral, with the formula Cu2CO32. This green-colored mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses. Individual crystals are rare but do occur as slender to acicular prisms...
) in the copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
system.