Corrosion Mechanisms of Solid Alkali Evaporation Equipment are mainly in the below ways;
- Corrosion by chlorates in alkaline solution: During the ion-exchange membrane process for alkali production, chlorates are generated as by-products in the anode chamber of the electrolytic cell. With the prolonged operation of the ion-exchange membrane, the side reaction leading to chlorates intensifies, resulting in an increase in their content in the alkaline solution. During alkali evaporation, the high-temperature concentrated alkali, along with the presence of chlorates, exhibits a certain level of corrosiveness towards nickel-based equipment. Chlorates in the alkaline solution gradually decompose above 250°C, releasing nascent oxygen that reacts with nickel materials to form a nickel oxide layer. This nickel oxide is easily soluble in concentrated alkali and gets carried away, leading to corrosion damage to nickel equipment.
- Thermal stress corrosion: During the falling-film evaporation process, thermal stress corrosion is a significant mechanism. In this process, the liquid forms a thin film flowing on the inner wall of the pipes, which experiences temperature variations and stress. When the film flows in high-temperature areas, the pipe wall may be affected by thermal expansion, resulting in thermal stress. Conversely, as the film flows into low-temperature areas, the pipe wall may rapidly cool down, creating cooling-induced contraction stress. These temperature gradients and stress variations cause stress concentration and initiate stress corrosion on the pipe wall.
Thermal stress corrosion may lead to the rupture and detachment of the oxide layer on the pipe wall surface, exposing the pipe wall to the corrosive medium. In an alkaline environment, thermal stress corrosion may also interact with the chemical components in the alkaline solution, further exacerbating the corrosion damage. Such corrosion damage will reduce the operational lifespan of falling-film evaporation equipment and may lead to equipment failure.