Effects of dissolution rate and flow characteristics on scalloping of pipe surfaces

Abstract

The phenomenon of flow-accelerated corrosion (FAC) is a significant problem with steels in water-cooling systems. The sculpting of surfaces that undergo FAC normally develops a characteristic described as “scalloping”. To obtain further insight into FAC it is of interest to understand the formation of scallops and their significance in the dissolution rate of steel piping. This study investigated how the dissolution rate and the flow characteristics lead to scalloping by altering water chemistry, temperatures and flow velocities. Experiments comprised of twelve- conditions were carried out on the dissolution of pipe coated with gypsum (CaSO42H2O). Scallop morphology was characterised with a digital camera. Atomic Absorption Spectroscopy (AAS) was used to analyse the dissolution rate. It was found that the large population of scallops develops with increasing water flow rate. The average dissolution rate increases with flow rate and temperature but is not significantly affected by the pH. The dissolution rate increases with time at pH 3 and 7 but decreases with time at pH 10. The dissolution rate of gypsum is controlled by diffusion transport mechanism at room temperature (25°C). At a lower temperature (10°C), the dissolution rate of gypsum is first controlled by the surface reaction mechanism but changes to diffusion transport mechanism after 2 hours into the experiment.