H2S Corrosion
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 H2S
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 H2S
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).
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Where Found
H2S corrosion can be found in production wells, flowlines, and
during drilling. Areas where H2S corrosion is common include sucker
rods
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Prevention / Mitigation
To reduce or prevent corrosion in an H2S environment:
Drilling - High pH, zinc treatments
Production - corrosion inhibitors
Flowlines - Corrosion inhibitors, H2S scavengers
Predicting corrosion
Sour gas wells may be corrosive if the pH is 6.5 or less, and H2S
concentration is 250 ppm or more.
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Pictures - Click on thumbnail to see larger picture
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Signs of hydrogen sulfide corrosion include shallow round pits with
etched bottoms. |
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H2S Attack on sucker rods followed by corrosion fatigue break,
caused by alternating stresses. |
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Sulfide stress cracking occurs when H2S corrosion is accelerated
by stresses. |
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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 stress. |
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