Indian Journal of Chemical Technology Vol. 6, March 1999, pp. 93-99

Corrosion inhibition of copper in aqueous chloride solution by diphenyl amine and cupric diphenyl dithiocarbamate

M M Singh, R B Rastogi, B N Upadhyay & M Yadav

Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221 005 , India

Received 30 September 1997; accepted II February 1999

The inhibition of copper corrosion in 1.5% sodium chloride solution was studied in the presence of different concentrations of diphenyl amine and its Cu(lJ) diphenyl dithiocarbamate complex . Both the compounds act as good corrosion inhibitors for copper. Their inhibitive action has been compared with those of diethyl analogues and contrary to the theoretical expectations these inhibitors have been found to exhibit lower inhibition efficiencies.

The study of corrosion inhibition of copper in aqueous sodium chloride solution is a comparatively recent phenomenon. Most of the investigations are related to the applications of common inhibitors like various deri vatives of aminothiazole I , benzotri­azole2

.4

, thidimidazole5, thiadiazole4

.6

, mercapto-S­triazole7

, pyrimidines8, dithiocarbamates9

.l o

,

xanthates3 etc . During a series of investigations, on corrosion inhibition of copper in aqueous sodium chloride solutions various forms of piperidine I I and diethylamine l 2 moieties have been found to act as highly efficient inhibitors. High potentiality shown by these inhibitors necessitated further extension of such hunt for better and better inhibitors. An improvement in the inhibition efficiency could be foreseen on the replacement of two ethyl groups in diethyl amine by two phenyl groups. It could serve two purposes; increase in the effective surface area of the molecule and greater adsorption due to involvement of 1t­

electrons in the phenyl rings. Thus, during the present investigation diphenyl

amine and cupric diphenyl dithiocarbamate h~ve been tested for their corrosion inhibition properties for copper corrosion in aqueous chloride solution.

Experimental Procedure

Copper specimens taken for experiments were supplied by Mis Goodfellow Metals Limited, England . The samples were 99.9% pure with the composition: Ag 500, Bi< 1 0, Pb<50 and other metals <300 ppm . The specimens ' dimensions for weight loss experiments were 3 x 2 cm and for electrochemical studies 2 x I cm. Polishing of the

samples was done with 110 to 4/0 grade emery papers successively. The polished samples were washed with benzene, followed by hot soap solution and lastly with distilled water. They were de greased by immersion in acetone for 1-2 min, dried and stored in vacuum desiccator. For weight loss experiments 300 mL of electrolyte (1.5% NaCl) was taken in 500 mL glass beakers with lids. Inhibition efficiencies (IE) were evaluated after 120 h using 10, 15,25, 50, 75 , 100 and 125 ppm of inhibitors. The specimens were removed from the electrolyte, washed thoroughly with distilled water, dried and weighed.

The electrochemical experiments were carried out in a three necked glass assembly containing 150 mL of 1.5% NaCI with different concentrations of inhibitors (from 10 to 125 ppm by weight) dissolved in it. Polarisation studies were performed in unstirred and undeaerated solutions using a Wen king potentiostat (POS-73). Starting from open circuit potential, the applied potential was increased manually in 10m V steps in the anodic or cathodic direction and the corresponding steady state currents were measured directly from the ammeter on the panel of potentiostat. All experiments were performed at T, ±0.2°C in an electronically controlled air thermostat where T stands for 25 , 35 and 45°C.

Analytical reagent grade NaCl , cupric sulphate and diphenyl amine were used in the present investigation. The aqueous solution of NaCI was prepared in doubly distilled water. Cu(JI) diphenyl dithiocarbamate was synthesized by stirring an equimolar mixture of diphenyl amine, carbon disulphide and NaOH at 5°C and then adding copper sulphate solution in excess .

94 INDIAN J. CHEM. TECHNOL., MARCH 1999

Tab le I-Inhibition efficiencies of diphenyl amine, calculated by weight loss and polarisation techniques at 25 , 35 and 45°C

Conc. of % I. E. inhibitor ppm 25°C 35°C 45°C

Wt. loss Polarisation Wt.loss Polarisation Wt. loss Polarisation

10 67.30 62.01 71.42 65 .00 82.91 77.91 (82 .69) (77.50) (87.14) (80.00) (92.46) (83 .28)

15 71.15 65.89 77.14 72.00 85.42 81.95 (83.65) (78.30) (90.00) (81.00) (94.47) (84.54)

25 73.07 69.76 80.00 76.00 90.45 84.85 (84.6 1 ) (79 .07) (91.42) (81 .50) (97.25) (85 .80)

50 76.92 71.32 82.85 78.50 91.45 86.75 (86.53 ) (79.84) (94.28) (83 .00) (97.48) (87.38)

75 80.76 76.74 85 .71 81.00 91.94 87.69 (88 .46) (80.62) (94 .28) (85.00) (97.98) (90.54)

100 84.6 1 79.06 88.57 84.50 92.96 88.45 (92.30) (81.39) (94.28) (86 .00) (97.98) (90.54)

125 84.6 1 79.84 88.57 84.80 92.96 87.69 (92.30) (81.39) (94.28) (86.00) (97.98) (89.27)

Note: The values in parentheses are the corresponding IE values for diethyl amine

Table 2- lnhibition efficiencies of cupric diphenyldithiocarbamate, calculated by weight loss and polarisation techniques at 25 , 35 and 45°C

Conc. of % I.E. inhibitor ppm 25°C

! 35 °C 45°C

Wt. loss Polarisation Wt loss Polarisation Wt.loss Polari sation

10 84.6 1 79.84 85.71 81.00 89.44 84.85 (9 1.34) (82 .95) (92 .85) (84.00) (94.97) (85.80)

15 86.53 82.17 88.57 8450 92.96 87.38 (92.30) (84.49) (93 .57) (86 .00) (95.4 7) (87.38)

25 88.46 84.49 91.42 85.50 96.48 87.69 (94.23) (86.82) (94.28) (88 .00) (97 .98) (89.90)

50 92 .30 85 .73 94.28 88.50 97.48 88.45 (96. 15) (87.59) (97.14) (89.00) (98.49) (91.17)

75 96.15 88.37 97.14 90.20 98.99 88.95 ( 100.00) (89 .92) (100.00) (90.00) (100 .00) (92.30)

100 96.50 89.14 97.14 90.40 99.49 90.15 ( 100.00) (90.69) (100.00) (91.50) ( 100.00) (92.42)

Note : The va lues in parentheses are the corresponding IE values fo r cupric diethyl dithiocarbamate

Results and Discussion

The inhibition efficiency of diphenyl amine and cupric diphenyl dithiocarbamate has been evaluated at di fferent temperatures by weight loss and electrochemical methods and the results obtained have been recorded in Tables 1 and 2. The data mentioned in the tabl es reveal that the inhibition effic iencies of both the inhibitors gradually increase wi th increase in their concentrations. The IE values of Cu(lJ) diphenyl dithiocarbamate are fairly higher than that of the corrcsponding amine. However, contrary to the assumption diphenyl amine does not show better IE than diethyl aminc at any of the concentrations and temperatures employed . Thus it appears that although the surface area of diphenyl amine' being undoubtedly

larger than that of diethyl amine, the projected surface area of adsorbed diphenyl amine on the corroding surface might be smaller than that of diethyl amine. Never the less the importance of diphenyl amine and eu(II) diphenyl dithiocarbamate cannot be denied since the relative difference at higher temperature and concentration is not more than 2%.

It is evident from the values given in Tables 1 and 2 that with increase in temperature there is invariably an increase in IE of the inhibitors. The increase in IE with rise in temperature may be due to sluggi sh diffusion of large inhibitor molecul es as compared to water molecules t3

. As a result , at higher temperature larger water free surface area will be available for the inhibitor molecules to get adsorbed on the metal

SINGH el a/.: CORROSION INHIBITION OF COPPER IN AQUEOUS CHLORIDE

4 .5

4. 0 N

E u

----., ~

"' -" c: .. u

" 3.0 .. L ... :> u

'" .9

4 .5

N

E 4 .0 u --C ~

~ .... ... ... c Co ." .. ~ L 3.0 ... :> u

'" .9

2·0 ' I I I II/

.' , " ' ,'

- 100

T = 25° C

0-----0 Blan k -- 25ppm Oiph",y lami n~ - 50 ppm -- 75ppm .. 0----0 100 ppm ---- 125 ppm

400

Pot"'t ial ( mV vs SCE )

Fig. I-Anodic polarization behaviour of copper in 1.5% sodium chl oride sol ution in absence and presence of diphenylamine at 25°C

T . 2SoC

0----0 Blank ---- 25ppm Oip~l dithio CuO!) compI4K ---- !iOppm

-----..A 75ppm .. 0----0 lOOppm .. - 125 ppm ..

Pot~nti a l ( mV V". SeE )

Fig. 2-Anodic po lari zation behaviour of copper in 1.5% sodium ch loride solution in absence and presence of diphenyl dithi o Cu(l l) complex at 25°C

..

..

95

1000

1000

96 INDIAN J CHEM. TECHNOL., MARCH 1999

0'---0 Blank

..--.-... 15pptn Oiph.."IQml.,..

.---. 5Opp,"

- tOOO - iCC - 800 - 100 - 600 - 500 - ' 00 -JOO - 200 -100

Pot~nt;ol (m V ¥s . 5 C E )

Fig. 3---Cathodic polarization behaviourof copper in 1.5% NaCI in absence and presence of diphenylamine at 25°C

7.5

N~ -... .. , ,

.f" i 1.5

" 10 ~ T t ' 25-C ~ u

0--0

1 Bla nk

~ SOOPm 1.0 ~ 7Spp",

0----0 100 ppm

____ 'f15ppm

06t I I '--:--_ '--:--_ L----'I . -L_ 1_ ---'_--'

- 1000 - 900 - BOO - 7()() - 600 -sao - .«)() - 300 - 200 - 100

Pahonti ol (mV v, S e E )

Fig 4---('alhodlc po lan /a ll on behaviour of copper in 1.5% NaCI in absence alld presence of diphen yl dith io Cu(ll ) complex at 25°C

surLlcc leading to Improved inhibition. The a \ailability or hi gher ac tivatIOn energy for adsorption of thc inhibitor mo lcc ules at elevated temperatures may bc another reason 14 for the enhancement in

percentage inhibition efficiency. Another explanation which can be offered to this bl~haviour is that chemisorption of the inhibitor molecules occurs during the inhibition process. However, in view of heat of adsorption values lying between -10.58 and -18.40 K cals/mol, this probability is completely overruled ..

The graphical presentation of anodic polarisation curves in the presence of diphenyl amine and cupric diphenyl dithiocarbamate respectively are illustrated in Figs I and 2. The nature of anodic polarisation curves in the presence of diphenyl amine (Fig. 1) is almost identical to that o\Jserved in the case of Cu(II) complex of diphenyl dithiocarbamate. It indicates that the inhibition of copper corrosion follows the same mechanism for both the inhibitors . However, there are certain specific elements of dissimilarity observed in the two figures . In the Tafel region, the anodic polarisation curves are very close to each other when diphenyl amine is used as inhibitor whereas for the complex the shift of polarisation curve from the blank curve is much more significant. Secondly, the curves in the post-current-minimum region for the complex as inhibitor are fair ly distingui shable at ea,~h

concentration and are spread over in a wider range of current density. These features are indicative of the fact that Cu(IJ) complex of diphenyl dithiocarbamate is a better inhibitor than diphenyl amine as it leads to greater polarisation of anode.

The cathodic polarisation curves of copper in the presence of diphenyl amine, shown in Fig. 3 do not differ much in nature to those of e u(II) complex of diphenyl dithiocarbamate, shown in Fig. 4 . However, there. are some distingui shable features, i.e., (i) the curves are much more widely spaced in the pre­plateau reg ion in Fig . 4 than in Fig. 3 and (ii) in the post plateau region the re verse is true. This shows that a lthough both the inhibitors are effective towards arrestlllg the cathodic reactions. each has got its own indi Vid ua l influence on the t.vo cathodic reactions viz. reduct ion of cuprous chloride and oxygen reduction IS.

Dlphenyl amllle seems to be better retarder of Cu ~ ion reduction process occ ulTing in pre-pl ateau region than the co mplex. In the post plateau region the shi ft of curve to\\"ards lower current density with Illcrease in concentrat ion is gradual in case o f diphenyl amine, however. It is abrupt and is not much affected by

further increase in concentration in case of the complex. Thus it may be inferred tha t even a very low concentration of the complex could retard the oxygen reduction process to a ve ry large exten t.

SINGH el al.: CORROSION INHIBITION OF COPPER 97

1.8

1.6

U

1.2

It> 1.0

GI 0 .!! 0.8

0.6

O.

0.2

0.0 ~. ~ .2 -4.0 -3 .8 -3.6 -3 • -3.2 -3.0

loge

Fig. 5--Variation of log 8/ 1-8 with log C for diphenylamine at different temperatures

2.5 r------------------------------------------------------------,

2.0

1.5 GI

~ co .£

1.0 ..

0.5

0.0 L-__________________________________________________________ ~

-4 .9 -4 .7 -4.5 -4.3 -4 1 ·3.9 -3.7 -35

love

Fi g. 6--Variation- of log 8/1 - 8 with log C for cupric diphenyl · dithiocarbamate at different temperatures

The mechanism of the inhibition in the earlier comrnunications ' I

, 12 using similar inhibitors has been explained in terms of the adsorption of these species on the surface of corroding metaL In Figs 5 and 6 Langmuir isotherm has been plotted for diphenyl amine and cupric diphenyl dithiocarbamate respectively_ It is evident from these curves that a linear relationship is obtained between log 8/ 1-8 and log C at all three temperatures. However, the slope

values are well be'low unity at temperatures 25 and 35°C and approach to unity at 45°C. Thus, it may be concluded that Langmuir isotherm is followed only at 45°C. Further, a linear relationship is observed fo r both inhibitors when the value of 8 is plotted against log C indicating the validity of Temkin's isotherm (Fig_ 7).

The mutual interaction between the adsorbed molecules, atoms or groups may affect the heat of

98 INDIAN J CHEM. TECHNOL., MARCH 1999

1.1

• Diphenygminc. • Cupric phenyl dKhiDC.Irbomote

1.0

O.u

~

o.e e

0.7

0.6 --0.5

~ .e ~ .6 ·44 ·4.2 ~ .O ·3.8 -36 ·3 4

log C

1

----I

•

·32

i i i I

I

·3 C

Fig. 7-Relationship between e and log C at 25°

2.5 r------'i---------------------...,.------~

2.0

1.5 <D

en co .!!

1.0

0.5

x

• x ;-

~

• Oiphenyt amine at 25 ppm

• Diphenyl Imine ot 100 ppm

.. Cupric diphenyl dKhiocarbomate a1 25 ppm

x Cupric diphenyl dithiocarbamlte at 100 ppm

~

0.0 "--_ ____________________________ J

310 · 3.15 3.20 3.30 3 .35 3 40

Fig. 8--Variation of log El/ l -El with l i T

adsorption and could lead to deviation from the ideal type of isothenn. A plot of log 811-8 vs liT for 25 and 100 ppm of diphenyl amine, eu(II) complex of diphenyl dithiocarbamate is shown in Fig. 8. The slope of the above plot yields the average heat of adsorption. It is evident that the heat of adsorption values for the two inhibitors selected for the present study lie between - 10.58 and - 18.40 Kcal mol-I which are approximately similar to those of their diethyl analogues. The above values further show that the complex of eu(II) gets adsorbed on the metal

surface more strongly than the corresponding parent amine. The inference from heat of adsorption data is in accordance with their respective inhibition efficiencies. Diphenyl amine has the lower heat of adsorption value which further confinns its lower inhibition efficiency.

Conclusions (i) Both the inhibitors are quite effective. (ii) The eu(II) complex is better inhibitor than the

corresponding amine because of the larger

SINGH et al. : CORROSION INHIBITION OF COPPER 99

effective surface area. (iii) Diphenyl amine and its Cu(II) complex show

lesser efficiency than their diethyl analogues;

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