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Four-layer aluminum brazing sheets (4343/3003/6111/3003) with honeycomb sandwich structure are used as candidates for floor of high-speed train and ship deck, which are often exposed to corrosive environments. Microstructure and surface conditions of optimized 6111 aluminum alloy, which serves as the main support layer of this four-layer brazing sheet, have a great effect on the corrosion properties, which were investigated by a set of 6111 aluminum alloy ground with sandpaper of different grits. The results show that the AlFeSi(Mn, Cu) phase acts as cathode, due to its higher potential than that of the matrix, and forms a multi-stage system with adjacent matrix, which aggravates the surface corrosion. A smoother surface exhibits better corrosion resistance. Specifically, when lowering surface roughness from 18.03 μm to 0.92 μm, the surface volume decreases from 0.629 mm³ to 0.029 mm³, and the average number of intermetallic particles AlFeSi (Mn, Cu) is reduced from 1631 mm^-2 to 917 mm^-2, with area fraction decreasing from 3.93% to 0.92%. As a consequence, the average corrosion depth decreases from 237 μm to 95 μm.