Anaerobic corrosion (also known as hydrogen corrosion) is a form of metal
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
occurring in
anoxic water. Typically following
aerobic
Aerobic means "requiring air," in which "air" usually means oxygen.
Aerobic may also refer to
* Aerobic exercise, prolonged exercise of moderate intensity
* Aerobics, a form of aerobic exercise
* Aerobic respiration, the aerobic process of cel ...
corrosion,
anaerobic
Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to:
* Anaerobic adhesive, a bonding a ...
corrosion involves a
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
reaction that reduces
hydrogen ions
A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...
and oxidizes a solid metal.
This process can occur in either
abiotic
In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them under ...
conditions through a thermodynamically
spontaneous reaction or
biotic conditions through a process known as
bacterial anaerobic corrosion. Along with other forms of corrosion, anaerobic corrosion is significant when considering the safe, permanent storage of chemical waste.
Chemical mechanisms
The overall process of corrosion can be represented by a
bimodal function, where the type of corrosion varies with time, including both oxygen-driven and anaerobic mechanisms.
The dominant process will depend on the given conditions. During oxygen-driven corrosion, layers of rust form, creating various non-homogenous anoxic niches throughout the metal's surface.
Within the niches the diffusion of oxygen is inhibited, leading to the ideal conditions for anaerobic corrosion to occur.
Abiotic
Under anoxic conditions, the mechanism for corrosion requires a substitute for oxygen as the
oxidizing agent
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
in the redox reaction.
For abiotic anaerobic corrosion, that substitute is the hydrogen ion produced in the dissociation of water and the proceeding reduction of the hydrogen ions into
diatomic hydrogen gas.
The
anodic
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
half-reaction involves the oxidation of a metal in
aqueous solution into a
metal hydroxide Metal hydroxides are hydroxides of metals. They are often strong bases. They consist of hydroxide anions and metallic cations. Some metal hydroxides, such as alkali metal hydroxides, ionize completely when dissolved. Certain metal hydroxides ar ...
.
A common reaction that represents this process is the transformation of solid iron in steel into
ferrous hydroxide
Iron(II) hydroxide or ferrous hydroxide is an inorganic compound with the formula Fe(OH)2. It is produced when iron(II) salts, from a compound such as iron(II) sulfate, are treated with hydroxide ions. Iron(II) hydroxide is a white solid, but eve ...
as visualized in the following overall redox reaction.
The ferrous hydroxide may be oxidized further by additional hydrogen ions in water to form the mineral
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
(Fe
3O
4) in the process called the
Schikorr reaction.
In general, the anaerobic corrosion of metals, such as iron and copper, occur at very slow rates.
However, when in chloride-containing aqueous environments, the rate increases because of the introduction of new mechanisms with the addition of a chloride anions.
Biotic
When in biotic conditions, anaerobic corrosion can be facilitated by the metabolic activity of microorganisms in the surrounding environment.
This process is known as microbiologically-influenced corrosion or
bacterial anaerobic corrosion. Most notably, the production of dissolved
sulfides by
sulfate-reducing bacteria
Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...
(SRB) react with solid metals and hydrogen ions to form metal sulfides in a redox reaction.
Environmental significance
The effects of anaerobic corrosion are evident when evaluating the safety of chemical waste disposal. Currently, the permanent disposal of nuclear waste is commonly in
deep geological repositories (DGR) that use copper coating to prevent metal corrosion.
In the DGR, four major types of corrosion are expected to occur, including oxygen-driven, radiation-influenced, anaerobic, and microbiologically-influenced corrosion.
Of these, the most notable process is the microbiologically-influenced corrosion in terms of the magnitude of corrosion.
The ability of microorganisms such as SRB to survive in a wide range of environments also lends to their relevance when considering the threat of corrosion to permanent chemical waste disposal.
See also
*
Corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
*
Bacterial anaerobic corrosion
*
Electrochemistry
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
*
Sulfate-reducing microorganism
*
Redox reaction
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
References
{{DEFAULTSORT:Anaerobic Corrosion
Corrosion
Hydrogen production