How exactly does phosphate erosion occur?

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I. Prerequisites for occurrence: In early pressurized water reactor steam generators, the secondary side was treated chemically with phosphates, with the aim of regulating the pH level of the boiler water and precipitating impurities. Additionally, there were areas within the equipment where flow rates were low, creating conditions that allowed for localized concentration of phosphates. II. Core formation process: Phosphates become highly locally concentrated in areas with low flow rates, such as the gaps in the tube sheet, the tube holes in the support plates, and slag accumulation zones. When the Na/PO4 molar ratio in the boiler water is below 2.5 and the temperature is above 177°C, an acidic phosphate environment is formed. Acidic phosphates destroy the protective oxide layer on the surface of the heat transfer tubes, undergoing a continuous and uniform reaction with the metal in the tube walls, gradually reducing the thickness of these walls and ultimately leading to erosion. When the unit load fluctuates, phosphates undergo a \"hidden-dissolution\" phenomenon, which further accelerates the process of local corrosion and intensifies the wear on the pipe walls.