Forms of Metallic Corrosion
59
Liquids or gases containing hydrogen sulfide can embrittle certain highstrength steels. Wet hydrogen sulfide environments are considered one of
the most effective in promoting hydrogen entry. In these cases, hydrogen
sulfide reacts with the steel to form atomic hydrogen:
Fe + H2S → FeS + 2H
The chemisorbed sulfur particularly poisons the hydrogen recombination
reaction and promotes hydrogen absorption; when the pH of the solution is
above 8, a protective iron sulfide film forms on the metal surface, which protects the steel and stops the corrosion. If cyanides are present, the protective
film will be destroyed. The unprotected steel corrodes rapidly and hydrogen damage results. Only a few parts per million (ppm) of hydrogen sulfide
are sufficient to cause embrittlement or cracking in steel. Hydrogen stress
cracking is a serious problem in petrochemical equipment used to store and
handle the sour or hydrogen-sulfide-containing fuels.
Exposure to process fluids containing hydrogen, as in catalytic cracking,
can result in hydrogen entry into the material. Exposure to hydrogen gas, or
molecular hydrogen under high pressure and temperature, enhances hydrogen entry and induces damage in iron alloys, nickel alloys, and titanium
alloys. Hydrogen gas, even at 1 atmosphere, is capable of causing cracking in
high-strength steel.
Regardless of the source of the hydrogen, the effect on the metal is the same.
3.10.2 Types of Hydrogen Damage
The specific types of hydrogen damage are as follows:
1. Hydrogen embrittlement, which may be further divided as:
a. Loss in ductility
b. Hydrogen stress cracking
c. Hydrogen environment embrittlement
d. Embrittlement due to hydride formation
2. Hydrogen blistering
3. Flakes, fisheyes, and shatter cracks
4. Hydrogen attack
3.10.2.1 Hydrogen Embrittlement
3.10.2.1.1 Loss of Ductility
The entry of hydrogen into a metal results in decreases in elongation and
reduction in area without the formation of any visible effects, chemical products, or cracking. The loss of ductility is only observed during slow-strain
rate testing and concentrated tensile tests. Tensile strength is also affected
59
Liquids or gases containing hydrogen sulfide can embrittle certain highstrength steels. Wet hydrogen sulfide environments are considered one of
the most effective in promoting hydrogen entry. In these cases, hydrogen
sulfide reacts with the steel to form atomic hydrogen:
Fe + H2S → FeS + 2H
The chemisorbed sulfur particularly poisons the hydrogen recombination
reaction and promotes hydrogen absorption; when the pH of the solution is
above 8, a protective iron sulfide film forms on the metal surface, which protects the steel and stops the corrosion. If cyanides are present, the protective
film will be destroyed. The unprotected steel corrodes rapidly and hydrogen damage results. Only a few parts per million (ppm) of hydrogen sulfide
are sufficient to cause embrittlement or cracking in steel. Hydrogen stress
cracking is a serious problem in petrochemical equipment used to store and
handle the sour or hydrogen-sulfide-containing fuels.
Exposure to process fluids containing hydrogen, as in catalytic cracking,
can result in hydrogen entry into the material. Exposure to hydrogen gas, or
molecular hydrogen under high pressure and temperature, enhances hydrogen entry and induces damage in iron alloys, nickel alloys, and titanium
alloys. Hydrogen gas, even at 1 atmosphere, is capable of causing cracking in
high-strength steel.
Regardless of the source of the hydrogen, the effect on the metal is the same.
3.10.2 Types of Hydrogen Damage
The specific types of hydrogen damage are as follows:
1. Hydrogen embrittlement, which may be further divided as:
a. Loss in ductility
b. Hydrogen stress cracking
c. Hydrogen environment embrittlement
d. Embrittlement due to hydride formation
2. Hydrogen blistering
3. Flakes, fisheyes, and shatter cracks
4. Hydrogen attack
3.10.2.1 Hydrogen Embrittlement
3.10.2.1.1 Loss of Ductility
The entry of hydrogen into a metal results in decreases in elongation and
reduction in area without the formation of any visible effects, chemical products, or cracking. The loss of ductility is only observed during slow-strain
rate testing and concentrated tensile tests. Tensile strength is also affected