內襯不銹鋼板表面裂紋肇因分析

Case Study of Crack Initiation and Propagation on Stainless Steel Cladding

覆面鋼板可依照製程條件選擇不同需求之內襯材質，兼顧設備良好的結構強度、耐高溫性或耐蝕性，因此覆面鋼板常被化工廠作為反應器或槽體的材料。本文即探討某化工廠採用碳鋼貼合不銹鋼內襯之覆面鋼板作為反應器胴體材料，經歷10年的運轉，於不銹鋼表面發生裂紋之成因。分析方法包括：背景資料蒐集、目視觀察、顯微組織分析及電子顯微鏡觀察等項目。分析結果顯示：由於內襯不銹鋼板材質中存有大量σ相，造成耐蝕性降低，長時間處在酸性及具有氯離子的操作環境下，易由表面發生蝕孔與沿晶腐蝕現象。表面缺陷形成後，環境中的微量氯離子得以進入缺陷中，並在缺陷末端處引發局部濃縮效應，導致後續以應力腐蝕破裂之機制成長。歸咎應力腐蝕破裂之起因係因σ相的形成，但其析出溫度與反應器操作溫度差異甚遠，研判σ相應與反應器商轉前經歷之高溫加工過程有關。

The clad steel reactors are frequently used in chemical processing industries primarily because of the clad material’s excellent compromise of cost, strength, and corrosion resistance. This paper investigated a failure case of chloride induced stress corrosion cracking of the stainless steel cladding found after the latest 10-years operation. The analysis included background information collection, visual inspection, chemical composition determination, microstructural examination, and microscopic analysis. The results revealed that the stainless steel cladding was exposed to a trace of chloride and acid environment for a prolonged period, and the sigma phase was confirmed to exist within the entire cladding layer. The sigma phase shall not exist in a good quality stainless steel because it reduces the corrosion resistance of stainless steel. Consequently, corrosion pits and inter-granular corrosion were initiated at sigma phase site in this case. After the formation of surface defects, and induced chloride can be migrated into and highly concentrated in the tip region of defects. Stress corrosion cracking. It was concluded that the cracking of stainless steel cladding was attributed to stress corrosion cracking and greatly enhanced by sigma phase precipitation during the improper hot rolling process.