Mechanisms hydrogen embrittlement

Cracking mechanisms in which corrosion is implicated include stress corrosion cracking, corrosion fatigue, hydrogen-induced cracking and liquid metal embrittlement. Purely mechanical forms of cracking such as brittle failure are not considered here. 

Owing to hydrogen embrittlement, the mechanical properties of metallic and nonmetal-lic materials of containment systems may degrade and fail resulting in leaks. Hydrogen embrittlement depends on many factors such as environmental temperature and pressure, purity of metal, concentration and exposure time to hydrogen, stress state, physical and mechanical properties, microstructure, surface conditions, and the nature of the crack front of material [23]. 

An embrittling effect, the mechanism of which is stiU debated, is possible with a hydrogen content of more than ca 3 ppm. The content of hydrogen and other gases can, however, be reduced by vacuum dagassing. 

Fusion Reactors. The development of fusion reactors requires a material exhibiting high temperature mechanical strength, resistance to radiation-induced swelling and embrittlement, and compatibUity with hydrogen, lithium and various coolants. One aUoy system that shows promise in this appHcation, as weU as for steam-turbine blades and other appHcations in nonoxidizing atmospheres, is based on the composition (Fe,Co,Ni)2V (30). 

Analysis methods for hydrogen absorbed in the deposit have been described (65), and instmments are commercially available to detect hydrogen in metals. Several working tests have been devised that put plated specimens under strain and measure the time to failure. A method for cadmium-plated work has been described (66) as has a mechanical test method for evaluating treatments on AlSl 4340 Steel (67). Additional information on testing for hydrogen embrittlement is also available (68). 

MSTMF519, Std. Methodfor Mechanical Hydrogen Embrittlement Testing of Plating Processes and Mircraft Maintenance Chemicals, American Society for Testing and Materials, Philadelphia, Pa., 1977. 

Certain anaerobic bacteria capable of producing hydrogen may, under special circumstances, contribute to hydrogen embrittlement of some alloys. Once again, if such mechanisms operate, they have very limited applicability in most cooling water systems. 

For many years hydrogen was considered as a deleterious impurity which deteriorates mechanical properties of materials. This is clearly illustrated by hydrogen embrittlement of ferrous metals and alloys. The main effort of the research was aimed therefore at the study of hydrogen embrittlement and at the ways to avoid it. ... 

Hydrogen-induced cracking (HlC) is most commonly encountered in steels but other metals are susceptible, as shown in Table 53.3. The presence of hydrogen atoms in a metal degrades some of its mechanical properties, especially its ductility, leading in some cases to embrittlement. 

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