Indian Journal of Chemistry Vol. 42A, December 2003, pp. 3027-3035

Ortho-substituted aniline-N-salicylidenes as corrosion inhibitors for zinc in sulphuric acid

M N Desai, J D Talati & Neesha K Shah*

Chemistry Department, School of Sciences, Gujarat University, Ahmedabad 380009, Gujarat, India

E-mail : mndesai23@yahoo.com

Received 16 January 2003

The corrosion of zinc in sulphuric acid containing different o-substituted aniline-N-salicylidenes has been studied with respect to inhibitor and acid concentration, period of exposure and temperature. All the five compounds studies show high (95-97%) inhibitive efficiency. At 0.5% inhibitor concentration in 0.5 M acid, the order of efficiency is: o-HNS (95.5%) < 0-

CNS (96.8%) < o-AnNS (99.7%) :5 o-TNS (99.8%) :5 ANS (99.9%). The activation energies are higher in inhibited than in uninhibited acid. It appears that an efficient inhibitor is characterized by a relatively greater decrease in the free energy of adsorption . Galvanostatic polarization studies indicate that these are mixed type inhibitors with predominant action on the local cathodes. Cathodic protection in the presence of these inhibitors has been studied. The inhibitors appear to function through adsorption due to the presence of the imine (-C=N-) group and conjugated double bonds.

Zinc is an electropositive metal (E'zn++rzn = -0.7619 V) and non-oxidizing acids dissolve it with the evolution of hydrogen I. Being amphoteric in nature it is attacked by alkalies also. Acids have long been used in removing the scale from metal surfaces and also in pickling. Hence to avoid attack on the base metal during scale-removal and cleaning zinc surfaces with acidic solutions, the use of inhibitors becomes necessary. A literature survey reveals that there are very few compounds, which may be considered as excellent inhibitors for corrosion of zinc in sulphuric acid. With many inhibitors the parameters affecting their performance have not been studied in detail. Compounds that have been reported as corrosion inhibitors for zinc in su.Iphuric acid include alkaloids2, thiourea3, benzenethiol4 and related derivatives, imidazole azo derivatives5

, quinoline derivatives6,

quaternary salts of pyridine bases 7, benzaldehyde and its derivatives8

, and Schiff bases of some aniline derivatives9

. Some of the Schiff bases have also been recommended as corrosion inhibitors for copper lO and aluminiumll.12 .

In the present work Schiff bases derived from some ortho-substituted anilines and salicylaldehyde have been reported as corrosion inhibitors for zinc in sulphuric acid. Due to the presence of an imine (-C=N-) group and conjugated double bonds, these should function as good inhibitors. Some research work l3 has revealed that the inhibitive efficiency of the Schiff bases is much greater than that of the corresponding amines or aldehydes.

The effect of inhibitor concentration, exposure period, temperature, and externally applied cathodic current on the behaviour of these inhibitors have been investigated.

Materials and Methods The Schiff bases were synthesized by condensation

of aniline and various ortho-substituted anilines with salicylaldehyde in the presence of ethyl alcohol. The reactants were mixed at 0° to 5°C. The mixture was then refluxed on a water bath for about an hour. After the reaction was completed, the reaction mixture was treated with ice-cold water, when crude solid separated. It was first washed with distilled water, then with very dilute hydrochloric acid, and finally with water. It was then purified by repeated crystallization from ethanol 14.

Rectangular specimens of electrolytic zinc of size 30x30 mm (thickness 28 SWG) with a small hole of about 2 mm diameter near the upper edge of the specimen were used for the determination of the corrosion rate. The specimens were polished using successively '0' to '0000' Oakey emery papers. The final polishing was done with a linen buffing wheel usi ng jeweller's rouge · followed by Tripoli composition which gave a mirror-look finish. The specimens were then degreased by carbon tetrachloride (sulphur free). The test specimens were exposed to 0.25 M and 0.5 M solutions of sulphuric acid containing controlled additions of various Schiff bases, viz., aniline-N-salicylidene (ANS), o-toluidine-

3028 INDI AN 1 CHEM, SEC A, DECEMBER 2003

N-salicylidene (o-TNS) , o-anisidine-N-salicylidene (o-AnNS), o-chloroaniline-N-salicy lidene (o-CNS) , and o-aminophenol-N-salicylidene (o-HNS).

One specimen only was suspended by a g lass hook in each beaker which contained 230 mL of the test

solution which was open to the air at 35±0.5°C. After the tests the specimens were cleaned with saturated ammonium acetate solution ls

. '6. Duplicate experiments were performed in each case and the mean values of the weight losses reported.

For polarization studies, metal coupons of c ircular design, di ameter 28.02 mm, with a handle 30 mmx 5 mm , were used. The handle and the back of the coupon and the auxiliary pl ati num electrode were coated with Perspex, leav ing only the circular position of apparent surface area, 615.6 mm2 exposed. The soluti on, 80 mL in each limb, was contained in a H­type cell with the Luggin capillary as near to the electrode surface as poss ible. The potenti al was measured with a microvoltmeter, using a saturated calomel reference electrode.

Results and Discussion A specimen of zinc immersed in 0.25 M sulphuric

ac id suffered a weight loss of 8770 mg dm-2 in 30 min

whereas that in 0.5 M acid, 22977 mg dm-2. The

results show that wi th a ll the inhibitors, except o-TNS and o-CNS at 0.1 % concentration, the weight loss due to corros ion decreases and the inhibitor efficiency increases with increase in inhibitor concentration . o-TNS and o-CNS show an increase in corros ion at lower (0. 1 %) concentrations but at higher concentrations they also confer protection, the extent of protection being dependent on the concentration of the inhibitor. Thus at 0. 1 % concentration in 0.25 M su lphuric acid , the efficiency of the various inhibitors increases in the order:

o-CNS (-26.9%) < o-TNS (- 1 %) < o-A nNS (11.3 %) < ANS (46.2%) < o- HNS (52.5 %) .

At 0 .3% concentration the inhibitor efficiency is found to range from 80.7% (o-CNS ) to 98 .7% (0-TNS) whereas at 0.5 % concentration all the five compounds confer more than 99.7% protection .

In 0.5 M acid at 0.1 % concentration, o-AnNS , 0-

H S, and o-C S show an increase in the extent of corrosion whereas o-TNS and ANS confe r 1 % and 21 % protection respectively. With increase in inhibitor concentrat ion, however, the efficiency of all the compounds is fo und to increase, the order of

increase in efficiency at 0.5% inhibiwr concentration being:

o-HNS (95.5 %) < o-CNS (96.8 %) < o-AnNS (99.7%) :s o-TNS (99.8%):S ANS (99.9%).

The effect of exposure period on inhibitive efficiency of different inhibitors at 35°C and 0.5 % concentration (1.0% in the case of 0 -TNS) was studied. From the results it is evident that as the exposure period is increased from 30 min to 180 min, the loss in weight in uninhibited 0 .25 M sulphuric acid increases from 8770 mg dm -2 to 17791 mg dm-2

and from 22977 mg dm-2 to 35913 mg dm-2 in 0.5 M acid. In inhibited 0 .25 M acid, all the five compounds considerably reduce the corros ion rate and almost complete (99.2-99 .9%) inhibition of corrosion could be achieved. The general order of increase in inhibitor effic iency in 0.25 M sulphuric acid is found to be:

o-CNS :S o-AnNS < ANS < o-HNS < o-TNS .

The difference in inhibitive efficiency of the different inhibitors is more effecti ve ly reflected in 0.5 M sulphuric acid which is more corrosive . Thus whereas the effic iency of the d lfferent inhibitors ranges from 95.5 % (o-HNS) to 9996% (o-TNS) for the 30 min exposure peri od, the ex tent of corrosion is found to increase with period of exposure, it being more so in the case of o-CNS fo llowed by o-HNS , e.g., the efficiency of o-CNS decreases from 96.8% for the 30 min period to only 3. 1 % for the 180 min period .

In 0.5 M ac id also the general order of increase in efficiency of different inhibitors is fou nd to be:

o-CNS < o- HNS < o-AnNS < ANS < o-TNS.

From the weight loss data, it may be generali zed th at where corrosion protection is desired for longer exposure periods, o-TNS and o-ANS show better performance. The results also show that the inco rporation of -CI , or -OH group in o-position in ani line-part of the Schiff base has a deleterious effect on the inhibitive efficiency of o-ANS, particularly for longer exposure periods. Even o-AnNS containing an - OCH3 group also shows a decline in effi ciency with time. In 0.5 M ac id also, the general order of inhibiti ve efficiency is found to be:

o-CNS < o-HNS < o-A nNS < ANS < o-TNS .

DESAI el al. : CORROSION INHIBITION OF Zn BY SALICYLIDENES 3029

To determine the effect of temperature on inhibitive efficiency, weight losses were determined in 0.25 M sulphuric acid containing 0.5% inhibitor (1.0% in the case of o-TNS and o-AnNS) at solution temperatures of 35°, 40°, 45°, 50° and 55°C. It is seen that the extent of corrosion in inhibited as well as uninhibited acid increases with a rise in temperature, the loss in weight being much higher in plain acid. The results also show that all the five compounds studied decrease the corrosion rate to an appreciable extent, the extent of inhibition ranging from 99.4% to 99.9%. From the weight losses it may be generalized that here also o-TNS appears to be the best inhibitor, while the chloro-compound which although confers more than 98% protection may be considered to be the least efficient of the five Schiff bases.

The values of energy of activation, Ea, were calculated with the help of the equation:

. .. (1)

and also from the plots of log p vs liT where p is the corrosion rate at temperature T(K) and R is the gas constant. From the Ea values, it is apparent that for the corrosion of zinc in uninhibited acid the Ea value is -8 kJ mor l whereas in inhibited acid the values are higher and range from 25 kJ mol- I (o-HNS) to 86 kJ mor l (o-CNS). In inhibited acid, the Ea values thus vary and depend on the inhibitive power of the inhibitor. It appears that the exponential term in the Arrhenius equation appreciably changes the Ea value with a slight change in the corrosion rate. The higher values of activation energy in inhibited acid suggest that the adsorption of the inhibitor on the metal surface may be physical or weak in nature 17

•

According to Putilova et al .ls, the behaviour of those inhibitors whose activity decreases with a rise in temperature and in whose presence the activation energies are higher in inhibited than in uninhibited acid may be compared with that of unstable catalyst poi sons whose adsorption decreases with increasing temperature. However, the very high inhibitive power (>99%) and almost a constant value of the extent of inhibition in the temperature range studied suggest that the adhesive bonds are stronger and the lower rate of hydrogen evolution in inhibited acid may not be able to cause any damage to the surface film or dislodge the protective layer.

Thermodynamic parameters If it is assumed that the inhibitor is adsorbed on the

metal surface in the form of a monolayer film, covering at any instant a fraction , 8, of the metal surface in a uniform random manner, then the heat of adsorption, QA, of the inhibitor can be calculated from the equation:

( 8 8 'J( TT? J QA = 2.303R log-2- -Iog~ ~

] -82 1 81 T2 ~ .. . (2)

where 8 1 and 82 are the fractions of the total surface covered by the inhibitors at temperatures TI and T2 (K) respectively with ,

8= (Wu ) - (WJ

(Wu) . . . (3)

where Wu is the weight loss in plain acid and WI the weight loss in inhibited acid . The val ues of free energy of adsorption (Go A) were calculated with the help of the following equation 19:

8 log C;nh =]og ] _ 8 - log B ... (4)

where 10gB = - 1.74 - (GoA/2.303RT) . .. (5)

From the values of QA and GOA, the values of the entropy of adsorption, S' A, were also calculated. The values of these thermodynamic functions are given in Table 1.

From the results it is evident that for all the five inhibitors, the heats of adsorption are negative. The values of free energy of adsorption are also negati ve which suggests a strong interaction20 of the inhibitor molecules and spontaneous adsorption on the metal surface2 1

•

For 0.25 M sulphuric acid the GOA values for all the inhibitors, like their inhibitive efficiencies, are almost the same (-34 kJ mol- I to -36 kJ mol- I). However, in 0.5 M acid , wherein the inhibitor efficiencies are somewhat more differentiated because of its higher corrosivity, the GOA values for a good inhibitor like 0 -

TNS are more negative than that for a less efficient inhibitor like a-CNS or o-HNS.

The positive values of the entropies of adsorption, S' A, also suggest the adsorption to be a spontaneous process. But the order of efficiency of the different inhibitors and the order of decrease or increase in

3030 INDIAN J CHEM, SEC A, DECEMBER 2003

Table I-Some thermodynamic parameters for adsorption of o-substituted aniline-N-salicylidenes for the corrosion of zinc in 0.25 M and 0.5 M sulphuric acid [Exposure period:30 min]

Inhibitor Temp 0.25M H2SO4

(%) (0C) G"A S'A (kJ mol- I) (J mol- I)

ANS 35 - 35.1 268 (0.5%) 40 - 36.0 264

45 - 36.1 259 50 - 35.6 255 55 - 35.1 251

o-TNS 35 - 36.3 343 ( 1.0%) 40 - 35.04 335

45 - 35.4 326 50 - 35.6 322 55 - 33.4 314

o-AnNS 35 - 35.1 289 (1.0%) 40 - 34.6 284

45 - 34.1 276 50 - 34.2 272 55 - 33.9 268

o-CNS 35 - 36.8 435 (0.5%) 40 - 34.6 423

45 - 33.8 414 50 -32.6 406 55 - 32.5 397

o-HNS 35 - 35.2 347 (0.5%) 40 - 35.3 343

45 - 35.8 339 50 - 36.1 326 55 - 36.6 318

entropy do not agree and thus suggest that the decrease in free energy is the controlling factor in the adsorption process.

Corrosion potentials and polarization behaviour In 0.25 M sulphuric acid, the corrosion potential of

zinc is -940 mY (SCE), the corresponding value in 0.5 M sulphuric acid is -880 mY (SCE). It is interesting to note that there is a distinct difference in the nature of the shift of corrosion potential of zinc in the presence of inhibitors. In 0.25 M sulphuric acid, in general, the shift of corrosion potential is in the posi ti ve direction indicating the immediate effect of the inhibitor on the anodic reaction; however all the inhibitors induce a significant increase in cathode polarization. The initial shift of the potential is the consequence of both polarization effect and of change in the ratio of anodic and cathodic areas. Thus the pronounced shift of potential in the negative direction indicative of cathode polarization by impressed currents need not be considered as inconsistent with the steady state potential shifting in the positive

0.5 M H2SO4

QA GOA S'A QA (35-55°C) (kJ mol- I) (J mol- I) (35-S5°C) (kJ mol- I) (kJ mol- I)

- 37.2 251 - 37.4 247

- 46.8 - 37.4 243 - 40.0 - 37.4 238

- 36.9 234

- 39.8 393 - 38.1 381

- 68.9 - 37.7 372 - 81 .2 - 37.3 368

- 37.0 360

- 38.0 372 - 36.4 364

- 54.1 - 37.6 356 -76.2 -36.0 347

- 35.5 339

- 31.0 427 -29.7 414

- 97.7 - 29.1 406 - 99.8 - 28.0 397

- 25.5 381

-29.1 159 - 29.3 155

- 13.7 - 28.9 155 -78.3 -27.6 155

-26.2 155

direction on the addition of the inhibitors. It may be that the effect of an inhibitor can be either a change in the exchange current or an increase in polarization or both.

In 0.5 M sulphuric acid the addition of aniline, N-salicylidene or o-toluidine, N-salicyJidene shifts the corrosion potential in the negative direction indicating the influence of these inhibitors on the cathodic reaction. This is confirmed by the significant increase in cathode polarization during galvanostatic polarization studies. In 0.25 M sulphuric acid in the presence of o-toluidine, N-salicylidene, an increase in inhibitor concentration does not change the initial shift in the corrosion potential. Thus the initial anodic/cathodic effect of the inhibitor does not change by an increase in inhibitor concentration indicating a strong adsorption even at low c~ncentrations. With o-anisidine, N-salicylidene, o-chloroaniline, N-salicylidene and o-amino phenol, N-salicylidene inhibitors initially there is a higher shift in the positive direction and with an increase in inhibitor concentration, there is a decrease, which shows that at

DESAI et at.: CORROSION INHIBITION OF Zn BY SALICYLIDENES 3031

low concentration of the inhibitor there is a greater initial effect of the inhibitor in the anodic direction but subsequently, with an increase in inhibitor concentration, its cathodic action also comes into play so that overall result is a decrease in the value of initial positive shift. Such a behaviour was observed also in 0.5 M sulphuric acid with o-chloroaniline, N­salicylidene or o-aminophenol, N-salicylidene.

In both 0.25 M and 0.5 M sulphuric acid, aniline, N-salicylidene increases cathode polarization significantly and this effect increases with an increase in its concentration which is in good agreement with loss in weight data. In 0.5 M sulphuric acid 0.5% aniline, N-salicylidene induces a significant increase in cathode polarization. At the current density of 1.62xlO-3 amp/cm2

, the shift in cathode potential during cathode polarization is 440 mY as compared to 5 mY in uninhibited 0.5 M sulphuric acid.

In 0.25 M sulphuric acid, at 0.1 % concentration of o-toluidine, N-salicylidene, the shifts in cathode and anode potentials are less than those in its absence which explains the acceleration of corrosion of zinc; however at 1.0% concentration, at the current density of 1.62xlO-3 amp/cm2

, the shift in cathode potential is 600 mY as compared to 0.0 mY in an uninhibited sulphuric acid. o-anisidine, N-salicylidene acts as an inhibitor by predominantly influencing the cathodic reaction and this effect increases with an increase in inhibitor concentration. At 0.1 % concentration in 0.25 M and 0.5 M sulphuric acid, o-chloroani line, N­salicylidene accelerates the corrosion of zinc. This behaviour is explained by polarization measurements which show that the shifts in both anode and cathode potentials are less than those in uninhibited 0.25 M and 0.5 M sulphuric acid. In 0.25 M sulphuric acid, at 0.5% concentration, o-chloroaniline, N-salicylidene is a mixed inhibitor with predominant action on the cathodic reaction, whereas in 0.5 M sulphuric acid it is a cathodic inhibitor.

In 0.25 M sulphuric acid, o-aminophenol, N­salicylidene acts as an inhibitor by influencing the cathodic reaction and this behaviour increases with concentration which is in good agreement with results obtained by loss in weight method. In 0.5 M sulphuric acid in the presence of 0.1 % of o-aminophenol, N­salicylidene, the shift in cathode and anode potentials are less than those in its absence which confirms its accelerating behaviour. At 0.5% concentration, this inhibitor acts as a cathodic inhibitor in 0.5 M sulphuric acid.

The efficiency of inhibitors investigated have been calculated from (1) extrapolation of the cathode Tafel line to open circuit potential, (2) from intersection of cathodic and anodic Tafel lines at open circuit potential. Both these methods show a good agreement in the efficiencies of the inhibitors as evaluated from loss in weight data.

Tafel parameters and efficiency of inhibitors for zinc in 0.5 M sulphuric acid is given in Table 2.

Cathodic protection According to Evans22

, when the corrosion of a metal is under cathodic control, the external current required to achieve cathodic protection is equal to the corrosion current which would be flowing if there were no protection. He also reports that if the control is even partly anodic, the current needed for complete protection will exceed the former value of the corrosion current. Antropov23 reports that under cathodic polarization, the adsorption of cationic and some non-ionic polar organic compounds is facilitated and the total protective effect may, therefore, be even more than the sum of the two individual effects.

Galvanostatic polarization studies with zinc in sulphuric acid reveal that the corrosion is under mixed control with predominance of the cathodic part. When increasingly higher current densities were applied to zinc as a cathode in sulphuric acid it was observed that the weight loss due to corrosion decreased with increase in the applied cathodic current (lapp) and complete protection of the metal could be achieved at a current density of 3.492 Aldm2 (Table 3). This value is higher than the theoretical current (viz., 2.335 A/dm2

) calculated from weight loss data. This is in conformity with Evans ' statement.

It is well-known that metal structures with organic coatings require lower currents for cathodic protection than uncoated ones. This is because the only areas which need protection are the defects or 'holidays ' in the protective layer24

. Machu25 has earlier suggested that lower current densities are required for calhodic protection in inhibited rather than in uninhibited solutions. According to Hackermann26 also, a combination of cathodic protection and an inhibitor which is adsorbed at strongly negative potentials may give greater protection than either system alone.

When studies on cathodic protection of zinc in 0.25 M sulphuric acid were extended to inhibited solutions it was observed that the loss in weight due to corrosion decreased as the cathodic current density was increased. However, the current densities

3032 INDIAN J CHEM, SEC A, DECEMBER 2003

required for complete protection were found to depend on the inhibitor itself as well as its concentration.

(i) If l obs > (Io + I I), the conjoint action is synergistic, (ii) If Jobs = (Io + II)' the conjoint ac tion is simply

additive, If 10 is the inhibiti ve efficiency without current, II

the per cent protection at various current densities in uninhibited acid, and l obs is the total protection due to the conjoint action of the current and inhibitor, then three cases of interest arise:

(iii) If Jobs < (Io + I I)' the conjoint action is antagonistic (i.e. , less effective).

The conjoint effect of the current and inhibitor is given in Table 4. From the results it is evident that in

Table 2-Tafel parameters and efficiency of inhibitors for zi nc in 0.5 M sulphuric acid (Temp: 350 ± v.S"C)

Inhibitor Con-osion Tafel sloEe b Corrosion current amE/cm2 Inhibi to r efticienc:t (%) potential Anodic Cathodic From ex trapolation From intersection From From From loss

E w rr (Y Idecade) (Y/decade) of cathodic Tafel of cathodic and (5) (6) in weight (mY vs SCE) line at E ,orr anodic Tafel lines method

at E corr

2 3 4 5 6 7 8 9

Nil 0.087 0.012 4.36xlO-3 4.67x lO-3

ANS (0.5%) 0.09 0.18 l.S85xlO-5 LOx 10-5 97 >99 >99

o-TNS ( 1.0%) 0.042 0.16 LOx 10-5 1.2x I0-5 >99 >99 >99

o-AnNS (1.0%) 0.043 0.11 I.Ox lO-5 1.25x 10-5 >99 >99 96-99

o-CNS (0.5%) 0.067 0.19 5.01 x I0-5 3.16x lO-5 >99 >99 3-99

o- HNS (0.5%) 0.064 0.23 4.467xI0-5 3.89x lO-5 99 >99 66-99

Table 3-Weight loss in the absence and presence of protective current, electrode potential s and efficiencies of inhibitors.

[Metal:ZincjAcid conc : 0.25 M sulphuric acid]

Inhibitor and its Without eXlerna l cathodic current Theoretical With external cathodic cun-ent Ratio concentration Weight Inhibitor E corr (mY) current for Current for % Reduction in Protective ipfi,

loss efficiency (vs SCE) complete complete current due to potential (mg/dm2

) (%) protec ti on protection (ip) inhibitor (Ep) (mY) amp/dm2 (i,) amp/dm2

( \IS SCE) 2 3 4 5 6 7 8 9

Nil 8770 - 940 2.335 3.492 .- 1240 1.496

ANS (0.2%) 3413 61 - 890 0.91 1.705 51 - 1120 1.88 (0.3%) 3 12 97 - 880 0.083 0.1786 95 - 1100 2.15

o-TNS (0.3%) III 99 - 880 0.03 0.04872 99 .- 1120 1.6

o-AnNS (0.15%) 4496 49 - 895 1.197 1.007 71 - 1220 0.841 (0.2%) 634 93 - 900 0.169 0.1949 94 - 1200 1.203

a-eNS (0.2%) 5 139 41 - 870 1.37 0.7308 79 - 1180 0.53 (0.3%) 16 19 8 1 - 880 0.45 0.1462 96 - 1180 0.325

o-HNS (0.2%) 2670 70 - 880 0.7 11 0.4222 88 - 1180 0.594 (0.3%) 188 98 - 890 0.05 0.0812 98 - 1140 1.624

DESAI el al.: CORROSION INHIBITION OF Zn BY SALlCYLlDENES 3033

Table 4-Conjoint effect of cathodic current and inhibitor on the corrosion of zinc in 0.25 M sulphuric acid

[Exposure period: 30 min ; Temp: 35±0.5°C]

Inhibitor and its Cathodic Weight loss Cathodic protection Actual protection 10 + I) Conjoint effect concentrati on c.d. (mg/d m2) due to curre nt in due to current + {lobs-(lo+I) }

applied pl ain acid inhibitor lobs (Ndm 2

) I)

Nil Nil 8770

ANS (0.2%) 0.0000 3413 61.1 (10) 0.3248 1423 7.4 83.8 68 .5 Synergistic (15 .3%) 0.6496 195 14.7 97.8 75.8 Synergistic (22%) 1.299 46 44.5 99.5 105.6 @

1.624 9.7 58 99.9 119. 1 @

1.705 0 62 100 123.1 @

(0.3%) 0.0000 312 96.4 (1 0) 0.0812 52 2.0 99.4 98.4 Additive (1.0%) 0.1624 8. 1 3.6 99.9 100 Additive (-0.1 %) 0.1786 0 4.0 100 100.4 Additive (-0.4%)

o-TNS (0.1%) 0.0000 8857 - 1 (10) 0.3248 669 > 99 92.4 98 (- 5.6%) 3.735 125 100 98.6 99 Additive (- 0.4%) 3.896 8.1 100 99.9 99 Additive (- 0.9%) 3.979 0 100 100 99 Additive (1 %)

(0.3%) 0.0000 III 98.7 (10) 0.01624 19.5 0.4 99 .8 99.1 Additive (0.7%) 0.03248 6.5 0.7 99 .9 99.4 Additive (0.3 %) 0.04872 0 1.1 100 99 .8 Additive (0.2%)

o-AnNS (0.15 %) 0.0000 4496 48.7 (10) 0 .3248 305 7.4 96.5 56.1 Synergistic (40.4%) 0.812 17.9 24.4 99.8 73.1 Synergistic (26.7%)

0.9744 8.1 29 99.9 77.7 Synergistic (22.2%) 1.007 0 30. 1 100 78 .8 Synergistic (22.2%)

(0.2 %) 0.0000 634 92.8 (10) 0.03248 140 0.7 98.4 93.5 Synergistic (4.9%) 0.1624 9.8 3.6 99.9 96.4 Synergistic (3.5 %) 0.1949 0 4.3 100 97.1 Synergistic (2.9%)

o-CNS (0 .2%) 0.0000 5139 41.4 (10) 0.3248 741 7.4 91.6 48.8 Synergistic (42 .8%) 0.6496 13 14.7 99.9 56.1 Synergistic (43.8%) 0.7308 0 19.9 100 61.3 Synergistic (38.7%)

(0.3%) 0.0000 1691 80.7 (10) 0.0812 330 2.0 96.2 82.7 Synergistic (13.5 %) 0. 1299 8. 1 2.9 99.9 83.6 Synergistic (16.3%) 0. 1462 0 3.2 100 83.9 Synergistic (16.1 %)

o-HNS (0.2%) 0.0000 · 2670 69.6 (10) 0.1624 152 3.6 98.3 73 .2 Synergistic (25.1 %) 0.3248 19.5 7.4 99.8 77.0 Synergistic (22.8%) 0.406 4.9 9.6 99.9 79.2 Synergistic (20.7%)

0.4222 0 10 100 79.6 Synergistic (20.4%)

(0.3%) 0.0000 188 97.9 (10) 0.03248 22.7 0.7 99.7 98.6 Additive (l.l %) 0.06496 6.5 1.6 99.9 99.5 Additive (0.4%) 0.0812 0 2.0 100 99.9 Additive (0.1 %)

@ See discussion

3034 INDIAN J CHEM, SEC A, DECEMBER 2003

absence of current the inhibitive efficiency of the five inhibitors at 0.2% concentration (and 0.15% in the case of o-ANS) increases in the order:

o-CNS (41.4%) < o-AnNS (48.7%) < o-TNS (60.7%) < ANS (61.1 %) < o-HNS (69.6%)

while the cathodic current densities (lp) required for complete protection in inhibited acid decrease in the order:

ANS (1.705) < o-CNS (0.7308) < o-HNS (0.4222) < o-AnNS (0.1949) < o-TNS (0.3%) (0.04872)

i.e. the values of protective current are lower than those for uninhibited acid.

From the results (Table 4), it is evident that in the case of o-CNS, o-AnNS, the conjoint effect is synergistic, in the case of o-TNS it is additive, while in the case of ANS and o-HNS the conjoint effect is dependent on the concentration of the inhibitor. The results also show that the conjoint action is greater at lower current densities. In the case of ANS, at higher current densities, although the total protection achieved is greater than the individual effects, the total of 10 + II is greater than l obs' This may be due to the fact that at higher current densities the adsorptive characteristics of the inhibitor are so changed under the influence of the applied current that higher current densities are required for cathodic protection. The synergistic effect in the case of some of the inhibitors shows that the results are in conformity with the views advanced by Evans and Antropov.

Mechanism of inhibition In general, most of the organic corrosion inhibitors

are compounds with at least one polar unit having atoms of nitrogen, sulphur, oxygen and in some cases selenium and phosphorous. The polar unit is regarded as the reaction centre for the establishment of the chemisorption process. In such a case the adsorption bond strength is determined by the electron density on the atom acting as the reaction centre and by the polarisability of the functional unit. Thus polar organic compounds acting as cOITosion inhibitors are chemically adsorbed on the surface of the bulk metal , M, forming a charge transfer complex between the polar atom/atoms and the metal:

M + Rn x: ~ M:X Rn ... (6)

According to Aramaki27, the metal and the compound are Lewis acid and base respectively and hence the

stability of the adsorption bond is closely related to the hard and soft acids and bases principle. The bulk metal is classified as the soft acid and a molecule or ion of the soft base is readily chemisorbed on the metal surface by forming a stable donor-acceptor bond. Those inhibitors which are in a distinct ionic form may also get attached to the metal surface of opposite polarity through electrostatic attraction. The adsorbed monolayer will then block the dissolution of the metal ions. The size, orientation, shape and electric charge on the molecule determine the degree of adsorption and hence the effectiveness of the inhibitor.

According to Hackerman28, amine type inhibitors have electron donating ability and their action is attributed to the adsorption of the molecule on the metal surface through an unshared pair of electrons belonging to the nitrogen atom. Simple ethylenediamine is believed to form chelate type metal complexes by bonding through the two nitrogens and the five-membered chelate formed is said to be stable. However, six-membered rings are the most stable for chelates which contain one or more double bonds29.

Acknowledgement Thanks of the authors are due to Prof. Y. K.

Agrawal, Head of the Chemistry Department, School of Sciences, Gujarat University, Ahmedabad, for keen interest and suggestions from time to time. Thanks are also due to UGC, for providing research grant to NKS

References I Greenwood M N & Earnshaw A, Chemistry of elements

(Butterworth and He inemann), (1997) 1206. 2 Subramanyan N & Ramkri shnaiah K, Proc 14th Seminar

Electrochem, C.E.C. R.I. , Karaikudi, India, 1973,387. 3 Shams EI, Dinn A M & EI Hosary A A Werkst Korros, 28

(1977) 26. 4 M S Abdel Aal, A A Abdelwahab & A EI Saied, Corrosion,

37(1981)557. 5 Ekilik Y Y & Grigorev Y P, Zashch Metal. 9 (1973) 736. 6 Kawai S, KalO H & Hayakawa Y, Denki Kagaku, 39 (1971)

288. 7 Antropov L I, Pogrebova I S & Dremova G I, Zashch Metal.

7 (1971) 3. 8 Grigorev Y P & Ek ilik Y Y, Zh Prikl Khim, 42 (1969) 141. 9 Desai M N, Chauhan P L & Shah N K, 7th Symposium on

Corrosion inhibitors, Ferrara, Italy, 1990; 8th Symposium on Corrosion Inhibitors, Ferrara, Italy, 1995 .

10 Li S. Chen S & Lei S, Corr Sci, 41 (1999) 1273. II Gomma G K & Wahdan M N, Mats chem and Phys, 39

(1995) 209. 12 Bansiwal, Anthony P & Mathur S P, Br Corros J, 35 (2000)

301

DESAI et al.: CORROSION INHIBITION OF Zn BY SALICYLIDENES 3035

13 Desai M N, Desai M B, Shah C B & Desai S M, Corros Sci, 26 (1986) 827.

14 Jaeger F M, Proc Acad Sci, Amsterdam, 23 (1920) 74.

15 Champion F A, Corrosion testing procedures (Chapman and Hall , London), 1964, 194.

16 Speller F N, Corrosion: Causes and prevention (McGraw Hill, New York), 1951 ,38.

17 Ghosh P K, Guhasarkar D K & Gupta V S, Br Corros J, 18 (1983) 287.

18 Putilova I N, Balezin S A & Barannik V P, Metallic corrosion inhibitors (Pergamon Press, New York), 1960, 31.

19 Abdel AM S & EL Saiyed A, Trans SAEST, 16 (1981 ) 197.

20 Rudresh H B & Mayanna S M, J electrochem Soc India , 31 (1982) 109.

21 Prasad D, Jha G S, Chaudhary B P & Sanyal S, J Indian chem Soc, 79 (2002) 264.

22 Evans U R, Corrosion and oxidation of metals, scientific principles alld practical applications, (Edward Arnold, London), 1960,877.

23 Antropov L I, Pro First International Congress on Metallic Corrosion, London 1961, (Butterworths, London), 1962, 146.

24 Fontana M G & Greene N D, Corrosion engineering (McGraw-Hill Book Co., New York), 1968, 207.

25 Machu W, Werkst Korros, 6 (1955) 247. 26 Hackerman N, NACE Basic corrosion course, edited by A

Brasunas (NACE, Houston) 1970, pp. 9. 27 Aramaki K, Mochizuki T & Nishihara H, Proc 10th

International Congress on Metallic Corrosion (Madras,. India, Oxford and IBH New Delhi), 1987,2759.

28 Hackerman N, Ind Eng Chern, 46 (1954) 553. 29 McGraw-Hill encyclopedia of science and technology, 4

(1987) 408.