The carbon steel is widely used in the petroleum industry because of its excellent mechanical properties, but it is exposed to different environments, Such as hydrochloric acid who are commonly used for pickling and also for the removal of rust and scales in petroleum industries. In this work, we have studied the protection against corrosion of carbon steel by applying the Ni-P alloy coating deposited on a steel substrate compared to another method of protection by adding the 4,5-dimethyl- 1,2-dithiol-3-thione inhibitor in an aggressive medium (1M HCL) using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The results obtained are shown that This organic compound inhibits the acidic corrosion even at low concentration, is good inhibitors and are in good agreement. The efficiency of the inhibition Is increase with the increase of inhibitor concentration to reach the optimal values of 8*10-5 mM and the best coating of nickel-phosphorus at current density 5 A dm-2. The obtained results showed that the Ni-P alloy coating Decreases the corrosion rate of carbon steel better than the inhibitor 4, 5-dimethyl- 1, 2-dithiol-3-thione.

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