Hydrogen permeation and diffusion in high strength steels
Hydrogen embrittlement in high-strength materials, such as structural steels and ultra-high-strength steels, is responsible for failures such as crack propagation at subcritical loads or fracture initiation, leading to a loss in mechanical properties such as ductility, hardness and toughness. First reported in 1875 by Johnson, he also observed that the loss of mechanical properties is reversed by eliminating the presence of hydrogen in the medium. This effect detected in isolated areas of application, such as in very specific marine environments or in the oil & gas industry, is increasingly evident and requires urgent solutions given the accelerated developments linked to the use of hydrogen as a fuel and to the generation of marine or offshore wind energy.
There are studies on the effect of Hydrogen on the toughness and fatigue behavior of metallic materials in which evidence is shown that mechanical embrittlement is controlled by the diffusion rate of atomic hydrogen in the steel. In general, the following processes can be found in a metal/hydrogen system: (i) the entry of atomic hydrogen into the metal produced by a cathodic process, (ii) the transport (diffusion) of hydrogen inside the metal, and (iii) the trapping of hydrogen in a solid solution or in structural defects.
We have seen that crystalline defects play a major role in the absorption and diffusion of hydrogen in the material. Theoretical studies supported by experimental observations suggest that the presence of hydrogen reduces the formation energy of crystalline defects, such as dislocations, vacancies, grain boundaries or cracking surfaces. Furthermore, hydrogen is concentrated around these defects, since they have a much greater hydrogen trapping capacity.
Another significant relationship to take into account is the interaction between hydrogen and the different precipitates of the material, whether they are carbides or nitrides used as grain refiners or precipitation hardeners, or secondary phases that have precipitated after some heat treatment. Hydrogen is trapped in fine precipitates (such as V, Nb or Ti carbonitrides), increasing the hydrogen absorption of the steel and decreasing its diffusivity. These fine precipitates (with a radius of tens of nm) improve the resistance to hydrogen embrittlement of steels, compared to the reference not micro-alloyed steels.
“We need to improve the understanding of the behavior of metallic materials in contact with hydrogen to design and manufacture more durable alloys and structures to be used in hydrogen-rich environments.”
Hydrogen storage systems up to 300 bars of pressure use seamless steels or aluminum as main elements. However, to reach high pressures above 350 bars, they require complex structures of metallic materials coated with layers of fiberglass and composite resins that not only make their manufacture harder, but also, and above all, hinder their recyclability at the end of their useful life.
As an alternative solution, multilayer composites, merged alloys that combine properties of the two materials and, in turn, have a multiplying effect on properties such as resistance to impact or mechanical resistance. Their use through the development of robust intermediate layers allows us to think of these materials as a future alternative to the complex combination proposed nowadays.