complexes are found at lower energies with respect to the free
ligand, and at ca. 1200 cm1 can be seen vibrations attributed to
the N–C–S group, showing that the sulfur atom is involved in the
coordination. The NMR spectra of the diamagnetic metal complexes
are slightly modified with respect to the free ligand ones;
it is noteworthy the disappearance in complex 3 of the NH signal
confirming the deprotonation of the ligand and the formation of
a neutral complex as a consequence. The simplicity of the spectra
shows the symmetry of the coordination around the metal. The
NMR spectra of the two metal complexes are quite similar, according
to the role of the d10 metal ion. In both cases, the coordination
environment around the metal involves the sulfur atom in its thionic
form and the iminic nitrogen as seen by the shift at lower ppm
values of the CHN signal. This coordination behaviour for complex
4 supports the cationic nature of the compound.
With the compounds synthesised in the present study, we have
investigated whether the introduction of molecules of new design
in acidic solutions could slow down the corrosion of steel. The corrosion
was monitored measuring the weight loss of steel exposed
to the corrosive agent (Table 3) and the amount of hydrogen that
evolves from the solution. The inhibition efficiency is expressed
using the equation I = [(w0 w1)/w0]100, where w0 and w1 are
the values of the corrosion rate in the absence and presence of inhibitors
respectively.
Looking at Table 3, it is apparent that for few compounds there
is a spread value within the series. The reason of this spread, especially
for Me-fbt in HCl 0.1 M (25% in terms of standard deviation),
for [Zn(fbt)2] in HCl 0.2 M (13% in terms of standard deviation) and
for Hfbt in HCl 0.5 M (15% in terms of standard deviation), can be
ascribed to the fact that the steel samples, even if of the same composition,
can present non homogeneous surfaces due, for instance,
to the presence of scratches, that could enhance the weight loss.
The inhibition efficiency increases in the sequence
[Zn(fbt)2] < Me-fbt < Hfbt < [Zn(Me-fbt)2]Cl2 (Table 4)
It is apparent from Table 4 that, as a general trend, inhibition
efficiency tends to decrease as HCl concentration increases and it
is also noteworthy that the metal complex [Zn(Me-fbt)2]Cl2 with
a concentration as low as 5.0 104 mol/L shows a good inhibition
efficiency for mild steel in hydrochloric acid. From the table it is also
apparent that there is a certain spread of the inhibition efficiency
arising from the variability of the weight loss. Nevertheless, the
general trend is not affected by this spread of values.
From the H2 evolution analysis (Fig. 2) it has been confirmed
that the compound [Zn(Me-fbt)2]Cl2 shows a good attitude in
inhibiting corrosion also in the presence of strongly acidic solution.
At the end of the observation time, [Zn(Me-fbt)2]Cl2 inhibits H2
Table 3
Weight loss data: the inhibitors are at 5.0 104 mol/L and each measurement was carried out in triplicate.
HCl
0.1 M
Average Std
dev
HCl
0.2 M
Average Std
dev
HCl
0.3 M
Average Std
dev
HCl
0.4 M
Average Std
dev
HCl
0.5 M
Average Std
dev
0.2193 0.3041 0.3105 0.4513 0.6284
No inhibitor 0.2358 0.2499 0.0395 0.2994 0.3047 0.3047 0.3210 0.3081 0.0142 0.4492 0.4309 0.0335 0.6931 0.6372 0.0521
0.2945 0.3105 0.2929 0.3923 0.5901
0.0222 0.0367 0.0411 0.0618 0.0894
Hfbt 0.0214 0.0246 0.0048 0.0341 0.0374 0.0374 0.0299 0.0383 0.0074 0.0591 0.0597 0.0019 0.0747 0.0885 0.0134
0.0301 0.0414 0.0439 0.0582 0.1014
0.0612 0.0950 0.0954 0.1584 0.2314
[Zn(fbt)2] 0.0704 0.0682 0.0062 0.1040 0.0930 0.0930 0.1053 0.0962 0.0087 0.1601 0.1514 0.0136 0.2594 0.2372 0.0200
0.0731 0.0799 0.0879 0.1357 0.2207
0.0422 0.0682 0.0738 0.1142 0.1597
Me-fbt 0.0485 0.0529 0.0135 0.0686 0.0690 0.0690 0.0784 0.0730 0.0059 0.1068 0.1067 0.0075 0.1754 0.1618 0.0127
0.0681 0.0701 0.0667 0.0992 0.1503
0.0122 0.0134 0.0241 0.0404 0.0582
[Zn(Mefbt)
2]Cl2
0.0101 0.0126 0.0027 0.0161 0.0162 0.0162 0.0252 0.0240 0.0013 0.0398 0.0377 0.0042 0.0648 0.0599 0.0043
0.0154 0.0192