退火爐熱交換器腐蝕破損原因分析

Failure Cause Analysis of Heat Exchanger tubes in Annealing Furnace

本文探討退火爐中兩種不同材質熱交換管腐蝕破損原因。某公司退火爐有316L及253MA兩種材質水平式熱交換器位於爐區頂部，因破管漏水滴落產品表面影響品質。為了解此兩種熱交換器之腐蝕破管成因，展開原因分析。分析方法包括外觀觀察、Spark及ICP成分分析、截面微硬度量測、草酸電解試驗、SEM金相分析、表面SEM/EDS分析、腐蝕生成物分析等。經分析結果顯示316L熱交換器之破孔由外而內發展，外壁表面附著明顯之氯離子，研判其破管原因應為停機冷卻後引發之孔蝕現象所造成。253MA材質因發生敏化現象，加上內壁沉積造成沿晶腐蝕以至於穿孔滲漏。建議目前線上使用之253MA熱交換器應檢查有無敏化現象，或是直接更換材質。定修時應清洗熱交換器外壁與內壁，以渦電流（ET）檢查熱交換管之缺陷狀況。冷卻水品質應加以控制，儘量避免沉積發生；操作時應防止熱交換器發生空燒，材質更換應以含有Mo之低碳不銹鋼或添加Ti、Nb之穩定化不銹鋼或鎳基合金為主。

The purpose of this paper is to study the failure cause of heat exchange tubes in the annealing furnace. There are two horizontal heat exchangers made of two different materials, 316L and 253MA, located at the top of annealing furnace. The failure of the heat exchanges cause water in the tubes dropped on the product surface and affected the product surface quality. For speculating the failure cause, some investigations and studies were performed. They included appearance examination, material composition and cooling water analysis, surface XRD analysis and SEM/EDS analysis etc. From the analysis results, it showed that the 316L tubes perforated from the outside and some chlorides were found on the surface. This implies that the failure might caused by pitting corrosion occurred in the periods of shut down. On the other hand, the leakage of the 253MA tubes was caused by the sensitization and intergranular corrosion due to the inner wall deposition. Some recommendations were proposed to avoid the failure, including sensitization examination, materials replacement, cleaning the tubes at shutdown and ET inspections, cooling water quality control, and avoid dry heating during operation.