Pitting corrosion
Encyclopedia
Pitting corrosion, or pitting, is a form of extremely localized corrosion
that leads to the creation of small holes in the metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic
while an unknown but potentially vast area becomes cathodic
, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions. The mechanism of pitting corrosion is probably the same as crevice corrosion
.
This kind of corrosion is extremely insidious, as it causes little loss of material with small effect on its surface, while it damages the deep structures of the metal. The pits on the surface are often obscured by corrosion products.
Pitting can be initiated by a small surface defect, being a scratch or a local change in composition, or a damage to protective coating. Polished surfaces display higher resistance to pitting.
s, nickel alloys, aluminum alloys. Metals that are susceptible to uniform corrosion in turn do not tend to suffer from pitting. Thus, a regular carbon steel will corrode uniformly in sea water, while stainless steel will pit. Additions of about 2% of molybdenum
increases pitting resistance of stainless steels.
s, e.g. in sea water, significantly aggravates the conditions for formation and growth of the pits through an autocatalytic process. The pits become loaded with positive metal ions through anodic dissociation. The Cl− ion
s become concentrated in the pits for charge neutrality and encourage the reaction of positive metal ions with water to form a hydroxide corrosion product and H+ ions. Now, the pits are weakly acidic, which accelerates the process.
Besides chlorides, other anions implicated in pitting include thiosulfate
s (S2O32−), fluoride
s and iodide
s. Stagnant water conditions favour pitting. Thiosulfates are particularly aggressive species and are formed by partial oxidation of pyrite, or partial reduction of sulfate. Thiosulfates are a concern for corrosion in many industries: sulfide
ores processing, oil well
s and pipelines transporting soured oils, Kraft paper
production plants, photographic industry, methionine
and lysine
factories.
Corrosion inhibitor
s, when present in sufficient amount, will provide protection against pitting. However, too low level of them can aggravate pitting by forming local anodes.
A single pit in a critical point can cause a great deal of damage. One example is the explosion in Guadalajara
, Mexico on April 22, 1992, when gasoline
fumes accumulated in sewer
s destroyed kilometers of streets. The vapors originated from a leak of gasoline through a single hole formed by corrosion between a steel
gasoline pipe and a zinc
-plated water pipe.
Firearms can also suffer from pitting, most notably in the bore of the barrel when corrosive ammunition is used and the barrel is not cleaned soon afterward. Deformities in the bore caused by pitting can greatly reduce the firearms accuracy. To prevent pitting in firearm bores, most modern firearms have a bore lined with chromium.
Pitting corrosion can also help initiate stress corrosion cracking
, as happened when a single eyebar
on the Silver Bridge
, West Virginia
and killed 46 people on the bridge in December, 1967.
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...
that leads to the creation of small holes in the metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
while an unknown but potentially vast area becomes cathodic
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions. The mechanism of pitting corrosion is probably the same as crevice corrosion
Crevice corrosion
Crevice corrosion refers to corrosion occurring in confined spaces to which the access of the working fluid from the environment is limited. These spaces are generally called crevices...
.
Mechanism
It is supposed by some that gravitation causes downward-oriented concentration gradient of the dissolved ions in the hole caused by the corrosion, as the concentrated solution is denser. This however is unlikely. The more conventional explanation is that the acidity inside the pit is maintained by the spatial separation of the cathodic and anodic half-reactions, which creates a potential gradient and electromigration of aggressive anions into the pit.This kind of corrosion is extremely insidious, as it causes little loss of material with small effect on its surface, while it damages the deep structures of the metal. The pits on the surface are often obscured by corrosion products.
Pitting can be initiated by a small surface defect, being a scratch or a local change in composition, or a damage to protective coating. Polished surfaces display higher resistance to pitting.
Susceptible alloys
Alloys most susceptible to pitting corrosion are usually the ones where corrosion resistance is caused by a passivation layer: stainless steelStainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
s, nickel alloys, aluminum alloys. Metals that are susceptible to uniform corrosion in turn do not tend to suffer from pitting. Thus, a regular carbon steel will corrode uniformly in sea water, while stainless steel will pit. Additions of about 2% of molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
increases pitting resistance of stainless steels.
Environment
The presence of chlorideChloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
s, e.g. in sea water, significantly aggravates the conditions for formation and growth of the pits through an autocatalytic process. The pits become loaded with positive metal ions through anodic dissociation. The Cl− ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s become concentrated in the pits for charge neutrality and encourage the reaction of positive metal ions with water to form a hydroxide corrosion product and H+ ions. Now, the pits are weakly acidic, which accelerates the process.
Besides chlorides, other anions implicated in pitting include thiosulfate
Thiosulfate
Thiosulfate is an oxyanion of sulfur. The prefix thio indicates that thiosulfate ion is a sulfate ion with one oxygen replaced by a sulfur. Thiosulfate occurs naturally and is produced by certain biochemical processes...
s (S2O32−), fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
s and iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
s. Stagnant water conditions favour pitting. Thiosulfates are particularly aggressive species and are formed by partial oxidation of pyrite, or partial reduction of sulfate. Thiosulfates are a concern for corrosion in many industries: sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
ores processing, oil well
Oil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
s and pipelines transporting soured oils, Kraft paper
Kraft paper
Kraft paper or kraft is paper or paperboard produced from chemical pulp produced in the kraft process.Pulp produced by the kraft process is stronger than that made by other pulping processes; acidic sulfite processes degrade cellulose more, leading to weaker fibers, and mechanical pulping...
production plants, photographic industry, methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
and lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
factories.
Corrosion inhibitor
Corrosion inhibitor
A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime...
s, when present in sufficient amount, will provide protection against pitting. However, too low level of them can aggravate pitting by forming local anodes.
Examples
1992 explosion in Guadalajara
The 1992 Guadalajara explosions took place on April 22, 1992, in the downtown district of Analco. Numerous gasoline explosions in the sewer system over four hours destroyed 8 kilometers of streets. Gante Street was the most damaged. Officially, by the Lloyds of London accounting, 252 people were...
, Mexico on April 22, 1992, when gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
fumes accumulated in sewer
Sanitary sewer
A sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...
s destroyed kilometers of streets. The vapors originated from a leak of gasoline through a single hole formed by corrosion between a steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
gasoline pipe and a zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
-plated water pipe.
Firearms can also suffer from pitting, most notably in the bore of the barrel when corrosive ammunition is used and the barrel is not cleaned soon afterward. Deformities in the bore caused by pitting can greatly reduce the firearms accuracy. To prevent pitting in firearm bores, most modern firearms have a bore lined with chromium.
Pitting corrosion can also help initiate stress corrosion cracking
Stress corrosion cracking
Stress corrosion cracking is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when...
, as happened when a single eyebar
Eyebar
In structural engineering and construction, an eyebar is a straight bar, usually of metal, with a hole at each end for fixing to other components...
on the Silver Bridge
Silver Bridge
The Silver Bridge collapsed in 1967, killing 46 people. The terms Silver Bridge or Silverbridge may also refer to:* Silver Memorial Bridge, the replacement for the above bridge, opened in 1969....
, West Virginia
West Virginia
West Virginia is a state in the Appalachian and Southeastern regions of the United States, bordered by Virginia to the southeast, Kentucky to the southwest, Ohio to the northwest, Pennsylvania to the northeast and Maryland to the east...
and killed 46 people on the bridge in December, 1967.
See also
- CorrosionCorrosionCorrosion 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...
- Crevice corrosionCrevice corrosionCrevice corrosion refers to corrosion occurring in confined spaces to which the access of the working fluid from the environment is limited. These spaces are generally called crevices...
- Micro pittingMicro pittingMicro pitting is a fatigue failure of the surface of a material commonly seen in rolling bearings and gears.It is also known as grey staining, micro spalling or frosting.-Pitting and micropitting:...
- Panel Edge StainingPanel Edge StainingPanel edge staining is a naturally occurring problem that occurs to anodized aluminium and stainless steel paneling and facades. It is semi-permanent staining that dulls the panel or facades surface , reducing the natural luster and shine produced by the anodizing processes used on the aluminium...
- Stress corrosion crackingStress corrosion crackingStress corrosion cracking is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when...
External links
- Pit Happens - Copper Corrosion in Household Plumbing.
- Pitting Corrosion - Corrosion Doctors http://www.corrosion-doctors.org/Forms-pitting/Pitting.htm
- Pitting Corrosion Pitting Corrosion Theory
- Chemical explanation of pitting corrosion