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|標題:||微結構與氧化鋯膜對 AISI 430 不銹鋼氫擴散之影響及管壁擴散之研究
Effects of Microstructures and Zirconium Oxide Films on Hydrogen Permeation in AISI 430 Stainless Steels and Diffusion in Hollow Cylinder Walls
|關鍵字:||Hydrogen Permeation;氫擴散;430 Stainless Steel;Zirconium Oxide Film;Blistering;Cold Work;Hollow Cylinder;Microstructures;430不銹鋼;氧化鋯膜;泡腫;冷加工;中空圓柱;微結構||出版社:||材料工程學研究所||摘要:||
三十年前，雖然Ash等人應用Frisch的方法對於n個重覆薄板（laminate）ABAB…求得其遲滯時間（time lag），其並不實用，因為在亨利定律（Henry's law）中之濃度比例常數K及擴散係數較難量測或計算。相反地，A或B的厚度容易被量得。利用薄板具三個不同體積分率（volume fraction）之三個遲滯時間實驗，本文提出求解這些參數的方法較適於應用在實際的系統上，諸如：複合材料（composite）、兩相合金（two-phase alloys）及晶粒晶界（grain/grain boundary）。
將厚度0.8 mm之AISI 430不銹鋼冷軋為0.4 mm並分別於800℃退火15、30及120分鐘以得到三個不同晶粒大小之試片，經由Devanathan氫穿透實驗及數學分析，求得晶界擴散係數
及晶粒擴散係數 ，結果顯示晶界為擴散障礙層（diffusion barrier），此與晶粒擴散遠大於晶界擴散之最重要假設相一致。
冷加工對於AISI 430氫擴散係數、氫濃度及氫捕捉密度 ( hydrogen trap density)之影響亦有探討。氫濃度及氫捕捉密度隨著冷加工的增加而增加，但擴散係數降低。捕捉密度的增加是因為冷加工使得擴散係數降低及氫濃度增加所造成。捕捉密度由 (冷加工試片)變為 (退火試片)。冷加工之鋼材由於其較高之氫濃度對於氫引發泡腫 ( hydrogen induced blistering) 較敏感，但退火鋼材並無發現泡腫的情形。
經由Devanathan氫穿透實驗及數學分析，電解沉積氧化鋯膜對於氫引發泡腫或氫進入之抑制效果也有探討，穿透實驗結果顯示氫在不銹鋼之擴散係數( )為 及在氧化鋯膜之擴散係數（ )為 ，而且金屬之表面氫濃度 被減為 ，此乃因氧化膜對金屬之高濃度比例常數( K=8022)及氫在氧化鋯膜之擴散係數非常低所造成。氫穿透實驗後發現無氧化鋯膜之試片具有泡腫而有氧化鋯膜之試片則無發現泡腫。電解沉積氧化鋯膜對於氫進入及泡腫之產生顯示有抑制效果。
藉由Carslaw 和Jaeger之方法的推導，對於中空圓柱(hollow cylinder)擴散之一般數學分析已有探討。本分析與中空圓柱具不同外內半徑比之質流或熱流有關，包括穩態(steady state)，建構暫態 (set up transient state)和衰退暫態(decay transient state)。在擴散係數為常數的條件下，對於一般方程式部分情況之濃度輪廓(concentration profile)及通量曲線(flux curve)已有顯示及討論。經由電化學穿透實驗，這些新方程式中之部分可用來量測管線(pipeline)之氫擴散係數及氫濃度已被提出。
Though Ash et al. use Frisch's method to obtain the time lag for n repeated laminates ABAB… 3 decades ago, it is not practical because the concentration ratio constant, K, in Henry's law and/or the diffusion coefficients are more difficult to measure or calculate. In contrast, the thickness of A or B is easily measured. The present treatment shows a method for determining these parameters from three lag time experiments using laminates with three different volume fractions. This treatment is more amenable to application to practical systems, such as composites, two- phase alloys, and grain/grain boundary.
An AISI 430 stainless steel sheet with thickness of 0.8 mm was cold-rolled to 0.4 mm and then annealed at 1073 K for 15, 30, and120 min respectively, to derive 3 different grain sizes. Through Devanathan hydrogen permeation tests and a mathematical analysis for diffusion in laminate structures, the grain boundary diffusion coefficient ( = ) and the lattice diffusion coefficient ( = ) were determined. The grain boundary revealed a diffusion barrier. The result is also consistent with the most important assumption that the lattice diffusivity is much greater than the grain boundary diffusivity.
Effects of cold work on hydrogen diffusivity, hydrogen concentration and hydrogen trap density in AISI 430 stainless steel were investigated. Hydrogen concentration and trap density were increased with increasing cold work, but diffusivity was decreased. The increase in trap density as a result of cold work was responsible for the decrease in hydrogen diffusivity and the increased in hydrogen concentration. Trap densities varied from for the cold-worked specimen to for the annealed. The cold-worked steel was more sensitive to hydrogen induced blistering due to its higher hydrogen concentration and trap density, but no blistering was found on annealed specimens. The critical hydrogen concentration and critical trap density for blistering were found between and , and , respectively.
Through Devanathan hydrogen permeation tests and a mathematical analysis, the retarding effect of electrolytic deposition of Zirconium (ZrO2) oxide films on the hydrogen-induced blistering or entry was investigated. The permeation test has indicated that the diffusion coefficient in a metal sheet of AISI 430 stainless steel ( ) was and that in ZrO2 ( ) was . Also, the subsurface hydrogen concentration on the metal was reduced to ( caused not only by a high concentration ratio (K; 8022) of the oxide film to the metal but also by the extremely low value of . Blistering was found on the uncoated specimen after the hydrogen permeation test but was not found on the coated specimen .The electrolytic deposition of film on the AISI 430 stainless steel demonstrated the retarding effect on hydrogen entry and the occurrence of blistering.
By a development of the method of Carslaw and Jaeger, a general mathematical analysis for diffusion in a hollow cylinder has been given. The analysis relates to flow of mass or heat in hollow cylinders with different outer / inner radius ratio, including steady, set up transient, and decay transient states. Under the condition where diffusion coefficient is constant, the concentration profiles, and flux curves indicated in general equations for some cases have been demonstrated and discussed. A few of these novel equations, which can be applied for measuring the diffusivity and the concentration profile of hydrogen in pipeline during the electrochemical permeation test, have been presented.
|Appears in Collections:||材料科學與工程學系|
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