Surface dissolution and formation and 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 producing a dimpled surface looking like orange peel, called “Scalloping”. This roughness plays an important role in the corrosion of pipes made of carbon steel and it seems that the formation of scallops are major factors in the thinning rate of the pipes. To characterize scallops, study the mechanisms of scallop formation and investigate how the formation of scallops and scallop phenomena affect the dissolution rate, experiments on the pressure drop and flow characteristics, of pipes made of plaster of Paris (CaSO₄-1/2H₂O) were performed. Atomic Absorption Spectroscopy (AAS) was used to analyze the dissolution rate of the plaster. The surface was photographed with a digital camera to observe the initiation of scallops. Pressure transducer was used to measure pressure drop. The size decreases with increasing flow rate whereas the population of scallops increases with increasing flow rate. Scalloping is believed to initiate from defect at the surface and it was found that size and population of scallops increase with increasing initial defect size and initial defect concentration respectively. The average dissolution rate increases with increasing flow rate, particle size, particle concentration and temperature. The dissolution rate of plaster is controlled by mixed kinetics. The entrance section affected the mechanism of the gypsum dissolution. It is found that concentration of defects on the plaster surface has greater effect on the dissolution rate than effect of defect size. Pressure drop increases with increasing flow rate and temperature but decreases with increasing initial defect size and concentration. This means that the diameter of the plaster pipe has a greater effect than the surface roughness.