A brief discussion on the causes of low-temperature wet hydrogen sulfide stress corrosion cracking in liquefied gas storage tanks in the petrochemical industry

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A brief discussion on the causes of low-temperature, wet hydrogen sulfide stress corrosion cracking in petrochemical liquefied gas spherical tanks. During physical and chemical inspections of such tanks, cracks are often found in the heat-affected zones of their welds, and large amounts of rust on the tank walls indicate that corrosion has occurred during their operation. 1. An environment prone to wet H2S stress corrosion cracking (SSCC): An environment is considered an SSCC environment when it meets all of the following conditions. ⑴ Temperature ≤ (60 + 2P)°C, where P is the operating pressure (gauge pressure, MPa) ； ⑵ The solubility of H2S in room temperature water is about 10PPm ； ⑶ The medium contains liquid water or is at a temperature below the dew point ； ⑷ The pH is less than 9, or cyanides are present in the medium. 2. Cause analysis: SCC is a common and severe damage phenomenon in engineering; it occurs as a result of hydrogen stress cracking caused by the generation of hydrogen atoms during the sulfidation corrosion process on metal surfaces. It is a form of equipment failure that poses severe hazards in fields such as petrochemicals. Stress-guided hydrogen-induced cracking occurs under stress-induced conditions, where the accumulation of hydrogen at inclusions and defects leads to the formation of rows of small cracks that develop in a direction perpendicular to the stress. It usually occurs in the heat-affected zone of welded joints and in areas with high stress concentration, such as at joint connections, locations of geometric changes, crack-like defects, or sites of stress corrosion cracking. The damage process of containers caused by wet hydrogen sulfide is as follows: Hydrogen sulfide gradually undergoes hydrolysis in water: H2S ≒ H+ + HS‑; HS‑ ≒ H+ + S2‑. When the hydrolyzed hydrogen sulfide solution comes into contact with the surface of steel, an electrochemical reaction occurs, as follows: Anodic reaction: Fe = Fe2+ + 2e; the products of this anodic reaction are Fe2+ + S2‑ = FeS. Cathodic reaction: 2H+ + 2e = 2H2 (gas evolution). The overall reaction is: Fe + H2S = FeS + 2H2 (gas evolution). During electrochemical corrosion of steel in a wet hydrogen sulfide environment, hydrogen atoms generated enter the steel and aggregate at internal defects within it (mainly at the interface between non-metallic inclusions and the metal matrix), resulting in a local pressure increase to 104 MPa. This will cause cracking at the internal defects in the metal. The reason why wet hydrogen sulfide stress corrosion cracking can occur even when the hydrogen sulfide levels are within acceptable limits is that, over time as spherical tanks are in use, rusting products form on the metal surface due to corrosion. These corrosion products can absorb a large amount of sulfides, increasing the concentration of sulfides in the corrosion products on the metal surface. This can lead to stress corrosion cracking. In other words, sensitivity to SSCC was also observed in water with such a low H2S concentration of 1 ppm.