A review of the effects of silica on stress corrosion cracking of steam generator tubes

Evaluation of operating chemistry and outside diameter stress corrosion cracking (ODSCC) data from steam generators in several PWR plants has shown that the combination of boric acid treatment of the secondary water and relatively high silica concentration in the tube bundle may inhibit ODSCC. This has led to a hypothesis that secondary side chemistry guidelines may be modified to take advantage of these findings. Beneficial effects of boric acid have been documented extensively. However, the effect of silica on corrosion has been controversial. Consequently, an extensive review of the available literature on the effects of silica on corrosion was conducted so as to make an informed judgment regarding its application as an ODSCC inhibitor. The results of the review are summarized here. A critical evaluation of the data is provided along with our conclusions. Inhibiting effect of silica is supported by laboratory tests, pulled tube examination and plant operating chemistry data. It acts as a buffer in both acidic and alkaline crevice environments. Based on the evaluation of the currently available data, we conclude that silica, in conjunction with boric acid, can inhibit ODSCC in PWR steam generator tubing. However, the inhibiting effects of silica and the available supporting data have not been adequately recognized by the nuclear industry. We recommend that consideration be given to the use of silica as an inhibitor. This review has not shed any light on the effect of silica on heat transfer. Hence we support additional evaluations to assess its impact on heat transfer and to modify secondary water chemistry guidelines.