Flow-accelerated corrosion is the dissolution of the normally protective iron oxide film into a flowing stream. FAC only occurs when the fluid is moving, contains water and is unsaturated in iron. FAC can be modeled as a turbulent mass transfer process as was done by Berge and his co-workers at Électricité de France.

Although reasonably well understood, in the 1990s a new effect was documented. The “entrance effect” occurs when flow passes from a resistant material to a non-resistant material. The effect causes a local increase in the corrosion rate manifested by a groove downstream of the attachment weld. Although observed and identified in the 1990s, investigators had speculated about its existence since the early 1980s.

Described in this paper are occurrences of the entrance effect in US nuclear plants.

Although often overlooked, this effect may be critical in determining the remaining service life of components and constitute an important vulnerability into a FAC program. FAC engineers should be aware of this effect to properly select inspection locations and to design repair measures.