Sulfide stress cracking (SSC) is a form of
hydrogen embrittlement
Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbe ...
which is a
cathodic
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
cracking mechanism. It should not be confused with the term
stress corrosion cracking
Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC ...
which is an
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 ...
cracking mechanism. Susceptible
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
s, especially
steels
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant t ...
, react with
hydrogen sulfide (), forming metal
sulfide
Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
s (MeS) and
atomic hydrogen
A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen consti ...
(H
•) as
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 engine ...
byproducts. Atomic hydrogen either combines to form H
2 at the metal surface or diffuses into the metal matrix. Since sulfur is a hydrogen recombination poison, the amount of atomic hydrogen which recombines to form H
2 on the surface is greatly reduced, thereby increasing the amount of diffusion of atomic hydrogen into the metal matrix. This aspect is what makes wet H
2S environments so severe.
[Foroulis Z.A ed., High-Temperature Metallic Corrosion by Sulfur and Its Compounds, The Electrochemical Society, Princeton, NJ. (1970) Conference proceedings]
Since SSC is a form of
hydrogen embrittlement
Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbe ...
, it is most susceptibile to cracking at or slightly below ambient temperature.
Sulfide stress cracking has special importance in the gas and
oil industry
The petroleum industry, also known as the oil industry or the oil patch, includes the global processes of exploration, extraction, refining, transportation (often by oil tankers and pipelines), and marketing of petroleum products. The largest ...
, as the materials being processed there (
natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon di ...
and
crude oil) often contain considerable amounts of hydrogen sulfide. Equipment that comes in contact with H
2S environments can be rated for sour service with adherence to
NACE NACE may refer to:
* NACE (region), North Atlantic and Central European region
* "Nace" (song), by Colombian pop singer Anasol, 2006
* NACE International
The Association for Materials Protection and Performance (AMPP), is a professional assoc ...
MR0175/ISO 15156 for oil and gas production environments or NACE MR0103/ISO17945 for oil and gas refining environments.
"
High Temperature Hydrogen Attack
High temperature hydrogen attack (HTHA), also called hot hydrogen attack or methane reaction, is a problem which concerns steels operating at elevated temperatures (typically above ) in hydrogen-rich atmospheres, such as refineries, petrochemical ...
" (HTHA) does not rely on atomic hydrogen. At high temperature and high hydrogen partial pressure, hydrogen can diffuse into
carbon steel
Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:
* no minimum content is specified or required for chromium, cobalt ...
alloys. In susceptible
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
s, hydrogen combines with
carbon within the alloy and forms
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on ...
. The methane molecules create a pressure buildup in the metal lattice voids, which leads to embrittlement and even cracking of the metal.
See also
*
Corrosion engineering
Corrosion engineering is an engineering specialty that applies scientific, technical, engineering skills, and knowledge of natural laws and physical resources to design and implement materials, structures, devices, systems, and procedures to mana ...
*
Crevice corrosion
Crevice corrosion refers to corrosion occurring in occluded spaces such as interstices in which a stagnant solution is trapped and not renewed. These spaces are generally called crevices. Examples of crevices are gaps and contact areas between pa ...
*
Pitting corrosion
Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic (oxidation r ...
*
Sulfidation
Sulfidation (British spelling also sulphidation) is a process of installing sulfide ions in a material or molecule. The process is widely used to convert oxides to sulfides but is also related to corrosion and surface modification.
Inorganic, mate ...
References
Corrosion
Materials degradation
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