High temperature hydrogen attack (HTHA), also called hot hydrogen attack or methane reaction, is a problem which concerns

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 ...

operating at elevated temperatures (typically above ) in hydrogen-rich atmospheres, such as

refineries A refinery is a production facility composed of a group of chemical engineering unit processes and unit operations refining certain materials or converting raw material into products of value. Types of refineries Different types of refineries ...

petrochemical Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable ...

and other chemical facilities and, possibly, high pressure

steam boilers Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporizatio ...

. It is not to be confused with

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 ...

. If a steel is exposed to very hot

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

, the high temperature enables the hydrogen molecules to dissociate and to diffuse into the

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, ductilit ...

as individual ''diffusible'' atoms. There are two stages to the damage: # First, dissolved carbon in the steel reacts with the surface hydrogen and escapes into the gas as methane. This leads to superficial

decarburization Decarburization (or decarbonization) is the process of decreasing carbon content, which is the opposite of carburization. The term is typically used in metallurgy, describing the decrease of the content of carbon in metals (usually steel). Deca ...

and a loss of strength in the surface. Initially, the damage is not visible. # Second, the reduction in the concentration of dissolved carbon creates a driving force which dissolves the

carbides In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of th ...

in the steel. This leads to a loss of strength deeper in the steel and is more serious. At the same time, some

hydrogen atoms 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 constit ...

diffuse into the steel and combine with

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...

to form tiny pockets of

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 Ear ...

at internal surfaces, such as grain boundaries and defects. This methane gas cannot diffuse out of the metal, and collects in the voids at high pressure and initiates cracks in the steel. This

selective leaching In metallurgy, selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from ...

of carbon is a more serious loss of strength and ductility. HTHA can be managed by using a different steel alloy, one where the carbides with other alloying elements, such as

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and h ...

and

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with le ...

, are more stable than

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

carbides. Surface oxide layers are ineffective as a protection as they are immediately reduced by the hydrogen forming water vapour. Later-stage damage in the steel component can be seen using ultrasonic examination, which detects the large defects created by methane pressure. These large defects in a stressed component are usually the cause of failure in service: which is usually catastrophic as hot flammable hydrogen gas escapes rapidly.

References

Corrosion Hydrogen Materials degradation {{corrosion-stub

See also

References