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標題: | 添加鋁對熱浸鍍及熱擴散鍍鋅層之性質影響 Effects of Al additions on the properties of hot-dip galvanized and zinc pack cementation coatings |
作者: | Tsung-Han Shen 沈宗翰 |
指導教授: | 林招松 |
關鍵字: | 鍍鋅層,熱浸鍍鋅,熱擴散法,鋁添加,抗蝕性, Zinc coatings,Galvanized,Pack cementation,Al addition,Corrosion, |
出版年 : | 2018 |
學位: | 博士 |
摘要: | 熱浸鍍鋅鋼板為一種應用廣泛的材料,與一般鋼板相比,其擁有優異的抗蝕性,因此大量應用於各個領域。雖然鋅鍍層具有優異的犧牲保護效果,鍍層自身的腐蝕速率卻相當快。因此,本論文第一部份,將對鍍鋅層添加鋁,進行鍍層本身抗蝕性的改善。設計不同鋁的添加量,對鍍層的抗蝕性與結構進行分析及評估。實驗結果顯示,在腐蝕前期,鋅鋁共晶結構以及晶界為發生腐蝕的起始點,並有腐蝕生成物堆積,或沿晶界往鍍層內部進行腐蝕,之後腐蝕擴展至整個鍍層表面。隨著腐蝕持續進行,低鋁含量(≤ 0.7 wt.%)的鍍層腐蝕速率明顯上升,而高鋁含量(≥ 2.5 wt.%)的鍍層腐蝕速率則變動不大。低鋁鍍層在鍍層腐蝕完畢後,鋼鐵底材的腐蝕隨即發生;而高鋁鍍層的試片則在試驗時間內(鹽霧試驗192小時)未觀察到底材發生腐蝕。所以,高鋁鍍層在鹽霧試驗的環境下發生腐蝕時,可以比低鋁鍍層產生更具保護性質的腐蝕生成物,因此在試驗後期可有效地阻止腐蝕因子的擴散,延緩底材發生腐蝕的時間。
以氣態金屬鹵化物與鋼鐵材料表面發生擴散反應之熱擴散法生成鍍層厚度均勻,相較於熱浸鍍鋅製程,更適合應用於複雜形狀工件之鍍鋅處理。本論文第二部分將探討添加鋁元素對於熱擴散鍍鋅層之影響。實驗結果顯示,熱擴散鍍鋅層主要是由δ鐵鋅相所構成,在接近鋼鐵底材處會有少量Γ相。當鍍鋅製程粉末中未添加氯化銨作為活化劑時,仍然可生成鐵鋅相鍍層,表示鐵鋅相的成長並非完全由氯化鋅前驅物反應的化學氣相沉積進行,部分是鐵與鋅之間的擴散。當製程粉末中添加鋁粉進行熱擴散製程時,熱擴散鍍層將轉變成雙層結構:外層為多孔的鐵鋁相,內層為鐵鋅相。和鐵鋅相不同的是,鐵鋁相之生成機制為氯化鋁前驅物參與的化學氣相沉積。因為底材與鋅之間可以直接擴散,鐵鋅相的成長在鐵鋁相之前;當氯化鋁的濃度上升到可以發生還原反應時,因鐵鋁間的親和力較鐵鋅高,鐵鋅相中鐵原子會被鋁搶走形成鐵鋁相,而鋅會藉由擴散和蒸發離開鍍層,使的鐵鋁相內部有許多孔洞。當鐵鋁相完整覆蓋鍍層表面時,製程環境中的鋅便不再參與擴散反應,當鐵鋅鍍層消耗完之後,鐵鋁相的鋁含量因擴散持續進行而上升,最後變成鋁含量更高且孔洞較少之鐵鋁相單層鍍層。在鍍層性能方面,鐵鋅+鐵鋁相鍍層因表層具有鐵鋁相,故具有較鐵鋅相鍍層優異之抗高溫氧化能力。在抗蝕性上,鐵鋁相鍍層的長時間抗蝕性較鐵鋅+鐵鋁相鍍層差。熱擴散鐵鋅+鐵鋁鍍層為鐵鋁相及鐵鋅相所構成之雙層結構,故其可兼具抗高溫氧化及抗蝕性兩種需求之保護效果。 Hot-dip galvanized steel is a widely used material due to its corrosion resistant properties. Although this coating performs excellent sacrificial protection, the corrosion rate of itself is fast. Therefore, we added different amounts of aluminum into the zinc coating to improve the corrosion resistance. It was found that during the early stages of corrosion, Zn-Al eutectic structure and -Zn grain boundaries were the initial position where corrosion occurred. As the corrosion test was continued, the corrosion rate of low Al content coatings (≤ 0.7 wt.%) increased, but that of the high Al content coatings (≥ 2.5 wt.%) hardly changed. Moreover, the steel substrate was corroded when the low Al content coatings were depleted. In contrast, the steel substrate with high Al content coatings remained intact after 192 h of the salt spray test (SST). It is likely that the corrosion products of the high Al content coating blocks the corrosion species diffusion and prevents the steel substrate from corrosion. Compared to hot-dip galvanizing, pack cementation technique, which uses gaseous metal halides to trigger the chemical reaction with the steel substrate, has the advantage of forming a uniform coating. It is used in the Zn coating process for small steel parts now. However, the technologies of Zn–Al co-cementation coatings, which possess better high-temperature oxidation resistance, has not been implemented yet. The relationship between the processing parameters and the properties of the coating is thus essential for industrial applications of Zn–Al co-cementation coatings. The second part of this dissertation thus aims to investigate the microstructural evolution of the Zn–Al co-cementation coating during the co-cementation process. Experimental results show that a double layer coating on steel was formed during the Zn–Al co-cementation reaction. The outer layer was mainly the Fe–Al alloy with a small amount of Zn and the inner layer was composed of Fe–Zn phases. The growth path of the Fe–Al layer was chemical vapor deposition. An Fe–Zn layer was formed during the early stages of the co-cementation reaction. As the elemental Al concentration was built up with continued reaction, pack cementation of Al started and reacted with the Fe–Zn to form an Fe–Al layer with voids resulting from vaporization of the resultant Zn atoms. The Fe–Al layer then grew at the expense of the Fe–Zn layer. After the Fe–Zn layer was depleted, the porous Fe–Al layer transformed into a compact coating as pack cementation proceeded. The Zn–Al co-cementation coating consisted of an Fe–Al overlay, which was resistant against high-temperature oxidations, and an Fe–Zn inner layer, which reduced the corrosion of the steel substrate in chloride solution. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/79075 |
DOI: | 10.6342/NTU201803120 |
全文授權: | 有償授權 |
顯示於系所單位: | 材料科學與工程學系 |
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