基于热动力学的Fe-Cr-Ni/Mo合金水汽氧化机理及延寿技术研究

To understand the formation and exfoliation reason of oxidation scale in the high temperature water vapor environment, the high temperature resistant steels in Fe-Cr-Mo/Ni system have been investigated in this project. With the CALPHAD method, the thermodynamic equilibrium phase diagram of Fe-Cr-Ni/Mo-O2-H2 will be calculated, and also the relationship between the H or O partial pressure and the standard free energy of the alloy oxides will be studied, in order to investigate the thermodynamic mechanism of high temperature oxidation behavior; By calculating the Ellingham diagram limited to certain types and compositions of oxides for the investigated alloy system, the influence of oxygen partial pressure on the formation driving force of oxides will be analyzed, and also on the formation order of the different oxide phases, finally the mechanism and effect of Oxygenated Treatment (OT) and Hydrogen Treatment (HT) on the formation and exfoliation of oxidation scale will be explained thermodynamically; Meanwhile, the metallic dynamic corrosion process under the high temperature water vapor condition is simulated in laboratory, in order to verify the thermodynamic calculation results. By adding certain amount of H2 to the experimental system, the morphology, thickness, composition, porosity and bonding force of the oxidation scale of the investigated steels will be measured and investigated. In this research, both thermodynamic calculation and the experimental methods are applied to study the influence of hydrogen treatment on the formation and exfoliation of oxidation scale for the Fe-Cr-Ni/Mo steels, and finally find the new technology to create the oxidation scale with better corrosion resistance on the material surface.

针对高温水汽环境下氧化皮的生成及剥落问题，本课题以Fe-Cr-Mo/Ni系耐热钢为研究对象,利用CALPHAD方法计算Fe-Cr-Ni/Mo-O2-H2体系的热力学平衡相图，建立H/O分压与合金氧化反应标准生成自由能的关系，揭示高温氧化的热力学机理；通过计算合金体系限定氧化物类型和成分的Ellingham图，研究氧分压对氧化相析出驱动力的影响，建立氧分压对体系氧化相析出序列的影响机制，揭示加氧/加氢工况对合金体系氧化皮生长及剥落的热力学影响机制；构建高温水汽环境金属腐蚀动态模拟台实验验证计算结果，通过向系统中添加定量H2，测量研究试验钢表面的氧化皮形貌、厚度、成分、孔隙率、结合力等参数变化。通过热力学计算方法及试验验证并行研究加氢工况对Fe-Cr-Ni/Mo合金钢体系氧化皮生长及氧化皮剥落的影响机制，并最终探索材料表面构建耐蚀氧化皮的新技术。

本项目以火电厂常用过热器管材T91、TP347H和TP304为研究对象，研究高温水汽环境下氧化皮的生成及剥落问题。通过采用CALPHAD方法构建了金属氧化反应序列的计算模型、热力学Ellingham图计算模型、水工况气体分压模型及分压作用下的复合氧化膜结构计算模型，分析并揭示了OT/HT工况对Fe-Cr-Ni/Mo合金体系氧化皮生长及剥落的热力学影响机制；构建高温水汽环境金属腐蚀动态模拟台，采用痕量技术定量向系统中添加H，通过测量试验钢表面的氧化皮的组成验证热力学计算结果。计算和实验结果证实典型材质的复合氧化膜均为三层结构，外层氧化膜（Fe3O4和Fe2O3复合氧化膜）、内层氧化膜（Spinel）和扩散层（Cr2O3）。通过水工况的氧分压在一定程度可调控氧化层的组成。计算结果显示常规的加氧工况OT、纯水工况、还原性全挥发处理工况AVT(R)对T91、TP347H和TP304的高温氧化控制效果相差无几，但基材在长时间的高温服役下会出现析出相（M23C6、sigma、laves及碳氮化物）的粗化，从而使基材劣化降低服役寿命。通过提高合金元素Cr含量可使表层致密氧化层Cr2O3的体积分数提高，而对于新型的Fe-18Ni-25Cr-xAl材料,当Al质量分数高于3%时表层会出现致密的Al2O3相，从而提高高温氧化保护效果。