火力發電廠廠用冷卻水系統微生物腐蝕之研究

Microbiologically Influenced Corrosion on A106 Carbon Steel in Water Cooling System of Thermal Power Plant

火力發電廠廠用冷卻水約300噸，因冷卻水中含多種的微生物而造成輸送鋼管（A106碳鋼）腐蝕，所以添加抑制劑、殺菌劑來改善此一現象。廠用冷卻水可分離出多種細菌包含中溫菌和高溫菌兩大類，將所培養之細菌進行腐蝕試驗，試驗方法包含電化學動態循環極化、交流阻抗及腐蝕電位趨勢圖、並用SEM觀察試片腐蝕前後之表面型態等。研究結果發現抑制劑（鉬酸鹽）和殺菌劑對中溫菌有抑制碳鋼腐蝕效果，但對高溫菌則大都無法發揮抑制腐蝕效果，經SEM觀察碳鋼在細菌腐蝕後，表面氧化層的孔洞增加且變大，若處於高溫菌的環境下則更加嚴重，不但氧化膜幾乎不見甚至碳鋼底材有孔蝕現象發生。由腐蝕電位趨勢圖可發現，當抑制劑或殺菌劑對碳鋼有抑制腐蝕效果時，腐蝕電位會處於較貴重的電位；反之則朝活潑的電位偏移。交流阻抗實驗結果顯示，當細菌層、氧化層與電雙層的阻抗較大時，碳鋼有較好的抗蝕能力。

The volume capacity of cooling water used in Taichung thermal power plant was about 300 m3. The amount of microorganism was increasing with increasing time and the microbiologically influenced corrosion (MIC) was also getting serious. Therefore, increasing the inhibitor concentration and/or adding some bactericide is a feasible method to control MIC. Various kinds of bacterial were found in the cooling water. In general, two categories including medium temperature germs and high temperature germs were separated. The corrosion characteristics were evaluated by potentiodynamic polarization tests, AC-impedance tests, corrosion potential trends, and SEM observation. It was found that the molybdate inhibitor and the bactericide was effective for medium temperature bacteria on controlling the corrosion of A106 carbon steel tubes but was not effective for high temperature bacteria. After corrosion test, the oxide film on carbon steel was degraded gradually with increasing corrosion time. With inhibitor and bactericide added, the oxide film was sustained in medium temperature bacteria environment but it was degraded and the steel was pitted in high temperature bacteria. Corrosion potentials of carbon steel were maintained at noble region for effective controls but shifted to active region for ineffective controls. Also, the much higher electrical resistance of germ layer, oxide layer, and double layer was found on effective controls and much lower on ineffective controls.