In-situ and ex-situ studies on early-stage scale establishment

• Main Author: Kang, Yihong
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2016
• Subjects: Chemistry; Molecular chemistry
• ISBN: 1369418051; 9781369418057

This thesis study is divided into two parts. In the first part, competitive oxidation during the nucleation-and-growth stage was investigated based on studies on Cu-Ni and Ni-Cr alloys. Two competing oxidation mechanisms were proposed to interpret the competitive oxidation behavior of a dilute Cu-Ni alloy during the nucleation-and-growth stage of oxidation under 10 -4 Torr oxygen pressure at a temperature range from 350°C to 600°C. NiO was found to be the preferred oxide phase to form initially on the Cu-Ni alloy surface rather than a copper oxide product. Depending on the time of the growth process, an individual NiO island could be limited by oxygen surface diffusion, diffusion of Ni in the Cu-Ni matrix towards the surface, or a combination of these two processes. In the competitive oxidation of dilute Ni-Cr alloys under low oxygen partial pressures (10-7 to 10-5 Torr) at 600°C, it was found that Cr2O3 is the preferred oxide to form on the alloy surface, with the growth of Cr 2O3 being limited by diffusion of Cr in the alloy towards the surface. Only when the amount of Cr2O3 formed initially on the surface was sufficiently high that the oxygen inward diffusion into alloy could be reduced to the extent that establishment of a continuous Cr 2O3 scale was possible. The second part of this study investigated the third-element effect on Al2O3-scale formation on Ni-based alloys. Two new perspectives of the third-element effect were proposed. The Cr addition to a relatively dilute Ni-Al alloy resulted in the formation of much coarser internal oxides, and decreased the internal oxidation rate significantly; on the contrary, Mn addition, which has the similar affinity to oxygen as Cr, to a similar Ni-Al alloy led to the formation of much finer rodlike internal oxides, and increased the internal oxidation rate significantly. Based on this experimental observation, it was proposed that the addition of a third element may change the internal oxide phase and morphology, such that the diffusion path of oxygen in the internal oxidation zone could be significantly affected, which in turn could affect the internal-to-external oxidation transition. The Cr addition into a relative dilute Ni-Al alloy was also found to enhance the Al grain-boundary diffusion at lower temperature (below 1000°C), which consequently enhanced the Al2O3-scale establishment during the heating process. Surface recrystallization, which occurs during the heating of alloys prepared with an abraded surface finish (i.e., 500-grit), could result in the formation of fine sub grains around 1 μm in size near the alloy surface, and provide necessary fast diffusion paths for Al. This Cr effect on the Al grain-boundary diffusion was much more evident during the oxidation of surface-abraded Ni-Al-Cr alloys than on the same alloys with a polished surface finish.