海氣象觀測塔螺栓斷裂原因分析

Failure Analysis of the Bolt Fracture in the Met Mast

由於溫室效應日益嚴重，發展綠色能源已成為全球的重要趨勢，台灣海峽為全球最佳離岸風電場域之一，因此，離岸風機成為政府發展綠能產業之重要項目。然因離岸與陸上風機於風力、溫濕度及腐蝕環境均存在著明顯之差異，故於發展先期建置海氣象觀測塔，用以監測未來離岸風力發電機可能遭遇的風況，同時記錄長期環境的變化，以評估環境對葉片、塔架材料及耐蝕塗層的影響。本文針對某海氣象觀測塔塔體法蘭螺栓發生斷裂的現象，進行原因調查與分析。分析結果顯示：破斷面表面可偵測到高濃度之Zn、P等元素，而此元素並不存在於材質本身與外界環境中，應為螺栓於製作後、於未施作表面皮膜處理前已發生裂縫，導致於進行磷酸鹽皮膜處理時，Zn、P等元素滲入裂縫鍍於破斷表面上。於螺栓鎖固完成後，此裂縫處即成為應力集中點，此時，螺栓於承受來自鎖固之扭力與外界風力之反覆應力下，加上來自海上環境之腐蝕性Cl離子侵入，而造成此區發生腐蝕疲勞現象。而隨著裂縫持續傳播，當剩餘之截面積無法承受外界之應力時，最終導致裂縫快速傳播而斷裂之結果，屬於螺栓加工不當或製程瑕疵，致使此應力集中處成為後續腐蝕疲勞發生之起始位置。

Green energy development has become the important tendency due to the Greenhouse effect getting worse and worse. Taiwan Strait is one of the best off-shore windfarm field in the whole world, therefore, off-shore wind power generation turns into the most important item of green industrial development for the Taiwan government. However, apparent differentials in wind, temperature, humility and environment are existed between off-shore and on-land wind generation, so it will be necessary to set up the met mast to monitor the variations of the wind condition and the long-term environment where occurred for the off-shore windfarm. The effect of environment on blade, structure materials, and coating will be thus evaluated. In this study, the failure analysis is employed to explore why and how the bolts of structure flange fail. Experimental results reveal that high content of zinc and phosphorous are detected on the fracture surface of the bolts, which are not essentially existed in both material and environment. It’s considered that these elements are coming from the phosphate film which is treated for the anti-corrosion purpose following the bolt producing. In other words, the cracks have formed before the phosphating process. These cracks would subsequently be the initiation site of the fracture as corrosion fatigue mode under the situation of repeated stress applied by outer wind power and environment of aggressive chloride ion, and then propagate quickly while residual cross section area too small to bear the outer stress. Which results in the fracture and the bolt failing. This failure is resulted from the improper processing or materials defect of bolt.