Effect of Temperature on the Pitting Behavior and Passive Film Characteristics of Alloy 600 in Chloride Solution

Abstract

The pitting behavior and passive film characteristics of Alloy 600 (UNS N06600) were investigated in deaerated 0.282 M sodium chloride (NaCl) solution in the temperature range from 90°C to 280°C using anodic polarization, x-ray photoelectron spectroscopy, and transmission electron diffraction. The pitting potentials decreased significantly with increasing temperatures up to 220°C and then decreased slowly at higher temperatures. Pit morphologies were also dependent on the test solution temperature; isolated pits were developed up to 220°C, while pit broadening occurred over a large surface area at 280°C. The film formed at 90°C consisted of mainly chromium hydroxide with a large amount of bound water and an amorphous structure. However, the concentration of non-protective, crystallized nickel hydroxide/oxide (Ni[OH]2/NiO) increased at 280°C. From the results of this study, it is suggested that the temperature dependency of pitting behavior is closely related to the changes in composition and structure of the passive films.