Surface Dissolution and the Development of Scallops

Abstract

Flow-assisted corrosion (FAC) is a significant problem with carbon steel components exposed to rapidly moving water or water/steam mixtures. Such components often develop distinctive patterns of surface damage called scalloping, so to gain further insight into FAC it is of interest to understand the formation and significance of scallops.

Experiments were carried out on the dissolution of pipes made of plaster of Paris (CaSO₄.½H₂O) to study the evolution of scalloping patterns as well as to explore the link between scalloping and hydrodynamics and scalloping and dissolution rate. The conductivity and pH of water flowing through the test sections were recorded and posttest examination was carried out. Scallops were observed along the plaster surface at the end of the tests. Their characteristics are strongly related to the flow rate; scallop size decreases with increasing flow rate whereas surface density of scallops increases with increasing flow rate. Imperfections such as voids on embedded particles seem necessary for scallops to develop at all.

Key Words:

• Scalloping
• Corrosion
• Dissolution
• Mass transfer