The corrosion of specimens of copper produced by electroplating on disc electrodes 10–500 μm in diameter from a pyrophosphate electrolyte is studied in this electrolyte (without added copper ions) using polarization curve measurements. It is found that the rate of corrosion of the microelectrodes with a radius of 5 μm is eight times higher than that of the microelectrodes with a radius of 25 μm; the measured rate of corrosion remains unchanged when varying the radius of the microelectrode in a range of 25–250 μm. It is shown that the process under investigation is corrosion with oxygen depolarization; the rate of reduction (of dissolved oxygen) increases eightfold when varying the radius of the microelectrode in a range of 5–25 μm. The experimental results are confirmed by the calculations of diffusion currents for the microelectrodes, which show that the size effect, i.e., an increase in the diffusion current density with decreasing area of the electrode surface, should be observed for electrodes with a radius less than 20–30 μm.