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11- Hydrogen embrittlement |
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The presence of hydrogen atoms in a metal
crystal lattice can be extremely detrimental, leading to a catastrophic
loss of mechanical strength and ductility.
It is generally accepted that the hydrogen is first of all adsorbed
on the metal surface before penetrating the lattice, where it diffuses
in ionic form (i.e. as protons).
The hydrogen atoms can have various origins the surrounding atmosphere
containing hydrogen or hydrogenated compounds (H2S, NH3, H2O, etc.),
electroplating processes during which the proton reduction reaction
occurs, electrochemical corrosion during which the cathodic reaction
is proton reduction.
Once they have penetrated the crystal lattice, hydrogen atoms can
cause several types of damage.
- Precipitation of brittle hydrides : this occurs in titanium and other metals with a high affinity for hydrogen (Ta, Zr, V, Pd, ...).
- Recombination to molecular hydrogen : when the metal contains macroscopic discontinuities or microscopic defects, these can represent sites for the recombination of hydrogen atoms. The hydrogen molecules are unable to diffuse away into the lattice and it is possible to build up high local pressures, leading to the formation of flakes and blisters, and "ladder-type" cracking.
- Hydrogen embrittlement : by interacting with lattice dislocations, hydrogen atoms cause a marked loss in the plastic strain capacity of the metal, which becomes brittle.
Means of preventing blistering and hydrogen embrittlement are
:
- for prevention of blistering : use sound steels containing few inclusions and defects, use coatings "impermeable" to hydrogen (nickel or enamel coated steel tanks, austenitic stainless steel cladding, rubber, polymers, etc.), employ inhibitors in the case of an aggressive medium operating in a closed circuit, etc.
- for prevention of hydrogen embrittlement : reduce the corrosion rate, modify the electoplating conditions, change the alloy, take appropriate precautions during welding and so on.
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