評估多種不銹鋼在模擬含硫化氫之氯鹽環境中應力庸蝕行為

Evaluating the Stress Corrosion Behavior of AISI 321 Stainless Steel in the H2S Simulated Cl- Containing Environment

本研究主要探討現場熱交換器所使用之321型不銹鋼發生破裂的原因。另外並以316、316L傳統沃斯田不銹鋼及2205雙相不銹鋼做為比較，以提供改善現場環境及選擇新材料的參考。為了模擬現場環境，本研究採用日本工程防蝕學會（JSCE），發展以硫代硫酸根代替硫化氫來評估材料在含H2S+Cl-環境之應力腐蝕行為，再以電化學極化及慢應變速率拉伸試驗，並配合高溫高壓U-Bend浸泡試驗，以期更接近現場熱交換器所處之腐蝕環境。結果顯示，溫度或溶液酸度的提升，均使鈍態膜更不具保護性，並且加速硫代硫酸根的分解，使孔蝕更易成核，進而誘發應力腐蝕破裂。高溫高壓U-Bend浸泡試驗則發現80℃比300 ℃破損程度更為嚴重，原因為腐蝕層的保護性和緻密度不同所致。四種材料在以上實驗中的比較均顯示雙相不銹鋼有最優異的抗孔蝕及應力腐蝕能力，316及316L型不銹鋼次之，321型不銹鋼則最差。

This study investigates the SCC failure of the heat exchanger of AISI 321 stainless steel. Other two austenitic stainless steels, 316, 316L, and one duplex stainless steel 2205 were used in the laboratory to affored the comparison results. Electrochemical polarizations, slow strain rate tensile tests, and U-bend immersion tests under high temperature and high pressure were used to study the corrosion behavior. Na2S2O3 solution, in accordance with JSCE standard(1), was used to simulate the practical environment of H2S containing and Cl- solutions. The results indicate that both temperature and acidity of the testing solution promote decomposition of Na2S2O3 and the nucleation of pits which induces SCC. In U-bend immersion tests under high temperature and high pressure, the number of cracks on the specimen in 80℃ is more than that in 300℃ due to the compactness and consequent protectiveness of the corrosion product layer. Duplex stainless steel shows the greatest SCC resistance, then 316 and 316L, and 321 the least.