Effect of inhibitors $(TiO_2,CeB_6 and ZnO)$ on metastable pitting and repassivation kinetics of Alloy 600

Abstract

Alloy 600 is prone to stress corrosion cracking (SCC) and intergranular attack (IGA) on secondary side of Steam Generator (SG) applications. It is reported that addition of small amount of inhibitors $(TiO_2, CeB_6 and ZnO)$ to secondary water of SG tubing reduced propagation rate of stress corrosion cracks hence increases lifespan of SG tubes. At high temperature, SCC of Alloy 600 occurs by repetitive processes of passive film breakdown-dissolution-repassivation. Repassivation is a film reforming process on the film broken surface. SCC of Alloy 600 is controlled by several variables. However, under certain loading conditions for an alloy/environment system, SCC is function of repassivation rate. Metastable pitting is also a process of film break down, dissolution and repassivation of an alloy. Thus, repassivation rate of Alloy 600 is of critical importance. However, there are very few studies on the influence of inhibitors on metastable pitting and repassivation kinetics of Alloy 600. Moreover, in spite of a couple of studies, the mechanism of inhibitors to reduce crack growth has not yet been elucidated. Objective of present study is to examine effect of inhibitors $(TiO_2, CeB_6 and ZnO)$ on repassivation kinetics and electrochemical noise of Alloy 600 in pH 10 borate buffer solutions and in 10 % NaOH solutions at 90 ℃ and at 200 ℃. To identify the optimum inhibitor among the candidate inhibitors $(TiO_2, CeB_6 and ZnO)$ for Alloy 600 and to understand why inhibitors such as $TiO_2$, $CeB_6$ and ZnO retard crack propagation of Alloy 600. Effect of inhibitors $(TiO_2, CeB_6 and ZnO)$ on metastable pitting properties and repassivation kinetics of Alloy 600 in 1 M $H_2SO_4$, pH 10 and 10 % NaOH solution were examined by electrochemical noise analysis (ENA) and rapid scratching electrode technique (RSET), respectively. Scope of repassivation kinetics was extended, and repassivation rate was measured by step potential technique (SPT) in pH10, 10 % NaOH and 1 M ...