The inhibiting action of Thai-bael fruit extract at room temperature on hot-rolled steel in 1M HCl
solution was studied. The chemical functional groups of the green inhibitors were characterized by
Fourier-transformed infrared spectroscopy. The electrochemical activities of steel surface were
investigated through linear polarization measurements, electrochemical impedance spectroscopy,
surface assessment techniques based on optical microscopy and X-ray absorption spectroscopy.
Electrochemical testing samples have been prepared in the form of square plates with the size 1 × 1
cm2. The organic corrosion inhibitor extract from Thai-bael fruit has shown the smallest corrosion
current density (Icorr) of 114.8 μA cm-2 and corrosion potential (Ecorr) of -424.6 mV, compared with
standard Ag/AgCl electrode potential. In comparison, similar tests in the bare HCl solutions yielded
Icorr = 882.4 mA cm-2 and Ecorr = -445.8 mV. The mixed-type corrosion inhibiting behaviour was evidenced
in the results of the polarization measurements. Electrochemical impedance spectroscopy reveals that
the resistance to charge transfer due to the presence of the extracts has been increased by about four
times that of the same test on the bare HCl solution, indicating the formation of a protective layer. The
adsorption of the organic molecules near the steel-electrolyte interface is evident in the decreasing
double-layer capacitance with the enhancing concentration levels of the extract. This latter finding
supports the displacement of the water molecules by means of the adsorption of the inhibitors on the
steel surface. The optical images of steel surface before and after being immersed in HCl solution also
showed pieces of evidence of corrosion retardation. XANES study as well as the linear combination
fitting revealed that the samples immersed in HCl solutions with Thai-bael fruit extract possess less
Fe3+ compositions. All tendencies across the four examinations indicate that Thai-bael fruit extract
could potentially inhibit the corrosion reaction of steel electrodes in the acidic solution.
