核电二回路热力系统的结构、外壳和管道多采用碳钢材料。由于核电站通常建在海岸附近，在基建、运行和停堆期间，碳钢设备会受到高湿度和高盐雾的大气环境的腐蚀影响，从而导致材料性能下降并缩短其使用寿命。为了减缓碳钢腐蚀，可采用氮气保护对核电站的二回路热力系统进行腐蚀防护。其中，深入研究氮气保养期间碳钢发生的腐蚀行为对于制定实际保养方案具有重要意义。因此，我们根据核电二回路热力系统实际的氮气保护要求和技术规范，设计并搭建了多工况实验平台，并采用多硬件集成系统，通过人机交互软件界面实现实验环境的相关参数的自动化控制，从而达到对实验条件的精确控制。采用腐蚀失重、扫描电镜、X射线衍射、能谱仪和电化学测试等方法研究了20#钢、Q235A、Q245R三种碳钢在盐雾环境下和氮气保护环境下的腐蚀行为，同时也讨论了腐蚀对力学性能的影响，并基于实验结果进行了腐蚀仿真分析，得出以下结论：

    在盐雾环境中，三种碳钢的腐蚀深度损失随时间呈近线性变化，平均腐蚀速率呈增长趋势。实验进行48 h后基体表面已被锈层完全覆盖，表明三种碳钢在盐雾环境下的腐蚀敏感性高。锈层中多为针片状和棉球状形貌，结构均疏松多孔，锈层对基体的保护性差。随锈层变厚，对基体有一定的保护作用，这与腐蚀速率的变化相一致。腐蚀产物的成分包括α-FeOOH、γ-FeOOH和γ-Fe₂O₃/Fe₃O₄，这与大气腐蚀产物一致，初始产物中活性相γ-FeOOH的含量较多，会加速腐蚀反应的发生，而γ-Fe₂O₃/Fe₃O₄则随腐蚀进行而增多，α-FeOOH含量相对稳定。

    在氮气保护工艺下，三种碳钢初期腐蚀深度随时间呈现幂函数规律变化，且幂指数n值均大于1，平均腐蚀速率随时间延长而加快，与盐雾下的腐蚀速率相比低两个数量级。腐蚀实验进行到35 d时试样表面出现了点蚀坑和不规则的局部腐蚀形貌，以局部腐蚀为主；而在80 d时，试样表面的裸露部分减少，锈层变厚且产物形状不规则，逐渐发展成均匀腐蚀；三种碳钢的腐蚀产物主要为β-FeOOH、α-FeOOH和Fe₃O₄。三种碳钢在3.5 wt% NaCl充氮驱氧溶液环境中的腐蚀电流密度相较于充饱和空气中的低，阳极活化极化区斜率变大，这表明氮气保护能有效抑制碳钢金属的阳极溶解。在盐雾环境中腐蚀240 h时，三种碳钢腐蚀严重，试样的整体厚度减薄，屈服强度与抗拉强度出现了大幅下降，而碳钢在氮气保护环境下腐蚀80 d时，其抗拉强度和屈服强度反而表现出略微的升高。

    在实验研究基础上，基于电化学腐蚀的基本原理，运用COMSOL Multiphysics多物理场仿真软件，建立了金属材料在大气腐蚀过程中的仿真模型，并对碳钢在不同氧气浓度下的腐蚀行为进行了仿真分析，进一步明确了腐蚀机理及影响腐蚀行为的因素。

关 键 词：碳钢；腐蚀模拟平台；核电二回路；氮气保护；腐蚀仿真

研究类型：应用研究

  Based on the basic principle of electrochemical corrosion, the structure, casing, and pipes of the secondary loop thermal system in nuclear power plants are often made of carbon steel. However, due to the high humidity and salt spray in the coastal areas where nuclear power plants are typically built, the carbon steel equipment is prone to corrosion during construction, operation, and shutdown, which can result in the degradation of material properties and a shortened service life. To mitigate carbon steel corrosion, nitrogen protection can be employed to provide corrosion protection for the secondary loop thermal system in nuclear power plants.And in-depth study of the corrosion behavior of carbon steel during nitrogen maintenance is of great significance for formulating practical maintenance plans. Therefore, based on the actual nitrogen protection requirements and technical specifications of the nuclear secondary circuit thermal system, we have designed and built a multi-operating condition experimental platform and adopted a multi-hardware integrated system. Through the human-machine interactive software interface, the relevant parameters of the experimental environment can be automatically controlled, which can achieve precise control of the experimental conditions. Using analysis methods such as corrosion weight loss, scanning electron microscopy, X-ray diffraction, energy spectrometer, and electrochemical testing, the corrosion behavior of three types of carbon steels, 20# steel, Q235A, and Q245R, was studied under salt spray and nitrogen protection environments， respectively. The study also discussed the impact of corrosion on mechanical properties and conducted a corrosion simulation analysis based on experimental results. The following conclusions were drawn:

  The corrosion depth loss of three types of carbon steel showed a nearly linear increase with time in a salt spray environment, with an average corrosion rate that increased over time. After 48 hours of exposure, the surface of the substrate was completely covered by a rust layer, indicating high corrosion sensitivity of the three types of carbon steel. The rust layer observed under scanning electron microscopy was mainly composed of needle-like and cotton ball-like structures, which were porous and provided poor protection to the substrate. With increasing thickness of rust layer, it provided a certain level of protection to the substrate, which is consistent with the changes in corrosion rate. The composition of corrosion products included α-FeOOH, γ-FeOOH, and γ-Fe₂O₃/Fe₃O₄, which were consistent with those found in atmospheric corrosion products. The initial corrosion product was mainly composed of the active phase γ-FeOOH, which accelerated the corrosion reaction. The content of γ-Fe₂O₃/Fe₃O₄ increased with the progression of corrosion, while the α-FeOOH content remained relatively stable.

  Under the nitrogen protection process, the initial corrosion depth of three types of carbon steel showed a power law change with time, with the power exponent n greater than 1, and the average corrosion rate increased with time. Compared with the corrosion rate in salt spray, a two magnitude lower corrosion rate was achieved. The corrosion products of the three types of carbon steel were mainly β-FeOOH, α-FeOOH, and Fe₃O₄. When the corrosion experiment lasted for 35 d, pitting and irregular local corrosion appeared on the sample surface, with the main form being local corrosion. However, at 80d, the exposed area of the sample surface decreased, and the rust layer became thicker with irregular product shapes, gradually developing into uniform corrosion. The corrosion current density of carbon steel in a 3.5 wt% NaCl solution under nitrogen deoxygenation was lower than that in air saturation, and the anode activation polarization region slope became larger, indicating that nitrogen protection could effectively inhibit the anodic dissolution of carbon steel. When the three types of carbon steel were corroded for 240 h in a salt spray environment, severe corrosion occurred, and the overall thickness of the samples decreased significantly, with a large decrease in yield strength and tensile strength. While for the carbon steels corrosion in a nitrogen-deoxygenated environment, a slight increase in both tensile and yield strength was observed when corroded for as long as 80 d.

  Based on experimental studies, a simulation model for the corrosion of metallic materials in atmospheric environments was developed using the principles of electrochemical corrosion and the COMSOL Multiphysics software. The corrosion behavior of carbon steel under different working conditions was simulated and analyzed, leading to a better understanding of the corrosion mechanism and the factors influencing the corrosion behavior.

Key words: Carbon steel; Corrosion simulation platform; Nuclear power second circuit; Nitrogen protection; Corrosion simulation

Thesis : Applied Research

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