膨脹接頭之波紋管腐蝕肇因分析

Corrosion Failure of a Stainless Steel Bellows

本文針對某石化工廠的膨脹接頭之波紋管（Bellows）腐蝕穿孔之肇因進行探討。分析檢測項目包括：目視檢查、管壁厚度量測、化學成分分析、電子顯微鏡分析、X光繞射分析、顯微組織觀察及不銹鋼敏化鑑定等。分析結果顯示：波紋管內壁表面因腐蝕性水溶液侵蝕，造成大面積減薄而破損，其腐蝕生成物主要為低溫形成的硫酸鐵及硫酸亞鐵。由於安裝此膨脹接頭的管線內容物含有SO2、SO3、H2O等混合腐蝕性氣體，於運轉不順期間管線曾多次昇降溫，使波紋管內壁表面溫度長時間處於40℃~ 160℃之間，因此造成硫酸與水的凝結，產生高腐蝕性水溶液，進而均勻侵蝕波紋管內壁，導致其厚度減薄蝕穿洩漏。

This study investigated the root cause of perforation corrosion of expansion joint bellows. The analysis included visual inspection, wall thickness measurement, chemical composition analysis, electron microscopic analysis, X-ray diffraction, metallographic examination and sensitization evaluation of stainless steel. The results showed that the inner wall of bellows was attacked by highly corrosive condensate solution, which further led to the thinning and perforation of the inner wall at the later stage of its service. The corrosion products were mainly composed of ferric sulfate and ferrous sulfate formed at low temperature. The flue gas inside pipeline was a mixture of SO2, SO3, H2O and other corrosive gases, and the gas temperature fluctuated between 40 ℃ ~ 160 ℃ during an abnormal operation. As a result, a very strong acidic condensate containing SO4 2- and SO3 2- ions was formed, and then caused the bellows inner wall being dissolved uniformly, leading to the wall thickness reduction and bellow perforation.