Hydrogen-embrittlement Flashcards
When and how does hydrogen-embrittlement happen?
Happens with some metals when they are stressed (applied or residual) and in a hydrogen rich environment. Hydrogen is absorbed through the surface of the metal, and migrates into the volume. When the hydrogen is inside the volume, different damage mechanisms can occur.
What is the affect of embrittlement
The affect metal will have reduced ductility and more brittle behavior.
The amount of captured hydrogen depends on:
Solubility, Diffusivity and Entrapment (the possibility for the hydodrogen to be entrapped at specific sites in the lattice). The solubility depends heavily on the phase state of the metal (increases a lot when moved from solid to liquid)
What are typical sources of embrittlement)
Electro chemical processes, ex. when hydrogen develops as a reaction product -or- when hydrogen develops as reactions related to a corrosive process. ALSO: Environmental absorption from air humidity.
Does higher stress give higher solubility of hydrogen in the metal?
Yes. Hence a crack tip is very exposed, and the hydrogen will contribute to crack growth.
Does higher stress give higher diffusivity of hydrogen in the metal?
The diffusivity is not affected much by the stress state. The small hydrogen atoms can diffuse quite rapidly both in and out of the metal surface.
Explain hydrogen entrapment
Hydrogen can be trapped when there exists a position where the atom can stay in a lower energy configuration. This can prevent rapid flow of hydrogen out of the surface/volume. The “traps” are often atoms that easily link with hydrogen. Detrapping requires an applied high activation energy (special heat treatments).
What is the effect of hydrogen in a metal?
The most common effect of this is brittle fracture (by intergranular decohesion) especially in high strength steels. Other effects can be hydrogen-induced blistering, hydrogen-induced cracking, cracking from hydride formation or hydrogen attack.
Why is Hydrogen so dangerous?
Hydrogen is very reactive with meny metals and its diffusivity and mobility through the lattices is very high. It decreases the cohesive strength(bindingsstyrke) of the crystals(Sigma_id).
Can hydrogen-embrittlement happen without applied stress?
Yes, it can happen due to residual stresses in the metal. Ex. internal gas pressure in pockets
Why does delayed fracture occur?
Delayed fracture happens when a metal is under high stress, but not as high as required for instant static failure (Upper Critical Limit).
If enough hydrogen is absorbed at a crack tip (or other high stress areas), the hydrogen-embrittlement will promote crack growth and fracture.
The fracture is delayed because it will take some time before enough hydrogen is diffused to the tip (or other high stress area). The lower the stress (but higher than Lower Critical Limit), the longer it takes for the fracture to occur.
