Hydrogen sulfide is a colorless, smelly, flammable, explosive, toxic and corrosive acid gas. In the petroleum industry, hydrogen sulfide in oil and gas not only comes from the formation and chemical additives but also comes from organic sulfide such as mercaptan and sulfide in sour crude oil. These organic sulfide compounds will be converted and decomposed into corresponding hydrogen sulfide when heated in the crude oil processing process. Dry H2S has no corrosive effect on metal materials and corrosive only when dissolved in water. According to NACE MR 0103 -- 2005 «Materials Resistant to corrosive Stress cracking in corrosive Petroleum», wet H2S corrosion environment is defined when the medium in contact with the container has free water in the liquid phase and meets one of the following conditions:
(1) The concentration of H2S dissolved in free water is greater than 50mg/L;
(2) The pH value of free water is less than 4.0, and H2S is dissolved;
(3) free water PH＞7.6 and water containing 20ppm of dissolved hydrocyanic acid (HCN) and some dissolved H2S;
(4) The partial pressure (absolute pressure) of H2S in the gas phase is greater than 0.3kPa;
Cracking types in wet H2S environments include hydrogen bubbling (Hb), hydrogen-induced cracking (HIC), sulfide stress corrosion cracking (SSCC), and stress-guided hydrogen-induced cracking (SOHIC).
Hydrogen bubble (HB)
Precipitation in the process of corrosion of hydrogen diffusion in steel, gathered to form molecular hydrogen in non-metallic inclusions, layering, and other discrete points, large molecular hydrogen is difficult to escape from inside the steel group, a huge internal pressure lead to the surrounding tissues to yield formation under the surface of flat face structure known as the hydrogen bubble, distributed parallel plate surface. It occurs without external stress and is closely related to defects such as inclusions in materials.
Hydrogen Induced Cracking (HIC)
HIC is a defect existing in pipeline steel and weld. Under the action of hydrogen pressure, adjacent hydrogen bubbling cracks on different layers are connected to each other, forming internal cracks with stepped characteristics. The occurrence of HIC does not require external stress and is generally related to large plane inclusions with high density in steel or irregular microstructure produced by segregation of alloying elements in steel. The results show that HIC has little influence on the conventional strength of steel, but more influence on toughness, which will increase the brittleness tendency of steel.
Sulfide Stress Corrosion Cracking (SSCC)
The hydrogen atoms produced by the corrosion in the wet H2S environment penetrate into the internal steel and dissolve in the crystal lattice to increase the brittleness of the steel. The cracking formed under the external stress or residual stress is called sulfide stress corrosion cracking. Sometimes, the brittle cracking of steel and alloy subjected to tensile stress in wet H2S and another sulfide corrosive environments is collectively called sulfide stress corrosion cracking.
The nature of SSCC is hydrogen embrittlement with fracture morphology characterized by brittleness mechanism. It usually occurs in medium and high strength steel or in areas with high hardness such as weld and heat-affected zone. Generally, intergranular fracture occurs in high-strength steel. SSCC and hydrogen-induced cracking is a kind of low-stress failure. Cracking may occur even under very low tensile stress. Most of the cracking occurs suddenly, and the crack is generated and propagated rapidly. In general, the higher the steel strength (hardness), the more likely hydrogen sulfide stress corrosion cracking will occur.
Stress Oriented hydrogen induced cracking (SOHIC)
Stress-induced small cracks in the inclusion or defect caused by hydrogen accumulation along the direction perpendicular to the stress is called stress-guided hydrogen-induced cracking, its typical feature is that the crack expands along with the "Z" shape. It is a special form of stress corrosion cracking (SCC). SOHIC also often occurs in weld heat-affected zones and other high-stress concentration zones. Different from SSCC, SOHIC is sensitive to inclusions in the steel. Stress concentration is usually caused by crack-like defects or stress corrosion cracks, and the coexistence of SSCC and SOHIC has been observed in many cracking cases.
Stress Corrosion Cracking (SCC)
Stress corrosion cracking (SCC) is a kind of environment-induced fracture (EAC), which is a common environment-induced crack dispersed on the outer surface of the steel pipeline. According to statistics, the stress corrosion cracking of stainless steel in wet state corrosion failure accident is even as high as 60%, ranking first in all kinds of corrosion failure accident. The formation of cracks may be due to a large number of factors around the pipeline, combined with environmental action, corrosion and tensile stress. Corrosion is related to the sensitivity of the material and the environment, regardless of the pH of the environment.