Corrosion and mechanical behaviors of Ni-base superalloys in high temperature steam environments = 니켈기 초합금의 고온 수증기 환경에서의 부식 및 기계적 특성

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Ni-base superalloys such as Alloy 617 and Haynes 230 were examined for VHTR intermediate heat exchanger applications. This study investigated the corrosion behaviors and mechanical characteristics such as tensile and creep properties in steam with and without hydrogen environments. Also, surface treatment was developed to improve the corrosion resistance in high temperature conditions. The corrosion rate followed the parabolic rate law because oxide growth was governed by diffusion of oxidant through oxide layer. In air condition, Cr-rich oxide layer exhibited the p-type characteristics. It means that dominant defect source is metal vacancy. Thus, oxide growth was occurred by outward diffusion of metal ion. Consequently, void and wrinkled surface were formed at oxide / metal interface due to metal vacancy condensation. On the other hands, that was metal interstitial and oxygen vacancy in steam with and without hydrogen condition. Therefore, voids were formed at upper oxide region by oxygen vacancy condensation. When hydrogen was added in steam environments, hydrogen gas permeated oxide / metal interface. The penetrated hydrogen was ionized and released an electron, which reduced the electron valence of metal ions. To maintain the charge neutrality, positively charged defects such as metal interstitials and oxygen vacancies were increased proportionally to the density of the ionized hydrogen. As a result, corrosion rate was accelerated and plate shaped oxide formed by increased diffusion of oxidant. Long-term aging at high temperature precipitation and coarsening of secondary phase such as M23C6 and M6C were developed. This phase leaded to increased tensile strength and decreased ductility. Also, sur-face and internal oxide acted as crack initiation site and consequently, decreased the tensile strength and ductility. However, ductile failure occurred at decarburization region and consequently increased the ductility. The creep rupture life in steam environmen...
Advisors
Jang, Chang-Heuiresearcher장창희
Description
한국과학기술원 : 원자력및양자공학과,
Publisher
한국과학기술원
Issue Date
2013
Identifier
565581/325007  / 020095016
Language
eng
Description

학위논문(박사) - 한국과학기술원 : 원자력및양자공학과, 2013.8, [ xii, 121 p. ]

Keywords

Ni-bas superalloys; 표면처리; 크립; Mott-Schottky analysis; 부식; 니켈기 초합금; Corrosion; Mott-Schottky analysis; Creep; Surface treatment

URI
http://hdl.handle.net/10203/197258
Link
http://library.kaist.ac.kr/search/detail/view.do?bibCtrlNo=565581&flag=dissertation
Appears in Collection
NE-Theses_Ph.D.(박사논문)
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