Aromatic Acid Derivatives as Corrosion Inhibitors for

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Aromatic acid derivatives as corrosion inhibitors for aluminium in acidic and alkaline solutions S.M. Hassan, M.N. Moussa*, M.M. El-Tagoury and A.A. Radi, Department of Chemistry, Faculty of Science, Damietta, Mansoura University, *Chemistry Department, Faculty of Science, Mansoura Univ., Mansoura, Egypt.

The inhibition of aluminium corrosion in solutions of hydrochloric acid or sodium hydroxide has been studied using weight-loss and hydrogen evolution methods. Benzoic acid and its derivatives inhibit acidic and alkaline corrosion effectively. The efficiency of inhibitors increases in the order: benzamide < benzaldehyde < acetophenone < benzoic acid < benzophenone (100%). The inhibition efficiency of acid anhydrides follows the sequence: pyromellitic > naphthalic > trimellitic. Inhibition takes place through adsorption by a one-step process with greater efficiency in alkaline than in acidic methanolic solutions. Introduction In our earlier communications1 it was found that aromatic acids are effective acid and alkaline corrosion inhibitors for aluminium. The benzene ring attached to the carboxyl group contributes to the inhibitor efficiency, which is further influenced by substituents in the benzene ring. Inhibition similar to previous cases2-7 occurs due to adsorption of these inhibitors in the molecular form. The aim of the present work was to investigate the effect of the molecular structure of some derivatives of aromatic acids on rate of corrosion of aluminium in hydrochloric acid and sodium hydroxide solutions. Experimental Materials and solutions The aromatic acid derivatives used as inhibitors are:

Aluminium pieces measuring 20 x 20 x 2 mm containing Al 99.535; Fe 0.19; Si 0.15; Mg 0.1; Cu 0.02 and Mn 0.005% were used. Each aluminium test sample was first mechanically polished on 4/0 emery paper, then immersed for 30 seconds at 80°C in the degreasing mixture8, rinsed thoroughly with distilled water and dried between two filter papers. ÷ 0.01 M solutions of the different inhibitor compounds were prepared by dissolving the necessary weighed quantities in 2N HCl or NaOH solutions. The corrosive acid or alkaline soltions were made 50% (v/v) with respect to methanol to ensure the presence of the inhibitors in solution. All inhibitors used were A.R. or chemically pure products and were used without further purification. Apparatus and working procedures Weight changes of aluminium in 100 ml of 2N hydrochloric acid and sodium hydroxide solutions were followed at different periods of immersion in the absence and in the presence of various concentrations of inhibitors. Inhibitors efficiency was determined as the percentage inhibition

where Wo and Wi are weight-losses in uninhbited and inhibited solutions respectively. In the hydrogen evolution method a weighed aluminium sample (20 x 20 x 2 mm) was immersed into 250 ml of 2N HCl at 25°C and the volume of hydrogen evolved was recorded as a function of time. The efficiency of a given inhibitor is evaluated as a percentage reduction in reaction rate, viz.

where R f r e e and R i n h , are the rates of aluminium dissolution in free acid and in presence of the given inhibitor, respectively, both measured at the same time. 8

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Results and Discussion The inhibitors under investigation can be classified into two groups: I) Benzene derivatives (benzoic acid, benzophene, phenylacetic acid, acetophenone, benzaldehyde, benzamide, phenol and mandelic acid). II) Aromatic acid anhydrides (pyromellitic anhydride, 1, 8-naphthalic anhydride and trimellitic anhydride). Fig. 1 represents weight loss-time curves obtained for benzene derivatives in 2N HCl solution. It is evident that all substances of group I inhibit the acid corrosion of aluminium as their curves fall below that of free acid. Similar curves were recorded for acid anhydrides. Table (1) shows that the percentage inhibition increases with the increase of the inhibitor concentration. The surface coverage (0) was calculated from the equation:

Where Wo and Wi are weight-losses in uninhibiting and inhibiting solutions, respectively. On plotting the relation between surface area coverage and the logarithm of molar concentration of different inhibitors the curves in Fig. 2 were obtained. These curves consist of an initial ascending portion which passes to a region of constancy. This constancy is an indication of completion of a monolayer of adsorbate. Hydrogen evolution measurements revealed the same behaviour. The volume of hydrogen evolved-time curves shown in Fig. 3 for 2N HCI media are characterized by an initial slow increase in the volume of hydrogen evolved with time followed by a sharp rise that indicates linear change. The first part of the curves as in weight-loss measurements may be due to the oxide film originally present on the metal surface. The dissolution of this film is not accompanied by hydrogen evolution, so that the rate of reaction increases only slowly until the oxide film completely dissolves. The corresponding curves for 2N NaOH solutions similar to those of weight loss are straight lines starting from the origin because the oxide film originally formed is easily soluble in NaOH solution and the hydrogen evolved gradually increases by increasing time. Curves for the inhibitor containing systems fall below that of the free acid or alkali. These curves indicate that the hydrogen evolution rate depends on both the type and concentration of the additives. Table (2) indicates that on increasing the inhibitor concentration the % R.R. increases and the effect depends also upon the type of inhibitor. Inhibition efficiency among members of group 1 follows the sequence: benzophenone > phenylacetic acid > benzoic acid; acetophenone > phenol > benzaldehyde > benzamide > mandelic acid. It is evident that molecular size plays an important role in the inhibition process as all the compounds, with the exception of phenol, are benzene derivatives which contain the same adsorption active centre; the carbonyl group. Benzophenone comes on top of the group owing to the large size of the benzoyl substituent which imparts better surface coverage property. Phenylacetic acid comes next because in this case the skeletal structure of the molecular Fig. 4 allows the phenyl group to lie flat on the aluminium surface9. The reverse situation is encountered in benzoic acid which occupies third place as the vertical orientation of the phenyl group is more probable. Acetophenone is situated next and it is ANTI-CORROSION

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obviously adsorbed through the carbonyl oxygen whose electron charge density is increased by means of the hyper-conjugation effect of the adjacent methyl group. On the other hand, the oxygen atom in phenol which is most probably the adsorption site, is a weaker electron donor and phenols are even known to be weaker bases than alcohols. Moreover, a probable reaction of phenol with aluminium similar to the colouring reaction with ferric chloride may lead to the formation of a soluble salt not integrated in the oxide layer and consequently renders the inhibitive action of phenol lower than that of acetophenone as shown from the above observed order phenol also has a smaller molecular size. Benzaldehyde can be expected to follow acetophenone as measured heats of hydrogenation10 of aldehydes are higher than these of ketones. This is due to the inductive effect of the hydrogen attached to the carbonyl carbon. Desorption of the aldehyde by the aggressive chloride ions is adied by the evolved higher heat of hydrogenation during reaction. There is also a tendency of aldehydes to condense and polymerize. Linear or cyclic association exists in amides and the lone pair of electrons on the nitrogen is shared strongly with the carbonyl carbon so that amides are nearly nonbasic. Perhaps for these reasons benzamide is less effective than benzaldehyde. Mandelic acid comes last in the sequence of inhibition efficiency among members of group (I) and this may originate from its being a racemic mixture of the optically active compound. Inhibition efficiency within group (II) follows the sequence: Pyromellitic anhydride > naphthalic anhydride > trimellitic anhydride. Skeletal structures in Fig. 5 favour vertical orientation of the compounds on the surface. Anhydrides react with electron donors like water to give

the corresponding acid and with alcohol to give the acid and ester. Therefore, inhibition efficiency of the studied anhydrides is expected to be comparable but less effective than that of the corresponding acids.

The results obtained support this view. However, the slight difference observed between pyromellitic and 1,8naphthalic anhydrides may be related to the extension of the ring system in the former thus providing better surface coverage. The inhibition efficiency is nearly reduced to half its value in case of trimellitic anhydride most probably due to reduction in molecular size and in the number of active carbonyl centers adsorbed on aluminium surface. In Conclusion, weight-loss and hydrogen evolution methods lead on the whole to approximately similar results. Corrosion inhibition takes place through adsorption of the inhibitor on the aluminium surface by a one-step process. This is supported by the decrease corrosion rate with increase in concentration of the inhibitor and the linear variation of weight loss or hydrogen evolution with time. Results of alkaline methanolic media generally show the same trends revealed by weight loss data. Efficiency is always higher than in acidic methanolic solutions. This was accounted for previously in the case of aromatic acids11.12. 10

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Table (1). Effect of concentration on the percentage inhibition in 2N HCl (50% v/v methanol) at 60 min. for first group (benzene derivatives).

Table (2). Effect of inhibitor concentration on the percentage reduction in rate in HCl (50% v/v methanol at 60 min. for the first group (benzene derivatives).

-4 1 x 10 -5 5x10-5 1 x 10 5 x 10 -4 1 x 10-3

1 Benzophenone 2 Polyacetic acid 3 Benzoic acid 4 Acetophenone 5 Phenol 6 Benzaldehyde 7 Benzamide 8 Mandelic acid

31.1

95.0

1.6

3.1

13.1 39.3 23.6 20.2

17.3 42.6 30.3 33.7 12.0 48.5

5.1 61.4

76.2 66.1 29.0 48.3 37.0 37.0 23.3 13.8

99.7 70.4 53.6 52.8 41.5 51.6 28.8 26.2

99.4 76.1 67.7 56.1 53.9 50.5 41.2 29.5

A solution of NaOH in methanol contains methoxide ions and since the basicity of these ions is less than that of hydroxide ions it would not be surprising that the rate of aluminium corrosion decreases in aqueous solution containing methanol in equal proportions as in present investigation. References 1 Moussa, M.N.; El-Tagoury, M.M.; Radi, A.A. and Hassan, S.M. 2nd Chemistry Conference (28-30 June 1988). Dept. of Chemistry, Faculty of Science, Alex. Univ., Alexandria 20C, 196. 2 Hassan, S.M.; Moussa, M.N.H.; Taha, F.I. and Fouda, A.E., Corros. Sci., 21, 439 (1981). 3 Hassan, S.M.; El-Awady, Y.A.; Ahmed, A.I. and Baghlaf, A.O.; Corros. Sci., 19, 951 (1979). 4 Dinnappa, R.K. and Mayanna, S.M.; J. of Applied

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1 x 10 -55 x 10 -5 1 x 10 -4 5 x 10 -4 1 x 10 -3

1 Benzophenone 2 Polyacetic acid 3 Benzoic acid 4 Acetophenone 5 Phenol 6 Benzaldehyde 7 Benzamide 8 Mandelic acid

36.9

4.6 16.9

9.2 22.3 11.5

7.6 6.1

40.0 10.0 21.5 31.5 26.9 20.0 23.8

76.9 47.6 40.7 34.6 41.5 26.1 27.6

3.8

9.2

98.4 53.8 56.1 37.6 53.0 29.2 31.5 10.0

99.2 61.5 64.6 46.1 61.5 36.9 38.4 13.8

Electrochemistry, 11, 111 (1981). Issa, I.M.; Moussa, M.N.H. and Gandour, M.A.A., Corros. Sci., 73,791 (1973). 6 Desai, M.N.; Patel, R.R. and Shah, D.K., J. Inst. Chem. Calcutta, 45, 87 (1973). 7 Issa, I.M.; El-Samahy, A.A. and Temerk, Y.M., J. Chem. U.A.R., 13,121 (1970). 8 Moussa, M.N.H.; Taha, F.I.M.; Gouda, M.M.A. and Singab, G.M.; Corros. Sci., 16, 379 (1976). 9 Aflaton G.M. Singab, Master degree thesis, Mansoura University (1975). 10 Moussa, M.N.H.; Ph.D. thesis, London University (1963). 11 Caldin, E.F. and Trichett, J.,Trans. Faraday Soc., 49, 722 (1953). 12 Caldin,E.F. and Long, G.J. Chem. Soc., 3737 (1954).

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