Hydrogen embrittlement (HE) also known as hydrogen assisted cracking or hydrogen-induced cracking, describes the embrittlement of a metal by diffusiblehydrogen. The essential facts about the nature of the hydrogen embrittlement of steels have now been known for 140 years.[1][2][3] It is diffusible atomic hydrogen that is harmful to the toughness of iron and steel.[4] It is a low temperature effect: most metals are relatively immune to hydrogen embrittlement above approximately 150°C.(302°F)[5]
In steels, diffusible hydrogen ions come from water that is typically introduced by a wet electrochemical process such as electroplating. It must be distinguished from the entirely different process high temperature hydrogen attack (HTHA) which is where steels operating at high temperatures above 400°C are attacked by hydrogen gas.[6]
For hydrogen embrittlement to occur, a combination of three conditions are required:
- the presence and diffusion of hydrogen atoms or ions
- a susceptible material
- stress
Diffusible hydrogen can be introduced during manufacture from operations such as forming, coating, plating or cleaning. The most common causes of failure in practice are poorly-controlled electroplating or bad welding practice with damp welding rods. Both of these introduce hydrogen ions which dissolve in the metal. Hydrogen may also be introduced over time (external embrittlement) through environmental exposure (soils and chemicals, including water), corrosion processes (especially galvanic corrosion) including corrosion of a coating and cathodic protection. Hydrogen atoms are very small and diffuse interstitially in steels. Almost uniquely amongst solute atoms they are mobile at room temperature and will diffuse away from the site of their introduction within minutes.[1]
https://en.wikipedia.org/wiki/Hydrogen_embrittlement
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 400 °C) in hydrogen-rich atmospheres: in refinery, petrochemical and other chemical facilities and, possibly, high pressure steam boilers. It is not to be confused with hydrogen embrittlement.[1]
If a steel is exposed to very hot hydrogen, the high temperature enables the hydrogen molecules to dissociate and to diffuse into the alloy 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 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 the steel. This leads to a loss of strength deeper in the steel and is more serious. At the same time some hydrogen atoms diffuse into the steel and combine with carbon to form tiny pockets of methane 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 of carbon is a more serious loss of strength and ductility.[2][3]
HTHA can be managed by using a different steel alloy, one where the carbides with other alloying elements, such as chromium and molybdenum, are more stable than iron carbides.[4] 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.[5][4] These large defects in a stressed component are usually the cause of failure in service: which is usually catastrophic as hot inflammable hydrogen gas escapes rapidly.
https://en.wikipedia.org/wiki/High_temperature_hydrogen_attack
https://en.wikipedia.org/wiki/Methane
https://en.wikipedia.org/wiki/Sabatier_reaction
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