Candle soot derived carbon dots as potential corrosion inhibitor for stainless steel in HCl medium

In the present work, functionalized carbon dots (CDs) derived from candle soot (CS) was used as a potential corrosion inhibitor for the corrosion of stainless steel (SS) in 1 M HCl medium. The carbon soot was collected from the waste candle which on further strong oxidation with conc. HNO 3 results in the formation of CDs. CS and CNs were characterized using FT-IR spectroscopy, XRD, UV–Visible spectroscopy, fluorescence spectroscopy, scanning electron microscopy (SEM), and transmission electron microscope (TEM). TEM analysis of CNs shows that the particles of CDs are spherical with an average size of 3 nm. Further, the corrosion inhibiting nature of CDs towards the corrosion of SS was evaluated using weight loss measurements, surface morphological analysis using SEM, potentiodynamic polarization studies, and electrical impedance spectroscopy. Calculation of activation energy using the Arrhenius equation shows the increase in activation energy while adding the CDs in 1 M HCl medium and the maximum activation energy (54.06 kJ mol −1 ) was observed for 200 mg L −1 CDs in an acid medium. The negative entropy obtained from the transition state equation suggested the associative mechanism in the formation of the activated complex. Langmuir adsorption isotherm suggested the type of interaction between CDs and SS is both physisorption and chemisorption. Electrical impedance spectral analysis and Tafel analysis show the high inhibiting nature of CDs towards the corrosion of SS in 1 M HCl medium due to the formation of a passive film of CDs on the surface of SS resulting in the protection of corrosion in acid medium. Further, Tafel analysis revealed the type of inhibitor as a mixed type inhibitor. Graphical abstract