Hydrogen Damages
There are three categories of hydrogen damages:
1-High temperature attack
This attack requires presence of atomic Hydrogen because of inability of
molecular Hydrogen to permeate steel at atmospheric temperature .At above
230 ° C and Hydrogen partial pressure above 100 psi ( 7 bar) , atomic
Hydrogen react Carbon which in the steel to form Methane .
Fe3C  4 H  3Fe  CH 4 The above reaction lead to
1. Remove carbon cause loss of strength
2. Accumulation of Methane inside builds up high internal pressure
inside the steel and creates fissures preferentially at grain boundary or
non metallic inclusions
The steel after Hydrogen attack may also be found to contain blisters in
addition to fissures , these blisters however , differ from low temperature
blisters because they contain Methane instead of Hydrogen
Prevention
Use the stabilized grades of steel 2-Hydrogen Blistering (Hydrogen Induced Cracking)
This is caused by atom Hydrogen diffusing into steel and being trapped at a
non metallic inclusion or at grain boundaries to produce molecular
Hydrogen , and this lead to high pressure is localized at the inclusions or rain
boundaries until the bulging occurs producing blisters or cracks .These
cracks are parallel to the surface along the original laminations generated at
various depths, these are finally connected together .Stepwise cracking
occurs when the steel link together to form a series of steps. Blistering is
very common name and it is often confused with high temperature Hydrogen
attack ,it is therefore recommended not use the name (blistering) , Hydrogen
induced cracking (HIC) can be used as an alternate to avoid confusion.
The formation of atomic Hydrogen is represented by the following reactions
H 2 S  2 H   S 2
Fe  2 H  Fe 2  2 H  2H   H 2 Prevention
1. Changing the corrosive environment
2. Coating or lining
3. Using steel resistance to Hydrogen induced cracking such as steel
containing Cu or cobalt
3-Hydrogen embrittlement
Hydrogen embrittlement occurs during the plastic deformation of alloys in
contact with Hydrogen gas and is strain rate dependent .It is a phenomenon
whereby hydrogen is absorbed in the metal (diffuses) , exerts local stresses
and leads to embrittlement of material .Hydrogen comes from two sources
,gaseous hydrogen and hydrogen released from cathodic reaction , and they
are differ from each other in the following:
1. Cathodic hydrogen is adsorbed on the surface as atomic hydrogen
(reduced), whereas gaseous hydrogen is adsorbed in the molecular
form and it then dissociates to form atomic hydrogen
2. The internal pressure produced by the gaseous hydrogen in much
lower than produced by the cathodic hydrogen
Examples of embrittlement
1. In plating operations
2. In pickling operations
3. In cleaning of high strength steels in chloride or fluoride solution
4. Manufacturing and fabrication processes
Materials most susceptible; Iron, titanium, zirconium high strength
aluminum alloys. Hydrogen embrittlement results in a brittle fracture
throughout the embrritled material as a result of hydrogen adsorption unless
the strength of the remaining material is less than the load applied .later
instantaneous final fracture occurs
Mechanism
It is a general opinion that the impurity segregations at the grain boundary
act as poisons and increase the adsorption of hydrogen at these sites.
Prevention
1. Select materials
temperature
resistant to hydrogen embrittlement for elevated
2. In pickling or plating operation submit the material to temperature
between 160-370° C to eliminate the effect of hydrogen embrittlement
3. Use inhibitors
4. Baking it is recommended that high strength steel be subjected to
backing at low temperature ( between 200-300 ° C) after plating
5. Change of design .avoid sharp corners as act stress raisers
6. Use low hydrogen welding rods
Difference between SCC and Hydrogen embrittlement
1. SCC begin at the surface , whereas hydrogen embrittlement begin
internally.
2. The magnitude of corrosion is higher at the origin of SCC than
observed with hydrogen embrittlement
It is possible to distinguish between them by applied currents, if on
applying a current ,a specimen becomes more anodic and cracking
accelerated the attack is SCC, whereas if cracking accelerated in the opposite
direction and hydrogen evolution is observed the attack is hydrogen
embrittlement.