Page 183 - CW E-Magazine (1-4-2025)

Special Report

Presence of Cl and carbon dioxide,
-
along with H S, such as that occurring
2
in oil wells, accelerates hydrogen crack-
ing through reduction of pH and remov-
ing protective corrosion product scale.

In addition to the above factors,
pickling of steels, use of wet welding
electrodes, and even simple corrosion
phenomena can introduce hydrogen
into the materials. Species such as
sulphides, cyanides, arsenic, antimony,
and phosphorus in the corrosive
environments or on the surface of the
material can enhance hydrogen permea-
tion into the material.

High temperature hydrogen damage/
decarburization
At elevated temperatures, well
above 150°C, both molecular and
atomic hydrogen, if present on steel
surface, can react with carbides leading
to decarburization and methane forma- Fig. 5: Micrographs of steel that suffered severe high temperature hydrogen attack
tion. The corresponding reactions are: (5a shows unexposed metal, 5(b) & 5(c) show depletion of carbides after 110,000 and 190,000 hours
C (Fe) + 2H → CH and C (Fe) + of exposure to steam).
4
2
4H →CH 4 When steel is exposed to hydrogen would have had only the tempera-
containing gaseous atmospheres at ture effect.
The first reaction occurs at the steel elevated temperatures it may suffer
surface, while the second reaction can- two types of permanent damage: Fig. 5 clearly depicts depletion of
occur either on the surface or in the carbides in 0.1C-1.1Cr-0.26Mo-0.17V
bulk of the steel. This is possible be- Internal decarburization (generally steel that suffered severe high tempera-
cause only atomic hydrogen can diffuse intergranular): The methane formed ture hydrogen attack.
into steel. within the steel cannot diffuse out
and hence generates internal voids, Hydrogen Blistering (HB)
The presence of hydrogen envi- which eventually coalesce to form In HB, atomic hydrogen diffuses in
ronment over industrial components micro-fissures and cracks. Features the materials that have some voids. In-
is sometimes intentional and in other of such type of attack are fissur- side these voids, atomic hydrogen can
times unintentional. Hydrocarbon ing, lamination, blistering, internal combine to form a hydrogen molecule.
industries deal with large quantities cracking and finally fracture. In this Because molecular hydrogen cannot
of hydrogen at various plants/units, type of damage, the tensile strength diffuse, tremendous pressure builds
which is intentional. On the other of steel is reduced to half its origi- up inside these voids. The equilibrium
hand, hydrogen generated as a result nal value and the material would pressure of molecular hydrogen in con-
of steam/water reacting/corroding be quite brittle. tact with atomic hydrogen is sufficient
with steel in a boiler (3Fe + 4H O → External decarburization, taking to rupture any material.

2
Fe O + 4H ) is unintentional. place at the surface, is qualitatively
4
3
2
similar to the decarburization Hydrogen blistering is one of the
The type, extent, and time for the occurring during heat treatment/ex- forms of corrosion and is dominant
damage depend on the temperature, posure to high temperature oxidiz- in the petroleum industry. It can take
pressure, and partial pressure of hydro- ing gases. This makes the surface place during refining and in storage
gen in the gas. softer and weaker than the bulk that tanks.
Chemical Weekly April 1, 2025 183

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