The corrosion mechanism and corrosion-resistances of Cu-40Ni alloy prepared by conventional casting(CA),mechanical alloy(MA) and hot-pressing processes with the great difference in grain size in acidic media were investigated by PARM273A and M5210 electrochemical apparatus through the potentiodynamic polarization method and the electrochemical impedance spectroscopy(EIS) technique.The results show that the corrosion potential of CA Cu-40Ni alloy in acidic medium moves negtively,while MA Cu-40Ni alloy moves positively with the increase of the H~+ concentration.There is no passive phenomenon in potentiodynamic polarization curve for the two alloys.With the increase of the H~+ concentration,the polarization resistance increases,and the corrosion current decreases,so the corrosion resistance increases for CA Cu-40Ni alloy,while the polarization resistance decreases,and the corrosion current increases,so the corrosion resistance decreases for MA Cu-40Ni alloy.The electrochemical impedance spectroscopy(EIS) composed of double capacitive loops for the two alloys show that the corrosion process is controlled by electrochemical reaction.The corrosion rate of MA Cu-40Ni alloy becomes faster than that of CA Cu-40Ni alloy because the reduction in the grain size of MA Cu-40Ni alloy produces large concentrations of grain boundaries and increases the number of reactive atoms in alloy surface.