火力電廠水牆管材質劣化原因分析

Investigation into Material Degradation of Waterwall Tubes in an Oil-Fired Power Plant

本文主要探討火力發電廠之水牆管於使用超過30年後材質劣化的肇因。分析方法包括背景資料蒐集、外觀觀察、電子顯微鏡分析、截面顯微組織及硬度量測等。分析結果顯示：由於歷年來曾多次發生冷凝器破管洩漏事件，導致微量海水侵入鍋爐爐水，並且使冷凝管材質所含的鋁、銅元素溶解；於鍋爐水牆管高熱通量區域，鋁離子易與水中溶氧逐漸在水牆管管壁上形成一層連續性厚實氧化物，使得管壁熱傳導能力下降，局部管壁溫度偏高，促使管壁腐蝕速率增加，進而發生溶氧孔蝕現象；另外，銅元素遷移且沈積至管壁表面時，加凡尼腐蝕效應亦促使管壁腐蝕速率增加。再者，經長時間使用過後之水牆管，利用超音波材質檢測後所得之數值與實際管內表面形貌於判讀上時常有所不同，主要原因應與檢測方法與設定條件有關。

The root cause of degradation in the waterwall tubes of an oil-fired power plant was investigated by visual inspection and ultrasonic testing and then further confirmed by electron microscopic analysis, metallographic examination, and hardness measurement. Numerous cases of leakage had occurred on the condenser tubes in recent years at this power plant. A minor amount of seawater penetration into the boiler water via the condenser tubes resulted in the dissolution of copper and aluminum in the waterwall tubes. The aluminum ions reacted with the dissolved oxygen in the boiler water and formed an aluminum oxide layer on the inner surface of the tubes. The thick oxide layer reduced the thermal conductivity of the tubes and resulted in an abnormally high temperature locally. The corrosion rate was thereby accelerated, and oxygen pitting caused by dissolved oxygen was observed. In particular, the corrosion rate of the tubes was further accelerated by the galvanic effect of copper deposits that migrated to the inner surfaces of the waterwall tubes. In addition, a prolonged operation of the waterwall tubes often led to differences in data obtained by ultrasonic testing (UT) and by actual morphology analysis, which could be attributed to detecting method and UT setup.