Hydrogen embrittlement (HE) also known as hydrogen assisted cracking or hydrogen-induced cracking, describes the embrittlement of a metal by diffusible hydrogen. The essential facts about the nature of the hydrogen embrittlement of steels have now been known for 140 years.It is diffusible atomic hydrogen that is harmful to the toughness of iron and steel. It is a low temperature effect: most metals are relatively immune to hydrogen embrittlement above approximately 150°C.(302°F)
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.
For hydrogen embrittlement to occur, a combination of three conditions are required:
the presence and diffusion of hydrogen atoms or ionsa susceptible materialstressDiffusible 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.
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