中文摘要

　　不銹鋼材料在高溫及高碳勢的環境下，可能發生金屬塵化，進而引起局部薄化或穿孔的現象。材料發生塵化的敏感性與表面是否生成具有保護性的鈍化膜有密切的關係。而不同的表面狀態(例如：表面加工、表面氧化、表面滲碳等)是否會影響塵化的現象，乃成為一有意義的課題。本研究針對304L不銹鋼經過研磨、噴砂及預氧化等方式的表面處理後，探討其在高碳勢的CO-CO2混合氣氛中的塵化敏感性之差異。另以預滲碳處理(包覆滲碳法)進行不銹鋼的表面滲碳，藉以探討表面奈米碳絲的生成與塵化發生的關係，和滲碳後對塵化之影響。

　　實驗結果顯示，經研磨、噴砂及預氧化處理後的304L不銹鋼三者試片，在熱重分析試驗後，其試片表面均無任何局部蝕孔的形成，其重量增加量均隨試驗溫度的升高而增加，且其重量增加量與時間皆呈線性關係，其試片重量增加主要是由表面碳沉積以及少部份由高溫氧化反應所造成。在長時間暴露試驗後，經研磨或噴砂處理後的不銹鋼其表面均可形成富鉻的氧化物銹皮，因而延緩塵化反應的發生。其中經噴砂處理者其表層氧化物生成的厚度較經研磨處理者為厚，對塵化有較佳的抑制性。而經預氧化處理後的304L不銹鋼，其原先的表面氧化銹皮容易產生破裂，且該破裂處下方基材為富鐵鎳金屬相，對碳的吸收相較於其它位置較敏感，故易遭受到金屬塵化侵蝕而破壞。局部侵蝕處其反應層內由三層不同的結構區域所形成，包括最外層的氧化物/金屬微粒複合層、中間層的石墨/氧化物/金屬微粒混合層以及最內層的內部氧化層等。經預氧化處理後的304L不銹鋼，其試片表面的局部侵蝕處數量、平均的最大侵蝕寬度和深度均隨試驗溫度的升高而增加。

　　304L不銹鋼在經預滲碳處理後，可造成其表層富鉻氧化物銹皮的生成，同時在表層銹皮下方可形成一滲碳的富鐵鎳金屬層(鉻缺乏)。在經熱重分析試驗後，實驗結果顯示，經預滲碳處理後的不銹鋼表面，無論是經保留銹皮或移除銹皮後，其重量增加量均隨溫度的升高而增加，其中移除銹皮後的試片其重量增加量均較大。試驗後，兩者試片均可發現其表面奈米碳絲的形成，且碳絲的端頭均富含鐵及鎳元素的金屬微粒，另在其生成位置的橫截面金相觀察與成份分析結果中，均顯現合金元素重整的情形，亦即該位置發生初期的金屬塵化反應。在經長時間暴露試驗後其結果顯示，預先經滲碳處理的確有加速不銹鋼塵化侵蝕破壞的作用。經預滲碳處理後的不銹鋼，在原先表面部份銹皮破裂處，將快速地達到碳過飽和而遭受塵化的侵蝕，即在熱重分析試驗結果中已呈現塵化初期的現象，在經長時間暴露試驗結果則有相當明顯凹陷的局部性蝕孔生成，其蝕孔數量、平均的最大侵蝕寬度和深度均隨溫度的升高而增加；而經過預滲碳處理後的不銹鋼，若將其表層的氧化物研磨除去後，則試片表面幾乎發生全面性的塵化現象。

Abstract

　　Stainless steels (SSs) could induce metal dusting damage and even lead the occurrence of the localized thinning/pitting phenomenon when they are exposed in high carbon-activity atmospheres at elevated temperatures. The relationship between the susceptibility to metal dusting of material and the surface to form the protective passive film is rather close. The surface state (eg. surface working, surface oxidation, and surface carburizing) of material on the susceptibility to metal dusting becomes the meaningful topic. In this investigation, the effect of surface pre-treatment (grinding. sand-blasting, and pre-oxidation) on the susceptibility to metal dusting of 304L SS was examined. Moremore, the pre-carburization treatment is expected to help the progress of carburizing on 304L SS. The process of metal dusting occurrence and nano-sized carbon filaments formation on 304L SS was investigated. The effect of carburizing for 304L SS on metal dusting was also examined. Consequently, the effect of surface pre-treatment on metal dusting susceptibility for 304L SS exposed in CO-CO2 atmosphere was investigated in this study.

　　For the as-ground, sand-blasted, and pre-oxidized 304L SS, the results showed that no localized pit was formed on specimen surface after TGA tests. A linear relationship of weight gain versus time was obtained for all specimens, and the weight gain increased with increasing temperature. More specifically, the decomposition of CO gas to form carbon was mainly responsible for the weight gain, and the contribution to the weight gain due to oxidation could not be ruled out. The as-ground and sand-blasted surfaces enhance the formation of oxide scale after long-term exposure. A thicker Cr-rich oxide layer was found and showed a significant resistance to metal dusting on 304L SS with the sand-blasted surface. The existence of an initial non-protective oxide on the pre-oxidized 304L SS surface, however, gave rise to the most susceptibility to metal dusting associated with localized attack. The Fe/Ni-rich metallic phase beneath the site of oxide breakdown was responsible to metal dusting attack. The direct carbon ingress could more easily induce the occurrence of metal dusting. The internal reaction layer of localized pitting site existed three different structure zones consisting of an outer oxide/metal composite layer, an intermediate graphite/oxide/metallic particle mixture, and an inner internal-oxidation layer. The number, average maximum width, and maximum depth of the localized pitting site observed on the pre-oxidized 304L SS were increased with increasing temperature.

　　The pre-carburization was performed using the pack-cementation process which gave rise to the formation of Cr-rich oxide scale on the 304L SS surface. On top of the metal surface, a carbon-rich metallic layer was produced (chromium depletion) beneath the oxide scale. The TGA experimental results showed a linear relationship of weight gain versus time was obtained for the pre-carburized 304L SS (reserved and descaled), and the weight gain increased as the temperature increased. Metal dusting associated with alloying element redistribution and Fe/Ni-rich particle formation could occur on the pre-carburized 304L SS. The results showed that the Fe/Ni-rich metal particles disintegrated from the substrate could promote the formation of nano-sized carbon filaments in the high carbon-activity atmosphere. It indicated that the formation of carbon filaments resulting from the initial stage of metal dusting was occurred. Pre-carburization treatment on SS could accelerate the damage from metal-dusting attack. After long-term exposure, the pitting with localized damage was formed distinctly on the pre-carburized 304 SS surface due to the metal dusting attack. The total number, average maximum width, and maximum depth of the attacked localized pit increased with increasing temperature. Moreover, uniform attack due to metal dusting occurrence was seen if the oxide scale formed on the pre-carburized 304 SS surface was removed by grinding.

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