氯離子應力腐蝕破損原因之探討與改善

接觸水的設備其材質常選用300系列不銹鋼以防銹蝕，但卻忽略水中含有微量氯離子，300系列不銹鋼在應力作用下，易於高濃度氯離子環境中發生氯離子應力腐蝕，本研究藉由轉爐廢氣引風機之304不銹鋼旋轉密封蓋破斷案例，提出易被忽略的高濃度氯離子環境，以及氯離子應力腐蝕之破損分析方法及其改善對策。本研究透過應力模擬、水質檢驗、抗拉強度檢驗、化學成分檢驗、金相組織檢驗、巨觀與微觀破斷面觀察等破損分析方法，確認密封蓋符合API RP 571規範所述之氯離子應力腐蝕特徵，研判破損機制為旋轉密封蓋工作環境水氣高，表面附著大量水垢，水垢氯離子濃度逐漸累積超過ASM Vol.13B Corrosion: Materials規範規定之容許值，再加上旋轉密封蓋因鑄造品質不良而使強度與耐蝕能力不足，導致旋轉密封蓋發生氯離子應力腐蝕而破斷。旋轉密封蓋因應力小、水垢累積慢，故氯離子應力腐蝕破損歷程緩慢，又因破損起始點為表面，因此可藉由定期檢查表面以掌握劣化程度，以預知維護降低破斷風險，並可藉由提升新品製作品質以延長壽命，原製作方式為鑄造，未來改採用市售品法蘭俥製成形，可大幅降低成本及提升品質。

關鍵字

破損分析；不銹鋼；水垢；氯離子應力腐蝕

並列摘要

300 series stainless steel is commonly utilized as a material for equipment exposed to water to prevent corrosion. However, the presence of trace amounts of chlorides in water is often neglected. In high chloride level environments and under stress, 300 series stainless steel is highly susceptible to chloride stress corrosion cracking. This study presents a case involving a broken 304 stainless steel rotating seal box in an induced draft fan within a BOF waste gas recovery system. The study highlights the easily overlooked high chloride level environment and provides failure analysis and improvement methods for preventing chloride stress corrosion cracking. This study confirms that the rotating seal box exhibits chloride stress corrosion characteristics as described in the API RP 571 standard through failure analysis methods such as stress simulation, water quality examination, tensile strength testing, chemical composition analysis, metallographic structure analysis and macroscopic and microscopic examination of fracture surfaces. The damage mechanism is attributed to an excessively wet environment leading to the deposition of scale on the surface. The concentration of chloride in the scale gradually accumulates beyond the allowable value specified in ASM Vol. 13B Corrosion: Materials standard. Additionally, poor casting quality results in insufficient strength and corrosion resistance of the rotating seal box leading to chloride stress corrosion and fracture. The process of chloride stress corrosion cracking to the rotating seal box is slow due to low stress and slow accumulation of scale. As the crack origins are on the surface, regular surface inspections can be used to monitor degradation and predict maintenance to reduce the risk of fracture. The lifespan can be extended by improving manufacturing quality. The original production method was casting; however, future production will use machining of a commercial flange which can significantly reduce costs and improve quality.