H2S, polysulfides, and sulfur Information
The maximum concentration of H2S in water is 400 ppm. Wells with large
amounts of H2S are usually labeled sour; however wells with only 10 ppm
or above can be labeled sour. Partial pressures of only 0.05 H2
S are considered corrosive.
The primary problem in the presence of H2S is metal embrittlement, caused
by penetration of H2
in metal. The attack mechanism is complex, with many postulated routes. May involve
SH- ion, since it is the only dissolved sulfur ion.
Hydrogen sulfide is a weak acid when dissolved in water, and can act as a catalyst
in the absorption of atomic hydrogen in steel, promoting sulfide stress cracking
(SSC) in high strength steels. Polysulfides and sulfanes (free acid forms of polysulfides)
may be formed when hydrogen sulfide reacts with elemental sulfur. These sulfanes
are produced along with other gaseous constituents. As pressure decreases up the
production tubing, the sulfanes dissociate and elemental sulfur precipitates, which
can cause plugging.
Iron sulfides are often formed from corrosion reactions, and can be important in
corrosion control, especially at lower temperatures and low H2
S partial pressures, where a protective film often forms. However, in order for
this protective film to form, the absence of oxygen and chloride salts is required.
In environments with hydrogen sulfide (H2S) corrosion, the most common
types include uniform corrosion, pitting corrosion, corrosion fatigue, sulfide stress
cracking, hydrogen blistering, hydrogen embrittlement, and stepwise cracking. For
more on the theory and mechanisms for each corrosion type, go to the theory page.
Corrosion products include black or blue-black iron sulfides, pyrite, greigite,
mackinwaite, kansite, iron oxide (Fe3O4), magnetite, sulfur
(S), and sulfur dioxide (SO2
H2S corrosion can be found in production wells, flowlines, and during
drilling. Areas where H2
S corrosion is common include sucker rods
Prevention / Mitigation
To reduce or prevent corrosion in an H2S environment:
Drilling - High pH, zinc treatments
Production - corrosion inhibitors
Flowlines - Corrosion inhibitors, H2
Sour gas wells may be corrosive if the pH is 6.5 or less, and H2
S concentration is 250 ppm or more.
- Click on thumbnail to see larger picture
Signs of hydrogen sulfide corrosion include shallow round pits with etched bottoms.
H2S Attack on sucker rods followed by corrosion fatigue break, caused
by alternating stresses.
Sulfide stress cracking occurs when H2
S corrosion is accelerated by stresses.
Hydrogen embrittlement fractures are caused by hydrogen entering the metal and concentrating
internally in high-stress areas, making the metal very brittle. Hydrogen induced
cracking can also occur if the metal is subjected to cyclic stresses or tensile