Investigation of the Mechanism of IGA/SCC of Alloy 600 in Corrosion Accelerating Heated Crevice Environments. Topical Report - Results for Mod of Heated Crevice (08/18/1999 - 08/31/2000).
The crevice formed by the tube/tube support plate (T/TSP) intersection in a pressurized water reactor (PWR) steam generator is a concentration site for nonvolatile impurities (referred to as hideout) in the steam generator water. The restricted mass transport in the small crevice volume prevents the species, which concentrate by a thermal/hydraulic mechanism during the generation of steam, from quickly dispersing into the bulk water. The presence of a porous scale corrosion product on the surface of the tube and deposits of corrosion products in the crevice further restrict mass transport. The objective of this program is to develop an understanding of the corrosion accelerating mechanisms, particularly IGA/SCC, in steam generator crevices. The important variables will be identified, including the relationship between bulk water chemistry and corrosion accelerating chemistries in a crevice. An important result will be the identification of water chemistry control measures needed to mitigate secondary side IGA/SCC in steam generator tubes. The approach uses an instrumented heated crevice, which is a replica of a PWR steam generator T/TSP crevice. While the system is operating at simulated steam generator thermal conditions, measurements can be made of the chemical, electrochemical, and thermal conditions in the crevice. Damage to the tube due to IGA/SCC and other corrosion processes will be monitored using electrochemical noise.
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