|4. Forms of Corrosion|
4.9 Sulfide Stress Corrosion Cracking
Sulfide Stress Corrosion Cracking (SSCC)
Sulfide stress cracking effects high strength effects high strength carbon steels above Rockwell hardness 22 especially in petroleum production and petroleum refining. SSCC is a specialized case of hydrogen cracking as it may be possible that sulfide stress corrosion cracking and hydrogen cracking proceed simultaneously.
Iron is oxidized to the ferrous form at the anode and hydrogen sulfide undergoes a two step dissociation at the cathode as shown below
At the anode F Fe2+ + 2e
At the cathode H2S + H2O H+ + HS- + H2O
HS- + H2O H+ +S- + H2O
Product combination 2e + 2H+ + Fe2+ + S- 2Ho + FeS (trolite)
The net reaction is Fe + H2S FeS + 2Ho
As shown by the above reaction, not only FeS is formed bu other sulfides such as FeS2 (Pyrite), Fe7S8 (pyrrhotite) and Fe9S8 (Kansite) may also be formed. At low H2S concentrations (0.0009 to 0.1 psi) the sulfide film is generally pyrite, trolite and some kansite and a lower rate of corrosion is observed. The increasing concentration of kansite leads to an increase in the corrosion rate.
Other factors affecting SSCC:
SSCC may occur if the partial pressure of H2S exceeds 0.01 atm for a 130 ksi yield strength steel.
pH: Lower pH adversely affects SSCC.
Chlorides: SSCC is accelerated by chlorides in 12% chromium steel but the effect is not significant in low alloy steels.
Temperation: SSCC occurs predominantly about 20%/
Hardness: The susceptibility to SSCC increases with increased strength. Steels with HRC over 22 are especially susceptible to SSCC.
Cold working: Cold working decreases the resistance to SSCC due to increase in the residual tensile stress.
Examples of Chloride induced SCC are shown in figures below. The samples are taken from a pump transporting hydrocarbons.
|A sleeve showing signs of Chloride induced stress corrosion cracking. The crack is at right angles to the retaining screw point which is the point of maximum stress.||
|Another example of Chloride induced SCC corrosion cracking.||
Select steels of proper strength in accordance with the pressure to be encountered.
Maintain appropriate pH
Select steels of hardness level below 22 HRC
Minimize cold working