Structure and Morphology of Electrodeposited CaCO3: X‐Ray Diffraction and Microscopy Studies

Abstract

A good understanding of the nucleation and growth of calcium carbonate phases is of considerable importance in a wide variety of applications, such as the control of scaling in cooling water systems, corrosion inhibition in marine systems, and geological mineraliza-tion and cementation processes. 1 Calcium carbonate deposition can be realized in two ways: (i) temperature-induced deposition due to changes in calcium carbonate solubility or (ii) pH change of a calcium salt solution resulting in CaCO 3 precipitation. Calcium carbon-ate has three crystal forms: calcite, aragonite, and vaterite. A number of studies 2-4 have been made to determine the conditions under which different calcium carbonate phases are formed. However, most studies 5-10 have been concerned with either spontaneous crys-tallization or the seeded crystal growth of calcium carbonate from saturated solutions. Electrochemical deposition provides a good way of simulating calcium carbonate crystallization at a metal surface. A number of works have appeared in the literature using this method. 11-13 They emphasize mostly the current-potential-time relationships during the deposition process. Here we present some results of X-ray diffraction (XRD) investigations to identify the crystal form and microcrystal orientation of the electrodeposited CaCO 3 phases. Scanning electron microscopy (SEM) was also employed to determine the deposit morphology. The influence of deposition time, temperature , magnesium ion concentration, electrochemical potential, and added organic inhibitors in solution has been investigated, and results are presented. Experimental A Princeton Applied Research (PAR) potentiostat (model 173), driven by a PAR universal programmer (model 179), provided the electrochemical control of the deposition process. A nickel foil or a vacuum-evaporated film of Ni on a silicon substrate was used as the working electrode; a platinum wire counter electrode was employed along with a saturated calomel electrode (SCE) as reference. The working electrode was polished to a 0.05 m finish and rinsed very well before use. Aqueous solutions containing 600 ppm calcium chloride plus 600 ppm sodium bicarbonate, with and without 300 ppm magnesium chloride or 20 ppm organic inhibitor added, were used for the experiments. Deposition at room temperature (25C) was carried out in a conventional H-type, three-compartment cell. Deposits were examined ex situ by XRD and SEM. A special cell was constructed for simultaneous electrodeposition and XRD measurements in situ at temperatures above ambient to about 95C. The cell was made of Teflon and shown schematically in Fig. 1. The cell body (A) measured about 5 cm in diam by 2 cm thick with a 2.5 cm bore. The plunger (B) was about 3 cm long and held the working electrode, as well as the counter and reference electrodes, thermocouple, and pH probes. Solution was introduced into the cell from an external reservoir, while the plunger was partially withdrawn (about 1 mm), so that electrodeposition could be carried out. The plunger was pushed toward the Kapton window for making XRD measurements. Two cartridge heaters embedded into the cell body heated the solution to the desired temperature. The aqueous solution flowed through the cell by a peristaltic pump. Deposition experiments under simulated cooling water conditions were also carried out at the Nalco Chemical Company using a benchtop unit. The setup was a flow system consisting of two concentric pipes with hot water running through the inside of an inner stainless steel pipe, while cold water flowed inside the annular space in the opposite direction. A polished silicon wafer with dimensions of 3 3 25 mm was used as the substrate for deposition; it was mounted on the outer wall of the stainless steel pipe and served as the heat exchange surface. The cooling water temperature was maintained at 50C. The deposits generated in the experiments were examined using XRD, X-ray rocking curves, and/or SEM. A 12 kW, direct-drive Rigaku rotating anode X-ray generator (model 4149C6) and a four-circle Huber diffractometer model (D422/511) were used for XRD measurements. The XRD measurement was done in a coupled theta-2 theta mode with respect to the substrate surface; hence, only crystal planes that are parallel to the substrate surface were probed. The XRD peak intensity was assumed to be proportional to the amount of crystal deposited on the substrate. The X-ray rocking curve method is a sensitive way to measure the crystal orientation with respect to the substrate surface. By fixing the X-ray detector at a certain angle that satisfies the Bragg condition of a specific crystal plane (that is, parallel to the substrate surface), and rocking the sub-strate plane around the Bragg angle, one observes a single sharp The structure and morphology of electrochemically deposited calcium carbonate have been investigated as a function of temperature , magnesium ion concentration, organic inhibitors, and deposition potential. At 25C, microcrystals of calcite grew with their [104] planes parallel to the electrode surface. In the presence of magnesium chloride, a mixed phase of calcite and magensium cal-cite, as well as aragonite, was formed. Organic inhibitors varied in their effectiveness in inhibiting calcite formation, e.g., 2-phos-phonobutane, 1,2,4-tricarboxylic acid (PBTC) performed better than ethanediphosphonic acid (EDPA). Figure 1. XRD/electrochemical cell for in situ studies. (A) cell body, (B) plunger, (W) Ni working electrode, (R) SCE reference electrode, (C) Au counter electrode, (H) cartridge heaters, (TC) thermocouple, (PH) pH sensor.