The effect of phosphonate anion (PHOS) on the corrosion of ordinary steel in simulated cooling water has been studied using weight loss, polarization curves and electrochemical impedance spectroscopy measurements. PHOS was studied in the concentration range from 7.5 x 10(-5) to 10(-3) M. The results obtained reveal that PHOS perform excellently as corrosion inhibitor for ordinary steel in simulated cooling water. The inhibition efficiency of PHOS was increased with increasing both its concentration and water circulation velocity. These two factors seem to promote the adsorption of phosphorus and oxygen ions on the metal surface, leading to the formation of a protective layer with a greater charge transfer resistance and lower permeability. The inhibition efficiency decreased slightly with temperature as well as hold time immersion under open circuit potential conditions. The adsorption of PHOS on ordinary steel surface obeyed Langmuir's isotherm. The effect of biocide, namely cetyltrimethylammonium bromide (CTAB), on the inhibition efficiency of PHOS has been studied, too. (C) 2010 Elsevier B.V. All rights reserved.