煤焦油工廠製程設備之腐蝕成因分析

Analysis of Corrosion Cause in the Coal Tar Process Equipments

煤焦油工廠經由分餾方式將輕油、萘油等物質分離提取，但從建廠以來即因設備管線腐蝕、熱交換器阻塞，導致必須時常停機處理，造成生產上之困擾。本文即在分析探討其製程之腐蝕原因，研究方法包括現場調查、製程破損件分析、管線腐蝕試驗片分析、設備阻塞物分析等。經由破損件分析結果，顯示其為硫化腐蝕與窩穴腐蝕合併作用，由管線試驗片表面銹層分析結果，顯示其為硫化鐵，由阻塞物分析鑑定結果亦為硫化鐵化合物。綜合上述之分析結果，推測煤焦油工廠之製程其腐蝕主要原因為硫化腐蝕，在煤焦油之流動沖刷及高溫時之窩穴現象作用下，硫化鐵剝落露出底材繼續硫化，使管線設備減薄。另外經由不同溫度及不同鋼材之煤焦油浸泡試驗結果，304不銹鋼腐蝕速率低，但於現場使用，腐蝕速率卻很高，顯示煤焦油之流動沖刷及高溫時之窩穴腐蝕現象對製程腐蝕扮演重要角色。

In the coal tar distillation process , the light oil, naphthalene oil are extracted from coal tar by high temperature distillation. But from the setup of the factory, it often be shutdown resulting from corrosion of equipment and pipe, or plug of heat exchanger, and be a troublesome for the production. The purpose of this paper is to analyze the root cause of corrosion in the process, by means of field investigation, equipment failure analysis, analysis of corrosion products on coupons’ surface, and the clogging materials in the equipments. From the analysis results of equipments failure , corrosion products on the coupons’ surface and the clogging materials of equipments, it showed that the failure cause was the synergistic effect of sulfide corrosion and cavitation or erosion. So, from the above results, we speculate that the main cause of corrosion in the process of the coal tar factory was sulfide corrosion, and in the effect of erosion and cavitation of coal tar, the iron sulfide layer was peeled off, then the substrate was emerged, and reacted with sulfide to form the iron sulfide, like that moved in circles to reduce the thickness of equipments. Separately, from the immersion test in the coal tar at different temperature, it showed that the corrosion rate of 304 stainless steel was very low, this result was contrary to the fact found in field, thus it can be seen the erosion and cavitation played an important roles in the failure of the processing equipments.