Abstract
Scaling, which is the formation of insoluble inorganic solids, is a major problem in water treatment plants and in the oil industry. Millions of dollars are spent each year to prevent the occurrence of scale. Calcium carbonate (CaCO3) and barium sulfate (BaSO4) are common types of scale encountered in these sectors. Scale predictions are typically based on thermodynamics and tend not to give a time-based quantification of surface scale growth. Recent work clearly demonstrated that scale deposition and precipitation are two different processes and that their rates should be evaluated separately. This paper initially compares the kinetics of CaCO3 precipitation in the bulk and deposition on the surface of austenitic stainless steel (316L) at 24°C and 70°C under a controlled turbulent flow regime. Secondly, precipitation and deposition data are combined to obtain the first kinetic surface deposition model giving a scale thickness in millimetres per year as a function of the saturation index for CaCO3 scale at these two temperatures. Experiments were performed using a Rotating Cylinder Electrode (RCE) device. Several techniques, such as Scanning Electron Microscopy (SEM) and Inductively Coupled Plasma (ICP), were used to investigate the precipitation in the bulk and deposition of CaCO3 scale on the stainless steel surface. The paper discusses the value and limitations of the model and how it can be used to build the understanding of surface scale deposition.
