Environmental Engineering Reference
In-Depth Information
Hydrogen solubility is higher in the high-temperature fcc structure of iron than
in the low-temperature bcc structure. Consequently, if the steel is cooled from
the level of 1100
C in a furnace containing hydrogen atmosphere, e.g., cracked
ammonia, entrapment of released hydrogen inside the metal results in hydrogen
damage.
During acid pickling or electroplating, and as a result of corrosion in service,
atomic hydrogen is generated on the metal surface as a cathodic reduction product
that diffuses in the bulk material. The diffusion rate is high particularly when
the material is stressed. In the pickling of steel, the level of hydrogen absorption
is dependent on both the bath temperature and the nature of the acid. The entry
of hydrogen is promoted by cathodic poisons that inhibit recombination of the
absorbed, discharged nascent hydrogen atoms. These poisons include cyanide
and ionic species of sulfur, arsenic, selenium, bismuth, tellurium, phosphorus,
iodine, and antimony. The role of sulfur as a poison is particularly important, as
sulfur is commonly encountered in steels as well as in the environment. Cathodic
protection of metals, by both galvanic coupling and impressed current, likewise
facilitates the production of cathodic nascent hydrogen on the metal surface and
can promote embrittlement of susceptible metals or alloys.
Gases or liquids containing hydrogen sulfide can embrittle certain high-
strength steels. Wet hydrogen sulfide environments are considered to be among
the most effective in promoting hydrogen entry. In these cases, hydrogen sulfide
reacts with the steel to form atomic hydrogen:
°
Fe
H
2
S
→
FeS
2H
(8.1)
The chemisorbed sulfur partially poisons the hydrogen recombination reaction
and promotes hydrogen absorption. The damage is absent in solutions with pH
values above 8 because the protective iron sulfide film formed on the metal sur-
face stops the corrosion of steel. Cyanides, if present in the solution, destroy this
protective film. The unprotected steel corrodes rapidly, and hydrogen damage is
encountered. Embrittlement or cracking in steel can develop in the presence of
only a few ppm of hydrogen sulfide. Hydrogen stress cracking is a serious prob-
lem in petrochemical equipment used to store and handle the sour or hydrogen
sulfide-containing oils.
Exposure to process fluids bearing hydrogen, as in catalytic cracking, can
cause hydrogen entry into the material. Exposure to hydrogen gas or molecular
hydrogen under high pressure and temperature facilitates hydrogen entry and
induces damage in iron alloys, nickel alloys, and titanium alloys, Hydrogen gas
even at one atmospheric pressure has been reported [2] to cause cracking in high-
strength steel.
The effect of hydrogen on the behavior of metal is the same irrespective of
the source of hydrogen. However, there are some basic differences between gas-
eous and cathodic hydrogen absorption processes. Cathodic hydrogen absorbs on
