Pit Initiation and Growth Behavior of Alloy 600 in Chloride Solutions at Temperature Range of 90∼280 .deg. C

Pit initiation and growth behavior of Alloy 600 have been evaluated in chloride solutions at temperature range of 90 ∼ 280 .deg. C by use of anodic polarization and immersion testing technique. The pit initiation resistance significantly decreased to 175 .deg. C with increasing temperature in terms of the pitting potential and pit density, whereas it showed slower decrease at higher temperature. The growth rate of stable pits, however, decreased with increasing temperature in chloride solutions containing cupric ion. The film formed in chloride solution at 90 .deg. C consisted of chromium oxyhydroxide having an amorphous structure, whereas a crystalline film of mixed chromium and nickel oxyhydroxides was formed at 280 .deg. C. The higher resistance to initiation at 90 .deg. C, therefore, could be due to the protective film of the amorphous chromium oxyhydroxde, and thus to low probability for pits to form. In this case, the pitted area is very small compared with the pit free area leading to increase in the growth rate. The pit initiation would, however, be facilitated at 280 .deg. C because of the formation of the defective crystalline film having nonprotective nickel oxyhydroxde, which in turn significantly increases anodic area. Consequently, the growth rate decreases by successive film breakdown around pit mouth and hindering effect of corrosion product on ion diffusion