The inhibition of steel-copper couple galvanic corrosion in petroleum waste water solution by Curcuma longa was studied at different temperature and different cathode to anode ratios using weight loss technique. Inhibition efficiency was increased with increase in inhibitor concentration but decrease with rise in temperature. Adsorption followed the Temkin isotherm with negative values of Delta G degrees(ads), suggesting a stable, and spontaneous inhibition process. Mathematical equations were suggested to represent the corrosion rate data with high correlation coefficients. Quantum chemical parameters were calculated for inhibitors using the AM1-SCF method to simulate the adsorption of the C. longa molecules on couple surface. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.