海水運輸管線Monel合金流量計破損原因分析

Failure Analysis of a Monel Alloy Flow Element in Seawater Pipeline

本文旨在分析某處海水輸送管線安裝之鎳銅合金Monel 400流量計發生腐蝕損壞的原因。該流量計安裝使用近3年，但配合系統運轉和維修時程，其間近1/3時間浸泡於靜止的海水中。觀察流量計的感測器外表呈現銅綠色，並帶有少量白色、土黃色與深楬色腐蝕生成物，且已形成腐蝕破孔。大量的綠色腐蝕生成物之化學組成及結晶結構與鹼式氯化銅相符。外表面除了孔蝕腐蝕，還伴隨著沿晶腐蝕和去合金腐蝕現象。由於腐蝕特徵與文獻描述的Monel 400受到硫酸根還原菌侵蝕之腐蝕形態相似，故微生物腐蝕可能是流量計損壞的主因之一，且由流量計操作歷史和局部區域腐蝕形態展現出均勻腐蝕、孔蝕腐蝕、沿晶腐蝕和去合金腐蝕等數種腐蝕現象推知腐蝕主因應為上述各種腐蝕現象之綜合效應。

The aim of this study was to investigate the failure cause of a Monel 400, one kind of nickel-copper alloy, flow element used in the seawater pipeline. The flow element has been used about three years, but it was immersed in stagnant seawater nearly one third of the period for the system operation and maintenance. From the observation of the sensor probe of flow element, it showed an aeruginous appearance with a small amount of white, khaki and brown corrosion products. Moreover, it has formed some corrosion broken holes. Besides, the chemical composition and crystalline structure of green corrosion products at sensor probe surface is in line with CuCl2 •3Cu(OH)2. In addition, the microstructure of outer surface of sensor probe shows pitting corrosion, intergranular corrosion and dealloying corrosion. Since the corrosion characteristics agree with Monel 400 corroded by sulfate-reducing bacteria described in the literature, microbiologically influenced corrosion may be one of the causes for this case. Finally, from the flow element operating history and corrosion morphology showing uniform corrosion, pitting corrosion, intergranular corrosion and dealloying corrosion, we believe that the flow element corrosion should be the combined effect of various phenomena.