Study of three new halogenated oxoquinolinecarbohydrazide N-phosphonate derivatives as corrosion inhibitor for mild steel in acid environment

•Three new halogenated oxoquinolinecarbohydrazide N-phosphonate derivatives.•Highly efficient corrosion inhibitors, at 0.48 mmol L-1, for mild steel in 1 M HCl.•Electrochemical studies prove the molecules as mixed-type adsorption inhibitors.•AFM images indicate the formation of a protective film on the metal surface.•Ab initio calculations show an excellent agreement with experimental data. Three new and original halogenated oxoquinolinecarbohydrazide N-phosphonate derivatives (DHOP-F, DHOP-Cl and DHOP-Br) were prepared from the corresponding 4-oxo-1,4-dihydroquinoline-3-carboxylates (81, 71 and 65% yields, respectively), characterized by IR, 1H, 13C and 31P NMR spectroscopy and HRMS and evaluated as corrosion inhibitors for mild steel AISI 1020 in hydrochloric acid media. Electrochemical Impedance Spectroscopy, Linear Polarization Resistance and Weight Loss Study revealed an anticorrosive efficiency of 93.0, 84.2 and 66.8%, respectively, for DHOP-Br, DHOP-Cl and DHOP-F, at a very low concentration (0.48 mmol L−1). Physical chemical calculations showed that the adsorption process is spontaneous, and all molecules obey the Langmuir theory. Data crossing between electrochemical and gravimetric measurements reveals a protection mechanism of adsorption by organic monolayer formation on the metal surface. Polarization curves proves that the organic molecules are all mixed-type corrosion inhibitors and Atomic Force Microscopy images suggest the formation of a protective layer on the metal surface. Ab initio calculations are in great agreement with the experimental results and, therefore, used to rationalize the reason why the bromide derivative is the best inhibitor.