Hydrogen embrittlement becomes a serious failure mode of many industrial metallic materials, especially on high-strength steels or alloys that experience high mechanical stress or fatigue, when subjected to high hydrogen content environment. Hydrogen embrittlement of metals is an important phenomenon related to the technological utilization of hydrogen, because excessive hydrogen can reduce the fracture strength and ductility of metals, leading to metals failure. This kind of phenomenon is more pronounced in various alloy materials and most lubricants, but it also occurs in many pure metals.
In recent years, rapid development of energy technology, has driven the application of many industry in the world, such as aerospace and nuclear energy industry. The use of hydrogen as energy source is regarded as the most desirable energy carrier in the near future due to its superior energy efficiency and environmental friendliness. However, the application of hydrogen in industrial applications also faces many technical challenges, especially the hydrogen embrittlement of metals. Thus, it is necessary to have a better understanding about the hydrogen embrittlement mechanisms in order to guarantee the safe operation of hydrogen systems.
Hydrogen embrittlement, in short, refers to the decrease in strength and ductility of metals or alloys when exposed to hydrogen. It was discovered in the late 19th century, when the presence of hydrogen was suspected after investigations into accidental combustor failures for several larger steam locomotive projects in Europe. Proper precautions were not taken, resulting in the brittle fracture of hundreds of engines. The loss of brittle fracture strength in the presence of hydrogen was first explained theoretically by Frederick Muhlhausen in 1905. Then, the phenomenon was studied mainly experimentally and the cause of hydrogen embrittlement has been the subject of extensive research since then.
To better understand the mechanism of hydrogen embrittlement, let us look first at the hydrogen-metal bonding. The hydrogen content in metallic materials can range from a few parts per million to several hundred times these levels. The hydrogen forms a variety of bonds with the metal surface such as H-H, H-O and H-M bonds, depending on the degree of coverage of the hydrogen atoms. Generally speaking, at low hydrogen concentration, the atoms are found in the form of hydrogen adsorbed onto the metal surface, forming a layer one or two molecules thick. At higher hydrogen concentrations, the hydrogen atoms form bulk-like solids within the metal.
The hydrogen-metal bonds cause a number of physical and chemical effects on the metals mechanical properties. These include changes in the surface free-energies, which result in a higher propensity for crack propagation, and lead to a decrease in fatigue and ductility. The hydrogen also reduces the stacking fault energy, which means that the dislocation motion required for plastic deformation is hindered and causes an increase in yield strength. Lastly, the hydrogen bonds reduce the surface diffusion rate, which affects the dynamic loading of a material and thus leads to an extended fatigue behaviour.
To prevent hydrogen embrittlement, a number of strategies have been developed. The most popular method is to use a suitable pretreatment process to reduce the hydrogen content. This involves the treatment of the metal surface with a special ion beam, usually a coating of platinum or palladium. However, this approach is very expensive and is not always practical. Alternatively, the hydrogen content can be reduced by applying a controlled cathodic protection system. This involves using a sacrificial anode to catalyze the formation of protons, which can then be removed from the metal surface by electrochemical processes.
In conclusion, hydrogen embrittlement is a serious failure mode of many industrial metallic materials and can
result in catastrophic fatigue or fracture. Good knowledge of hydrogen-metal bonding and the corresponding mechanism is essential for avoiding embrittlement under different hydrogen concentration levels. This paper provided a brief overview of the hydrogen embrittlement phenomenon and how it can affect mechanical properties of metals. Various strategies for mitigating the hydrogen embrittlement such as pretreatment and cathodic protection were also presented.
