INTRODUCTION:
Among the metals available for use as galvanic anodes, aluminium has the maximum theoretical ampere – hour capacity. A highly negative potential can be obtained if it is used in alkaline media, but the efficiency is very poor because of wastage of aluminium by local cell corrosion1. The possibility of improving the utilization of aluminium in alkaline media by incorporation of addition agents like Ca (OH)2 in combination with complexing agents like citric and tartaric acids, has been reported 2,3. It has been pointed out that phenols can inhibit the corrosion to aluminium in HCl4, although phenol as such is corrosive to aluminium at high temperatures5. However, it has been found 6 that a by-product of a coke-plant containing, among other things, phenol, pyrocatechol and resorcinol is a good inhibitor of aluminium in NaOH solution.
In the present study reports the inhibition of aluminium corrosion in sodium hydroxide (1.0 N) solution by the salicylic acid in conjunction with calcium acetate whose effects on the corrosion inhibition of metals and alloys in corrosive media have not been reported previously
EXPERIMENTAL METHOD:
The commercially available pure aluminium coupons were used in our studies.
All the chemicals used in the present investigation, were of Analar grade. The double distilled water, one distillation of which was conducted from alkaline potassium permanganate in all-glass pyrex unit, was used for preparing solutions.
For determination of corrosion inhibition, rectangular coupons of size, 2.0 x 1.0 x 0.15cm3, polished with wax coated emery papers of different rates were used. Then they were washed with distilled water and dried.
Weight Loss Studies:
The aluminium metals were weighed immersed vertically in 80 ml of aerated, unstirred 1.0 N NaoH solution with and without the inhibitor for a stipulated period of time. The coupons were removed from the solution and then they were cleaned by brushing under running tap water to remove the corrosion products, dried and reweighed to determine the weight loss. The experiments were carried out in triplicate to ensure reproducibility and the mean values of weight losses in uninhibited and inhibited corroding solutions respectively.
Gasometric Measurements:
Hydrogen evolution measurements were carried out with the aluminium coupons immersed in 1.0 N NaOH solution, with and without the inhibitor. The hydrogen gas evolved during the course of the reaction was measured by the specially designed instrument, based on the principle of gasometry. The hydrogen gas evolved would displace equal volume of air and the air is measured at atmospheric pressure and temperature for different intervals of time. The corrosion rates were evaluated from the volume of hydrogen evolved at the laboratory temperature and pressure.
The measured volume of hydrogen is converted into the volume at STP using the following equation.
P1 V1 =
PO VO
T1 TO
P1 V1 TO
VO =
PO T1
According to the stoichiometric equation for the corrosion process,
A1 + H2O + OH-
A1O2- + 3 / 2 H2,
33, 600 ml of H2 gas at STP is formed by the dissolution of 27 g of A1.
Therefore VO ml of H2 at STP is formed by the dissolution of A1 equal to
27g x VO, g
33,600
Using the above relationship for the measured volume of hydrogen, the corresponding loss of weight of aluminium in the given period of time, was computed and from these values corrosion rates and hence the percentage inhibition efficiencies were found out.
Open Circuit Potential Measurements
The potentials of aluminium coupons, immersed in the test solutions, were measured under open – circuit condition as a function of time against a saturated calomel electrode used as a reference electrodes. The measurements were made until a steady state potential value was attained.
Polarisation Studies
The galvanostatic polarization studies were carried out in a beaker with a platinum electrode as the counter electrode, a standard calomel electrode as the reference electrode and the aluminium electrode as the working electrode.
To minimize the IR ohmic drop, the aluminium electrode and Luggin capillary were kept at a constant distance throughout the experiment. The potential was measured by means of high impedance multimeter.
The aluminium electrode was kept immersed in the test solution without passing current for 45 minutes so that a steady open circuit potential was reached. After measuring the steady OCP value the aluminium electrode was cathodically polarized by passing known values of current in the increasing steps of 10 mA min-1. The steady potential was measured at each current density. The aluminium electrode was agin brought to the open circuit condition and kept in this condition for about 45 minutes so that a steady value of potential was reached. It was then anodically polarized as detailed above. The measurements were carried out in unstirred test solutions. Corrosion current, I Corr., was evaluated by the extrapolation of the cathodic Tafel plot. The corrosion potential, Ecorr., was also determined. Each experiment was duplicated and the mean value was computed.
RESULTS AND DISCUSSION:
In the Pourbaix’s diagram of aluminium 7 given below, points on line (1) represent the potential of aluminium metal, A1O, in contact with an aqueous solution of A13+ of activity 10-6. It can be seen that this potential is not pH dependent. Below this line occurs the immunity domain, where the metal is stable and immune to corrosion at these potentials. If a potential, more negative than the value on line (1 ), is applied to aluminium it will act cathodically and gets protected.
A13+ + 3e-
Al
Conversely, if a more positive potential is applied, such as by the presence of an oxidant like O2, then aluminium acts anodically and gets corroded with the O2 , then aluminium acts anodically and gets corroded, with the O2 acting cathodically.
The rate of corrosion of aluminium in 1.0 N NaOH and the effect of various concentrations of salicylic acid as well as calcium acetate on it are determined by weight loss method (Table 1). The inhibition efficiency of salicylic acid increases with its concentration but the increase is not significant. The maximum percentage inhibition is only 6.87. The percentage inhibition of corrosion of aluminium in 1.0 N NaOH by 0.8 x 10-3 M calcium acetate was found to be 19.31. However the inhibition efficiency of various concentrations of salicylic acid, in conjunction with calcium acetate of concentration 0.8 x 10-3 M, showed a significant increase (Table 2).
Table 1: Effect of salicylic acid as well as that of calcium acetate on the rate of corrosion of aluminium in 1. 0 N NaOH at 30 ± 1o C
|
[Salicylic acid] x 103, M |
Rate of Corrosion x 105, gcm-2 h-1 |
Percentage inhibition |
|
0 |
13.10 |
-- |
|
11.3 |
12.41 |
5.26 |
|
13.5 |
12.35 |
5.72 |
|
15.8 |
12.24 |
6.56 |
|
20.3 |
12.20 |
6.87 |
|
[Calcium acetate] x 103, M |
|
|
|
0.4 |
12.10 |
7.6 |
|
0.6 |
11.61 |
11.3 |
|
0.8 |
10.57 |
19.31 |
Thus the increasing concentration of salicylic acid along with calcium acetate of particular concentration 0. 8 x 10 -3 M, (Table 2) had the greater inhibitive influence than the effect of salicylic acid alone (or) calcium acetate alone in the NaOH solution corroding aluminium (Table 1). Thus the synergistic inbition by calcium acetate and salicylic acid appears to operate.
Table 2: Effect of salicylic acid as in conjuction with calcium acetate on the rate of corrosion of aluminium in 1. 0 N NaOH at 30 ± 1o C
|
[Salicylic acid] x 103, M |
Rate of Corrosion x 105, gcm-2 h-1 (Weight Loss) |
Rate of Corrosion x 104 g cm-2 h-1 (gasometry) |
|
0 |
13.10 |
14.15 |
|
4.5 |
12.50 |
13.5 |
|
9.0 |
11.43 |
12.75 |
|
13.5 |
10.96 |
12.0 |
|
18.0 |
9.75 |
10.6 |
|
22.5 |
9.11 |
10.00 |
|
27.0 |
8.52 |
9.35 |
|
31.5 |
7.74 |
8.25 |
|
36.0 |
7.06 |
7.65 |
|
40.5 |
6.75 |
7.20 |
[ Ca (CH3 COO)2] x 103, M = 0.8
The Ca2+ and salicylate ion, probably, from a complex and the complex compound get physically adsorbed, with the calcium disposed towards the aluminium surface having highly negative potential 8, 9 in alkaline medium.
The inhibition efficiency is not appreciable and any specific reason for this observation could not be found out.
Gasometric Measurements
In alkaline solution the OH- ions, which are specifically adsorbed on the aluminium surface, introduce partial negative charge at the point of interaction and these charges develop into large negative charge of the metal surface 8,9. Thus the dissolution of aluminium in the alkaline medium occurs at a very high negative potential.
A1 A1 3+ +
3e-
The Al3+ is readily complexed 10 by OH- resulting in the formation of AlO2-
Al 3+ + 4OH-
Al (OH)4-
Al (OH) 4-
A102- + 2H2O
Due to the vigorous reaction of Al3+ with OH- , the concentration of Al3+ is so reduced that the potential of Al(s) - A13+ (aq) electrode is decreased much below that of H+ (aq) – H2 (g) electrode i.e. a highly negative potential for A1(s) - A13+ (aq) electrode is possible.
The reduction of water occurs as the reaction controlling corrosion of metals in the alkaline solution.
H2O + e-
H + OH-
H + H H2
Due to low hydrogen over – voltage on aluminium there is intense evolution of hydrogen when aluminium corrodes in NaOH solution. The gasing is however reduced considerably by the addition of salicylic acid along with calcium acetate .
Open Circuit Potential Measurements:
The increasing addition of the inhibitor caused small shift in the values of the steady state open circuit potential and therefore attempt was made to confirm the nature of inhibition from the analysis of complete polarization curves.
Polarisation Measurements:
The polarization behaviour of aluminium, functioning as cathode as well as anode in the test solutions and the electrochemical data obtained from the studies are shown in Tables 3 and 4. It is evident that salicylic acid, in conjunction with calcium acetate, brings about the considerable polarization of the cathode as well as the anode. It was thus inferred that the inhibitive action is of mixed type. The cathodic and anodic Tafel slopes increased with increasing inhibitor concentrations and the increase was predominant in the case of the former indicating that the cathodic inhibition was dominating though the inhibitive action is of mixed nature. The non-constancy of Tafel sloes for different inhibitor concentrations reveals that the inhibitors act through their interference in the mechanism of the processes at the cathode as well as the anode.
Table 3: Effect of salicylic acid as in conjuction with calcium acetate on the percentage inhibition of aluminium in 1. 0 N NaOH at 30 ± 1o C
|
[Salicylic acid] x 103, M |
Percentage inhibition (Weight Loss) |
Percentage inhibition (gasometry) |
Percentage inhibition (Polarisation technique) |
|
-- |
13.1 |
-- |
-- |
|
4.5 |
4.58 |
6.80 |
4.49 |
|
13.5 |
16.33 |
17.24 |
16.85 |
|
18.0 |
25.57 |
16.89 |
30.79 |
|
22.5 |
30.45 |
31.03 |
33.90 |
|
27.0 |
34.96 |
35.51 |
35.94 |
|
31.5 |
40.91 |
43.10 |
41.11 |
|
36.0 |
46.10 |
47.24 |
47.52 |
|
36.0 |
46.10 |
47.24 |
47.52 |
|
40.5 |
48.47 |
50.34 |
52.14 |
[ Ca (CH3 COO)2] x 103, M = 0.8
Table 4: Corrosion parameters for aluminium in 1. 0 N NaOH solution without and with salicylic acid in conjunction with calcium acetate
|
[Salicylic acid] x 103, M |
O C P V |
Ecorr, V |
I Corr, mA cm-2 |
|
-- |
1.55 |
1.50 |
15.13 |
|
4.5 |
1.56 |
1.55 |
14.45 |
|
9.0 |
1.57 |
1.56 |
13.80 |
|
13.5 |
1.56 |
1.54 |
12.58 |
|
18.0 |
1.59 |
1.55 |
10.47 |
|
22.0 |
1.59 |
1.55 |
10.47 |
|
22.5 |
1.59 |
1.54 |
10.00 |
|
27.0 |
1.55 |
1.56 |
9.54 |
|
31.5 |
1.60 |
1.55 |
8.91 |
|
36.0 |
1.54 |
1.53 |
7.94 |
|
40.5 |
1.54 |
1.55 |
7.24 |
[ Ca (CH3 COO)2] x 103, M = 0.8
The corrosion currents were evaluated from the cathodic polarization curves. It was found to decrease with the increasing concentration of the inhibitor. The inhibition efficiencies were determined from the values of corrosion current and the inhibition efficiency values were found to show good agreement with those obtained from mass loss and gasometric measurements (Table 3).
CONCLUSION:
The inhibitive influence of salicylic acid, in conjucation with calcium acetate, on the corrosion of aluminum in 1.0 N NaOH was studied by weight loss, gasometric and polarization measurements. The inhibition efficiency values, determined by these techniques, showed close agreement. The combination of salicylic acid and calcium acetate was found to show synergistic inhibition effect. The steady OCP value for aluminium electrode dipped in 1.0 N NaOH, was found less shifted by the increasing addition of the inhibitor. The polarization studies revealed that the corrosion of aluminium in 1.0 N NaOH was predominantly under anodic control, in the presence as well as in the absence of the inhibitor. The nature of inhibition was found to be of mixed type.
REFERENCES:
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Received on 17.11.2009 Modified on 29.12.2009
Accepted on 30.01.2010 © AJRC All right reserved
Asian J. Research Chem. 3(2): April- June 2010; Page 351-354