不銹鋼沼氣輸送管腐蝕劣化原因分析

Failure Analysis of a Marsh Gas-carrying Stainless Steel Pipe

本文旨在分析探討不銹鋼沼氣輸送管線發生破損導致漏水之原因。分析方法包括外觀觀察、材質成分分析、光學顯微鏡與掃描式電子顯微鏡金相分析、破斷面及裂縫分析、腐蝕生成物分析、草酸電解試驗及微生物觀察等。分析結果發現輸送管內壁表面有大小不等之孔洞，截面蝕孔內有明顯之氯離子，銲道則有熔接不良、不對心等缺陷，底材或銲道部份其組織皆正常，草酸電解試驗顯示無敏化現象。由上述結果推測沼氣輸送管漏水主要原因為氯離子侵蝕所造成，其來源經追查為酸洗廢液混入所導致。銲道則因銲接缺陷再加上孔蝕作用，使銲道更易穿孔漏水。

The purpose of this paper is to investigate the failure cause of a stainless steel pipe for carrying marsh gas. Experimental analysis methods included examination of appearance, chemical composition analysis by spark, metallography observation by optical microscopy and scanning electron microscopy, fractured surface analysis, corrosion product analysis, oxalic acid electrolytic test and observation of microorganism. The analysis results showed that there were different size cavities on the inside wall surface, some chlorides inside the pitting hole of the section, incomplete fusion and misalignment in the weld, normal microstructures of substrate and weld, and no sensitization of substrate. From the results, we speculated that the leakage of stainless steel pipe should be resulted from pitting by chloride which came from waste hydrochloride acid for pickling. Pitting and weld defects both accelerated the perforation of weld, and finally leaded to leakage.