The effect of diethanolamine (DEA) and hexamethylenetetramine (HMTA) on the corrosion of carbon steel (CS) in concentrated aqueous solution of ammonium nitrate and urea (ANU) was studied by weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy. Both amines showed high inhibition efficiency >99% above a concentration of 0.5 wt% at 25 and 50 °C. The addition of 0.1–0.5 wt% sebacic acid to the tested solution containing amines did not enhance the inhibition efficiency. A positive synergistic effect was observed when 0.1–0.5 wt% of cinnamic acid was added to the ANU solution containing 0.3–1 wt% of amines. These results clearly demonstrate the potential of using CS for storage and transport of ANU solutions under ambient conditions.