ing that this compound adsorbs according to Langmuir
adsorption isotherm where,
h¼KC
KC þ1:ð10Þ
With
K¼1
55:5
exp DGads
RT
;ð11Þ
where Kis the equilibrium constant for the adsorption process,
Cis the concentration of the inhibitor.
The value of equilibrium adsorption constant obtained
from this isotherm is about 4.21 ·10
4
L mol
1
suggesting a
chemically adsorbed film (Villamil et al., 2002).
It is well known that values of DG
ads
of the order of
20 kJ mol
1
or lower indicate a physisorption mechanism;
those of the order of 40 kJ mol
1
or higher involve charge
sharing or a transfer from the inhibitor molecules to the metal
surface to form a co-ordinate type of bond (Donahue and
Nobe, 1965; Kamis et al., 1991).
The largest negative value of DG
ads
(36.94 kJ mol
1
) ob-
tained here indicates that this inhibitor is strongly adsorbed
onto the copper surface and is typical to chemisorption.
4. Conclusions
The main conclusions drawn from this study are:
– MMI inhibits the corrosion of copper in 0.5 M H
2
SO
4
.
– The inhibition is due to adsorption of the inhibitor mole-
cules on the copper surface.
– The adsorption of MMI on the copper obeys the Langmuir
adsorption isotherm model.
– Results obtained from dc polarization, ac impedance and
weight loss techniques are in reasonably good agreement
and show increased inhibitor efficiency with increasing
inhibitor concentration.
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Inhibiting effects of 2-mercapto-1-methylimidazole on copper corrosion in 0.5 M sulfuric acid 235