Causes of stress corrosion cracking in the acidic water tank of the petrochemical acidic water stripping unit

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Reasons for stress corrosion cracking in the acidic water tank of petrochemical acid water stripping units 1. Operating conditions and usage of the acidic water tank: Due to the relatively harsh operating conditions of this tank, its main operating media include raw water with a H2S content of 1.0–2.8 g/L and an NH3 content of 5000 ppm/L; it also contains CO2, CN–, phenols, oils, and other substances. The pH level is around 10±5, while the temperature of the raw water is 65–70°C. When carbon steel operates under such conditions, its metal surface suffers significant corrosion. Its form of corrosion is electrochemical corrosion, and stress corrosion cracking is likely to occur at welds as well as in the base metal where bending forces are applied (such as at the areas where steel plates are compressed and deformed near the bottom of columns in tanks). At the same time, uniform corrosion of the metal surface occurs. If no anti-corrosion measures are taken on the surface of the metal (carbon steel), stress corrosion cracking in the weld seams and their heat-affected zones typically occurs within less than 1 year of operation. While using ordinary coatings does not provide good corrosion protection. 2. Causes of metal stress corrosion cracking: As H2S dissolves in water, it undergoes a corrosive reaction with metals. H2S + Fe → FeS + H2 ↑. FeS reacts with HN3 to form HN3HS, which deposits on the metal surface and leads to underfilm corrosion. It can also cause sulfide stress corrosion cracking (SCC) in areas with stress concentration. H3N + H2S —— NH4HS. Ammonium is highly soluble in water (with a solubility volume ratio of 700:1); when ammonia combines with water, it forms relatively stable crystalline hydrates, NH3 + H2O, under low temperature conditions. However, its melting point is low at –78.85°C, and its electrolysis reaction is as follows: NH3 + H2O —— NH4+ + OH–, which generates ions and electrolyte, leading to electrochemical corrosion. Furthermore, the combined effect of hydrogen sulfide and ammonia exacerbates the corrosion. In the presence of cyanide (CN－), at a pH greater than 7.5, cracking increases as the concentration of CN－ in the medium rises. When HN3HS reacts with H3N: HN4HS + HN3 → (HN4)2S. Ammonium sulfide, (HN4)2S, increases the solubility of H2S in water, thereby raising the HS– concentration. On the other hand, when ammonia dissolves in water, it raises the pH of the water, creating more favorable conditions for the reaction between CN– and FeS. However, the concentration of NH3 in aqueous solutions at 6000 mg/L is far above the acceptable range (the concentration of NH3 should generally be kept below 1000 mg/L). Electrochemical heterogeneities always exist on the surface of metal materials. Defective areas or weak points on the metal surface, having a lower potential than other areas, act as active sites that provide crack initiation sites for stress corrosion. If the material already has scratches, holes, or gaps, they are the current sources of cracks. Therefore, in terms of corrosion phenomena, most cases of corrosion cracking occur at the welds on the tank walls, in the heat-affected zones, and at the areas where the columns at the bottom of the tank are under stress.