Quartz Crystal Microbalance Evaluation of Inhibitors for Inorganic Scale

In this paper, the use of a quartz crystal microbalance (QCM) with a quartz crystal sensor coated with iron oxide is proposed to evaluate the efficacy of inhibitors in the prevention of scale formation. The quartz crystal was first iron-plated by electrodeposition over the original gold film on the outer side of the crystal and then oxidized. The iron oxide layer is more representative for an evaluation of the inhibitor's effectiveness because tubing and equipment in oil-industry facilities are made of low carbon steel that is coated with an iron oxide layer. The scale formation was conducted under a steady supersaturation condition. The experiments were performed with slow addition (0.2 cm(3)/min) of 40 cm(3) of Na2CO3 solution (1,000 ppm) to 200 cm(3) of synthetic formation water (AF-W2). The performance at 10 ppm concentration of two commercial scale inhibitors, diethylenetriamine penta(methylenephosphonic acid) (DETPMPA) and polyphosphonocarboxylic acid (PPCA), was evaluated. The mass variation on the iron oxide plated QCM crystal sensor, caused by CaCO3 deposition, is related to supersaturation, pH value, and efficacy of scale inhibitor. Scanning-electron-microscope (SEM) images show that besides calcite crystals, there are also deposits in the form of spherical lenses, which is characteristic of the polymorph vaterite.