Newly Synthesized Morpholinyl Mannich Bases as Corrosion Inhibitors for N80 Steel in Acid Environment

New Mannich bases, 3-morpholino-1-phenylpropan-1-one (MPO) and 3-morpholino-1-phenyl-3-(pyridin-4-yl) propan-1-one (MPPO), were synthesized, characterized, and studied as corrosion inhibitors for N80 steel in 1 M hydrochloric acid (HCl) solution using weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and FT-IR spectroscopy. The inhibition efficiency increases with increasing inhibitor concentrations, and the corrosion inhibition efficiency of the MPO and MPPO could reach 90.3% and 91.4%, respectively, at a concentration of 300 ppm at 305 K. The effect of the temperature on the corrosion inhibition behavior of inhibitors was discussed. Electrochemical tests showed that the synthesized inhibitors are mixed. The EIS test results showed that the presence of MPO and MPPO reduced the double-layer capacitance in the corrosion process, thereby reducing the charge transfer resistance. The SEM and EDX results showed that the MPO and MPPO formed a uniform adsorption film on the surface of the N80 steel. The adsorption mechanism of the inhibitors was simulated with different adsorption models and the results showed that the inhibitors were the chemisorbed type. The results of the FT-IR spectroscopy proved that the inhibitor interacted with metal atoms on the steel surface.