A brief history of the development of the rotating cage as a promising and reliable laboratory methodology for inhibitor evaluation is presented. The influence of the geometry of the rotating cage on the flow pattern as well as on corrosion rates has been investigated. The importance of vessel length and diameter, rotating cage length (and, as a consequence, the sample length), rotating cage diameter, rotation speed, volume of liquid, and flow pattern in determining the corrosion rates, and hence, inhibitor efficiency has been established.

In 1990, the rotating cage was introduced as a promising laboratory methodology, to simulate pipe flow in the laboratory by rotating the specimens at speeds up to 1500 rpm ~4. In the literature, rotating cage experiments are also reported as high-speed autoclave tests (HSAT) 6'7 or rotating probe 4'5 experiments.

In 1999, the atmospheric pressure rotating cage was described, together with a systematic analysis of flow patterns 8'9. Depending on the rotation speeds and liquid level, flow is classified in one of four categories, homogeneous, side-wall affected, top-cover affected, and turbulent. In the homogeneous zone, the vortex dimensions (length and width) increase with rotation speed. Equation (1) is used to determine the wall shear stress in the homogeneous zone.

EQUATION (1)

where z is the wall shear stress; Re is the Reynolds number; p is the density of the fluid; r is the radius of the rotating cage; and co angular velocity. In the side-wall affected zone and top-cover affected zone, the wall shear stress is less than that calculated by Eqn. 1, due to the movement of fluids in the opposite direction. In the turbulent zone, on the other hand, the wall shear stress may be higher than that calculated by Eqn. 1, due to the penetration of the vortex through the cage.

In the results of a study published in 2001, the rotating cage was identified as the preferred methodology for evaluating corrosion inhibitors 1°. This assessment was based on a quantitative comparison of field and laboratory data on general corrosion rates, pitting corrosion rates, and percentage inhibition (calculated from general and pitting corrosion rates) under three different field conditions using three continuous and three batch inhibitors. The rotating cage methodology was also identified as an inexpensive and relatively simple methodology to carry out.

In 2001, ASTM published a new standard, ASTM G 170-0 la, "Evaluating and Qualifying Oilfield and Refinery Corrosion Inhibitors in the Laboratory". This ASTM Standard describes three methodologies to evaluate the efficiency of inhibitors in the laboratory: Rotating Cylinder Electrode (RCE), Rotating Cage (RC), and Jet Impingement (JI). They are compact, inexpensive, hydrodynamically characterized, and scalable, i.e., they can be carried out under various flow conditions. Using these methodologies, several variables that influence inhibitor performance in the field can be simulated, including composition (of the steel, brine, oil, and gas); temperature; pressure; and flow.

In 2002, the application of the atmospheric pressure rotating cage for simultaneous determination of inhibitor efficiency and drag reduction properties of chemicals was demonstrated ~5. In this study, the experiments to determine the inhibitor efficiency were carried out at 140 F (60°C) and 1,725 kPa (250 psi) (20% HzS , 2.5% CO2, balance argon), with a rotation speed of 500 rpm. On the other hand, the experiments for determining drag reduction properties were carried out at atmospheric pressure. The experimental setup was the same as that for …