Inhibition Effect of Anodic Oxidation of Aluminum Heat Sink Surface on Scaling of Platinum Grading Electrodes in Converter Valves

GENG Zhi; PENG Rui; WU Jie; WU Tianbao; LIU Xuewen; WANG Jie; WANG Zhigao; FAN Jun; WU Yuewei; LIU Lei

doi:10.16490/j.cnki.issn.1001-3660.2026.02.004

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Surface Technology ›› 2026, Vol. 55 ›› Issue (2) : 38-49. DOI: 10.16490/j.cnki.issn.1001-3660.2026.02.004

Inhibition Effect of Anodic Oxidation of Aluminum Heat Sink Surface on Scaling of Platinum Grading Electrodes in Converter Valves

GENG Zhi^1,*, PENG Rui¹, WU Jie¹, WU Tianbao¹, LIU Xuewen¹, WANG Jie², WANG Zhigao², FAN Jun², WU Yuewei^3,*, LIU Lei³

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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 (R_p) 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.

Key words

high-voltage direct current / converter valves / aluminum heat sink / platinum grading electrode / corrosion / scaling / anodic oxidation

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GENG Zhi, PENG Rui, WU Jie, WU Tianbao, LIU Xuewen, WANG Jie, WANG Zhigao, FAN Jun, WU Yuewei, LIU Lei. Inhibition Effect of Anodic Oxidation of Aluminum Heat Sink Surface on Scaling of Platinum Grading Electrodes in Converter Valves[J]. Surface Technology. 2026, 55(2): 38-49

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