低H2S、高CO2分压条件下双相不锈钢应力腐蚀敏感性研究

目的针对IS15156标准中对双相不锈钢使用条件的限制,研究双相不锈钢2205在不同温度、不同低H_2S分压条件下的开裂敏感性。方法通过模拟我国西部酸性油田低H_2S、高CO_2工况环境,利用高温高压设备,进行了三点弯曲试验,结合失重法测试腐蚀速率,并使用SEM和EDS进行微观形貌观察和腐蚀产物分析。结果双相不锈钢2205的腐蚀速率较低,未超过0.014 mm/a,且硫化氢分压对腐蚀的影响较小,但发现了由氧化铝等夹杂导致的点蚀。双相不锈钢2205在低硫化氢分压的中温(100℃)区发生应力腐蚀开裂,同时发生了选择性腐蚀,铁素体相优先于奥氏体相腐蚀,其他温度条件下仅发现点蚀。硫化氢分压升高时,开裂敏感性有一定程度的降低。结论双相不锈钢2205在低硫化氢分压条件下的开裂类型为氢脆型应力腐蚀开裂。氧化物夹杂诱发点蚀,氢在应力集中区域聚集,发生氢脆。当硫化氢分压从6 k Pa增加到165 k Pa时,局部腐蚀敏感性的增加使氢脆得到缓解,开裂敏感性降低。双相不锈钢2205无法在低硫化氢的中温井口环境中使用,标准中以H_2S分压作为使用限制并不十分完善。

Strain Corrosion Sensibility of Duplex Stainless Steel in Environment with Low H2S and High CO2 Pressure

ABSTRACT:Objective Aimed at the service restrictions of duplex stainless steel in standard IS15156, cracking sensibility of duplex stainless steel 2205 at different temperatures and diverse low H2S partial pressures was studied.Methods The experi-ments of weight loss and three point bending test were carried out in the simulated corrosion environment based on acid oilfield containing low H2S and high CO2 in West China. High temperature and pressure autoclave was used during the test. SEM com-bined with EDS was joined to analyze the surface morphologies and corrosion scales.ResultsThe corrosion rates of duplex stainless steel 2205 were low in our experiments. All of the results were below 0.014 mm/a. The effect of H2S partial pressure was not evident. But pitting corrosion caused by oxide inclusions such as Al2O3 was discovered. Stress corrosion cracking hap- pened to duplex stainless steel 2205 during the test at 100℃ under low H2S partial pressure. Meanwhile, selective corrosion occoured. Ferrite phase corroded prior to austenite phase. Pitting corrosion occurred at other test temperatures. And also, the cracking sensibility diminished to a certain extent with the elevation of H2S partial pressure. Conclusion Under low H2S partial pressure, the cracking type of duplex stainless steel 2205 was hydrogen embrittlement stress cracking according to the analysis on the pattern of fissure. The oxide inclusion induced pitting corrosion which led to the assembling of H. And then, hydrogen embrittlement happened. With the lifting of H2S partial pressure from 6 kPa to 165 kPa, the raising sensibility of local corrosion relieved the hydrogen embrittlement. The cracking susceptility reduced. Duplex stainless steel 2205 was not suitable for the mesotherm condition containing low H2S in wellhead. The service restrictions based on H2S partial pressure was not perfect.