低壓汽機釋壓膜片腐蝕肇因分析與改善評估

Corrosion Failure and Corrosion Resistance Enhancement of Pressure Relief Diaphragm

本文主要探討低壓汽機釋壓膜片於法蘭夾緊位置發現破裂之原因，並且評估其釋壓膜片壽命延長的表面處理技術可行性，以及對機械性能的影響程度。破損肇因分析包含目視檢查、化學組成分析、截面顯微組織及電子顯微鏡分析。由於破裂處位於安裝位置低處容易積水，加上外界腐蝕物質沈積，造成膜片表面出現孔蝕現象，孔蝕依膜片之軋製方向而沿著晶界成長，使膜片產生剝落腐蝕；此外，法蘭鋼材與鋁材膜片直接接觸，兩者的電位差可加速膜片的腐蝕速率。另外，進行釋壓膜片耐蝕性能提升的可行性評估，經由拉伸試驗及鹽霧試驗之模擬試驗，結果證實釋壓膜片表面經過鉻酸鹽皮膜處理後，防蝕能力可大幅提升，而且無需擔心膜片產生尺寸變異及機械性質改變的負面現象。

In this study, breakage cause(s) of the aluminum alloy pressure relief diaphragm was investigated by visual inspection, chemical composition analysis, metallographic examination and electron microscopic analysis. The lower portion of diaphragm was vulnerable to pitting corrosion and severe wall thinning because of an accumulation of moisture and air-laden chlorides. Numerous corrosion pits were formed and propagated along the grain boundaries, so the diaphragm was finally perforated by exfoliation corrosion. The direct contact between the carbon steel clamping flange and aluminum diaphragm can accelerate the corrosion rate of aluminum by the effect of galvanic corrosion. To enhance the corrosion resistance of aluminum alloy diaphragm, a chromate conversion coating treatment was applied and the potential influence on mechanical properties was also evaluated. Tensile test and salt spray test had confirmed that the corrosion resistance of aluminum alloy after chromate conversion coating treatment can be improved and has no effect of losing its function.