Environmental Engineering Reference
In-Depth Information
8.3.4 Hydrogen Attack
Hydrogen attack is a form of damage that occurs in carbon and low-alloy steels
exposed to high-pressure hydrogen gas at high temperatures for extended time.
The damage may manifest itself as loss in strength of the alloy or formation of
cracks and fissures, and is prevalent at temperatures above 200
C. Under these
conditions the reaction occurs between absorbed hydrogen and the iron carbide
or the carbon in solution resulting in the formation of a hydrocarbons, generally
°
2H
2
Fe
3
→
CH
4
(g)
3Fe
(8.2)
The methane produced does not dissolve in iron lattice and internal gas pres-
sures lead to the formation of fissures or cracks. The generated defects or the
decarburization itself may lower the strength and ductility of the steel. The de-
carburization may take place internally or at the surface. In the latter case, the
decarburized layer grows to increasing depths as the reaction continues. Cracking
may develop in the metal under tensile stress, or the progressive weakening of
the metal may result in failure by some other mechanism. The damage is depen-
dent on temperature and hydrogen partial pressures. Surface decarburization takes
place at temperatures above 540
°
C and the internal decarburization at tempera-
tures above 200
C. Hydrogen attack can take several forms within the metal
structure depending on the severity of attack, stress, and the presence of inclu-
sions in the steel. In the absence of stress, a component may undergo a general
surface attack. The fissures formed are parallel to the metal surface. Areas of
high-stress concentration are often the initiation point of hydrogen attack because
of preferential diffusion of hydrogen to these areas. Isolated decarburized and
fissured areas are often found adjacent to weldments. Blisters and laminations
may also result from severe hydrogen attack.
Resistance of steels to hydrogen attack is related to the stability of carbides.
The addition of carbide-stabilizing elements such as vanadium, titanium, chro-
mium, and molybdenum has beneficial effects. Hydrogen attack does not occur
in austenitic stainless steels. The extent of hydrogen attack is a function of expo-
sure time. Nelson [7] related the susceptibility of the failure of carbon and low-
alloy steels from the formation of methane gas inside the steel, with the partial
pressure of the hydrogen in contact with steels and temperature. The operating
limits of these steels are often represented by Nelson curves (Fig. 8.14).
Decarburization has also been reported in nickel alloys during heat treatment
at 1100
°
C in hydrogen atmospheres. Hydrogen reacting with foreign elements
other than carbon in the matrix at high temperatures and thus producing gaseous
reaction products may also cause damage in some materials. Examples are the
formation of steam (H
2
O) in welded steels, Cu, Ni, and Ag by reaction with
oxygen; and formation of ammonia (NH
3
) in molybdenum by reaction with nitro-
°
