Three imidazolidine derivatives, namely, 1, 4-diazaspiro [4, 5] decane (DSD), 6-methyl-1, 4-diazaspiro [4, 5] decane (MDSD) and 2-methyl-2-phenethyl imidazolidine (MPI) have been synthesized and investigated herein as novel corrosion inhibitors for API 5L X60 steel alloy in 1 M HCl solution using electrochemical techniques (OCP, EIS, LPR and PDP) complemented with quantum chemical calculations. At room temperature under naturally aerated conditions, the imidazolidine derivatives do not alter the corrosion mechanism of the steel alloy but greatly enhance the resistance to the acid corrosion by strongly lowering the capacitance of charge at the steel-solution interface. This results in increase resistance to charge transfer leading to decrease in both anodic and cathodic half-reactions. The imidazolidine derivatives functioned as mixed-type inhibitors during the electrochemical acid corrosion of the steel. MPI with an alkylphenyl substituent exhibited the highest corrosion inhibition efficiency and the attachment of a methyl group to the cyclohexyl substituent of DSD provided improved efficiency than MDSD. Quantum chemical descriptors such as the energy of the highest occupied molecular orbital (, the energy of the lowest occupied molecular orbital (, the energy gap , dipole moment (, electronegativity , global hardness , softness , fraction of electrons transferred , and electrophilicity index have been calculated and discussed. With inhibition efficiency reaching 80%, the imidazolidine molecules prove to be promising inhibitors for the acid corrosion of steel in 1 M HCl solution.