Results inhibitor concentrations. The structure of inhibition

Results and  Discussion

Weight loss process

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                            Table 1 delivers the
standards of the inhibition efficiency and corrosion rate with different
concentration (0.2, 0.4 and 1.0)  of few alkyl
substituted 2,6-diphenylpiperidin-4-ones with thiosemicarbazone 21 on mild-steel
corrosion in 1 N sulfuric acid media have been computed by a weight
loss process at 303 K.

 

 

 

 

 

Table 1

Corrosion parametric
quantity of Alkyl substituted 2,6-diphenyl piperidin-4-ones with thiosemicarbazone
on mild-steel in 1 N sulfuric acid from weight loss measurements (303K)

SL NO.

Name of the inhibitor and concentration

Weight Loss   (gram)

Corrosion Rate (milli miles per year)

Inhibition efficiency                              (%)

Surface Coverage          (?)

1

Blank

0.0890

79.96

2

01TS

0.2

0.0177

15.9

80.11

0.8011

0.4

0.0091

8.2

89.78

0.8978

1

0.0014

1.26

98.43

0.9843

3

02TS

0.2

0.0233

20.93

73.82

0.7382

0.4

0.0172

15.45

80.67

0.8067

1

0.0061

5.48

93.15

0.9315

4

03TS

0.2

0.0245

22.01

72.47

0.7247

0.4

0.0179

16.08

79.89

0.7989

1

0.0094

8.45

89.44

0.8944

                                                The
weight loss process indicates the inhibition efficiency (IE) raises with an
increment in inhibition concentration and also the corrosion rate of the
inhibitor decreases than to blank solution, it conveys that the alkyl substituted
2,6-diphenyl piperidin-4-one with thiosemicarbazones react as a corrosion
inhibitor. The surface courage (?) of the corrosion inhibition  raised to raise in the different concentration
of the inhibitor.  The optimum inhibition
efficiency of these compounds achieved even at very low concentration. The effect
of concentration of alkyl substituted 2,6-diphenyl piperindin-4-one with
thiosemicarbazones on weight loss processes disclose that the metal-loss gently
lowered with rising inhibitor concentrations. The structure of inhibition
efficiencies (IE) of these inhibitors at 303 K is (01TS) > (02TS) >
(03TS). Examination of the inhibition efficiencies of inhibitors (01TS), (02TS)
and (03TS) explains that inhibition efficiency, decrease when it is substituted
with various groups at 3-position of the piperidin-4-one ring. This tendency can be suggested by
the conformations of substituted piperidin-4-ones and steric hindrance formed
by the substitutes (20).

Potentiodynamic polarization method

                                                Figure 2 to 4
indicates the potentiodynamic polarization plots of the alkyl substituted 2,6-diphenyl
piperidin-4-ones of  thiosemicarbazone in
1N sulfuric acid at different concentrations at 303K. The kinetic  parameters 
of  inhibition of corrosion acquired
from these plots are  disclosed  in Table 2.

Figure
– 2

                Potentiodynamic polarization
graph for mild-steel in 1 N sulfuric acid in the before and after addition of
(01TS)

I    (Ampere per square
centimeter)

Figure – 3

                Potentiodynamic polarization graph for mild-steel in
1 N sulfuric acid in the before and after addition of (02TS)

I  (Ampere per square
centimeter)

 

 

Figure
– 4

                Potentiodynamic polarization graph for mild-steel in
1 N sulfuric acid in the before and after addition of (03TS)

I  (Ampere per
Square centimeter)

Table
2

 Potentiodynamic polarization parametric
quantity of the mild-steel in 1 N sulfuric acid with and without Alkyl
substituted piperidin-4-ones with thiosemicarbazone

 

SL NO.

Name of the Inhibitor and concentration

Icorr
(in
micro amper per cm2) x10-2

Ecorr
(milliVolts vs Saturated calomel electrode)

ba

(milliVolts/Decade)

Bc
(milliVolts/Decade)

Corrosion Rate
(milli
miles per year)

Inhibition Efficiency (%)

1

Blank

 

4.57

-524

120

-137

104.71

 

2

01TS

0.2

0.69

-547

46

-147

15.81

84.90

0.4

0.35

-498

39

-131

8.02

92.34

1.0

0.26

-469

35

-122

5.96

94.31

3

02TS

0.2

1.30

-533

79

-136

29.79

71.55

0.4

0.48

-532

55

-133

11.00

89.50

1.0

0.31

-495

32

-135

7.10

93.22

4

03TS

0.2

1.07

-525

48

-139

24.52

76.57

0.4

0.46

-494

30

-140

10.54

89.93

1.0

0.39

-478

35

-116

8.94

91.47

 

From
the table 2, it can  identify that as the
inhibitor concentration increases, the Icorr
value reduces and the inhibitor efficiency increases apparently and this shows
the inhibiting character of the inhibitor. The least corrosion rate in
potentiodynamic polarization examination exhibit that the inhibitor has
definitely performed on the mild-steel metal surface. A distinction in the
values of Tafel constant of ba  and bc
concluded the character of the inhibitor. An increment in the bc values in the
concentration of the organic inhibitor is a typical aspect of a cathodic
inhibitor, which reveals a higher in the energy barrier for proton discharge
prominent to less gas evolution. The variation of ba and bc
values in the several concentrations of inhibitor in 1 N sulfuric acid  are presented in table 2. Examination of
the results exposes that ba
and bc values do not shift
appreciably with the concentration of Alkyl substituted Piperidin-4-ones with thiosemicarbazone
and slight variation is observed. The
Ecorr value shifts in the
direction of  less negative region when
the concentration of the inhibitor is improved. This is associated to enhanced
inhibitor adsorption on the mild-steel metallic surface. Thus, some alkyl
substituted 2,6-diphenyl  Piperidin-4-ones with thiosemicarbazone may be
measured as a mixed type character of organic inhibitors having more cathodic
character.

AC impedance method

Nyquist graphs for mild-steel metallic corrosion
inhibition in 1 N sulfuric acid for alkyl substituted 2,6-diphenyl piperidin-4-ones
of  thiosemicarbazone  exhibited in the figure 5 to 7. The Impedance
range of the Nyquist plots is described by fitting the experimental data to a
simple equivalent circuit model as exposed in Figure 4. It exists of the
solution resistance (Rs)
and the double layer capacitance (Cdl)
which is organized in parallel to the charge transfer resistance (Rct) (12). Table 3 provides
the significance of double layer 
capacitance (Cdl),
charge transfer resistance  (Rct) and  inhibition efficiency.

                                                                Fig.
4 Equivalent circuit model for fitting
impedance spectra

                The
Charge transfer resistance Rct
values are determined from the variation in impedance at low and high
frequencies. The Rct value
is a determined of electron transfer over the surface of mild-steel, that is
inversely proportional to the rate of the corrosion. The double layer
capacitance Cdl is
estimated at the frequency fmax  at which the imaginary component of the
impedance is maximal using the equation (Ref.18 and 20).

                                                                Cdl =

                                                                                       (5)

Figure 5

Nyquist graph of mild-steel
corrosion in 1 N sulfuric acid in the absence and presence of (01TS)

Figure 6

Nyquist group of mild-steel
corrosion in 1 N sulfuric acid in the absence and presence of  (02TS)  

 

Figure 7

Nyquist graph of mild-steel
corrosion in 1 N sulfuric acid in the absence and presence of (03TS)

      Z’ 
(ohm)

 

Table 3

Impedance parametric
quantity of the mild-steel in 1 N sulfuric acid  with and without Alkyl substituted
piperidin-4-ones with thiosemicarbazone

SL NO.

Name of the inhibitor and concentration

Rct
(ohms)

Cdl 
 (µ faraday)

Inhibition   efficiency (%)

1

Blank

 

32.13

0.14

 

2

01TS

0.2

167.75

0.19

80.85

0.4

332.28

0.10

90.33

1.0

541.12

0.17

94.06

3

02TS

0.2

107.52

0.48

70.12

0.4

161.49

0.15

80.10

1.0

356.79

0.11

90.99

4

03TS

0.2

89.82

0.17

64.29

0.4

257.06

0.83

87.50

1.0

341.22

0.71

90.58

The  data from
Table 3  shows  that  Rct value rises with  a rise in concentration of inhibitor. The
addition of inhibitor decreases the Cdl
values, because the absorption of inhibitors on the metal surface (expansion in
the surface courage of inhibitor). The impedance diagrams are almost semi-circular,
illustrate that the corrosion of mild-steel is chiefly inhibited by a charge transfer
method, and the existence of inhibitor does not influence the dissolution
mechanism of mild-steel 1 and 3. The data on inhibition efficiency attained
from impedance measurements are good agreements with the weight loss process
and potentiodynamic polarisation measurements

The inhibitor using
in the present research has two groups of anchoring sites, namely ring Nitrogen
and thiosemicarbazone group. Usually, these types of compounds exist either in
a boat or in chair conformation. The Alkyl substituted  2,6-diphenyl Piperidin-4-ones with thiosemicarbazone
chair conformation is the preferred conformation as phenyl and alkyl
substitutions are in equatorial orientations. The inhibition efficiency of
these compounds is higher. The interaction of these compounds with mild metal
surface could appear either over a carbonyl group or through ring nitrogen, but
not over  both of carbonyl group or
nitrogen as they are in para position to one another. The simultaneous
participation of both the groups is ruled out, because it would be possible
only through the attainment of boat conformation that is highly strained. Further,
the less electro negativity of nitrogen than sulfur support, ring nitrogen to
be the inhibiting site 16 and 20. The inhibition efficiency is further
increased by converting carbonyl group to thiosemicarbazone group. Here we also
expect the involvement of either thiosemicarbazone group or ring nitrogen because
of the presence of substitutes. The high inhibition may be on account of the
attraction between nitrogen and sulfur atom of thiosemicarbazone and metal
surface. The present investigation indicates that all the three corrosion
monitoring techniques complement with each other. Analysis of inhibition
efficiency of the three different inhibitors shows the following trends: (01TS)
> (02TS) > (03TS)

Adsorption
isotherm

             Organic molecules are helping to
inhibit corrosion as they adsorbed on the metal-solution integrates. The
adsorption anticipate on the chemical sharing of the solution, chemical skeleton
of the inhibitor, temperature, the environment of the metal surface, and
electrochemical potential at the metal-solution integrates. The adsorption procedure
provides awareness about the adsorbed molecules themselves in addition to their
cooperation with the metal surface. The standards of surface coverage (q ) equivalent to several
concentrations of an inhibitor (C)
are utilized to attain the finest adsorption isotherm. The q
values have been computed using the following relationships

             q  =  

                 (From weight loss process) and

             q  =    

           (From impedance method)

             The
Langmuir adsorption isotherm is indicated as q =   

    

             Where
C is the concentration and K is the equilibrium constant for the adsorption procedure.
The above equation might be modified as   

  =

  + C                                                                   

                                                                                                Figure 8

Langmuir isotherm for
the adsorption of compounds (01TS), (02TS), (03TS) on the surface of mild-steel
in

1 N H2SO4

             A straight line with slope equal to unit is obtained
when plotting C/? vs C when the experimental data follow
Langmuir adsorption isotherm. The existing study Langmuir adsorption isotherm
was observed throughout all the inhibitors on mild-steel in 1 N sulfuric acid.
Langmuir plots are disclosed in Figure 8 for which ? values are attained from weight loss measurements. Equilibrium
adsorption constants are intended from the Langmuir plot and specified in Table
(4). The high value of  K communicates that the inhibitor is
firmly adsorbed on mild-steel metallic surfaces.

Table 4

Equilibrium
constants from Langmuir adsorption isotherms

 

Sl.No

Name of the inhibitor

K

1

01TS

20.66

2

02TS

13.05

3

03TS

12.50

 Quantum chemical calculations

             Quantum chemical estimations were
shown from experimental results that it is possible to receive a better
performance with inhibitors as a corrosion organic inhibitor. It had been
expressed that the energy of highest occupied molecular orbital (EHOMO) regularly pooled with
the electron donating ability of the molecules. Greater values of EHOMO
mark a type of the molecule to donate electrons to perform with acceptor
molecules with empty molecular orbital or low energy unfilled.
Accordingly, the energy of lowest unoccupied molecular orbital (ELUMO) shows the intelligence
of the molecule to accept electrons. The least value of ELUMO suggests the molecule accepts electrons more possible.

             The electronic characteristics like
energy of the highest occupied molecular orbital (EHOMO) and the energy of the lowest unoccupied molecular
orbital (ELUMO), energy
gap (E) between EHOMO and ELUMO
on the backbone atoms observed by optimization. The optimized molecular
skeleton of inhibitor was given in figure 9. The HOMO and LUMO surfaces for
inhibitor given in figure 10. The electronic properties are detailed in table 4

Figure 9

Optimized molecular
structure of the inhibitors

·        
N-(amino-l2-methyl)nitrous amide compound with 1l3-ethane and
3-methyl-4-methylene-2,6-
diphenylpiperidine
(1:1:1)
 

N-(amino-l2-methyl)nitrous
amide compound with 1,3-dimethyl-4-methylene-2,6-diphenylpiperidine
and 1l3-ethane (1:1:1)
 

N-(amino-l2-methyl)nitrous
amide compound with 3-methyl-4-methylene-2,6-diphenylpiperidine
and l3-methane (1:1:1)
 

 

 

 

 

 

 

Figure 10

HOMO and LUMO surface
of the inhibitors(01TS, 02TS and 03TS)

Table 5

Quantum parameters of
(01TS), (02TS) and (03TS)

Quantum chemical parameters

01TS

02TS

03TS

EHOMO

-0.28202

-0.28038

-0.28200

ELOMO

0.10952

0.10774

0.10712

DE=
(ELOMO – EHOMO)

0.39154

0.38812

0.38912

I= – EHOMO (ev)

0.28202

0.28038

0.28200

A= -ELOMO

-0.10952

-0.10774

-0.10712

? (Debye)

3.5729

3.4763

3.2728

                               

                                   The results
seem to reveal that, charge transmission from the molecules of inhibitor takes
place during the adsorption of the inhibitor molecules to the metal surface. When
the chemisorption reaction appears, one among the reacting species perform as an
electron pair donor and the rest as an electron pair acceptor 22. It is clear
from Table 5 illustrates that the EHOMO
of inhibitors are almost the same. The conclusions are in excellent concurrent
with that of the experimental values. The energy gap (?E) is a valuable parametric quantity as a function of inhibitor
molecule reactivity in the direction of the adsorption on the metal face
20. As the energy gap (?E)
decreases, the reactivity of the molecule rises, dominant to rise in the
percentage (%) of inhibition efficiency (IE)
of the molecule. The least value of the energy gap (?E) will be supported a enhanced inhibition efficiency, because the
energy essential for the rejection of an electron from the finally occupied
orbital will be low 23.

                                The dipole moment
? (Debye) is a further value
electronic parameter in quantum chemical calculation studies that analysis from
non uniformed sharing of charges on the a mixture of atoms in the molecule. The
larger values of the dipole moment (?),
possibly increases the absorption between chemical compounds and metal surface
24. The energy of the deformability increases respectively, with the rise of
dipole moment ? (Debye) developing the
molecule easier to adsorb on the mild-steel metal surface. The inhibitor
molecules volume further rises with the increase of Debye. This increases the approaching area surrounded
by the molecule and the surface of the mild-steel in extension to the corrosion
inhibition capability of inhibitors.             

CONCLUSION:

1)      
The inhibition nature of
mild-steel metal in 1 N sulfuric acid by alkyl 
substituted 2,6-diphenyl piperidin-4-one with thiosemicarbazone has been
examined by quantum chemical calculation, AC impedance spectroscopy, weight
loss process and potentiodynamic polarization method.

2)      
The alkyl  substituted 2,6-diphenyl piperidin-4-one with thiosemicarbazones
present greatest efficiency towards inhibition of corrosion of mild-steel in 1 N
H2SO4 medium. This has possibly induced  by the progress of a complex on the mild-steel
metal surface at lower concentrations because adsorption on the surface.

3)       The
higher value corrosion inhibition efficiency (IE) of these inhibitor compounds
gained even at very low concentration of the inhibitor. The rate of corrosion inhibition gets decreased
with the increment in the inhibitor concentration.

4)       The adsorption of a number of alkyl substituted
2, 6-diphenyl piperidin-4-one with thiosemicarbazones at mild-steel corrosion in
acid solution (1 N H2SO4) pursued the Langmuir adsorption
isotherm model.

5)       The deviation of  Tafel constants ba and bc
and Ecorr values with the
rise in inhibitor concentration suggest that these compounds perform as an inhibitor
of mixed characteristics with more cathodic character. This is attested by the electrochemical
AC impedance spectroscopy, which shows a transform in the charge transfer
resistance and double layer capacitance signifying  the inhibitors adsorption on the mild-steel
surface. The some alkyl substituted 2,6-diphenyl piperidin-4-one with
thiosemicarbazone adsorb to the mild-steel surface mainly by chemisorption
mechanism.

6)      
Even though these
compounds containing ring nitrogen, the ultimate inhibition efficiency (IE) are
due to

>N-NH-CS-NH2.

7)      
Computed quantum
chemical characteristics such as HOMO-LUMO energy gap (?E) and dipole moment  (?) were formed in good correlation with
experimentally resolved corrosion  inhibition efficiency.

8)      
Analysis
of inhibition efficiency of the three distinctive inhibitors displays the
successive trends. (01TS) > (02TS) > (03TS).