A Surface Kinetic Scaling Model For CaCO₃ On a Stainless Steel Surface (316 L)

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. INTRODUCTION Scaling is a major issue for the oil industry. There are several different types of scale such as calcium sulfate and barium sulfate; however, the most common is undoubtedly calcium carbonate.^{1, 2} The water used to enhance oil recovery brings large amounts of ions, such as calcium ions, from the carbonates reservoir. The water composition, in addition to the elevated temperature, promotes scale formation.