Corrosion inhibition of mild steel by 3-amino-4-(4'-substituted benzeneazo)pyrazoline-5-ones in artificial seawater

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Indian Journal of Chemical Technology Vol. 7, September 2000 pp.207-212

Corrosion inhibition of mild steel by 3-amino -4-( 4' -substituted benzeneazo )- pyrazoline -5-ones in artificial seawater

G S Suresha·* & LK Ravindranathb

"Department of Chemistry, SSMRV College, 1 P Nagar, Bangalore 560 078, India bDepartment of Chemistry, Sri Krishnadevaraya University, Anantapur 515 003, India.

Received 17 January 2000; accepted 22 June 2000

The corrosion of mild steel in artificial seawater containing various concentrations of 3-amino-4-(4'-substituted benzeneazo)-Pyrazoline-5-ones has been studied by polarization technique. These compounds inhibit corrosion effectively even in trace concentration. The degree of corrosion inhibition is a function of temperature, concentration and nature of the inhibitor. The corrosion current density, percentage inhibitor efficiency and thermodynamic parameters for the surface adsorption of inhibitors were calculated for the inhibition process. The IR spectra of the firm formed on the corroding surface indicated the formation of a complex between the inhibitor molecule and the mild steel. The inhibitors appeared to function through a general adsorption mode following the Frumkin adsorption isotherm and Langmuir adsorption isotherm. The thermodynamic parameters of adsorption showed a strong interaction of these inhibitor molecules with the mild steel sur-

face. The results revealed that these compounds act as mixed inhibitors for mild steel in artificial seawater.

Mild steel due to its versatility is a very important component of most of the objects used in marine envi- ronments. The unrelenting corrosive action of the ocean environment may result in the destruction of mild steel in a matter of hours. The consequences of this destructive process are often quite high maintenance costs, increased down time, lack of availability and even a concern for safety of personnel and equip- ment. Some examples of the serious consequences of marine corrosion may be the collapse of an offshore drilling platform, bridge failure, leaking of pipe joints and corrosion product bleeding on coastal buildings.

This problem has been attributed to a higher concentration of chloride ions present in seawater.

The highly corrosive nature of seawater towards most metals requires some degree of control on its corrosivity to achieve economic maintenance, minimum loss of chemical product and maximum safety conditions. Corrosion technology has been evolved over the past several decades to prevent the corrosion.

One way of protecting mild steel from corrosion is to use corrosion inhibitors. A variety of organic compounds has been tried as corrosion inhibitors for mild steel in several corrosive environment ¹·5

. It is well known that the corrosion inhibition is a surface process, which involves specific adsorption⁶of inhibitors.

The objective of the present work is to study the

* For correspondence

inhibition effect of some 3-amino-4-( 4' -substituted benzeneazo)-Pyrazoline-5-ones (Fig. 1) on the corrosion of mild steel in artificial seawater. Polarization technique was used to evaluate corrosion data. An attempt was also made to know the nature of interaction between inhibitor and mild steel surface. The adsorption behaviour of compounds 1-5 was studied to determine the appropriate adsorption isotherm and the thermodynamic parameters.

Experimental Procedure

Compounds 1 to 5 were prepared by a method de- scribed in the literature⁷. The structures of the compounds were confirmed by spectral studies. The ho- mogeneity and purity of compounds were tested through thin layer chromatography. A cylindrical mild steel rod embedded in teflon (exposed surface area

=

0.21 cm²) was used as working electrode, saturated

Where R = (1)-H, (2) -CH3, (3)-0CH3, (4)-0H, (5)- Cl

Fig.l--Compounds used in the present study.

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208 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2000

Table !--Corrosion current density at various concentrations of inhibitors in artificial seawater (pH=8.8) at 298 K.

Artificial Corrosion current density J..LA/cm²

Concentration Seawater I 2 3 4 5

w·2

8.51 0.95 0.55 1.78 1.19 2.04

(88.12) (93.12) (77.75) (84.03 (74.50)

10"3 8.51 1.46 1.01 1.82 1.58 2.19

(81.75) (87.37) (77.25) (80.25) (72.62)

w ·4

8.51 1.70 1.35 2.13 1.86 2.59

(78.75) (83.12) (73.37) (76.75) (67.62)

w · s

8.51 2.30 1.83 2.98 2.57 3.16

(7 1.25) (77.12) (62.75) (67.87) (60.50)

w·6

_8.51 ^3.08 ^2.87 ^3.60 ^3.41 ^3.85

(61.50) (64.12) (55.00) (57.37) (51.87)

The values of% inhibitor efficiency are given in the parenthesis

. (2) (I) (4) (3) (~) (A)

Fig.2-Anodic and Cathodic Tafel plots for mild steel in simulated sea water with 10-³M various inhibitors (I to 5) and without inhibitor (A) at 298 K. (Curve numbers refer to the compounds as listed in Fig. I )

calomel and a platinum foil (1 x 1 cm2

) were used as reference and counter electrodes respectively. Artifi- cial seawater (pH=8.8) was. prepared⁸by dissolving (gL-¹) NaCI, 23.00; MgCh, 4.88; Na2S04,3.83; CaCl2, 0.925; KCI, 0.65; KBr, 0.09; H3B03, 0.024; SrCl2, 0.023; NaF, 0.0028; NH4Cl, 0.094; K2HP04, 0.0648;

Na2C0₃, 0.189 in distilled water.

The surface to be exposed to the corroding medium was mechanically polished with 4/0 grade emery pa- per using ethyl alcohol and water as lubricant. Further it was polished on chemois leather and then washed

thoroughly by running distilled water. Finally it was exposed to corrosive media. Anodic and cathodic polarization of mild steel was carried out in 100 mL artificial seawater without and with various concentrations (10.2M - 10·⁶M) of inhibitors under stirred con- dition using potentiostat I galvanostat (PAR model 362 over a potential range of 10- 500 mV at different temperatures (298 - 318 K). The current was recorded on a Digital Multimeter (Systronic type 435). The IR data was recorded on pure samples of a particular inhibitor and on scrapings of the film formed on mild steel surface after it was exposed to seawater containing that compound for a period of 72 h using Nicolet impact 400D, Ff-IT spectrometer with a resolution of 4 cm·¹^•

Results and Discussion

Fig. 2 shows typical anodic and cathodic poten- tiostatic polarization curves (Tafel plots) carried out at 298 K in artificial seawater and seawater containing 10·³M concentrations of various inhibitors. Both the anodic and cathodic Tafel plots in each case shifted towards higher polarisation level with reference to uninhibited seawater in the presence of inhibitors without any appreciable change in the slope. From the polarization diagrams, the corrosion current density was calculated using the relation,

· _ ba

I

^be

I

leorr-

2.302 (ba + bJ Rp

where Rp is the polarization resistance (!1£/!1/), b. and be are the anodic and cathodic Tafel slopes, respectively. The inhibitor efficiency was calculated by using the equation

I.E(%)= (1- i/i_{0) X}100

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SURESH & RA VJNDRANATH : CORROSION INHIBITION OF MILD STEEL 209

Table 2---(:orrosion current density at various concentrations of inhibitors in artificial seawater (pH=8.8) at 308 K

Artificial Corrosion current density J.!Ncm2

Concentration

Seawater 1 2 3 4 5

10·2 19.2 2.28 1.76 4.45 3.05 4.96

(88.1 0) (90.75) (76.57) (83.94) (73.89)

10·3 19.2 3.15 2.56 4.89 3.74 5.60

(83.40) (86.52) (74.26) (80.31) (70.52)

10·4 19.2 4.44 4.08 5.66 4.97 6.16

(76.63) (78.52) (70.21) (73.84) (67.57)

10.s 19.2 5.24 4.74 6.43 5.87 6.96

(72.42) (75.05) (6.15) (69.10) (03.36)

10·6 19.2 7.64 7.26 8.55 7.94 8.78

(59.78) (61.78) (55.00) (58.21) (53.78)

Table 3---(:orrosion current density at various concentrations of inhibitors in artificial seawater (pH=8.8) at 318 K Corrosion current density J.!Ncm2

Concentration Artificial Sea-

2 3 4 5

water

10-2 35.2 6.99 5.32 10.72 9.76 12.45

(80.02) (84.80) (69.37) (72.14) (64.42)

10-3 35.2 7.42 6.83 12.37 10.67 13.42

(78.80) (80.48) (64.65) (69.51) (61.54)

10-4 35.2 9.95 8.90 13.91 12.34 15.07

(71.57) (74.57) (60.25) (64.74) (56.94)

10-s 35.2 11.57 10.62 14.96 13.81 15.97

(66.94) (69.65) (57.25) (60.54) (54.37)

10·6 35.2 15.88 14.86 17.67 17.19 18.34

(54.62) (57.54) (49.51) (50.88) (47.68)

Table 4---(:orrosion current density at various concentrations of inhibitors in artificial seawater (pH=8.8) at 328 K Corrosion current density J.!Ncm2

Concentration Artificial

Seawater 2 3 4 5

w ·2

66.5 15.15 13.10 21.64 16.31 23.88

(77.04) (80.15) (67.21) (75.28) (63.81)

10-3 66.5 17.38 14.22 23.10 20.05 25.36

(73.66) (77.69) (65.00) (69.62) (61.57)

10-4 66.5 19.98 18.58 24.69 22.15 26.69

(69.72) (71.84) (62.59) (66.43) (54.56)

10-s 66.5 26.02 23.36 29.82 28.02 31.81

(60.57) (64.60) (54.81) (57.54) (51.80)

10-6 66.5 85.09 32.38 37.35 36.01 38.66

(46.83) (50.93) (43.40) (45.43) (41.42)

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210

INDIAN J. CHEM. TECHNOL., SEPTEMBER 2000

Table 5--Yalues of free energy of adsorption during the adsorption of inhibitors (10.³M) on mild steel in artificial sea water (pH= 8.8)at different temperatures

-LlGads KJ mor¹

Temperature (K) 2 3 4 5

298 30.63 31.75 30.03 30.47 29.38

308 31.96 32.50 30.13 30.63 30.12

318 32.20 32.52 30.37 30.97 30.53

328 32.48 33.06 31.45 31.94 30.98

Table 6--Values of enthalapy of adsorption during the adsorption of inhibitors (10.³M) on mild steel in artificial sea water at different temperatures

-LlHaos KJ mor¹

Temperature (K) 2 3 4 5

298 19.40 13.44 8.97 10.46 16.41

308 20.05 13.89 9.27 10.81 16.97

318 20.70 14.34 9.57 11.16 17.52

328 21.35 14.79 9.87 11.51 18.07

Table 7-Yalues of entrophy of adsorption and activation energy during the adsorption of inhibitors (10.³M) on mild steel in artificial seawater

Thermodynamic Property LlS JK.¹mor¹

-Ea KJ mor¹

65 70.32

__.,..._. (2)

0 _ . - . - - ( 1 ) (.4)

0 ;;;_..---<>-... (3)

~~,.

2 45 72.52

3 30 71.55

4 35 72.02

5 55 67.46

Log [inhibitor] (mol dm-3)~

Fig.3--Frumkin adsorption isotherms of 3-amino-4-(4'- substituted benzene azo)- pyrazoline-5-ones studied on the surface of mild steel in simulated sea water at 298 K. (curve numbers refer to the compounds as listed in Fig. I ).

Log [inhibitor) (rnol dm-~~

Fig.4--Langmuir adsorption isotherms of 3-amino-4-(4'- substituted benzene azo)-pyrazoline-5-ones on the surface of mild steel in simulated sea water at 298 K. (Curve numbers refer to the compounds as listed in Fig. I).

where i₀and i are the COITosion current densities in the absence and in the presence of inhibitor, respectively.

The corrosion current density in artificial seawater decreased considerably in the presence of trace

amounts of the inhibitors. Tables 1-4 show the varia- tion of corrosion current density and percentage inhibition efficiency at different temperatures with the inhibitor concentrations. The extent of corrosion inhibition was found to depend on the nature and concen-

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SURESH & RAVINDRANATH : CORROSION INHIBITrON OF MILD STEEL 211

g

--,

5

22L_L_ ____ ^{_ L}^{_ _ _ _}~L---~----~~~

290 300 310

T(K)~

Fig.5--The dependence of tl.Gaus on temperature in

--·

simulated sea water containing 10-³M concentration of inhibitors.(Curve numbers refer to the compounds as listed in Fig. I).

tration of the inhibitor. This trend was similar for all the inhibitors. The corrosion cunent density decreased considerably with an increase in concentration of each inhibitor and reached a minimum value at 10-2 M concentration. At a particular concentration of the inhibitor, inhibitor efficiencies are in the order 2>1>4>3>7>5. The percentage inhibition efficiency reached the highest values at highest concentrations (10-2

M) and the lowest temperature (25°C).

The surface coverage (8) of inhibitors was calculated by using the equation³:

8

=

^(1-ili0 )

where i and i0 are corrosion current densities with and without the inhibitor, respectively.

The values of surface coverage 8 conesponding to different concentrations of inhibitors at different temperatures were used to determine which isotherm best describes the adsorption process. The most frequently used isotherms include; Langmuir, Frumkin, Parsons, Temkin, Flory-Huggins, Dhar-Flory-Huggins &

Blockris-Swinkels. Figs 3 and 4 show experimental results at 298 K fitting Frumkins⁹and Langmuir's10 isotherm, respectively.

The inhibitor action may be interpreted 11 in terms of adsorption of the compound on the metal surface through functional group containing atoms with lone pair of electrons. In the present system, -N=N-, and - NH2 groups are probably responsible for the interaction with the surface. Supplementary evidence for this

r

E

u

~

0>

0 -'

3.0 3.2 3.3

+X 10-3 ( K-~~

Fig.6--Arrhenius plots for the corrosion of mild steel in simulated sea water in the presence of 10-³M inhibitors.(Curve numbers refer to the compounds as listed in Fig. I).

is obtained from IR spectral data of pure sample of a particular inhibitor and that of the scrap of the film formed on mild steel surface exposed to seawater containing inhibitors for a period of 72 h. The char- acteristic vibrational frequencies of -NH2 (3370 cm-1 ) in pure compounds disappear in the iron - complex suggesting direct involvement of -NH2 group in the interaction of the inhibitor with the metal surface.

The adsorbed inhibitor or its complex reinforces the protective film of mild steel offered by the ferric oxide layer by thickening the existing film and I or by covering the unprotected areas. This trend has been

. I . d . l h d. ¹²¹³ previOus y notice w1t ¹ot er me 1a. · .

From the surface coverage obtained at different temperatures, the thermodynamic parameters were evaluated. Adsorption isotherms obtained by plotting log (8/1-8) versus log C were I inear. Hence, from the equation log C=log (8/ 1-8) - log [3, L1Gacts was calcu-

lated14.

log

f3

= 1.74 -~Gact/2.303 RT

The low values of -~Gacts are generally consistent with a physiosorption. Those having higher values involve charge sharing or a transfer from the inhibitor molecules to the metal surface to form a coordinate

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212 ^{INDIAN J.}^CHEM^{. TECHNOL.}, SEPTEMBER 2000

type of bond¹⁵^• Thus, the lowest values of - L1Gads (Table 5) indicated that the adsorption was dominated by physiosorption which was a more favourable phenomenon than chemisorption ¹⁶• The relative increase of -L1Gads with temperature also suggested a chemisorption. Adsorbed inhibitors may impede corrosion by blocking the anodic and cathodic reaction sites by which Tafel lines shift towan1s higher polarisation levels without changing the slopes.

This is in accordance with the observed results.

Value¹⁴of L1Sacts was obtained from the slope of the plot of L1Gads versus temperature. !JHads was obtained from Gibb's Helmholtz equation and Ea was evaluated from Arrhenius plots. These data are shown in Tables 6 and 7.

The corrosion inhibition by these compounds can also be explained in terms of the relative electron donor properties of the anchoring atom of the functional group. These compounds are anchored on to the metal surface by the nitrogen atom of the -NHz- group. The electron density at the nitrogen atom in these inhibitor molecules and also inhibitor efficiency varies in the order 2>1>4>3>5. The compound 2 has an additional electron releasing group -CH₃which probably may be anchored additionally, accounting for the high inhibition efficiency.

Conclusion

All the compounds used in the present system were found to be effective inhibitors for mild steel in artificial seawater. Temperature and concentration of the inhibitor affect the inhibitor efficiency. These inhibitors act as mixed inhibitors. TheIR spectral data of the film formed on the surface of the metal exposed

to the inhibited seawater indicated the formation of a complex between the inhibitor molecule and the surface of the corroding metal. The adsorption behaviour followed Frumkim's and Langmuir's adsorption isotherm. The change in - L1Gads with temperature suggested both physiosorption and chemisorption of the inhibitor.

Acknowledgement

One author (GSS) wishes to thank the Principal, SSMRV College, Bangalore for his encouragement.

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