Lawsonia inermis also known as henna was studied as a corrosion inhibitor for aluminum alloy in seawater. The inhibitor has been characterized by optical study via Fourier transform infrared spectroscopy (FTIR). The FTIR proves the existence of hydroxyl and carbonyl functional groups in Lawsonia inermis. Aluminum alloy 5083 immersed in seawater in the absence and presence of Lawsonia inermis was tested using electrochemistry method, namely, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PP). EIS and PP measurements suggest that the addition of Lawsonia inermis has caused the adsorption of inhibitor on the aluminum surface. The adsorption behavior of the inhibitor follow Langmuir adsorption model where the value of free energy of adsorption, ????, is less than 40 kJ/mol indicates that it is a physical adsorption. Finally, it was inferred that Lawsonia inermis has a real potential to act as a corrosion inhibitor for aluminum alloy in seawater.

This study utilizes optical measurements, thermo-impedance analysis, potentiodynamic polarization studies and morphology observations of henna leaves extract (HLE) incorporated in an acrylic resin coating. The acrylic resin coating with 0.2 wt/vol% HLE (AC2) had the best performance protecting metal from corrosion. XRD and DSC analysis demonstrate that an increase in the crystallite size limits the close packed structure, which increases the free volume and reduces the Tg of the coating. Open circuit potential (OCP) measurements demonstrate that the AC2 coating has a uniform potential due to the lower rate of coating barrier destruction. Electrochemical impedance spectroscopy (EIS) indicates that AC2 has the highest coating resistance, Rc (4.79 × 108 Ω), and lowest coating capacitance, Cc (3.32 × 10−9 F/cm2). An elevation in temperature caused coating deterioration for all of the coatings. AC2 has the lowest dielectric constant, εr, indicating less water uptake and lower ionic conductivity. An additional study of potentiodynamic polarization demonstrates that AC2 has shifted to the noble potential and gives the lowest corrosion current density, icorr, reading. The corrosion rate is the lowest for AC2 (3.93 × 10−7 mm/year), while the polarization resistance is the highest at 7.44 × 107 Ω. An SEM morphology study indicates that AC2 has lesser delamination and greater coverage of HLE in the coating.