目的 解决高压直流输电系统换流阀铝散热器腐蚀导致的均压电极结垢问题。方法 采用硫酸和硫酸+草酸两种电解液体系对换流阀铝散热器进行阳极氧化处理,通过X射线衍射、扫描电子显微镜、X射线光电子能谱对氧化膜物相与形貌进行分析,通过电化学阻抗谱对氧化膜耐蚀性能进行评估,并采用循环冲刷试验装置模拟研究了散热器表面阳极氧化对均压电极结垢的影响。结果 铝合金在硫酸或硫酸+草酸体系中处理后表面生成无定形Al2O3和AlOOH（或Al(OH)3）组成的致密氧化膜,在模拟内冷水环境中的腐蚀速率可降低1个数量级,耐蚀性显著提高且随温度升高不会明显减弱。散热器在泄漏电流流入的区域（阳极）比流出区域（阴极）更容易发生腐蚀,经过阳极氧化预处理后腐蚀明显减弱。均压电极结垢主要来源于散热器腐蚀,电流流出方向更容易发生结垢;散热器经过阳极氧化预处理后,在外加电压下腐蚀速率显著降低,循环水中的金属离子浓度下降,最终均压电极的结垢速率降低96%。结论 换流阀铝合金散热器表面阳极氧化能够有效抑制均压电极结垢。
Abstract
To mitigate the scaling on platinum grading electrodes caused by corrosion of aluminum heat sinks in high-voltage direct current (HVDC) converter valves, the work aims to carry out anodic oxidation treatment on aluminum heat sinks with two electrolyte systems, namely sulfuric acid and sulfuric-oxalic acid. The phase composition and morphology of the oxide films were analyzed through X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the oxide films was evaluated via electrochemical impedance spectroscopy (EIS). A cyclic flushing test apparatus was employed to simulate the actual service conditions of aluminum heat sinks and platinum grading electrodes in HVDC converter valves to investigate the effect of anodic oxidation of aluminum heat sink surface on scaling formation on grading electrodes. The results of EIS, XRD, SEM and XPS showed that after treatment in sulfuric acid or sulfuric-oxalic acid, a dense oxide film composed of amorphous Al2O3 and AlOOH/Al(OH)3 formed on the aluminum alloy surface, which could reduce the corrosion rate in simulated cooling water by one order of magnitude. The polarization resistance (Rp) from EIS exhibited an initial rise followed by a decline as sulfuric acid concentration increased at both room temperature and elevated temperatures. The corrosion inhibition efficiency of the oxide film was up to 98% when the concentration of sulfuric acid was 160 g/L at room temperature and 96% when the concentration was 120 g/L. Compared with the pure sulfuric acid, the corrosion inhibition efficiency of the oxide film formed in the mixed sulfuric-oxalic acid electrolyte was less lower. The corrosion inhibition efficiency of the oxide film was up to 98% when the concentrations of sulfuric acid and oxalic acid were 180 and 70 g/L at room temperature and 95% when the concentrations was 140 and 50 g/L. The oxide film on 6063 aluminum alloy after sulfuric treatment was smooth and defect-free, characterized with uniformly dense porous structure along with honeycomb-like dimples under all the concentrations. The oxide film on 6063 aluminum alloy after sulfuric-oxalic treatment also displayed a porous structure but with smaller, irregularly shaped pores. The results of simulated cyclic flushing test showed that the base 6063 aluminum alloy material suffered severe corrosion after 7-d flushing and corrosion under applied voltage of about 17 V, especially at the anode where the leakage current flowed in, with abundant flocculent precipitating on the surface. However, the 6063 aluminum alloy after anodic oxidation treatment in both type of acids retained the original oxide film morphology with only minor flocculent precipitates under the same test conditions. The corrosion severity showed a trend of base material >> sulfuric-oxalic acid > sulfuric, consistent with the corrosion inhibition efficiency results from EIS. Consistent with the corrosion severity of simulated aluminum heat sinks, the scaling rate on simulated grading electrodes when the leakage current was 4 mA reduced 96.8% and 96.0% under the conditions of sulfuric-oxalic acid and sulfuric treatment, respectively. The scaling on simulated grading electrodes was mainly composed of Al, Mg and O, which verified that the scaling on grading electrodes came from the corrosion of simulated aluminum heat sinks. Therefore, it can be concluded that the anodic oxidation on aluminum heat sink surface in converter valves effectively inhibits scaling rate on platinum grading electrodes.
关键词
高压直流输电 /
换流阀 /
铝散热器 /
均压电极 /
腐蚀 /
结垢 /
阳极氧化
Key words
high-voltage direct current /
converter valves /
aluminum heat sink /
platinum grading electrode /
corrosion /
scaling /
anodic oxidation
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