Environmental Engineering Reference
In-Depth Information
mental globular sulfides. On the similar consideration, hot-rolled or annealed
steel is preferred to cold-rolled steel.
2.
Material selection
. Rimmed steels contain numerous voids and are much
prone to blistering or flaking. Killed steels show much less susceptibility.
Silicon-killed steels are preferable to aluminum-killed steels. The occurrence
of corrosion-generated hydrogen blistering can be minimized by the use of
metals that are chemically resistant to the environment. Nickel-containing
steels, including austenitic stainless steels and nickel-base alloys, have very
low hydrogen diffusion rates and are often recommended to prevent hydro-
gen blistening.
3.
Use of coatings
. The use of inside coating or liner that is impervious to
hydrogen penetration and resistant to the medium can avoid hydrogen blis-
tering of steel tanks or containers. The coatings may be metallic, inorganic,
or organic. Cladding of steel with austenitic stainless steel or nickel or a
rubber lining on steel is often used.
4.
Removal of poisons
. Removal of hydrogen evolution poisons such as sulfides,
arsenic compounds, cyanides, and phosphorus-containing ions from the envi-
ronment greatly reduces the incidence of hydrogen blistering. In petroleum
process streams these poisons are quite prevalent. The basic approach to
reduction of corrosion-induced hydrogen blistering in catalytic cracking units
is to reduce the concentration of sulfur and bisulfide ions in water condensate.
5.
Use of inhibitors
. The corrosion-generated hydrogen blistering can be mini-
mized by use of inhibitors, as the cathodic reduction of hydrogen ions also
thereby gets retarded. However, the use of inhibitor can be economical only
in closed recirculating systems.
6.
Improvements in design
. Modifications in design for improved performance
of a component need careful consideration. For example, in services in which
blistering is expected, external support pads should not be continuously
welded to the vessel itself to prevent hydrogen entrapment at the interface.
7.
Proper heat treating procedure
. The decreased solubility of hydrogen in bcc
structured steel compared to the fcc structure leads to flaking and fish-eye
formation in steels when they are cooled in hydrogen atmospheres from high
temperatures (more than 1100
C). The damage is aggravated if the cooling
is rapid because this results in hydrogen-sensitive martensitic microstructure.
A reduced cooling rate inhibits the formation of martensite and also allows
hydrogen to be slowly released from the steel, thereby eliminating the
damage.
°
Hydrogen embrittlement
may be prevented or minimized by the following
measures:
1.
Material selection
. In general, the susceptibility of steels to hydrogen embrit-
tlement increases with the tensile strength of the material. The threshold
