Quinoline quaternary ammonium salts 1-benzylquinoline bromide (1) and 1-benzylquinoline chloride (2) have been synthesized and then developed as corrosion inhibitors in acidic HCl solution. Electrochemical impedance, potentiodynamic polarization and gravimetric measurements, as well as SEM, indicate that both can efficiently protect the mild steel from corrosion. The fact that 1 shows better performance than 2 under the same conditions could be attributed to the cooperative effect of counterions on the corrosion process. The results of thermodynamic and electrochemical studies were in agreement with the observed inhibition efficiency, indicating that counterions can affect the interactions between organic cations and the metal surface, and influence the stability of the protective film, which leads to the detected differences in their inhibition behavior.