A brief discussion on the analysis of corrosion causes in the acidic water tank of petrochemical acidic water stripping units

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The last edit of this post was made by Wang Wei2 on 2020-6-7 at 09:56. A brief analysis of the causes of corrosion in the acidic water tanks used in petrochemical acid water stripping units: 1. Damage to the coating surface. The main reason sewage causes corrosion to conventional epoxy, furan, and phenolic coatings that cure at room temperature is that small phenolic molecules can penetrate these coatings, leading to swelling and breakdown of the molecular structure of the organic coatings. Furthermore, in sewage at temperatures of 65–70°C, the high temperature, in combination with other factors, causes damage to the anti-corrosion coating. Due to this property of organic coatings, they can be used at higher temperatures in pure water. It has a high glass transition temperature. When used in an aqueous solution containing certain corrosive agents, the glass transition temperature drops significantly. For example, epoxy enamel can be used at around 80°C for a long time, but in an aqueous solution under such conditions it can only be used below 60°C, and long-term use cannot be guaranteed. In other words, in an aqueous solution containing corrosive agents, smaller molecular gases and these agents can easily penetrate into the organic coating, disrupting its original molecular structure and reducing its heat resistance. The surface coating softens, bubbles form, the coating hardens, and it breaks down, losing its function. 2. Metal corrosion beneath the coating: 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, leading to corrosion beneath the scale. It can also cause sulfide stress corrosion cracking (SCC) in areas with stress concentration. H3N + H2S = NH4HS. Ammonia is highly soluble in water (with a solubility volume ratio of 700:1); when it combines with water, it forms the relatively stable crystalline hydrate NH3●H2O at low temperatures. However, its melting point is low at -78.85°C, and its electrolysis reaction is as follows: NH3●H2O = NH4+ + OH-, with the generation of ions leading to electrochemical corrosion in the electrolyte. 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 increases 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 of the corrosion cracks occur at the welds on the tank walls, in the heat-affected zones, and at the areas under stress on the tank bottom. In one 3000 m3 tank that had been in use for over a year, more than 300 cracks were detected during inspections. In summary, the corrosion in acid water tanks is primarily stress corrosion cracking in the weld seams and their heat-affected zones. The main factors are caused by a high temperature, a relatively alkaline high pH value, and acidic corrosive substances present in the medium. This medium is still quite corrosive to metals, so the anti-corrosion coating required has to be of high quality.