Root Cause Analysis of the Localized Corrosion on a Liquid Chlorine Container

[[abstract]]鹼氯( Chlor-Alkali )產品 -「液氯」，在工業原料與民生用品方面的用途十分廣泛，由於是化學性質非常活潑的毒性化學物質，一旦發生洩漏，除了本身的毒性對人體、社區環境會造成重大的危害與污染之外，氯氣並幾乎可與所有金屬反應，造成對設備、管線嚴重的腐蝕損壞。國內的液氯包裝容器，以噸裝鋼筒( Ton Container )之使用最為普遍，鋼筒製作設計的使用壽命約在30~35年之間，如今發現不到15年之鋼筒，其瓶閥及安全熔塞 ( Fusible Plug ) 之螺孔螺牙( Hole Threads )，已有嚴重破損現象，且從外觀上看不出明顯的損壞瑕疵，螺牙嚴重的局部缺損，甚至無法繼續使用，除了增加營運成本，在安全管理上也存在著無法預期的斷裂風險，與毒性化學物質洩漏的潛在危害，此一問題實不容忽視。 由於文獻上的缺乏，本研究經蒐集近500支鋼筒檢查發現；鋼筒胴身之腐蝕率甚低，螺孔螺牙則已嚴重破損，同一只鋼筒盛裝相同的內容物，在相同的環境條件下使用，產生不同程度的腐蝕差異。透過本研究從鋼筒中各種腐蝕因子可能形成之腐蝕機制，與鋼筒灌裝、使用及製程操作控制等實務上之相關性，以根源分析、非破壞檢測、金相試驗等方法，進行分析、研判，歸納出螺孔與胴體之間腐蝕模式與生成機制之差異，提出在鋼筒設計製作或材質選用上的多重考量，對使用中之鋼筒提出有效之改善與防制措施，建立完善的維修作業程序，提昇鋼筒之使用安全性，防止螺牙持續損壞，延長其使用壽命，降低容器之洩漏風險，防止意外事故之發生。[[abstract]]Liquid Chlorine, Chlor-Alkali’s product, is widely used in the industry. As its is an active chemical and toxic substance, chlorine, once leaking out, may not only react to all metals and causes serious damage to facilities and pipe lines but also contaminate the surrounding environment. In Taiwan, the ton container is the most common vessel for transporting liquid Chlorine. The life time of the ton containers is designed to be appropriately 30 to 35 years. However, it has found that some containers show a life span of less-than-15-years, in which the hole threads around the block valves and fusible plugs appear to be corroded seriously. The rest of the ton container shows minor corrosion, but the localized corrosion around the hole threads determine the life span of the container. The study collected almost 500 ton containers, the phenomena of localized corrosion was observed in large portion of the containers. The study explores the relationship among all the possible corrosion mechanisms including operation conditions such as loading, usage, operation, and control. With Root-Cause Analysis, Non-Destructive Testing, and Metallurgical experiment, the corrosion was found to be a combination of several corrosion mechanisms working jointly for a long time. The corrosion found includes chlorine, galvanic, and crevice corrosion. The study also investigated possible prevention schemes to slow down the corrosio