A brief discussion on the causes of corrosion in sulfur-containing wastewater in industry

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1. Overview: With the rapid development of the oil industry, and the increasing number of production containers and pipelines used in oil extraction and water injection in oil fields, corrosion problems have become increasingly severe. The sulfur-containing wastewater tanks, sewage containers, and sewage pipelines in various refineries also face severe corrosion problems. The wastewater contains large amounts of H2S, NH3, HCN, and CO2, as well as numerous harmful ions such as Cl- and SO42-, along with a high volume of bacteria; moreover, its temperature is quite high, ranging from 70 to 80°C. Therefore, the corrosion of low-carbon steel equipment is quite severe. In particular, in oilfield sulfur-containing wastewater, the corrosion depth after 2–3 years reaches 3–4 mm; in many cases corrosion leads to perforations, resulting in an increasing number of wastewater tanks and pipelines that become unusable. The corrosion problem associated with sulfur-containing wastewater is a current challenge in oil fields and refining systems, and it has drawn attention from the field of corrosion protection. The sulfur-containing wastewater from the refinery’s wastewater treatment plant originates mainly from the wastewater generated during the production processes in the atmospheric and vacuum distillation units as well as the catalytic units. Due to the large amount of corrosive substances present in the wastewater, sewage tanks, containers, and pipelines suffer severe corrosion, resulting in numerous spots and pits. In some cases, stress corrosion cracking occurs in the heat-affected zones of the welds, and welding repairs are unable to eliminate these cracks. The corrosion mechanism and protection methods are described in detail below. 2. Analysis of corrosion conditions 2.1 Corrosion mechanism As crude oil is processed more deeply, the sulfides and nitrides present in it are further decomposed. Therefore, sulfur-containing wastewater contains large amounts of H2S, NH3, and HCN. These substances, or the reaction products between them, become the main corrosive agents. Sulfur-containing wastewater contains large amounts of corrosive substances such as H2S, NH3, and HCN, creating a H2S-NH3-HCN-H2O corrosive environment. Fe + H2S → FeS↓ + 2(H2O). In an H2S-containing aqueous solution, steel undergoes not only normal corrosion resulting from the electrochemical formation of FeS, but the hydrogen atoms generated also penetrate and diffuse into the steel. The hydrogen atoms that penetrate into steel are distributed partly within the metal lattice, while the rest diffuse toward metal defects (dislocations, vacancies, or submicroscopic or larger voids such as pores and interlayers in welds), where they accumulate to form hydrogen molecules. And these hydrogen molecules do not easily escape from the steel, causing permanent embrittlement of the steel. Due to the continuous accumulation of hydrogen molecules, the pressure in that area increases, and the resulting high internal stresses cause the steel plate to delaminate, form bubbles, and even crack. Welding the crack with electric welding only lengthens the original crack, which is a characteristic of hydrogen embrittlement. At a pH of 6, the steel surface is covered with FeS, providing good protective properties. However, in the presence of active ions such as CN- and Cl-, it dissolves the FeS protective layer, accelerating the corrosion reaction: FeS + 6CN- → 4- + S2-; 4- + 2Fe2+ → Fe2↓. Additionally, since wastewater contains large amounts of NH3, CN-, and Cl-, these can react with H2S to form NH4HS and (NH4)2S, which are substances with strong corrosive properties. H2S+NH3→ NH4HS; H2S+2NH3 → (NH4)2S. At the same time, this leads to an **increase in the solubility of H2S in water, that is, an increase in the solubility of HS-. Furthermore, the pH of NH3 when dissolved in water is around 10 or higher. Thus, it provides excellent pH conditions for the reaction between FeS and CN-. The corrosion of carbon steel by sulfur-containing wastewater is characterized by thickness reduction and local perforation, as well as a high tendency to cause bubbling, cracking, and stress corrosion cracking at welds. For the corrosion of metals and non-metals caused by the above sulfur-containing wastewater, anti-corrosion treatment is mostly carried out using coating materials.