Surface Finishing Tutorial

#003 Hydrogen Embrittlement - Absorbed Hydrogen

Category : Hydrogen embrittlement

July15, 2009

Hydrogen atom is the smallest of all atoms, and it's size is 1.06 angstrom = 1.06 x 10-10m. On the other hand, the lattice structure spacing of metal is 2~3 angstrom so the hydrogen atoms can easily penetrate into metals.
For example, sulfuric acid and hydrochloric acid used for pickling will dissolve the surfaces of steel alloys. The dissolution of the steel and hydrogen ion generation occur simultaneously, and the hydrogen ions will electrically discharge and become hydrogen atoms at the steel's surface. Much of the hydrogen atoms will be dissipated into air as hydrogen gas, but some will penetrate the steel.

As for the plating process, some hydrogen will also penetrate the steel if cathode current efficiency is low due to hydrogen generated by electrolysis of water. In some cases, hydrogen may penetrate the coating and increase the hardness of the coating, but this hydrogen may also penetrate the steel under certain conditions.
The hydrogen absorbed by steel exists in various places such as interstitial lattice, transitional region, grain boundary, and inclusions. The hydrogen in the interstitial structure maybe ignored as the cause of hydrogen embrittlement induced breakage since the existing amount is negligibly small. It is said that the hydrogen more closely related is "defective hydrogen".
With high strength steel alloys, hydrogen embrittlement induced breakage/crack occurs at grain boundaries. Hydrogen concentrates at grain boundaries and weakens the metal's atomic binding.
With low strength steel alloys, stress loadings cause transitions to gather around inclusions, and hydrogen is attracted towards the same area and result in destruction.

Hydrogen embrittlement induced destruction cases are organized as below.

（1）	Destruction occurs at the point of tensile stress. Does not occur at the point of compressive stress.
（2）	Destruction is likely to occur where stress is concentrated around notches.
（3）	The destructive hydrogen in steel is of diffusible in nature and can freely move about within the steel and concentrate around notched features. The amount of absorbed hydrogen and hydrogen embrittlement risk do not necessarily coincide.
（4）	Steel material's susceptibility to hydrogen embrittlement mainly depends on the material's strength, and is not dependent on the alloy's atomic composition. Most destruction incidents occur with high tensile strength steel alloys with high hardness of HRC40 or more.
（5）	The breakage type is either inter-granular or trans-granular. The absorbed hydrogen congregates at inter-crystalline region and weakens the metal's atomic bound.
（6）	Hydrogen embrittlement induced delayed failure does not typically prevail against high strain rate shocks. For example, aircraft landing gear usually does not fracture with shocks from landing, but typically fails while the craft is static or during taxiing.
（7）	Steel alloys are more insensitive to hydrogen embrittlement as the material is more thermodynamically stable.
（8）	Hydrogen embrittlement fracture is temperature sensitive. More often occurs at temperatures of −10〜120 deg. C.
（9）	It takes time for hydrogen to diffuse and accumulate for the destruction to occur.

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