SYNTHESIS OF SOME NEW 1, 2, 4-TRIAZOLE THEIR …shodhganga.inflibnet.ac.in/bitstream/10603/72595/8/chapter 6.pdf · THEIR ANTICORROSION PROPERTIES ON MILD STEEL IN HYDROCHLORIC ACID

Chapter VI Synthesis and anticorrosion property of triazoles

211

SYNTHESIS OF SOME NEW 1, 2, 4-TRIAZOLE DERIVATIVES AND

THEIR ANTICORROSION PROPERTIES ON MILD STEEL IN

HYDROCHLORIC ACID MEDIUM

6.1. Introduction

The presence of three nitrogen atoms in five-membered ring systems with

molecular formula C2H3N3 defines an interesting class of compounds, the triazole. This

may be of two types, the 1,2,3-triazoles and the 1,2,4-triazoles. 1,2,4-Triazoles are

amphoteric in nature, forming salts with acids as well as bases. 1,2,4-Triazole and its

derivatives are an important class of compounds which possess diverse agricultural,

industrial and biological properties, including anti-microbial, sedative, anticonvulsant,

anticancer, anti-inflammatory, diuretic, antibacterial, hypoglycemic, antitubercular and

antifungal. In recent years, the synthesis of these heterocyclic compounds has received

considerable attention. In addition to these diverse applications of many substituted

triazole derivatives over biological system, a number of triazole derivatives were found

application in corrosion inhibition of various metals.

Tao et al. [1] synthesized oxo-triazole derivative and studied its inhibiting action

on the corrosion of mild steel in sulphuric acid using weight loss, potentiodynamic

polarization, EIS and SEM. Wang [2] synthesized 3,5-Bis (n-pyridyl)-4-amino-1,2,4

triazoles and studied their corrosion inhibition performance on mild steel in phosphoric

acid solutions been investigated by weight loss and polarization methods. Corrosion

inhibition of mild steel in molar perchloric acid by 3,5-bis(n-pyridyl)-4-amino-1,2,4-

triazoles (n-PAT, n = 2, 3 and 4) was studied by Lebrini et al. [3] at 30 ºC using

gravimetric and electrochemical impedance spectroscopy techniques. A new corrosion

inhibitor namely, 3, 5-bis(2-thienyl)-4-amino-1,2,4-triazole has been synthesised and its

inhibiting action on the corrosion of mild steel in acid baths has been investigated by

Bentiss et al. [4] using various corrosion monitoring techniques such as corrosion

weight loss tests and electrochemical impedance spectroscopy.

Ramesh and Rajeswari [5] investigated the newly synthesised triazole

derivatives namely 3-benzylidene amino 1,2,4-triazole phosphonate, 3-para nitro

benylidene amino 1,2,4-triazole phosphonate, 3-salicylalidene amino 1,2,4- triazole

phosphonate on the corrosion of mild steel in acid medium. The corrosion inhibition

efficiencies of some triazole derivatives for steel in the presence of acidic medium have

http://en.wikipedia.org/wiki/Molecular_formula


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been studied by El Sayed et al. [6] using AM1, PM3, MINDO/3 and MNDO semi-

empirical SCF molecular orbital methods. Three triazole derivatives viz., (4-chloro-

acetophenone-O-1’-(1’,3’,4’-triazolyl)-metheneoxime, 4-methoxyl-acetophenone-O-1-

(1’,3’,4’-triazolyl)-metheneoxime and 4-fluoro-acetophenone-O-1’-(1’,3’,4’-triazolyl)-

metheneoxime have been synthesized by Li et al [7] as new inhibitors for the corrosion

of mild steel in acid media.

The corrosion behaviour of mild steel in 0.1 M HCl solution without and with 5-

amino-1,2,4-triazole, 5-amino-3-mercapto-1,2,4- triazole, 5-amino-3 methylthio-1,2,4-

triazole and 1-amino-3-methylthio-1,2,4-triazole was studied by Hassan [8] as a

function of immersion time and solution temperature. A comparative study of 5-amino-

1,2,4-triazole, 5-amino-3-mercapto-1,2,4-triazole, 5-amino-3-methylthio-1,2,4-triazole

and 1-amino-3-methylthio-1,2,4-triazole as corrosion inhibitors was investigated by

Hassan et al. [9] for mild steel in 0.1 M HCl solution at 20 ºC. The corrosion inhibition

of mild steel in 1 M HCl by 3,5-di(m-tolyl)-4-amino-1,2,4-triazole and 3,5-di(m-tolyl)-

4H-1,2,4-triazole has been investigated by Mehdi et al. [10] at 30 ºC using

electrochemical and weight loss measurements. Ouici et al. [11] reports the corrosion

inhibition of mild steel using 5-(2-methoxyphenyl)-1,2,4-triazole 3-thione and 5-(3-

methoxyphenyl)-1,2,4-triazole 3-thione in 5 % hydrochloric acid.

The effect of addition of 2 [5- 2-pyridyl -1,2,4-triazol-3-yl] phenol on mild steel

dissolution in 1 M hydrochloric acid was studied by Bentiss et al. [12] through

electrochemical impedance spectroscopy (EIS) , potentiodynamic polarisation curves

and gravimetric measurements. The inhibitive action of some thiadiazole derivatives,

namely 2,5-bis(2-thienyl)-1,3,4-thiadiazole and 2,5-bis(3-thienyl)-1,3,4- thiadiazole

against the corrosion of mild steel in 0.5 M H2SO4 solution has been investigated by

Lebrini et al. [13] using weight loss measurements, Tafel polarisation and

electrochemical impedance spectroscopy (EIS) techniques. The inhibition effect of 3,5-

diphenyl-4H-1,2,4-triazole on the corrosion of mild steel in hydrochloric acid solution

was investigated by Bentiss et al. [14] at 30 ºC using electrochemical and weight loss

measurements. Two triazole derivatives namely 1-[2-(4-nitro-phenyl)-5[1,2,4]triazol-1-

yl-methyl-[1,3,4]oxadiazol-3-yl]-enthanone and 1-(4-methoxy-phenyl)-2-(5-

[1,2,4]triazol-1-yl-methyl-4H[1,2,4]triazol-3-ylsulfanyl)-ethanone were studied by

Zhang et al.[15] as corrosion inhibitors on mild steel in 1 M hydrochloric acid solution.


213

Bentiss et al.[16] reports the use of some 4H-triazole derivatives, namely 3,5-

diphenyl-4H-1,2,4-triazole, 3,5-bis(4-pyridyl)-4H-1,2,4- triazole and 3,5-bis(4

methyltiophenyl)-4H-1,2,4-triazole for the dissolution protection of mild steel in normal

hydrochloric acid solution. The corrosion inhibition properties of three triazoles

derivatives, namely 4-amino-5-phenyl-4H-1, 2, 4-triazole-3-thiol, 4-amino-5-(2-

hydroxy) phenyl-4H-1, 2, 4-triazole-3-thiol and 4-amino-5-styryl-4H-1,2,4-triazole-3

thiol, on mild steel corrosion in 1 M HCl solution was studied by Quraishi et al. [17]

using electrochemical and gravimetric methods. Thus, in the light of above, the present

chapter reports the antioxidant activity and corrosion inhibition behaviour of newly

synthesized compounds such as 8-bromo-5-morpholino-3-(4-propylphenyl)-

[1,2,4]triazolo[4,3-c]pyrimidine (8-BMPTP), 8-bromo-3-(2-fluoro-3-methoxyphenyl)-

5-morpholino-[1,2,4]triazolo[4,3 c]pyrimidine (8-BFMMTP) and 8-bromo-3-(2-fluoro-

4,5-dimethoxy-phenyl)-5-morpholin-4-yl-[1,2,4]triazolo[4,3-c]pyrimidine (8-

BFDMTP). The synthesized compounds were characterized by FTIR, elemental

analyses and 1H-NMR spectral studies. Antioxidant activity was studied by assaying

diphenylpicrylhydrazyl (DPPH), nitric oxide and hydroxyl radicals. Mass loss and

electrochemical techniques were used to elucidate the corrosion inhibition mechanism

of 8-BMPTP, 8-BFMMTP and 8-BFDMTP on MS in 0.5 M HCl. The thermodynamic

activation and adsorption parameters were calculated and discussed.

6. 2. Synthesis of inhibitors

6.2.1. Synthesis of 1-(5-bromo-2-chloropyrimidin-4-yl)hydrazine (2)

A solution of 5-bromo-2,4-dichloropyrimidine (1) (0.01 mol) in ethanol was

taken and cooled to 0–5 °C in an ice bath. Trietheylamine (0.01 mol) was added to the

cold reaction mixture and then hydrazine hydrate (0.02 mol) was added slowly at 5-10

°C. The reaction mass was allowed to stir at room temperature for 1h. The solid thus

obtained was filtered, washed with chilled water and dried to afford compound 2. 1H

NMR (DMSO-d6, 400 MHz) δ: 8.06 (s, 1H, NH), 7.85 (s, 1H, py-H), 4.34 (s, 2H, NH2).

6.2.2. Synthesis of 1-(5-bromo-2-morpholinopyrimidin-4-yl) hydrazine (3)

A solution of 1-(5-bromo-2-chloropyrimidin-4-yl)hydrazine (2) (0.01 mol) in

ethyl acetate (50 ml) was taken and morpholine (0.021 mol) was added to it. The

contents were refluxed on a water bath for 1 h. The solvent was evaporated on a steam


214

bath, water was added into crude mass and stirred for 15 min. The solid thus obtained

was filtered, washed with chilled water and dried to afford compound 3. 1H NMR

(DMSO-d6, 400 MHz) δ: 8.06 (s, 1H, py-H), 7.85 (s, 1H, NH), 4.38 (s, 2H, NH2), 3.63-

3.60(m, 8H, 4CH2).

6.2.3. 2-(4-Propylbenzylidene)-1-(5-bromo-2-morpholinopyrimidin-4-yl) hydrazine

(4a)

The product obtained (3, 2.74 g, 0.01 mol) and 4-propylbenzaldehyde (1.48 g,

0.01 mol) were taken in EtOH and stirred at room temperature about 1 h. The reaction

completion was confirmed by TLC. 1H NMR (DMSO-d6, 400 MHz) δ: 10.39 (s, 1H,

NH), 8.40 (s, 1H, Py-H), 8.09 (s, 1H, CH), 7.59-7.56 (d, 2H, Ar-H), 7.25-7.23 (d, 2H,

Ar-H, J = 9.0 Hz), 3.66-3.64 (m, 8H, 4CH2), 2.58-2.48 (t, 2H, CH2), 1.62-1.54 (m, 2H,

CH2), 0.90 (t, 3H, CH3).

6.2.4. 2-(2-Fluoro-3-methoxybenzylidene)-1-(5-bromo-2-morpholinopyrimidin-4-yl)

hydrazine (4b)

The product obtained (3, 2.74 g, 0.01 mol) and 2-fluoro-3-methoxybenzaldehyde

(1.54 g, 0.01 mol) were taken in EtOH and stirred at room temperature about 1 h. The

reaction completion was confirmed by TLC. 1H NMR (DMSO-d6, 400 MHz) δ: 10.67

(s, 1H, NH), 8.69 (s, 1H, Py-H), 8.12 (s, 1H, CH), 7.48 (t, 1H, Ar-H), 7.17-7.15 (d, 2H,

Ar-H, J = 9.0 Hz), 3.84 (s, 3H, CH3), 3.66-3.31 (m, 8H, 4CH2).

6.2.5. 2-(2-Fluoro-5-methoxybenzylidene)-1-(5-bromo-2-morpholinopyrimidin-4-yl)

hydrazine (4c)

The product obtained (3, 2.74 g, 0.01 mol) and 2-fluoro-5-methoxybenzaldehyde

(1.54 g, 0.01 mol) were taken in EtOH and stirred at room temperature about 1 h. The

reaction completion was confirmed by TLC. 1H NMR (DMSO-d6, 400 MHz) δ: 10.76

(s, 1H, NH), 8.62 (s, 1H, Py-H), 8.13 (s, 1H, CH), 7.41 (d, 1H, Ar-H, J=9.0 Hz), 7.19 (s,

1H, Ar-H), 7.00 (d, 1H, Ar-H, J = 9.0 Hz), 3.75 (s, 3H, CH3), 3.67-3.62 (m, 8H, 4CH3).


215

6.2.6. 8-Bromo-5-morpholino-3-(4-propylphenyl)-[1,2,4]triazolo[4,3-c]pyrimidine (8-

BMPTP)

The product obtained (4a, 4.04 g) and iodobenzene diacetate (IBD) (3.86 g)

were heated at 15-20 °C for 2h. The reaction completion was confirmed by TLC. Yield:

74 %. m.p.: 139-141 °C. FT-IR (KBr, cm−1

) ν: 2959 (C-H), 1648 (C=N), 1463 (C=C),

1376 (C-N), 1112 (C-O), 519 (C-Br). 1H NMR (DMSO-d6, 400 MHz) δ: 7.85 (s, 1H,

Py-H), 7.64-7.61 (d, 2H, Ar-H, J = 12 Hz), 7.36-7.33 (d, 2H, Ar-H, J = 12 Hz), 3.27-

3.02 (m, 8H, 4CH2), 2.73-2.68 (t, 2H, CH2), 1.74-1.67 (m, 2H, CH2), 1.01-0.96

(t,3H,CH3). MS: m/z, M+ 404.0. Anal. calcd. for C18H20BrN5O (in %): C, 53.74; H,

5.01; N, 17.41. Found C- 53.55, H-5.19, N-17.56.

6.2.7. 8-Bromo-3-(2-fluoro-3-methoxyphenyl)-5-morpholino-[1,2,4]triazolo[4,3-c]

pyrimidine (8-BFMMTP)

The product obtained from (4b, 4.10 g) and iodobenzene diacetate (3.86 g) were

heated at 15-20 °C for 2h. The reaction completion was confirmed by TLC. Yield: 84

%. m.p.: 188-190 °C. FT-IR (KBr, cm−1

) ν: 2922 (C-H), 1648 (C=N), 1463 (C=C), 1376

(C-N), 1305 (C-F), 1119 (C-O), 518 (C-Br). 1H NMR (DMSO-d6, 400 MHz) δ: 7.90 (s,

1H, Py-H), 7.37 (d, 1H, Ar-H, J = 8.0 Hz), 7.33 (m, 1H, Ar-H, J = 8.0 Hz), 7.28 (m, 1H,

Ar-H), 3.97 (s, 3H, OCH3), 3.34 (s, 2H, CH2), 3.00 (s, 6H, CH2). MS: m/z, M+ 408.0.

Anal. calcd. for C16H15BrFN5O2 (in %): C, 47.07; H, 3.70; N, 17.16. Found C- 47.23,

H-3.62, N-17.32.

6.2.8. 8-Bromo-3-(2-fluoro-4,5-dimethoxy-phenyl)-5-morpholin-4-yl-[1,2,4]triazolo

[4,3-c]pyrimidine (8-BFDMTP)

The product obtained from (4c, 4.10 g) and iodobenzene diacetate (3.86 g) were

heated at 15-20 °C for 2h. The reaction completion was confirmed by TLC. Yield: 75

%. m.p.: 194-196 °C. FT-IR (KBr, cm−1

) ν: 2926 (C-H), 1638 (C=N), 1462 (C=C), 1376

(C-N), 1304 (C-F), 1112 (C-O), 515 (C-Br). 1H NMR (DMSO-d6, 400 MHz) δ: 7.90 (s,

1H, Py-H), 7.30 (s, 1H, Ar-H), 7.18 (s, 1H, Ar-H), 3.95 (s, 3H, OCH3) 3.87 (s, 3H,

OCH3), 3.37-3.05 (m, 8H, 4CH2). MS: m/z, M+ 437. Anal. calcd. for C16H15BrFN5O2

(in %): C, 47.07; H, 3.70; N, 17.16. Found C- 47.15, H-3.66, N-17.26.


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8-Bromo-5-morpholino-3-aryl-1,2,4-triazolo[4,3-f]pyrimidines were prepared

by the method summarized in Scheme 6.1.

Scheme 6.1: Scheme for the synthesis of 8-bromo-5-morpholino-3-aryl-1,2,4-

triazolo[4,3-f]pyrimidines . Reagents and conditions: (i) NH2NH.H2O, MeOH, TEA, 5-

10 °C, 1 h. (ii) morpholine, EA, r.t., 1 h. (iii) ArCHO, EtOH, r.t., 1 h. (iv) IBD, MeOH,

15-20 °C, 2 h.

6.3. Results and discussion

6.3.1. Characterization of inhibitors

The synthesized inhibitors were purified and characterized by FTIR, 1H NMR,

13C NMR and Mass spectral studies. The

1H NMR spectrum of 8-BFMMTP is shown in

Fig. 6.1. The 1H NMR spectra of 8-BFMMTP showed multiplet at around 3.34 - 3.00 is

due to eight protons of 4 CH2 groups. The singlet at δ 3.97 is due to three protons of

benzene ring protons. The presence of multiplets at δ 7.26 is due to one aromatic ring

proton. The presence of doublet peak at 7.30 and 7.37 is due to two aromatic ring

protons. The singlet at δ 7.90 is due to one proton of pyridine ring. The LC-MS of 8-

BFDMTP (Fig. 6.2) showed molecular ion peak which is in agreement with the

molecular formula.

The FTIR spectrum of 8-BFMMTP is shown in Fig. 6.3. The absorption band

around 2926 cm-1

is assigned to the C–H stretch. The strong bands at around 1638 cm-1

and 1462 cm-1

are assigned to C=N and C=C stretch, respectively. New bands appeared


217

at 1376 cm-1

, 1304 cm-1

and 1112 cm-1

corresponding to C-N, C–F and C-O stretching

frequencies. The clear band at 515 cm-1

is due to C-Br stretch which confirmed the

synthesis of 8-BFDMTP inhibitor.

The spectral data (FTIR, 1H NMR and mass) of all the synthesized compounds

are in full agreement with the proposed structures. The elemental analyses data showed

good agreement between the experimentally determined values and the theoretically

calculated values within ±0.4 %.

Fig

. 6.1

: 1H

NM

R s

pec

trum

of

8-b

rom

o-3

-(2

-flu

oro

-3-m

ethoxyphen

yl)

-5-m

orp

holi

no

-[1,2

,4]t

riaz

olo

[4,3

-

c]pyri

mid

ine

(8-B

FM

MT

P).


218

Fig. 6.2: LCMS of 6-bromo-2-(3, 4-dimethoxy-phenyl)-1H-benzoimidazole (8-

BFDMTP).

Fig. 6.3: FTIR spectrum of 8-bromo-3-(2-fluoro-3-methoxyphenyl)-5-morpholino

[1,2,4]triazolo[4,3-c]pyrimidine (8-BFMMTP).

N

N

NN

N

O

OF

Br

N

N

NN

N

O

O

F

Br

O


219

6.3.2. Antioxidant activity

The in vitro scavenging assay was performed spectrophotometrically with BHT

as positive control. Natural antioxidants are characterized by their ability to scavenge

free radicals. Proton-radical scavenging action is an important attribute of antioxidants,

which is measured by the DPPH• scavenging assay. DPPH, a protonated radical has

significant absorbance maxima at 517 nm which decreases in the presence of

antioxidant due to the scavenging of the proton radical [18]. Hydrogen donating ability

of the antioxidant molecule contributes to its free radical scavenging potential. The

DPPH radical scavenging activity shown by the synthesised compounds is because of

its H- donating capacity.

Hydroxyl radical (OH•) scavenging assay shows the ability of the compounds

and standard BHT to inhibit hydroxyl radical mediated deoxyribose degradation in an in

vitro system that consists of Fe3+

-EDTA-ascorbate and H2O2. In this system, hydroxyl

radicals generated by the Fenton’s reaction attack deoxyribose and degrade into

fragments that react with thiobarbituric acid (TBA) on heating to form a pink colour.

Hydroxyl radical (OH•) scavenging capacity of the compounds is directly related to its

antioxidant activity.

In addition to reactive oxygen species, nitric oxide is also used to test the

antioxidant activity of the synthesised compounds. Sodium nitroprusside (SNP) in

aqueous solution at physiological pH spontaneously generates nitric oxide which

interacts with oxygen to produce nitrite ions that can be estimated using Griess reagent.

Scavengers of nitric oxide compete with oxygen, leading to the reduced production of

nitrite ions. Suppression of the released NO may be partially attributed to direct NO

scavenging. The in vitro antioxidant activity of 8-BMPTP, 8-BFMMTP, and 8-

BFDMTP are determined spectrophotometrically by DPPH•, HO• and NO

• scavenging

activity and the results are givin in Table 6.1. The synthesised compounds showed good

radical scavenging activity in all the antioxidant scavenging methods and are

comparable with BHT. However, increase in activity was marginal with increase in

concentration.


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Table 6.1. IC50 values for evaluated antioxidant assays of 8-BMPTP, 8-BFMMTP, and

8-BFDMTP

Compounds IC50 (μg/mL)

DPPH• HO• NO

•

8-BMPTP 14.2±14 15.6±21 14.3±42

8-BFMMTP 14.0±23 14.3±11 14.0±18

8-BFDMTP 13.8±64 14.0±19 13.6±14

AAa 11.8±0.44 - -

BHAb - 13.1±0.56 -

6.3.3. Weight loss measurements

6.3.3.1. Effect of inhibitor concentration

The corrosion inhibition efficiencies (IE %) of the inhibitors 8-BMPTP, 8-

BFMMTP and 8-BFDMTP after 6 h of immersion at 30 – 60 ºC are evaluated by

weight loss method are listed in the Table. 6.2. From Table 6.2, it is apparent that the

inhibition efficiency increased with increasing concentration (0.1 g/L – 0.5 g/L) of each

inhibitor. This observation can be attributed to an increase in the number of inhibitor

molecules adsorbed on the metal surface, which separate the mild steel from the acid

solution, resulting in the retardation of metal dissolution [19]. The inhibition efficiency

of the inhibitors followed the order 8-BMPTP >8-BFMMTP > 8-BFDMTP. The

corrosion rate (CR) was calculated from the following equation:

(6.1)

where, ΔW is the weight loss (gm cm-2

h-1

), S is the surface area of the specimen (cm2)

and t is the immersion time (h). The corrosion inhibition efficiency IE (%) was

calculated according to the equation (6.2)

(6.2)

where (CR)a and (CR)p are corrosion rates in the absence and presence of the inhibitor,

respectively. The corrosion parameters namely, the corrosion rate (CR), and inhibition

efficiency IE (%) of mild steel in 0.5 M HCl in the presence and absence of inhibitors at


221

different temperatures obtained from weight loss measurements are listed in Table 6.2.

The inhibitor was found to attain the maximum inhibition efficiency at 0.5 g/L for all

the studied inhibitors. This is due to the fact that, adsorption and the degree of surface

coverage of inhibitor on the mild steel increases with the inhibitor concentration, thus

the mild steel surface gets efficiently separated from the medium [20]. The protective

property of these compounds is probably due to the interaction between π-electrons and

hetero atoms with positively charged steel surface [21]. In the absence of any inhibitor,

the corrosion rate of mild steel increased sharply with the increase in temperature (30 –

60 ºC), whereas in the presence of inhibitors, the corrosion rate variably decreases for

all three studied inhibitors. The corrosion rate was much lower in the presence of

inhibitor than in its absence at each temperature.


222

Table 6.2. Weight loss data of mild steel corrosion in 0.5 M HCl in the presence of different concentrations of the inhibitors at different

temperature

Inhibitor C

(mM)

30 °C 40 °C 50 °C 60 °C

CR

(mg cm-2 h-1)

IE

(%)

CR

(mg cm-2 h-1)

IE

(%)

CR

(mg cm-2 h-1)

IE

(%)

CR

(mg cm-2 h-1)

IE

(%)

8-BMPTP Blank 0.452 - 0.765 - 0.796 - 1.260 -

0.1 0.189 58.2 0.582 55.4 0.384 51.7 0.639 49.3

0.2 0.159 64.8 0.648 61.4 0.325 59.2 0.572 54.6

0.3 0.118 73.9 0.739 67.9 0.269 66.2 0.453 64.0

0.4 0.102 77.4 0.774 77.0 0.209 73.7 0.421 66.6

0.5 0.064 85.8 0.858 82.0 0.192 75.9 0.351 72.1

8-

BFMMTP

0.1 0.115 74.6 0.218 71.5 0.262 67.1 0.469 62.8

0.2 0.105 76.8 0.196 74.4 0.241 69.7 0.442 64.9

0.3 0.103 77.2 0.181 76.3 0.221 72.2 0.439 65.1

0.4 0.085 81.2 0.166 78.3 0.195 75.5 0.398 68.4

0.5 0.061 86.5 0.124 83.8 0.172 78.4 0.335 73.4

8-

BFDMTP

0.1 0.086 80.9 0.178 76.7 0.222 72.1 0.429 65.9

0.2 0.071 84.3 0.156 79.6 0.201 74.7 0.412 67.3

0.3 0.063 86.1 0.141 81.6 0.181 77.3 0.384 69.5

0.4 0.045 90.0 0.126 83.5 0.156 80.4 0.361 71.3

0.5 0.031 93.1 0.094 87.7 0.132 83.4 0.285 77.4


223

6.3.3.2. Thermodynamic and activation parameters

Thermodynamic and activation parameters play important role in understanding

the inhibition mechanism. The weight loss measurements were performed in the

temperature range of 30 – 60 ºC in the absence and presence of different concentrations

of inhibitors (0.1 g/L – 0.5g/L) during 6h of immersion time in 0.5 M HCl for mild

steel. The CR gets increased with the rise in temperature in the uninhibited solution, but

in the presence of inhibitor, CR gets highly reduced (Table 6.2). Hence, inhibition

efficiency decreases with the rise in temperature. It may be due to the fact that, higher

temperature accelerates hot-movement of the organic molecules and weakens the

adsorption capacity of inhibitor on the metal surface [22, 23].

The activation energy (Ea*) for dissolution of MS can be expressed using the following

Arrhenius equation:

(6.3)

An alternative formulation of the Arrhenius equation is,

(6.4)

where, k is Arrhenius pre-exponential factor, h is Planck’s constant, N is Avogadro’s

number, T is the absolute temperature and R is the universal gas constant. Using

equation (6.3), and from a plot of the ln CR versus 1/T (Figs. 6.4 - 6.6), the values of Ea*

and k at various concentrations of 8-BMPTP, 8-BFMMTP and 8-BFDMTP were

computed from slopes and intercepts, respectively (Table 6.3). Further, using equation

(6.4), plots of ln (CR/T) versus 1/T gave straight lines (Figs. 6.7 – 6.9) with a slope of (-

∆Ha*/2.303R) and an intercept of [log (R/Nh) + ∆Sa

*/2.303R], from which the values of

∆Ha* and ∆Sa

* were calculated and are listed in Table 6.3. Generally, the values of the

activation energy for the inhibited solutions are lower than that of the uninhibited

solution, indicating a chemisorption process of adsorption [24], whereas higher values

of Ea* indicates a physical adsorption mechanism [25]. In the present study, the values

of Ea* in inhibited solution are increases when compared to uninhibited acid solutions

(Table 6.3). This supports physisorption of 8-BMPTP, 8-BFMMTP and 8-BFDMTP on

mild steel surface.


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The positive sign of the activation enthalpy (∆Ha*) reflects the endothermic

nature of the mild steel dissolution process [26, 27]. The negative value of ∆Sa* for all

the three inhibitors indicates that the formation of the activated complex in the rate-

determining step represents an association rather than a dissociation step, meaning that a

decrease in disorder takes place during the course of the transition from reactants to

activated complex [28].

Fig. 6.4: Arrhenius plots for the corrosion of mild steel in 0.5 M HCl in the absence and

presence of different concentrations of 8-BMPTP.


presence of different concentrations of 8-BFMMTP.

-9

-8.5

-8

-7.5

-7

-6.5

-6

-5.5

-5

2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35

ln C

R/T

(mg c

m-2

h-1

K-1

)

103/T (K-1)

8-BMPTP

Blank

0.1 g/L

0.2 g/L

0.3 g/L

0.4 g/L

0.5 g/L

-0.004

-0.004

-0.003

-0.003

-0.002

-0.002

-0.001

-0.001

0.000

0.001

2.90 3.00 3.10 3.20 3.30 3.40

ln C

R (

mg c

m-2

h-1

)

103/T (K-1)

8-BFMMTP

Blank

0.1 g/L

0.2 g/L

0.3 g/L

0.4 g/L

0.5 g/L


225


presence of different concentrations of 8-BFDMTP.

Fig. 6.7: Alternative Arrhenius plots for mild steel corrosion in 0.5 M HCl in the

absence and presence of different concentrations of 8-BMPTP.

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35

ln C

R(m

g c

m-2

h-1

)

103/T(K-1)

8-BFDMTP

Blank

0.1 g/L

0.2 g/L

0.3 g/L

0.4 g/L

0.5 g/L

-9

-8.5

-8

-7.5

-7

-6.5

-6

-5.5

-5

2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35

ln C

R/T

(mg c

m-2

h-1

K-1

)

103/T (K-1)

8-BMPTP

Blank

0.1 g/L

0.2 g/L

0.3 g/L

0.4 g/L

0.5 g/L


226


absence and presence of different concentrations of 8-BFMMTP.


absence and presence of different concentrations of 8-BFDMTP.

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

2.90 3.00 3.10 3.20 3.30 3.40

ln C

R/T

(m

g c

m-2

h-1

K-1

)

103/T (K-1)

8-BFMMTP Blank

0.1 g/L

0.2 g/L

0.3 g/L

0.4 g/L

0.5 g/L

-11

-10

-9

-8

-7

-6

-5

-4

2.9 3.0 3.1 3.2 3.3 3.4

ln C

R/T

(m

g c

m-2

h-1

K-1

)

103/ T(K-1)

8-BFDMTP

Blank

0.1 g/L

0.2 g/L

0.3 g/L

0.4 g/L

0.5 g/L


227

Table 6.3. Activation parameters for mild steel in 0.5 M HCl in the absence and

presence of different concentrations of 8-BMPTP, 8-BFMMTP and 8-BFDMTP

Inhibitor C

(mM)

E*a

(kJ mol-1)

k

(mg cm-2 h-1)

∆Ha*

(kJ mol-1)

∆Ha*=E a

*-RT

(kJ mol-1)

∆Sa*

(J mol-1 K-1)

8-BMPTP 0 26.15 15336 23.50 23.38 -173

0.1 42.28 1760309 39.64 39.52 -134

0.2 46.39 7422303 43.75 43.62 -122

0.3 47.59 10574965 44.95 44.82 -119

0.4 54.27 112560340 51.63 51.50 -99

0.5 58.83 480337721 56.19 56.06 -87

8-

BFMMTP

0 26.15 15336 23.50 23.38 -173

0.1 36.93 279288 34.29 34.16 -149

0.2 37.90 369904 35.26 35.13 -147

0.3 38.08 377377 35.44 35.31 -146

0.4 40.16 733072 37.52 37.39 -141

0.5 45.61 4583621 42.97 42.84 -126

8-

BFDMTP

0 26.15 15336 23.50 23.38 -173

0.1 31.68 57584 29.04 28.91 -162

0.2 33.04 83116 30.40 30.27 -159

0.3 34.73 125617 32.09 31.96 -156

0.4 37.02 244507 34.38 34.25 -150

0.5 45.66 5060622 43.02 42.90 -125

6.3.3.3. Adsorption isotherm

The basic information on the interaction between an organic inhibitor and a mild

steel surface can be obtained from various adsorption isotherms. The most commonly

used adsorption isotherms are the Langmuir, Temkin and Frumkin isotherms. The

surface coverage (θ) for different concentrations of inhibitor in 0.5 M hydrochloric acid

was tested graphically to determine a suitable adsorption isotherm. Plots of C/θ versus

C yielded straight lines (Figs. 6.10 – 6.12) with correlation co-efficient (R2) values close

to unity. This indicates that the adsorption of these inhibitors can be fitted to the


228

Langmuir adsorption isotherm. According to Langmuir adsorption isotherm there is no

interaction between the adsorbed inhibitor molecules, and the energy of adsorption is

independent on the degree of surface coverage (θ). Langmuir isotherm assumes that the

solid surface contains a fixed number of adsorption sites and each site occupies one

adsorbed species. According to this isotherm, θ is related to the inhibitor concentration,

C and adsorption equilibrium constant Kads, as

(6.5)

From the intercepts in Figs. 6.10 – 6.12, the values of Kads were calculated. The large

values of Kads obtained for all three studied inhibitors imply efficient adsorption, and

hence, good corrosion inhibition efficiency. Using the calculated values of Kads, ΔGads

was evaluated according to the equation (6.6).

(6.6)

where R is the gas constant and T is the absolute temperature (K). The value of 55.5 is

the concentration of water in solution (mol L-1

). Using the plot of ∆Gads vs T (Fig. 6.13)

the values of ∆Sads and ∆Hads were computed from slope and intercept, respectively. The

calculated ∆Gads, ∆Sads and ∆Hads values of the studied 1,2,4-triazole derivatives are

tabulated in Table 6.4.

It is generally accepted that, the values of ΔGads up to -20 kJ/mol, the adsorption

can be regarded as physisorption, in which case inhibition results from the electrostatic

interaction between the charged molecules of the inhibitors and the charged metallic

surface. In contrast, for values above -40 kJ/mol, the adsorption is regarded as

chemisorption which is due to charge sharing or transfers from the inhibitor molecules

to the metal surface to form a covalent bond [29]. The values of ΔSads and ΔHads give

information about the mechanism of corrosion. The negative value of ΔHads indicates

that adsorption process is exothermic. An exothermic adsorption process may be

chemisorption or physisorption or mixture of both [30], whereas endothermic process is

attributed to chemisorption [31]. In exothermic adsorption process, physisorption can be

distinguished from chemisorption on the basis of ΔHads values. For physisorption

process, the magnitude of ΔHads is around - 40 kJ mol-1

or less negative while its value

-100 kJ mol-1

or more negative for chemisorption [32].


229

In the present work, the calculated ∆Gads values (Table 6.4) lie between -16.2 to

-19.9 indicated that the adsorption mechanism of the synthesized 1, 2, 4-triazole

derivatives on mild steel in 0.5 M HCl solution is physisorption. The negative values of

ΔHads again confirmed that 1, 2, 4-triazole derivatives adsorbed on the mild steel surface

through physisorption. The values of ∆Sads are negative for all the inhibitors implies

that the activated complex in the rate determining step represents an association rather

than a dissociation step, meaning that a decrease in disordering takes place on going

from reactants to the activated complex [33].

Fig. 6.10: Langmuir isotherm for the adsorption of 8-BMPTP on mild steel in 0.5 M

HCl at different temperatures.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6

C/θ

C (g/L)

8-BMPTP

303 K

313 K

323 K

333 K


230

Fig. 6.11: Langmuir isotherm for the adsorption of 8-BFMMTP on mild steel in 0.5 M


Fig. 6.12: Langmuir isotherm for the adsorption of 8-BFMMTP on mild steel in 0.5 M


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6

C/θ

C (g/L)

8-BFMMTP

303 K

313 K

323 K

333 K

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6

C/θ

C(g/L)

8-BFMMTP

303 K

313 K

323 K

333 K


231

Fig. 6.13: Plots of ΔG ads vs. absolute temperature of 8-BMPTP, 8-BFMMTP and 8-

BFDMTP.

Table 6.4. Thermodynamic parameters for adsorption of 8-BMPTP, 8-BFMMTP and 8-

BFDMTP on mild steel in 0.5 M HCl at different temperatures from Langmuir

adsorption isotherm

-21

-20

-19

-18

-17

-16

-15

-14

-13

-12

300 305 310 315 320 325 330 335

ΔG

(k

J m

ol-1

)

T(K)

8-BMPTP 8-BFMMTP 8-BFDMTP

Inhibitor T

(K)

R2 Kads

(L mol-1

)

∆Gads

(kJ mol-1

)

∆Hads

(kJ mol-1

)

∆Sads

(J mol-1

K-1

)

303 0.998 35714 -19.12 -25.20 -11.70

8-BMPTP 313 0.997 36363 -19.80

323 0.997 30395 -19.95

333 0.992 23980 -19.91

303 0. 993 28735 -18.57 -42.50 -4.80

8-BFMMTP 313 0.995 28490 -19.16

323 0.997 26595 -19.59

333 0.992 23419 -19.85

303 0.987 11325 -16.23 -42.40 -3.31

8-BFDMTP 313 0.984 10060 -16.46

323 0.993 10373 -17.07

333 0.991 9813 -17.44


232

6.3.4. Potentiodynamic polarization

The potentiodynamic polarization curves obtained from the corrosion behaviour

of mild steel in 0.5 M HCl in the absence and presence of 8-BMPTP, 8-BFMMTP and

8-BFDMTP are shown in Figs. 6.14 - 6.16. The electrochemical parameters such as

corrosion potential (Ecorr), corrosion current density (Icorr), Tafel slopes i.e., cathodic (bc)

and anodic (ba) obtained from the polarization measurements are listed in Table 6.5.

The IE % was calculated using the following equation:

(6.7)

where (Icorr)a and (Icorr)p are the corrosion current density (mA cm-2

) in the absence and

presence of the inhibitor, respectively. From the potentiodynamic polarization curves, it

can be clearly seen that Icorr decreases and IE % increases with increasing inhibitor

concentration. The cathodic and anodic Tafel slope values changed with the inhibitor

concentration, indicating that 1,2,4-triazole derivatives controlled both the cathodic

hydrogen evolution and anodic mild steel dissolution reactions [34]. The corrosion

potential Ecorr values do not show any appreciable shift i.e., not more than 85 mV with

respect to the corrosion potential of blank solution, which suggest that all the three

inhibitors acted as mixed type [35]. From the Tafel slopes it is evident that cathodic

reaction is more polarized when an external current density is applied (bc >ba), which

indicates more pronounced cathodic inhibition. Among the synthesized 1,2,4-triazole

derivatives, 8-BMPTP shows highest inhibition efficiency. The inhibition efficiency of

the studied inhibitors follows the order, 8-BMPTP > 8-BFMMTP > 8-BFDMTP.


233

Fig. 6.14: Potentiodynamic polarization curves for MS in 0.5 M HCl containing

different concentration of 8-BMPTP.


different concentration of 8-BFMMTP.

-8

-7

-6

-5

-4

-3

-2

-1

0

-1 -0.8 -0.6 -0.4 -0.2 0

log i

(A

cm

2)

Ecorr Vs SCE (V)

8-BMPTP

Blank 0.1g/L 0.2 g/L 0.3 g/L 0.4 g/L 0.5 g/L

-8

-7

-6

-5

-4

-3

-2

-1

0

-1 -0.8 -0.6 -0.4 -0.2 0

log i

(A

cm

2)

Ecorr Vs SCE (V)

8-BFMMTP

Blank 0.1 g/L 0.2g/L 0.3 g/L 0.4 g/L 0.5 g/L


234


different concentration of 8-BFDMTP.

-8

-7

-6

-5

-4

-3

-2

-1

0

-1 -0.8 -0.6 -0.4 -0.2 0

log

i (

A c

m2)

Ecorr Vs SCE (V)

8-BFDMTP

Blank 0.1 g/L 0.2 g/L 0.3 g/L 0.4 g/L 0.5 g/L


235

Table 6.5. Potentiodynamic polarization parameters for the corrosion of mild steel in

0.5 M HCl in absence and presence of different concentrations of 8-BMPTP, 8-

BFMMTP and 8-BFDMTP inhibitors at 30º C

6.3.5. Electrochemical impedance spectroscopy

Nyquist plots of mild steel in 0.5 M HCl solution in the absence and presence of

different concentrations of 8-BMPTP, 8-BFMMTP and 8-BFDMTP at 30 ºC are shown

in Figs 6.17 - 6.19. Nyquist impedance plots were analyzed by fitting the experimental

data to a simple circuit model (Fig. 6.20) that includes the solution resistance (Rs),

charge transfer resistance (Rct) and double layer capacitance (Cdl). The values are

presented in Table 6.6. The IE % was calculated using the charge transfer resistance by

the following expression:

Inhibitor C

(mM)

Ecorr

(mV )

Icorr

(mA cm-2

)

ba

(mV dec-1

)

bc

(mV dec-1

)

IE

(%)

Blank -496 0.2730 13.155 9.909 -

0.1 -496 0.0543 7.879 15.411 80.11

0.2 -489 0.0432 9.677 14.383 84.17

8-BMPTP 0.3 -461 0.0383 4.674 18.513 85.95

0.4 -453 0.0300 8.708 10.374 88.99

0.5 -237 0.0139 4.629 7.772 94.92

0.1 -488 0.0676 7.855 15.231 75.22

0.2 -432 0.0612 4.251 18.143 77.57

8-BFMMTP 0.3 -481 0.0572 6.539 15.338 79.02

0.4 -466 0.0509 5.478 17.024 81.33

0.5 -441 0.0357 4.381 16.103 86.90

0.1 -455 0.0850 7.01 17.162 59.20

0.2 -464 0.0933 5.742 15.677 65.80

8-BFDMTP 0.3 -468 0.0687 5.74 16.27 74.82

0.4 -492 0.0589 7.375 15.022 78.40

0.5 -493 0.0409 8.732 14.397 85.02


236

(6.8)

The impedance spectra (Figs. 6.17 - 6.19) exhibit semicircle which can be

attributed to the charge transfer that takes place at electrode/solution interface and this

process controls the corrosion of mild steel. On the other hand, some Nyquist plots are

not perfect semicircle, which is attributed to surface inhomogeneity and roughness [36].

It is evident from these plots that the impedance response of MS in uninhibited acid

solution has significantly changed after the addition of the inhibitor in the aggressive

solution. It is apparent from Table 6.6 that the value of Rct increased with increasing

concentration of the inhibitors. The increase in Rct values is attributed to the formation

of an insulating protective film at the metal/solution interface. It is also clear that the

value of Cdl changed upon the addition of each of the inhibitor, indicating a decrease in

the local dielectric constant and/or an increase in the thickness of the electrical double

layer, suggesting that the inhibitors function by forming a protective layer at the metal

surface [37]. Therefore, the changes in Rct and Cdl were caused by the steady

replacement of water molecules by the adsorption of inhibitor on the mild steel surface,

reducing the extent of metal dissolution [38]. The results obtained by potentiodynamic

polarization, electrochemical impedance spectroscopy (EIS) and weight loss

measurements are in good agreement with each other.

Figs. 6.21 – 6.23 show the Bode plots recorded for the mild steel electrode

immersed in 0.5 M HCl in absence and presence of inhibitor 8-BMPTP, 8-BFMMTP

and 8-BFDMTP. A new phase angle shift in the higher frequency range and a change in

the phase angle shift with increase in concentration of the inhibitors were observed.

This phase angle shift resulted from the formation of an inhibitor film which changed

the electrode interfacial structure. The continuous change in the phase angle shift is

obviously correlated with the progress of surface coverage by inhibitor molecules [39].


237

Fig. 6.17: Nyquist plots in the absence and presence of different concentrations of 8-

BMPTP in 0.5 M HCl.


BFMMTP in 0.5 M HCl.

-750

-650

-550

-450

-350

-250

-150

-50

50

-50 450 950 1450 1950

Z '

'(oh

m c

m2)

Z' (ohm cm2)

8-BMPTP

Blank 0.1g/L 0.2g/L 0.3g/L 0.4g/L 0.5g/L

-550

-450

-350

-250

-150

-50

50

-50 450 950 1450

Z '

'(oh

m c

m2)

Z' (ohm cm2)

8-BFMMTP

Blank 0.1g/L 0.2 g/L 0.3g/L 0.4g/L 0.5g/L


238


BFDMTP in 0.5 M HCl.

Fig. 6.20: Electrochemical equivalent circuit used to fit the impedance.

-650

-550

-450

-350

-250

-150

-50

50

-50 450 950 1450

Z''

(oh

m c

m2)

Z' (ohm cm2)

8-BFDMTP

Blank 0.1 g/L 0.2 g/L 0.3 g/L 0.4 g/L 0.5 g/L


239

Fig. 6.21: Bode plots for mild steel in 0.5 M HCl in the absence and presence of

different concentration of 8-BMPTP.


different concentration of 8-BFMMTP.

0

0.5

1

1.5

2

2.5

3

3.5

-2 0 2 4 6

log Z

(O

hm

cm

2)

log frequency (Hz)

Blank

0.1 g/L

0.2 g/L

0.3 g/L

0.4 g/L

0.5 g/L

-80

-70

-60

-50

-40

-30

-20

-10

0

-2 0 2 4 6

Ph

ase

(d

eg)

log frequency (Hz)

0

0.5

1

1.5

2

2.5

3

3.5

-2 0 2 4 6

log Z

(O

hm

cm

2)

log frequency (Hz)

Blank

0.1 g/L

0.2 g/L

0.3 g/L

0.4 g/L

0.5 g/L

-80

-70

-60

-50

-40

-30

-20

-10

0

-2 0 2 4 6

Ph

ase

(d

eg)

log frequency (Hz)


240


different concentration of 8-BFDMTP.

Table 6.6. Impedance parameters for the corrosion of mild steel in 0.5 M HCl in

presence of different concentration of the 8-BMPTP, 8-BFMMTP and 8-BFDMTP

Inhibitor C

(mM)

Rct

(Ω cm2)

Cdl

(µF cm -2

)

IE

(%)

Blank 170 162 -

8-BMPTP 0.1 874 329 80.51

0.2 1090 336 84.37

0.3 1192 233 85.70

0.4 1295 226 86.84

0.5 1353 261 87.40

8-BFMMTP 0.1 676 291 74.80

0.2 754 199 77.42

0.3 817 296 79.15

0.4 895 143 80.96

0.5 1140 329 85.05

8-BFDMTP 0.1 452 400 62.37

0.2 506 508 66.34

0.3 582 424 70.74

0.4 770 263 77.88

0.5 1079 240 84.21

0

0.5

1

1.5

2

2.5

3

3.5

-2 0 2 4 6

log Z

(O

hm

cm

2)

log frequency (Hz)

Blank

0.1 g/L

0.2 g/L

0.3 g/L

0.4 g/L

0.5 g/L

-80

-70

-60

-50

-40

-30

-20

-10

0

-2 0 2 4 6

Ph

ase

(d

eg)

log frequency (Hz)


241

6.3.6. Morphological investigation

The SEM micrographs obtained for mild steel surface in the absence and

presence of optimum concentration (0.5 mM) of the inhibitors in 0.5 M HCl at 6 h

immersion time and 30 ºC are shown in Figs. 6.24a - 6.24e. The image of the polished

mild steel is shown in Fig. 6.24a. The mild steel surface in the absence of inhibitors

exhibited a highly corroded surface with pits and cracks (Fig. 6.24b). This is due to the

attack of mild steel surface with aggressive acid medium. However, in the presence of

inhibitors the mild steel surface could be observed with a thin layer of the inhibitor

molecules (Figs.6.24c - 6.24e), giving protection against corrosion. The inhibited mild

steel surface was smoother than the uninhibited surface indicating the presence of a

protective layer of adsorbed inhibitors on the metal surface. The formed surface film has

higher stability and lower permeability in aggressive solution than uninhibited mild

steel surface. Hence, they show an enhanced surface property which seemed to provide

corrosion protection to the mild steel beneath them by restricting the mass transfer of

reactants and products between the bulk solution and the mild steel surface.

(a)

(d) (c)

(b)


242

Fig. 6.24: SEM images of mild steel in 0. 5 M HCl after 6 h immersion at 30o C (a)

before immersion (polished) (b) without inhibitor (c) with 0.5 mM of 8-BMPTP (d)

with 0.5 mM of 8-BFMMTP (e) with 0.5 mM of 8-BFDMTP.

6.3.7. Antioxidant activity and corrosion inhibition

The in vitro antioxidant activity of the compounds 8-BMPTP, 8-BFMMTP and

8-BFDMTP were determined spectrophotometrically by DPPH radical, hydroxyl radical

and nitric oxide radical methods. The reduction capacity of DPPH, hydroxyl and nitric

oxide radical methods were determined by decrease in its absorbance at 517, 535 and

546 nm respectively, which is induced by antioxidants. On the other hand, it is well-

established that organic molecules incorporating an electron donating groups (hydroxyl

and methoxy) can act as free radical trapping agents. The compounds 8-BMPTP, 8-

BFMMTP and 8-BFDMTP present the good scavenging activity on all the reported

assays. The compound 8-BFDMTP bearing a two methoxy groups (electron donating

group) in the aromatic ring of substituted aldehyde showed dominate scavenging

activity. Antioxidants from different origin have high bioavailability, therefore high

protective efficiency against free radicals [40]. Free radicals and singlet oxygen

scavengers (antioxidants) were found to have metal and alloy corrosion inhibition

character, which depend to a greater extent on the structural feature of the antioxidant

added and to its accepting–donating hydrogen or electron behaviours [41]. In this

connection, and on the basis of available results obtained by antioxidant activity

measurements, we undertook examination of corrosion inhibition studies of compounds

8-BMPTP, 8-BFMMTP and 8-BFDMTP. The results showed that compounds 8-

BMPTP, 8-BFMMTP and 8-BFDMTP are excellent corrosion inhibitors with maximum

inhibition efficiency IE (%) values of 94.9 %, 86.9 % and 85.0 %, respectively. The

(e)


243

greater antioxidant activity and corrosion inhibition behaviour of compound 8-BMPTP

is linked to the electron donating effect of the two methoxy groups attached to the

aromatic ring at fourth and fifth positions, which increases the electron density on the

benzene ring. The increase in delocalization electron density in the molecule makes

more reactive towards scavenging reactive oxygen as well as inhibiting corrosion

process. The adsorption of inhibitor molecules is further stabilized by participation of p-

electrons of benzene ring. The electronegative oxygen and nitrogen atoms present in

compounds 8-BMPTP, 8-BFMMTP and 8-BFDMTP facilitate more efficient adsorption

on mild steel surface. Reduction of oxygen availability in the corroding system and the

presence of a barrier between the electrode surface and oxygen in the medium retard the

rate of metal corrosion [42].

6.4. Conclusion

A new 1,2,4-triazoles were synthesized in good yield, characterized by different

spectral studies and their antioxidant activity have been evaluated. Compounds 8-

BMPTP, 8-BFMMTP and 8-BFDMTP demonstrated good antioxidant activity. The

synthesised compounds have shown excellent inhibition property for the corrosion of

mild steel in 0.5 M HCl solutions, and the inhibition efficiency increases with

increasing concentration of the inhibitors. The studied inhibitors are effective at 30 °C,

beyond 30 °C, it is found that inhibition efficiency decreases with the increase of

temperature. The optimum concentration for all the studied inhibitors was 0.5 mM. The

inhibition ability of these compounds follow the order 8-BMPTP > 8-BFMMTP > 8-

BFDMTP, and the inhibition efficiencies determined by polarization, EIS and weight

loss methods are in good agreement with each other. The adsorption of all the studied

molecules obeys the Langmuir isotherm model. The negative values of free energy of

adsorption indicated that the adsorption of triazole molecules is spontaneous process.

The results obtained from potentiodynamic polarization indicated that the synthesized

inhibitors represent a mixed-type of inhibitors. The calculated ∆Gads and ΔHads values

indicated that the adsorption mechanism of the synthesized triazole derivatives on mild

steel in 0.5 M HCl solution is physisorption. SEM analysis shows that the formed

surface film has higher stability and lower permeability in aggressive solution than

uninhibited mild steel surface. Hence, they show enhanced surface properties. The

antioxidant activity and anticorrosion property of the synthesized triazole derivatives

are correlated with each other.


244

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(2008) 473.

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