Corrosion inhibition of mild steel in hydrochloric acid solution by salicylidene-4,4'-dimorpholine (SDM) and SDM assembled on nickel oxide nanoparticles (NiONPs) has been studied with gravimetric, electro-chemical impedance spectroscopy (EIS) and polarization techniques. Inhibition was found to increase with increasing concentration of the inhibitors. While studying the temperature effect on corrosion behaviour of SDM and SDM assembled on NiONPs, the inhibition efficiency decreases for SDM only but increases for SDM assembled on NiONPs. The adsorption of both the inhibitors on the mild steel surface obeys the Langmuir adsorption isotherm. The activation energy as well as other thermodynamic parameters (Delta H* and Delta S*) for the inhibition process was calculated. EIS analysis results showed that the capacitive loops for SDM assembled on NiONPs were far away from blank when compared with SDM only. Polarization curve shows that the inhibitors are of mixed type. Further, the protective layer formation was confirmed from atomic force microscopy (AFM) results. Various methods such as EIS-MS, H-1 NMR, XRD, FTIR, and DLS were performed for the confirmation of the structure, interaction of SDM with NiONPs and size of NiONPs. (C) 2015 Elsevier B.V. All rights reserved.