decreased and inhibition efficiency IE (%) is increased
by increasing inhibitor concentration [15]. The extract
causes changes in the anodic, cathodic Tafel slopes and
the Ecorr values in the presence of different concentrations,
suggesting that these compounds behave as
mixed-type (anodic/cathodic) inhibitors [16-18]. Increasing
inhibition efficiencies with increasing concentrations
of the extract shows that the inhibition actions are due
to its adsorption on the steel surface [19,20]. The difference
in inhibition efficiency between WE and AE may
be due to the effect of the high water-soluble part, which
includes the glycoside and some polysaccharide. The
inhibition efficiencies obtained from potentiodynamic
polarization were different from those calculated from
weight-loss measurements, which may be attributable to
the fact that the weight-loss method gives average corrosion
rate, whereas electrochemical method gives instantaneous
corrosion rates. These variations may also arise
because of the difference in the time required to form
an adsorbed layer of inhibitors on metal surface that can
inhibit corrosion [21,22].