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Nitrate has been used to control Sulphate Reducing Bacteria (SRB) in oil and gas reservoirs. Nitrate is thermodynamically a strong oxidizer, which could increase carbon steel corrosion rates yet; literature documenting the effect of nitrate on corrosion is scarce and contradictory. This study investigated the effect of nitrate on carbon steel corrosion under simulated oilfield produced water (rich in carbon dioxide with pH~5) and simulated seawater (minimum carbon dioxide and a pH~7). CO2 corrosion tests were conducted with carbon steel X65 exposed to uninhibited brines at 25°C, 60°C, and 80°C via rotating cylinder electrode (RCE). Inhibited synthetic brine was tested under acidic environments at 80°C. Changes in nitrate, nitrite, ammonium and ferrous ions were monitored using ion chromatography. The results provide experimental evidence to demonstrate that the corrosivity of nitrate strongly depends on pH. Nitrate tends to increase CO2 corrosion rates in the presence of synthetic produced brine (pH~5) and is relatively benign when added to synthetic seawater (pH~7). At the conditions tested, nitrite impurities contained within the source of nitrate tend to increase carbon steel corrosion exposed to synthetic brines. In the presence of metallic iron and under CO2 environments, nitrate reduces to ammonium, which is thermodynamically stable in acid solutions. The reduction of nitrate can be modelled based on first order kinetics. The main factors promoting the reduction of nitrate are pH, iron to nitrate ratio, and temperature.