The electrochemical behavior of copper in copper sulfate solutions is studied under isothermal conditions and during heat transfer in various hydrodynamic regimes. The violation of the Nernst dependence of the metal potential on the activity of its ions in solution under certain conditions of medium motion and heat transfer was revealed. The effect of the illumination of the electrode surface on the metal potential is found. The effects are associated with the formation of an oxide film on the metal surface under the given temperature and hydrodynamic conditions. It is shown that the film has protective properties and inhibits the anodic dissolution of the metal.