Mass transfer effects in feeder flow-accelerated corrosion wall thinning

Flow conditions play a dominant role in Flow-Accelerated Corrosion (FAC) under certain conditions, e.g., in CANDU feeders. While chemistry and materials set the overall potential for FAC, flow conditions determine the local distribution of wall thinning. Recent plant data of feeders and laboratory tests confirms that there is a close relationship between local flow conditions, expressed by mass transfer coefficient, and FAC rate in CANDU feeder bends. The knowledge of local effects can be useful for minimizing the number of inspected components, predicting the location of the highest FAC rate for a given piping component, and determining what components or feeders should be replaced. A similar evaluation applies also to FAC in heat transfer equipment such as heat exchangers and steam generators. The objective of this paper is to examine the relationship between FAC rate and local mass transfer parameters. For FAC where the flow is dominant, the FAC rate is proportional to mass flux of ferrous ions. The mass flux is the product of the mass transfer coefficient and the concentration difference, or degree of saturation. The mass transfer coefficient describes the intensity of the transport of corrosion products (ferrous ions) from the oxide-water interface into the bulk water. Therefore, this parameter can be used for predicting the local distribution of FAC rate in the mass-transfer controlled FAC. The degree of saturation reduces the mass flux, thus reducing the FAC rate. This effect can be significant in long piping, e.g., in outlet feeders. The paper presents plant and laboratory evidence for the relationship between local mass transfer conditions and the FAC rate. It shows correlations for mass transfer coefficient in components that are highly susceptible to FAC and most important flow parameters that affect mass transfer coefficient. The role of surface roughness, wall shear stress, and local turbulence is also discussed. (author)