A brief discussion on the corrosion of acidic water tanks in petrochemical acidic water stripping units

Thread Content

This post was last edited by Wang Wei2 on 2020-6-6 at 19:06. A brief discussion on the corrosion of the acidic water tanks in the acidic water stripping units in petrochemical industries. 1. Situation: The acidic water treatment units in petrochemical refineries are primarily used to treat sulfur-containing wastewater from units such as atmospheric and vacuum distillation, catalytic processing, and hydrogenation processes. A single-tower side-stream, pressurized system is employed, taking advantage of the higher volatility of CO2 and H2S compared to NH3 and their lower solubility than NH3, to remove NH3, H2S, and CO2 from wastewater. The function of the acidic water tank is primarily to buffer and store the sulfur-containing wastewater discharged by the aforementioned devices; after oil removal, this wastewater is pumped into the tower by the feed water pump for further treatment. Due to the complex composition of the raw water, which contains various substances such as H2S, NH3, CO2, CN-, phenols, and oils, it has strong corrosivity. For the use of epoxy-based coatings and epoxy-modified anti-corrosion coatings in the tanks during the initial stage, these have a significant destructive effect on the anti-corrosion coating. The coating developed problems within less than 3 months of use: bulging, hardening, and cracking, failing to serve as an anti-corrosion coating. Thus, it causes corrosion of the exposed metal surfaces. The main areas of corrosion are at the bottom and walls of the tank; the surface defects appear as penetrating cracks near the welds, which is a form of stress corrosion cracking. Some acidic water tanks were scrapped after being in use for nearly two years. During the leakage process, it not only affects production but also causes certain pollution to the surrounding environment. 2. Causes of corrosion: The operating conditions in the acidic water tank of acidic water gas treatment systems are relatively harsh. The main components of the working medium 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 tends 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. Furthermore, using ordinary anti-corrosion coatings provides very poor protection. Therefore, which type of anti-corrosion coating to use for corrosion protection is an issue that our anti-corrosion engineers need to consider carefully.