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 absorbed, hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement. Hydrogen embrittlement occurs in steels, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminium, and stainless steels are generally less susceptible. The essential facts about the nature of hydrogen embrittlement have been known since the 19th century. Hydrogen embrittlement is maximised at around room temperature in steels, and most metals are relatively immune to hydrogen embrittlement at temperatures above 150 °C. Hydrogen embrittlement requires the presence of both atomic ("diffusible") hydrogen and a mechanical stress to induce crack growth, although that stress may be applied or residual. Hydrogen embrittlement increases at lower strain rates. In general, higher-strength steels are more susceptible to hydrogen embrittlement than mid-strength steels. Metals can be exposed to hydrogen from two types of sources: gaseous dihydrogen and atomic hydrogen chemically generated at the metal surface. Atomic hydrogen dissolves quickly into the metal at room temperature and leads to embrittlement. Gaseous dihydrogen is found in pressure vessels and pipelines. Electrochemical sources of hydrogen include acids (as may be encountered during pickling, etching, or cleaning), corrosion (typically due to aqueous corrosion or cathodic protection), and electroplating. Hydrogen can also be introduced during manufacturing by the presence of moisture during welding or while the metal is molten. The most common causes of failure in practice are poorly controlled electroplating or damp welding rods. Hydrogen embrittlement as a term can be used either specifically for embrittlement in steels and similar metals at relatively low hydrogen concentrations, or more broadly to encompass all embrittling effects that hydrogen has on metals. These broader effects include hydride formation, which occurs in titanium and vanadium but not in steels, and hydrogen-induced blistering, which generally occurs at high hydrogen concentrations and does not require the presence of stress. However, hydrogen embrittlement is usually distinguished from high temperature hydrogen attack (HTHA), which occurs in steels at temperatures above 204 °C and involves the formation of methane pockets. The mechanisms by which hydrogen causes embrittlement in steels are not comprehensively understood and continue to be actively investigated. Hydrogen-related degradation is also a significant concern in large energy systems, particularly in hydrogen-cooled turbogenerators, where prolonged exposure to hydrogen atmospheres may influence the structural integrity of rotor steels and associated components.

Mechanisms

Hydrogen embrittlement is a complex process involving a number of distinct contributing micro-mechanisms, not all of which need to be present. It is now widely accepted that hydrogen embrittlement depends on material and environment, with no single mechanism exclusively applicable. The mechanisms include the formation of brittle hydrides, the creation of voids that can lead to high-pressure bubbles, enhanced decohesion at internal surfaces, and localised plasticity at crack tips that assist in the propagation of cracks. A wide variety of mechanisms have been proposed and investigated as causes of brittleness once diffusible hydrogen has dissolved into the metal.

Internal pressure: At high hydrogen concentrations, absorbed hydrogen species recombine in voids to form hydrogen molecules (H2), creating pressure from within the metal. This pressure can increase to levels where cracks form, commonly designated hydrogen-induced cracking (HIC), as well as blisters forming on the specimen surface, designated hydrogen-induced blistering. These effects can reduce ductility and tensile strength. Hydrogen-enhanced localised plasticity (HELP): Hydrogen increases the nucleation and movement of dislocations at a crack tip. HELP results in crack propagation by localised ductile failure at the crack tip, with less deformation occurring in the surrounding material, which gives a brittle appearance to the fracture. Hydrogen-decreased dislocation emission: Molecular dynamics simulations suggest a ductile-to-brittle transition caused by the suppression of dislocation emission at the crack tip by dissolved hydrogen. This prevents crack-tip blunting and promotes brittle cleavage-like failure. Hydrogen-enhanced decohesion (HEDE): Interstitial hydrogen lowers the stress required for metal atoms to fracture apart. HEDE can occur when the local concentration of hydrogen is high, such as at crack tips, stress concentrators, or in the tension field of edge dislocations. Metal hydride formation: The formation of brittle hydrides allows cracks to propagate in a brittle manner. This is particularly important in vanadium alloys, while most structural steels do not readily form hydrides. Phase transformations: Hydrogen can induce phase transformations in some materials, and the new phase may be less ductile.

Hydrogen embrittlement in tribological systems Hydrogen embrittlement may also influence degradation processes in steels subjected to frictional and contact loading. Absorbed hydrogen can accumulate at dislocations, grain boundaries and other lattice defects, facilitating localized plasticity and promoting crack initiation in near-surface layers. Under sliding and rolling contact conditions, hydrogen-assisted changes in microstructure and stress state may reduce wear resistance and modify the morphology of wear debris formed during friction. Experimental studies on high-nitrogen manganese steels have demonstrated that hydrogen charging can affect tribological behaviour and fracture mechanisms under both dry sliding and rolling contact conditions. Similar hydrogen-assisted fracture and embrittlement phenomena in steels have also been discussed within the framework of hydrogen-enhanced localized plasticity mechanisms.

… excerpt ends here. Continue reading the full article.