Recent interest has centred on the application of nitrate injection to control reservoir souring. In the present study, the impact of nitrate injection on the corrosion of ASTM A572-50 carbon steel was evaluated. The steel samples were exposed to corrosive production water (high chloride and sulphate) for 21 days at 50 degrees C. Crude oil and NaNO3 were added as bacterial nutrients. Indigenous bacterial isolates were used and four different conditions were evaluated: control cell (no bacteria), nitrate reducing bacteria (NRB) inoculated cell, sulphate reducing bacteria (SRB) inoculated cell and mixed bacteria (NRB+SRB) inoculated cell. Open circuit potential (E-corr) and Electrochemical impedance spectroscopy (EIS) were employed to monitor the corrosion process throughout the immersion time. It is shown that the corrosion rates in the cells inoculated with bacteria are lower than the control cell. Both NRB and SRB may offer beneficial corrosion protection to the steel. However, the microscopy results show that localized corrosion in the presence of bacteria is inevitable. Additionally, nitrate promotes the growth of NRB and suppresses the growth of SRB: hence H2S production can be eliminated. NRB oxidize the produced sulphide by SRB and maintain E-h in an inhibitory level to SRB. (C) 2012 Elsevier Ltd. All rights reserved.