Inhibitive action of four thiadiazole derivatives namely 2-amino-5-methyl-1, 3, 4-thiadiazole (AMTD), 2-amino-5-methylthio-1, 3, 4-thiadiazole (ATTD), 2-amino- 5-(4-tert-butyl)-1, 3, 4-thiadiazole (ABTD), 2-amino-5-(4-ethhylphenyl) -1, 3, 4-thiadiazole (AETD) on the corrosion of copper-nickel alloy in natural sea water has been studied. Mass loss, polarisation and impedance measurements have been employed to analyse their inhibition behavior of substituted thiadiazole derivatives. Polarization measurements showed that the thiadiazoles inhibits the corrosion of copper-nickel alloy by blocking the active sites of the metal surface. Impedance parameters such as charge transfer resistance (Rct) and double layer capacitance (Cdl) are related to adsorption of inhibitors on the metal surface, leading to the formation of a protective film. The adsorption of thiadiazole derivatives on the alloy surface in natural seawater obeys the Langmuir adsorption isotherm.