What are the sources and basic characteristics of phosphate scale?

In circulating cooling water systems, polyphosphates are added as corrosion inhibitors or scale inhibitors. In water, polyphosphates hydrolyze to form orthophosphates, resulting in the presence of PO in the water. When it combines with calcium ions, calcium phosphate is formed, which has very low solubility and thus precipitates and adheres to the surface of the substrate, leading to the formation of calcium phosphate scale. This type of scale affects heat transfer and is difficult to remove. Therefore, in circulating cooling water systems where polyphosphate chemicals are used, attention must be paid to the formation of calcium phosphate scale. (Salt scale made of phosphates appears grayish-white and is relatively loose in texture. Scale containing both carbonates and phosphates is also grayish-white. This is because apatite is gray in color; if corrosion products are present, it takes on a grayish-red or reddish-brown appearance.) When oxidants are added to the boiler water or feed water, the color of the scale is usually grayish-black

Carbonate scale forms in areas of the equipment where heat is applied and the water used as a working fluid becomes concentrated; the more intense the heating, the more severe the scaling. Basic properties of carbonate scale: Carbonate scale appears white or grayish-white in color. If the device is corroded, it will take on the color of the corrosion products. In the presence of abundant oxygen, the corrosion products are mainly iron oxide trioxide, and the scale appears pink or red ； When the oxygen supply is insufficient, the corrosion products are mainly iron tetraoxide, and the scale appears grayish-white or gray-brown. Carbonate scale is hard and brittle, adheres firmly, and is difficult to remove or scrape off. In nature, carbonates exist in various forms and compositions, and carbonate scales correspond to these as well. Scale is similar to marble; its fracture surface is granular. When it is thick and contains corrosion products or other impurities, a layered deposition pattern can be observed at the fracture site.