液/固双相流条件下钝性材料冲刷腐蚀的临界流速现象、规律和机理

Erosion-corrosion in liquid-particle two-phase flow is one of the main reasons causing the failures of various flow-handling components in petroleum, chemical engineering, hydraulic, power, ship, metallurgy and minning industries. For erosion-corrosion of passive materials, there is often a phenomenon of critical flow velocity (CFV), beyond which the degradation of materials increases sharply. CFV is an important criterion to evaluate the erosion-corrosion performance of materials. Up to now, not only the data accumulation on CFV, but also the understanding on the mechanism of CFV are far from enough. In this project, several typical and common-used passive metallic materials contaning different oxides are selected as test materials. The systematical and comparative data of CFV for above-mentioned passive materials can be obtained after the erosion-corrosion tests by using the same test method under the same test conditions. The effects of various enviromental factors such as sand content, the diameter of particles and impact angle on CFV will be investigated and reliable empirical equation and mathematical model on CFV will be established. The correlation of CFV with the composition, structure, thickness, repassivation ability and mechanical properties of passive films will be also conducted. These will be beneficial to understanding the inherent mechanism of CFV and chemo-mechanical synergistic mechanism of erosion-corrosion. Theoretical fundermantals on the materials selection, the developments of erosion-corrosion resistant materials and coatings as well as the controll of enviromental factors of erosion-corrosion will be also achieved.

液/固双相流冲刷腐蚀是造成石油、化工、水利、电力、船舶、冶金和采矿等行业中过流设备失效的主要原因之一，钝性材料冲刷腐蚀常存在临界流速现象，即只有达到临界流速以后，材料损伤才随流速急剧增大。冲刷腐蚀临界流速是反映材料耐冲蚀性的重要判据。至今，无论是对冲刷腐蚀临界流速的数据积累，还是对临界流速的内在机理阐述，都还远远不够。本项目拟选取几种常见的、含有不同氧化物的典型钝性材料作为研究对象，选择统一的方法对这些材料在相同条件下的临界流速进行测试，建立一套系统的、可参照性强的临界流速数据；探究含砂量、颗粒度和攻角等环境参数对材料临界流速的影响规律，建立可靠的经验公式及数学模型；深入研究钝化膜的成分、结构、厚度、再钝化能力及力学行为与临界流速的关系，揭示临界流速变化的内在机理，深化对冲刷腐蚀力学-化学交互作用机理的理解，为过流部件的选材、耐冲刷腐蚀材料或涂层的开发、冲刷环境条件控制提供基础理论支撑。

石油石化、船舶和电力等行业用各种过流部件，例如泵、阀、管道、螺旋桨等均承受流体的机械冲刷和海水等腐蚀介质的电化学腐蚀的共同作用，其服役寿命往往很短。这些过流部件的损伤不仅造成更换部件的直接成本，更重要的是这些过流部件往往起到控制舰船动力或工艺参数的作用，因此其造成的间接损失往往更大。过流部件服役安全性的一个重要指标就是流速流态的控制。在冲刷腐蚀过程中，材料的损伤随流速的提高常存在所谓的临界流速现象，即当流速达到某个临界值时，材料的损伤会突然增大。因此，准确确定临界流速的大小无疑对指导材料的使用有重要的应用价值，同时研究临界流速及其影响因素对于深入理解冲刷腐蚀机理也有重要的理论意义。本项目首先研究并提出了几种精确确定临界流速的方法，如失重法、粗糙度法、恒电位极化、开路电位等，然后研究了含沙量、颗粒度和攻角等环境参数对材料临界流速的影响规律，并探讨了喷射不同冲蚀区域临界流速的差异。采用统一的方法，比较了工程应用中常见的典型钝性材料（6种不锈钢、3种镍基合金和3种钛合金）的临界流速差异，建立了一套系统的、可参照性强的常用钝性材料液/固双相流冲刷腐蚀临界流速数据，对这些材料的耐冲刷腐蚀性能进行排序，为过流部件的选材提供了理论参考。采用离子标记法和颜色标记法证实了钝性材料的临界流速行为取决于材料表面由固体颗粒冲击引起的去钝化过程与再钝化过程之间的竞争，出现临界流速现象的临界条件就是钝化膜的去钝化时间与再钝化时间相等。为了进一步阐明临界流速的本质，研制了一套单颗粒冲击实验装置，以便实验模拟冲刷腐蚀的单元损伤过程。研究了临界流速与材料再钝化性能之间的关系，研究了点蚀和临界流速之间的交互作用。最后还探索了利用表面处理和涂层封孔来提高临界流速的可行性。