Zeta Potential of Deposit Components at Elevated Temperatures

One of the major concerns in fossil fuel, nuclear, and geothermal power plants is the formation of deposits of corrosion products in various areas of the water steam cycle. This process causes overheating and rapture of boiler tubes, loss of efficiency of turbines, damages other segment of power units and, in the case of nuclear plants, it can cause radioactive contamination in cooling-water systems. Deposits can also enhance the corrosion processes in all listed types of plants. The damage caused by the deposits can significantly influence the performance of power plants. The deposits are mainly composed of various oxide phases, such as silica (SiO2), and particulate corrosion products, such as magnetite (Fe3O4). Dissolved or colloidal oxides tend to precipitate on the inner surfaces of the process lines [1-4]. Understanding the electrokinetic behavior of the oxide particles at the parameters of power plant environments is very important for understanding the mechanism of the deposit formations and further finding ways for its control and prevention. Examination of numerous power plants made evident the presence of crud particles that generally behave as colloids [4-7]. The colloidal characteristics of crud particles may have a decisive role in the mechanism of deposit formation. Accordingly, the deposition process can be explained by the electrostatic interaction between the metal surface of the pipe walls and the colloidal particles based on the electrical charge present on their surfaces. Electrostatic and chemical interaction at the solid/solution interface results in the formation of the electrical double layer (EDL). One of the important parameters of the EDL is the zeta potential (ζ, or ZP), which is defined as the surface potential at the plane of shear where the bulk solution “slips” along the particle surface. Hypothetically, the shear plane divides the hydrodynamically immobile fluid layer adsorbed at the solid surface and the bulk (mobile) fluid. The zeta potential can be experimentally obtained using a variety of electrokinetic techniques that induce relative movement of solution against the solid surface. Ultimately, it is the magnitude of the zeta potential that determines the extent of the interaction between colloids and other surfaces. That is, high zeta potential values provide greater colloidal stability (by way of increased electrostatic repulsion between charged surfaces). The sign of the zeta potential depends on the balance of the adsorbed species. The point at which the zeta potential switches its sign from negative to positive is called the isoelectric point (IEP), and it is in this area that the suspensions possess the lowest stability, and hence the greater chance of aggregation or deposition. By knowing the magnitude and sign of the zeta potential of the deposit components at elevated temperatures, the Zeta Potential of Deposit Components at Elevated Temperatures