This review paper examines the potential of using Vernonia amygdalina (bitter leaf) extracts as a
green corrosion inhibitor for mild steel and aluminum in various aggressive corrosive environments.
A survey of previous literature revealed that increasing the concentration of the extract significantly
reduced the corrosion rate and enhanced the corrosion inhibition efficiency for various mild steel
and aluminum samples studied under different operating conditions. Vernonia amygdalina extracts
effectively protected the substrate even at low concentrations due to the presence of phytochemicals
such as saponins, terpenoids, tannins, glycosides, flavonoids, alkaloids, and phenolics, as well as the
adsorption of the extract onto the surface of the mild steel and aluminum substrates, thereby reducing
the corrosion rate by decreasing the available sites for the dissolution reaction. Results from various
studies indicate that the extract functions as a mixed-type inhibitor, effectively inhibiting both anodic
and cathodic reactions. Physical adsorption is widely reported as the mechanism by which Vernonia
amygdalina extracts adhere to the surfaces of mild steel and aluminum. Findings from most studies
reveal that the surface coverage of the extract aligns with the Langmuir isotherm model. Various
methods, including weight loss measurements (WL), gasometry, thermometry, potentiodynamic
polarization (PDP), electrochemical impedance spectroscopy (EIS), Fourier transform infrared
spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy
(EDS), gas chromatography-mass spectrometry (GC-MS), and density functional theory (DFT),
were employed by different researchers. This paper also discusses the various phytochemicals and
functional groups present in Vernonia amygdalina.