Compatibility of C1020 carbon steel with biodiesel in terms of molecular structure and interface properties involves analysis of chemical reactivity of the steel exposed to the fuel matrix. Molecular reactant–interface shielding (MRIS) analysis is used to determine the level of compatibility of C1020 carbon steel with biodiesel from jatropha oil (\JOB), biodiesel from neem oil (\NOB), and biodiesel from waste cooking oil (\WCOB). The test is to verify whether the steel compatibility is influenced by the degree of total unsaturation, indicated by the iodine number, or by a combination of polyunsaturated ester load, acidic/water activation, and electrochemical charge transfer shielding. Numerical assessment includes fatty acid content, physical property data, potentiodynamic polarization constants, corrosion rates, and impedance characteristics of the three steel–biodiesel pairs. The molecular block differentiates the monounsaturated and polyunsaturated molecules via polyunsaturated ester content and quadratic unsaturation density, whereas the interface block encompasses corrosion current density, corrosion rate, charge transfer resistance, constant phase element modulus, and capacitance non-ideality. Iodine number is not adequate for determining compatibility. Biodiesel from waste cooking oil has the highest iodine number of 86.40 g I$_2$ / 100 g oil, but has the least polyunsaturated component, 0.420\%, and the highest charge transfer resistance, 601 $\Omega$ cm$^2$. Biodiesel from jatropha oil contains the maximum proportion of polyunsaturated molecules, 61.064\%, highest quadratic unsaturation density, 325.777, highest corrosion current density, 16.23 $\mu$A cm$^{-2}$, and lowest charge transfer resistance, 120 $\Omega$ cm$^2$. This results in MRIS risk factor scores of 96.88 for \JOB, 54.10 for \NOB, and 2.22 for \WCOB, indicating the compatibility sequence \WCOB $>$ \NOB $>$ \JOB. Thus, the compatibility of C1020 steel depends on the reaction of molecular oxidation pressure and interfacial resistance, not only the degree of unsaturation in the bulk phase.

C1020 carbon steel,biodiesel corrosion,fatty-acid methyl esters,jatropha biodiesel,neem biodiesel,waste-cooking-oil biodiesel,charge-transfer resistance,oxidation stability,electrochemical impedance spectroscopy