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Citric acid as corrosion inhibitor for aluminium pigment
Corrosion Science 46 (2004) 159–167 www.elsevier.com/locate/corsci
Citric acid as corrosion inhibitor for aluminium pigment B. M€ uller
*
Fachhochschule Esslingen––Hochschule f€ur Technik, Chemieingenieurwesen/Farbe-Lack-Umwelt, Kanalstrasse 33, D-73728 Esslingen, Germany Received 6 February 2003; accepted 2 July 2003
Abstract Aluminium pigments corrode in aqueous alkaline media (e.g., water-borne paints) with the evolution of hydrogen. The hydrogen corrosion of aluminium pigment can be inhibited by the chelating agent citric acid, which is a renewable raw material and absolutely non-toxic. At pH 8 more hydrogen is evolved as at pH 10 which can be explained with the isoelectric point of aluminium oxide (pH 9) and a chemical reaction of citrate with aluminium. The product of this reaction should be an aluminium(III)–citric acid-chelate which is assumed to be the actual corrosion inhibitor. This assumption can be corroborated by tests with aluminium(III)–citric acid-chelates. With increasing addition of citric acid the hydrogen volumes evolved increase, which could also be explained with a chemical reaction of citrate with aluminium. Agaric acid (a-hexadecyl-citric acid) can be considered as a surfactant with the chelating agent citric acid as hydrophilic group and inhibits the corrosion reaction of aluminium pigment better when compared to citric acid. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: A. Aluminium; Inhibition; Measurement of evolved hydrogen
1. Introduction Aluminium pigments are mainly used in metallic decorative topcoats or printing inks. Water-borne paints significantly reduce the emission of organic solvents to the atmosphere during paint application. The pH-values of water-borne paints are about
*
Tel.: +49-711-397-3515; fax: +49-711-397-3502. E-mail address: [email protected] (B. M€ uller).
0010-938X/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0010-938X(03)00191-4
160
B. M€uller / Corrosion Science 46 (2004) 159–167
8–9. A severe problem of water-borne metallic paints is the hydrogen evolution caused by aluminium pigments in aqueous alkaline paint media. A detailed study of the corrosion reactions of different types of aluminium pigments in aqueous alkaline media has been presented elsewhere [1]: 2Al þ 6H2 O ! 2AlðOHÞ3 þ 3H2
ð1Þ
The hydrogen evolution during this corrosion reaction may lead to dangerous pressure-build ups in containers. Therefore, corrosion inhibition is necessary. Commonly established stabilization methods (chromatic treatment and stabilization with organic phosphorus compounds) show some disadvantages like reduced intercoat adhesion after humidity test (organic phosphorus compounds) [2]; the chromatic treatment is problematic because chromium(VI) is carcinogenic. So, the investigation of alternative and especially non-toxic methods for the inhibition of this corrosion reaction is of interest. Citric acid is a renewable raw material and absolutely non-toxic; citric acid is naturally occurring, e.g., in lemon juice. Moreover, citric acid is a tetradentate chelating agent (Fig. 1). Citric acid has been mentioned previously as corrosion inhibitor for aluminium pigments in different publications [3–6]. If these results scattered in the literature are summarized and combined with unpublished experiments a full view about corrosion inhibition of aluminium pigment by citric acid appears. The subject of the present study is a comparative discussion of citric acid as corrosion inhibitor for aluminium pigment in aqueous alkaline media.
Fig. 1. Simplified presentation of the possible chemical reaction of citrate anion with aluminium leading to an aluminium(III)–citric acid-chelate.
B. M€uller / Corrosion Science 46 (2004) 159–167
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2. Experimental method An unstabilized pigment paste for solvent-borne paints of a lamellar non-leafing aluminium pigment (35 wt% hydrocarbon solvent; specific surface about 5 m2 /g solids, average particle diameter 16 lm, aluminium content of the metal >99.9%) was used [1]. The corrosion medium was usually a mixture of desalinated water and butyl glycol in the ratio 9:1; butyl glycol is the most common cosolvent in water-borne paints. To improve the wetting of the hydrophobic aluminium pigment paste by the medium, 2.0 weight% (wt%) of a wetting agent (adduct of 10 mol of ethylene oxide to nonylphenol) was added. Additionally, some tests were carried out in water and butyl glycol in the ratio 1:1 without wetting agent. Citric acid was dissolved in concentrations of 0.35–6.0 mmol/100 ml in the aqueous solvent mixture. The pHvalues of the solutions were raised to 8.0 and 10 with dimethylethanolamine (DMEA); DMEA is a commonly used neutralizing agent in water-borne paints. Then 5.0 g aluminium pigment paste were dispersed for five minutes with a magnetic stirrer in 100 ml of the corrosion media. The progress of the corrosion reaction was determined daily by volumetric measurement of the evolved hydrogen over a period of 21 days at room temperature. With regard to practical applications of aluminium pigments in water-borne paints the total hydrogen volume evolved in a certain period of time is the most important measured quantity. Previous studies showed that neither the wetting agent [7] nor DMEA [8,9] inhibits the corrosion of aluminium pigment. The hydrogen volume by complete reaction of the assumed pure aluminium metal pigment was calculated to 4.32 l when measured at 20 °C [4]. In a previous study the hydrogen volume was determined to 4.24 l (average value out of 29 gas volumetric tests; mean deviation 3.3%; maximum deviation 8%) [4]. So, it can be concluded that there is no significant reduction of oxygen.
3. Experimental results and discussion Fig. 2 shows that the hydrogen evolution of the standard aluminium dispersions (without inhibitor) was faster at pH 10 as at pH 8 [4,5]. In the first days of the corrosion reaction no hydrogen evolution was observed; after this latency period the corrosion reaction initiated. With addition of citric acid (Fig. 2) hydrogen formation was observed immediately after the dispersion of the aluminium pigment. The citrate anion is supposed to convert aluminium to aluminium(III) (high rate of hydrogen evolution in the first days) which then lead to the build-up of a protective layer (corrosion inhibition: plateau of the hydrogen–time-curve in Fig. 2). So, a reaction product of aluminium and citric acid (possibly a chelate complex) should be the actual corrosion inhibitor (Fig. 1). At the pH value of 8 more hydrogen was evolved as at pH 10 (Fig. 2); similar pHdependencies of the hydrogen evolution have been observed previously also with other corrosion inhibiting chelating agents [4,5]. This surprising pH-dependency can
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Fig. 2. Time dependency of the hydrogen evolution from dispersions of the aluminium pigment in water/ butyl glycol ¼ 9:1 without (standard) at pH 10 and with addition of 2.4 mmol/100 ml citric acid at pH 8 and 10.
be explained with the isoelectric point of aluminium oxide (pH 9, [10]); the isoelectric point of lamellar aluminium pigments may be lower than 9 because of the lubricating agents (fatty acids). At pH 8 (below the isoelectric point of aluminium oxide) the aluminium oxide surface is positively charged; so, more of the negatively charged citrate anion is adsorbed as above the isoelectric point at pH 10 (negatively charged aluminium oxide surface) which lead to increased hydrogen formation at pH 8 because of the chemical reaction of the adsorbed citrate ion (Fig. 1). The fact that below the isoelectric point more polyanions are adsorbed as above has been observed by different authors (with different analytical methods) on aluminium oxide (alumina) [11,12], aluminium pigment [4,13] and aluminium metal sheets [14]. The assumption, that an 1:1-aluminium(III)–citric acid-chelate (Fig. 1) is the actual corrosion inhibitor, is justified by previous results with other aluminium(III)chelates [15]. To corroborate this assumption with citric acid, soluble aluminium(III)–citric acid-chelates were prepared by combining aqueous solutions of citric acid and different aluminium(III) salts (see [15,16]). The actual concentrations were 1.8 mmol/100 ml of aluminium(III) salts and a small excess of 2.0 mmol/100 ml of citric acid, to be sure that all aluminium(III) is chelated. The results with aluminium(III)–citric acid-chelates are recorded in Fig. 3 (for clarity only the hydrogen volumes after 21 days are plotted). It is obvious, that the aluminium(III)–citric acidchelates reduce the hydrogen formation when compared to citric acid (Fig. 3). The conversion of aluminium to aluminium(III) can be calculated from the hydrogen volumes evolved according to Eq. (1), respectively, Fig. 1. The reduction of the calculated aluminium conversion (0.5–0.9 mmol; Fig. 3) is smaller than the added amount of aluminium(III) salts (1.8 mmol). So, the 1:1-aluminium(III)–citric acidchelates improve corrosion inhibition when compared to citric acid but not in the extent as expected.
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Fig. 3. A comparison of hydrogen volumes evolved within 21 days from dispersions of the aluminium pigment in water/butyl glycol ¼ 9:1 at pH 10 with addition of 2.0 mmol/100 ml of citric acid only and with citric acid plus 1.8 mmol/100 ml of aluminium(III)-nitrate, -chloride respectively -sulfate. The tests were carried out isochronously.
Fig. 4. Hydrogen volumes evolved within 21 days from dispersions of the aluminium pigment in water/ butyl glycol ¼ 9:1 at pH 10 and calculated conversion to aluminium(III) versus addition level of citric acid. The tests with 2.0 mmol/100 ml of citric acid was carried out three times.
In the next series of experiments the influence of the concentration of citric acid was examined (Fig. 4). The hydrogen volumes evolved increase with increasing addition of citric acid (linear correlation; Fig. 4). Moreover, the repeated tests with 2.0 mmol of citric acid show the good reproducibility of the test method (Fig. 4). But
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these results cannot be explained with the formation of 1:1-aluminium(III)–citric acid-chelates only because always more aluminium is converted to aluminium(III) as citric acid is added (molar ratios). Now the concentrations of soluble aluminum(III) in filtered corrosion media were measured by atomic absorption spectroscopy (AAS). Aluminium(III) is only very slightly soluble in water/butyl glycol ¼ 9:1 at pH 8 and 10; in the media of the standards (complete conversion) the measured aluminium(III)-concentration is only 0.02 mmol/100 ml [5]. With addition of citric acid the solubility of aluminium(III) becomes much higher (Fig. 5). But again these results cannot be explained with the formation of soluble 1:1-aluminium(III)–citric acid-chelates only, because always more aluminium(III) is detected in the filtered media by AAS than citric acid is added (molar ratios). A possible explanation could be the assumption of an electrostatic stabilization of aluminium hydroxide crystal nuclei by the citrate polyanion; this phenomenon is known as precipitation, scale or threshold inhibition [17]. Colloidal aluminium hydroxide cannot be filtered and is therefore detected by AAS. Moreover, with addition of citric acid the amount of soluble aluminum(III) is always lower than the calculated conversion to aluminum(III) (Fig. 5). This indicates that a significant amount of the formed aluminum(III) is located on the surface of the pigment as part of the non-soluble protective layer [5,15]. As a provisional result can be stated, that a reaction product of citrate and aluminium is the actual corrosion inhibitor. But the question about the exact chemical composition of this reaction product is still open. The lowest hydrogen evolution (best corrosion inhibition) was observed in Fig. 4 (above) with the lowest citric acid addition of 0.35 mmol. So, we examined this test over a longer period as the usual 21 days (Fig. 6). In water/butyl glycol ¼ 9:1 after the
Fig. 5. From the hydrogen evolution calculated conversion to aluminium(III) compared with measured aluminium(III) concentration (AAS) in water/butyl glycol ¼ 9:1 after 21 days.
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165
Fig. 6. Long term time dependency of the hydrogen evolution from a dispersion of the aluminium pigment in water/butyl glycol (W/BG) ¼ 9:1 and 1:1 with addition of 0.35 mmol/100 ml of citric acid at pH 10. In water/butyl glycol ¼ 1:1 the hydrogen volume is constant up to 142 days.
Fig. 7. Structural formula of agaric acid (a-hexadecyl-citric acid).
initial hydrogen evolution in the first 3–5 days a plateau of the hydrogen–time-curve is observed in Fig. 6 (corrosion inhibition). But after about 45 days the corrosion rate increases which leads to complete reaction of the aluminium pigment after 55 days (Fig. 6). So it is assumed, that the lowest examined citric acid addition produces initially a thin protection layer on the aluminium pigment surfaces which is destroyed, respectively, dissolved by the medium in the course of time. In water/butyl glycol ¼ 1:1 however, there is a stable plateau of the hydrogen–time-curve up to 142 days (Fig. 6). There is now the question how to improve corrosion inhibition of aluminium pigment by citric acid, e.g., by an appropriate derivative of citric acid. One possibility is to introduce a long-chain hydrophobic alkyl group in the citric acid molecule [6]. Such a derivative is agaric acid (a-hexadecyl-citric acid; Fig. 7). Agaric acid can be considered as a surfactant with the chelating agent citric acid as hydrophilic group. The experimental results with agaric acid in comparison to citric acid are presented in Fig. 8. It is obvious, that agaric acid is a better corrosion inhibitor than citric acid. So, the hydrophobic group improves corrosion inhibition. Other examples for corrosion inhibition of aluminium pigment with chelating agents with hydrophobic groups have been presented elsewhere [6,18].
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Fig. 8. A comparison of hydrogen volumes evolved within 21 days from dispersions of the aluminium pigment in water/butyl glycol ¼ 9:1 and 1:1 at pH 10 with addition of 2.0 mmol/100 ml of citric acid and agaric acid. The tests were carried out isochronously.
4. Conclusions The hydrogen corrosion of aluminium pigment in aqueous alkaline media can be inhibited by the chelating agent citric acid. At pH 8 more hydrogen is evolved as at pH 10 which can be explained with the isoelectric point of aluminium oxide (pH 9) and a chemical reaction of citrate with aluminium. The product of this reaction should be an aluminium(III)–citric acid-chelate which is assumed to be the actual corrosion inhibitor. This assumption can be corroborated by tests with aluminium(III)–citric acid-chelates. With increasing addition of citric acid the hydrogen volumes evolved increase, which could also be explained with a chemical reaction of citrate with aluminium. Agaric acid (a-hexadecyl-citric acid) can be considered as a surfactant with the chelating agent citric acid as hydrophilic group and inhibits the corrosion reaction of aluminium pigment better when compared to citric acid. Acknowledgements The author is grateful to his former students M. Gampper, M. Kurfeß, D. Mebarek, M. M€ uller and T. Schmelich for their skillful experimental work and to Eckart-Werke for the aluminium pigment. References [1] B. M€ uller, Farbe Lack 103 (8) (1997) 24. [2] R. Besold, W. Reißer, E. Roth, Farbe Lack 97 (1991) 311.
B. M€uller / Corrosion Science 46 (2004) 159–167 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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B. M€ uller, M. M€ uller, I. L€ ohrke, Farbe Lack 100 (1994) 528. B. M€ uller, M. Gampper, Werkst. Korros. 45 (1994) 272. B. M€ uller, K. Franze, D. Mebarek, Corrosion 51 (1995) 625. B. M€ uller, S. Kubitzki, Mater. Corros. 48 (1997) 755. B. M€ uller, J. Coat. Technol. 67 (846) (1995) 59. B. M€ uller, Br. Corros. J. 31 (1996) 315. B. M€ uller, React. Funct. Polym. 39 (1999) 165. G.A. Parks, Chem. Rev. 65 (1965) 177. D. Balzer, H. Lange, Colloid Polym. Sci. 253 (1975) 643. K. Ishiduki, K. Esumi, Langmuir 13 (1997) 1587. B. M€ uller, Mater. Corros. 49 (1998) 753. B. M€ uller, Surf. Coat. Int. Part B 85 (2002) 111. B. M€ uller, K. Franze, Werkst. Korros. 45 (1994) 467. B. M€ uller, W. Kl€ager, G. Kubitzki, Corros. Sci. 39 (1997) 1481. W.M. Hann, Dispersants, in: J.I. Kroschwitz, M. Howe-Grant (Eds.), Kirk–Othmer, Encyclopedia of Chemical Technology, vol. 8, fourth ed., Wiley, New York, 1993, p. 293. [18] B. M€ uller, Tenside, Surf. Det. 38 (2001) 273.
Citric acid as corrosion inhibitor for aluminium pigment B. M€ uller
*
Fachhochschule Esslingen––Hochschule f€ur Technik, Chemieingenieurwesen/Farbe-Lack-Umwelt, Kanalstrasse 33, D-73728 Esslingen, Germany Received 6 February 2003; accepted 2 July 2003
Abstract Aluminium pigments corrode in aqueous alkaline media (e.g., water-borne paints) with the evolution of hydrogen. The hydrogen corrosion of aluminium pigment can be inhibited by the chelating agent citric acid, which is a renewable raw material and absolutely non-toxic. At pH 8 more hydrogen is evolved as at pH 10 which can be explained with the isoelectric point of aluminium oxide (pH 9) and a chemical reaction of citrate with aluminium. The product of this reaction should be an aluminium(III)–citric acid-chelate which is assumed to be the actual corrosion inhibitor. This assumption can be corroborated by tests with aluminium(III)–citric acid-chelates. With increasing addition of citric acid the hydrogen volumes evolved increase, which could also be explained with a chemical reaction of citrate with aluminium. Agaric acid (a-hexadecyl-citric acid) can be considered as a surfactant with the chelating agent citric acid as hydrophilic group and inhibits the corrosion reaction of aluminium pigment better when compared to citric acid. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: A. Aluminium; Inhibition; Measurement of evolved hydrogen
1. Introduction Aluminium pigments are mainly used in metallic decorative topcoats or printing inks. Water-borne paints significantly reduce the emission of organic solvents to the atmosphere during paint application. The pH-values of water-borne paints are about
*
Tel.: +49-711-397-3515; fax: +49-711-397-3502. E-mail address: [email protected] (B. M€ uller).
0010-938X/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0010-938X(03)00191-4
160
B. M€uller / Corrosion Science 46 (2004) 159–167
8–9. A severe problem of water-borne metallic paints is the hydrogen evolution caused by aluminium pigments in aqueous alkaline paint media. A detailed study of the corrosion reactions of different types of aluminium pigments in aqueous alkaline media has been presented elsewhere [1]: 2Al þ 6H2 O ! 2AlðOHÞ3 þ 3H2
ð1Þ
The hydrogen evolution during this corrosion reaction may lead to dangerous pressure-build ups in containers. Therefore, corrosion inhibition is necessary. Commonly established stabilization methods (chromatic treatment and stabilization with organic phosphorus compounds) show some disadvantages like reduced intercoat adhesion after humidity test (organic phosphorus compounds) [2]; the chromatic treatment is problematic because chromium(VI) is carcinogenic. So, the investigation of alternative and especially non-toxic methods for the inhibition of this corrosion reaction is of interest. Citric acid is a renewable raw material and absolutely non-toxic; citric acid is naturally occurring, e.g., in lemon juice. Moreover, citric acid is a tetradentate chelating agent (Fig. 1). Citric acid has been mentioned previously as corrosion inhibitor for aluminium pigments in different publications [3–6]. If these results scattered in the literature are summarized and combined with unpublished experiments a full view about corrosion inhibition of aluminium pigment by citric acid appears. The subject of the present study is a comparative discussion of citric acid as corrosion inhibitor for aluminium pigment in aqueous alkaline media.
Fig. 1. Simplified presentation of the possible chemical reaction of citrate anion with aluminium leading to an aluminium(III)–citric acid-chelate.
B. M€uller / Corrosion Science 46 (2004) 159–167
161
2. Experimental method An unstabilized pigment paste for solvent-borne paints of a lamellar non-leafing aluminium pigment (35 wt% hydrocarbon solvent; specific surface about 5 m2 /g solids, average particle diameter 16 lm, aluminium content of the metal >99.9%) was used [1]. The corrosion medium was usually a mixture of desalinated water and butyl glycol in the ratio 9:1; butyl glycol is the most common cosolvent in water-borne paints. To improve the wetting of the hydrophobic aluminium pigment paste by the medium, 2.0 weight% (wt%) of a wetting agent (adduct of 10 mol of ethylene oxide to nonylphenol) was added. Additionally, some tests were carried out in water and butyl glycol in the ratio 1:1 without wetting agent. Citric acid was dissolved in concentrations of 0.35–6.0 mmol/100 ml in the aqueous solvent mixture. The pHvalues of the solutions were raised to 8.0 and 10 with dimethylethanolamine (DMEA); DMEA is a commonly used neutralizing agent in water-borne paints. Then 5.0 g aluminium pigment paste were dispersed for five minutes with a magnetic stirrer in 100 ml of the corrosion media. The progress of the corrosion reaction was determined daily by volumetric measurement of the evolved hydrogen over a period of 21 days at room temperature. With regard to practical applications of aluminium pigments in water-borne paints the total hydrogen volume evolved in a certain period of time is the most important measured quantity. Previous studies showed that neither the wetting agent [7] nor DMEA [8,9] inhibits the corrosion of aluminium pigment. The hydrogen volume by complete reaction of the assumed pure aluminium metal pigment was calculated to 4.32 l when measured at 20 °C [4]. In a previous study the hydrogen volume was determined to 4.24 l (average value out of 29 gas volumetric tests; mean deviation 3.3%; maximum deviation 8%) [4]. So, it can be concluded that there is no significant reduction of oxygen.
3. Experimental results and discussion Fig. 2 shows that the hydrogen evolution of the standard aluminium dispersions (without inhibitor) was faster at pH 10 as at pH 8 [4,5]. In the first days of the corrosion reaction no hydrogen evolution was observed; after this latency period the corrosion reaction initiated. With addition of citric acid (Fig. 2) hydrogen formation was observed immediately after the dispersion of the aluminium pigment. The citrate anion is supposed to convert aluminium to aluminium(III) (high rate of hydrogen evolution in the first days) which then lead to the build-up of a protective layer (corrosion inhibition: plateau of the hydrogen–time-curve in Fig. 2). So, a reaction product of aluminium and citric acid (possibly a chelate complex) should be the actual corrosion inhibitor (Fig. 1). At the pH value of 8 more hydrogen was evolved as at pH 10 (Fig. 2); similar pHdependencies of the hydrogen evolution have been observed previously also with other corrosion inhibiting chelating agents [4,5]. This surprising pH-dependency can
162
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Fig. 2. Time dependency of the hydrogen evolution from dispersions of the aluminium pigment in water/ butyl glycol ¼ 9:1 without (standard) at pH 10 and with addition of 2.4 mmol/100 ml citric acid at pH 8 and 10.
be explained with the isoelectric point of aluminium oxide (pH 9, [10]); the isoelectric point of lamellar aluminium pigments may be lower than 9 because of the lubricating agents (fatty acids). At pH 8 (below the isoelectric point of aluminium oxide) the aluminium oxide surface is positively charged; so, more of the negatively charged citrate anion is adsorbed as above the isoelectric point at pH 10 (negatively charged aluminium oxide surface) which lead to increased hydrogen formation at pH 8 because of the chemical reaction of the adsorbed citrate ion (Fig. 1). The fact that below the isoelectric point more polyanions are adsorbed as above has been observed by different authors (with different analytical methods) on aluminium oxide (alumina) [11,12], aluminium pigment [4,13] and aluminium metal sheets [14]. The assumption, that an 1:1-aluminium(III)–citric acid-chelate (Fig. 1) is the actual corrosion inhibitor, is justified by previous results with other aluminium(III)chelates [15]. To corroborate this assumption with citric acid, soluble aluminium(III)–citric acid-chelates were prepared by combining aqueous solutions of citric acid and different aluminium(III) salts (see [15,16]). The actual concentrations were 1.8 mmol/100 ml of aluminium(III) salts and a small excess of 2.0 mmol/100 ml of citric acid, to be sure that all aluminium(III) is chelated. The results with aluminium(III)–citric acid-chelates are recorded in Fig. 3 (for clarity only the hydrogen volumes after 21 days are plotted). It is obvious, that the aluminium(III)–citric acidchelates reduce the hydrogen formation when compared to citric acid (Fig. 3). The conversion of aluminium to aluminium(III) can be calculated from the hydrogen volumes evolved according to Eq. (1), respectively, Fig. 1. The reduction of the calculated aluminium conversion (0.5–0.9 mmol; Fig. 3) is smaller than the added amount of aluminium(III) salts (1.8 mmol). So, the 1:1-aluminium(III)–citric acidchelates improve corrosion inhibition when compared to citric acid but not in the extent as expected.
B. M€uller / Corrosion Science 46 (2004) 159–167
163
Fig. 3. A comparison of hydrogen volumes evolved within 21 days from dispersions of the aluminium pigment in water/butyl glycol ¼ 9:1 at pH 10 with addition of 2.0 mmol/100 ml of citric acid only and with citric acid plus 1.8 mmol/100 ml of aluminium(III)-nitrate, -chloride respectively -sulfate. The tests were carried out isochronously.
Fig. 4. Hydrogen volumes evolved within 21 days from dispersions of the aluminium pigment in water/ butyl glycol ¼ 9:1 at pH 10 and calculated conversion to aluminium(III) versus addition level of citric acid. The tests with 2.0 mmol/100 ml of citric acid was carried out three times.
In the next series of experiments the influence of the concentration of citric acid was examined (Fig. 4). The hydrogen volumes evolved increase with increasing addition of citric acid (linear correlation; Fig. 4). Moreover, the repeated tests with 2.0 mmol of citric acid show the good reproducibility of the test method (Fig. 4). But
164
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these results cannot be explained with the formation of 1:1-aluminium(III)–citric acid-chelates only because always more aluminium is converted to aluminium(III) as citric acid is added (molar ratios). Now the concentrations of soluble aluminum(III) in filtered corrosion media were measured by atomic absorption spectroscopy (AAS). Aluminium(III) is only very slightly soluble in water/butyl glycol ¼ 9:1 at pH 8 and 10; in the media of the standards (complete conversion) the measured aluminium(III)-concentration is only 0.02 mmol/100 ml [5]. With addition of citric acid the solubility of aluminium(III) becomes much higher (Fig. 5). But again these results cannot be explained with the formation of soluble 1:1-aluminium(III)–citric acid-chelates only, because always more aluminium(III) is detected in the filtered media by AAS than citric acid is added (molar ratios). A possible explanation could be the assumption of an electrostatic stabilization of aluminium hydroxide crystal nuclei by the citrate polyanion; this phenomenon is known as precipitation, scale or threshold inhibition [17]. Colloidal aluminium hydroxide cannot be filtered and is therefore detected by AAS. Moreover, with addition of citric acid the amount of soluble aluminum(III) is always lower than the calculated conversion to aluminum(III) (Fig. 5). This indicates that a significant amount of the formed aluminum(III) is located on the surface of the pigment as part of the non-soluble protective layer [5,15]. As a provisional result can be stated, that a reaction product of citrate and aluminium is the actual corrosion inhibitor. But the question about the exact chemical composition of this reaction product is still open. The lowest hydrogen evolution (best corrosion inhibition) was observed in Fig. 4 (above) with the lowest citric acid addition of 0.35 mmol. So, we examined this test over a longer period as the usual 21 days (Fig. 6). In water/butyl glycol ¼ 9:1 after the
Fig. 5. From the hydrogen evolution calculated conversion to aluminium(III) compared with measured aluminium(III) concentration (AAS) in water/butyl glycol ¼ 9:1 after 21 days.
B. M€uller / Corrosion Science 46 (2004) 159–167
165
Fig. 6. Long term time dependency of the hydrogen evolution from a dispersion of the aluminium pigment in water/butyl glycol (W/BG) ¼ 9:1 and 1:1 with addition of 0.35 mmol/100 ml of citric acid at pH 10. In water/butyl glycol ¼ 1:1 the hydrogen volume is constant up to 142 days.
Fig. 7. Structural formula of agaric acid (a-hexadecyl-citric acid).
initial hydrogen evolution in the first 3–5 days a plateau of the hydrogen–time-curve is observed in Fig. 6 (corrosion inhibition). But after about 45 days the corrosion rate increases which leads to complete reaction of the aluminium pigment after 55 days (Fig. 6). So it is assumed, that the lowest examined citric acid addition produces initially a thin protection layer on the aluminium pigment surfaces which is destroyed, respectively, dissolved by the medium in the course of time. In water/butyl glycol ¼ 1:1 however, there is a stable plateau of the hydrogen–time-curve up to 142 days (Fig. 6). There is now the question how to improve corrosion inhibition of aluminium pigment by citric acid, e.g., by an appropriate derivative of citric acid. One possibility is to introduce a long-chain hydrophobic alkyl group in the citric acid molecule [6]. Such a derivative is agaric acid (a-hexadecyl-citric acid; Fig. 7). Agaric acid can be considered as a surfactant with the chelating agent citric acid as hydrophilic group. The experimental results with agaric acid in comparison to citric acid are presented in Fig. 8. It is obvious, that agaric acid is a better corrosion inhibitor than citric acid. So, the hydrophobic group improves corrosion inhibition. Other examples for corrosion inhibition of aluminium pigment with chelating agents with hydrophobic groups have been presented elsewhere [6,18].
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B. M€uller / Corrosion Science 46 (2004) 159–167
Fig. 8. A comparison of hydrogen volumes evolved within 21 days from dispersions of the aluminium pigment in water/butyl glycol ¼ 9:1 and 1:1 at pH 10 with addition of 2.0 mmol/100 ml of citric acid and agaric acid. The tests were carried out isochronously.
4. Conclusions The hydrogen corrosion of aluminium pigment in aqueous alkaline media can be inhibited by the chelating agent citric acid. At pH 8 more hydrogen is evolved as at pH 10 which can be explained with the isoelectric point of aluminium oxide (pH 9) and a chemical reaction of citrate with aluminium. The product of this reaction should be an aluminium(III)–citric acid-chelate which is assumed to be the actual corrosion inhibitor. This assumption can be corroborated by tests with aluminium(III)–citric acid-chelates. With increasing addition of citric acid the hydrogen volumes evolved increase, which could also be explained with a chemical reaction of citrate with aluminium. Agaric acid (a-hexadecyl-citric acid) can be considered as a surfactant with the chelating agent citric acid as hydrophilic group and inhibits the corrosion reaction of aluminium pigment better when compared to citric acid. Acknowledgements The author is grateful to his former students M. Gampper, M. Kurfeß, D. Mebarek, M. M€ uller and T. Schmelich for their skillful experimental work and to Eckart-Werke for the aluminium pigment. References [1] B. M€ uller, Farbe Lack 103 (8) (1997) 24. [2] R. Besold, W. Reißer, E. Roth, Farbe Lack 97 (1991) 311.
B. M€uller / Corrosion Science 46 (2004) 159–167 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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