Ni-Al/Re-Ni复合涂层在高温碳化气氛下的阻扩散行为及失效机理研究

Iron based alloys are great ideal materials for petrochemical plants because of their good plasticity and desirable mechanical properties at medium temperatures. However, carburization corrosion including coking and carburizing occurs when iron based alloys are exposed to carbonaceous atmosphere in the petrochemical environment, which would shorten the service life of the alloys. The aim of this study is to investigate the elemental diffusion, corrosion scale growth mechanism on the Ni-Al/Re-Ni composite coating, as well as the interface stress and adhesion. High temperature corrosion mechanism of the coating exposed to the carbonaceous atmosphere will be clarified. The impact of mutual diffusion on the concentration of Al, Ni and Fe and microstructure along the coating thickness direction will be explored, and the diffusion-reaction kinetics model will be set up. The influence of environmental factors (temperature and atmosphere et al.) and mutual diffusion on the service life of the composite coating will be studied. Coating failure mechanism and main factors will be proposed. This project lays the foundation for developing the material and structure design theory and method for the protective coating against carburization corrosion on the iron based alloys.

铁基高温合金由于具有较好的中温力学性能和良好的热加工塑性，是制造石油化工装置的理想材料之一,但铁基合金在石油化工环境下会产生表面和内部结焦、渗碳等碳化腐蚀，大大缩短材料的使用寿命。本项目针对石油化工等领域用铁基合金表面Ni-Al/Re-Ni抗碳化复合涂层，研究Ni-Al/Re-Ni复合涂层高温碳化过程中元素扩散和腐蚀产物生长机制、碳化过程中界面应力的变化及界面间的粘附性，揭示复合涂层的抗高温腐蚀机理；研究互扩散导致沿涂层厚度方向Al，Ni和Fe的浓度及相组成等组织结构的演化，建立铁基合金与Ni-Al/Re-Ni复合涂层扩散反应动力学模型；研究环境因素（如温度和气氛等）及涂层与基体互扩散对复合涂层服役寿命影响规律，揭示复合涂层的失效机理及主要影响因素，为发展铁基合金表面具有良好抗高温碳化防护涂层材料与结构的设计理论与方法奠定基础。

本项目采用电镀法+包埋渗铝法在Fe基合金表面制备具有阻扩散性能的Ni-Al/Ni-Re涂层。该涂层呈现两层结构：Ni2Al3外层和Ni-Re阻扩散层内层。采用Boltzmann-Matano法计算了650℃扩散渗铝过程中Al元素在Ni镀层中的扩散系数比在Ni/Ni-Re镀层中的扩散系数低5-6个数量级，由此说明Ni-Re阻扩散层在渗铝过程中有效降低了Al的扩散速度；研究了Ni2Al3/Ni-Re涂层和Ni2Al3涂层在650℃干湿CO2气氛下的腐蚀热力学、动力学及氧化产物的组织结构，并阐明了涂层的抗腐蚀机理。结果显示Ni2Al3/Ni-Re涂层抗CO2和CO2+H2O气氛氧化腐蚀性能均优于Ni2Al3涂层。其原因为Ni-Re阻扩散层的加入优化了Ni2Al3涂层的化学成分和组织结构，阻止了腐蚀过程中内氧化和柯肯达尔孔洞的发生，从而提高了涂层在干湿CO2气氛下的抗氧化腐蚀性能。水蒸气的加速氧化作用体现在通过增加氧空位浓度和促进Al3+离子的扩散来促进表面氧化铝的生成。研究了Ni-Re层在650℃，CO2气氛下对涂层与基体互扩散行为的抑制效果及机理。结果显示，无Ni-Re内层的Ni2Al3涂层与基体间发生了互扩散，并且互扩散区内发生了明显的内氧化。相同条件腐蚀后的Ni2Al3/Ni-Re涂层无明显的互扩散行为，其原因为Ni-Re层中Re与涂层中的Al形成了Al11Re4金属间化合物，有效阻碍Al,Ni,Cr和Fe等合金元素的扩散，对涂层与基体间互扩散行为起到了良好的抑制效果。研究了在600℃，50%CO-4%H2O-46%H2气氛下Ni2Al3/Ni-Re涂层抗循环腐蚀性能。由于Ni2Al3/Ni-Re复合涂层整体具有Al含量较高的外层结构，Ni-Re阻扩散层抑制了高碳活度元素Fe、Cr的扩散，使得涂层处于较低的材料碳活度，从而抗碳化腐蚀性能优异。