Please explain: What is the reason for the corrosion of centrifugal flow channels caused by the sludge water discharged from water treatment plants?
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Seek an explanation: The reasons for the corrosion of centrifugal flow channels caused by sludge water discharged from water treatment plants. It would be best if the chemical properties of this sludge water could also be explained. The sludge discharge contains about 0.015 of polyaluminum chloride.
The sludge water contains a large amount of colloidal particles, suspended solids, coagulants, as well as hydroxides produced by these coagulants, all of which corrode metal pipes
We know that metals are prone to electrochemical corrosion in aqueous solutions. When mud and water are present, underfilm corrosion of dirt is likely to occur. When the pH value is less than 7, it is acidic; in such conditions, a dense oxide film cannot form on the metal surface, which exacerbates metal corrosion. Since polyaluminum chloride is an acidic substance, its pH value ranges from 3.8 to 4.5. Therefore, when a metal surface is exposed to an acidic solution for an extended period of time, its surface remains in an activated state, preventing the formation of a passivation film that provides corrosion protection. However, the electrolyte formed on the metal surface causes electrochemical corrosion of the metal. Under such conditions of use, due to the activation effect of chloride ions in the aqueous solution, it plays a significant role in both the disruption and establishment of the passive state. The theoretical view on phase-forming films suggests that, due to its small radius and strong penetration ability, chloride ions can most easily pass through the extremely small pores in the film; moreover, when it interacts with metals, soluble compounds are formed. The fundamental reason why chloride ions destroy the passivation film is that Cl- ions have a strong ability to be adsorbed by metals. They are preferentially adsorbed by metals, displacing oxygen from the metal surface. We already know that oxygen determines the passivation state of metals. Chloride ions and oxygen compete for adsorption sites on metal surfaces, and can even replace the already adsorbed \"passivation ions\". Due to the fast reaction rate between chlorides and metals, the adsorption is not stable, resulting in the formation of soluble substances. Such a reaction accelerates metal corrosion.
Chloride intergranular corrosion. It is recommended to use stainless steel pumps, preferably double-directional steel ones