Amorphous and nanocrystalline alloys of aluminum–transition metal–rare earth metal have been actively studied in recent years, due to the increased values of corrosion and mechanical properties compared to crystalline counterparts. For a wider industrial use of Al-PM-REM amorphous alloys, it is necessary to increase their thermal stability and glass-forming ability. The simultaneous use of two transition metals – nickel and cobalt in different proportions, and, accordingly, the creation of Al–Ni–Co–REM compositions, makes it possible to obtain amorphous alloys with higher GFA and thermal stability compared to ternary compositions. In the present work, the effect of the cobalt content on the corrosion behavior of Al–Ni–Co–Gd(Yb) amorphous tapes obtained by spinning from metal melts is studied. The higher the solubility of REM hydroxide, the lower the corrosion resistance and, accordingly, the higher the corrosion rate of the material. According to the gravimetric and chemical analyzes, it was shown that compositions with a high content of cobalt (4 at %) have a higher corrosion resistance under the studied conditions than compositions containing 2 at % cobalt. Al86Ni4Co4R6 alloys turned out to be more stable than Al86Ni6Co2R6 alloys, despite the slightly higher electronegativity of cobalt compared to nickel, which may be due to the lower solubility of cobalt(II) hydroxide. The studied compositions have high corrosion resistance and can be considered promising for the development of aluminum-based protective coatings.