Corrosion Inhibitors for Aluminum Student Experiments Bodo Muller Fachhochschule fur Druck Stuttgart, Fachbereich Farbe-Lack-Kunststoff, Nobelstr. 10, D-70569 Stuttgart, Germany
The inhibition of the electrochemical corrosion of metals in aaueous media is of meat economic and scientific interest (1,2).Aspecial application of corrosion inhibitors is the stabilization of aluminum ~ i e m e n t in s waterborne paints. e.g., metallic basecoats for'tKe automotive industry (3).A general overview regarding aluminum pigments is given in reference ( 4 ) .These waterborne metallic basecoats reduce significantly the emission of organic solvents to the atmosphere during the paint application (3). One of the problems of waterborne metallic basecoats is the corrosion of the aluminum pigments in alkaline (pH about 8) aqueous paint medium, which causes the formation of hydrogen (51:
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A disadvantageous consequence of this corrosion reaction is that the color of the oaint chanees from silver to grey ( 5 ) .Areview about different stabiliration methods for alummum pigments is presented in reference (5).An additional possibility for the inhibition of corrosion of aluminum pigments is the application of chelating agents (6). Therefore, a simple and reliable test method is applied in the present studv to investigate the corrosion inhibitinr effects of v a r i o u ~ c h e l a t i nagents ~ on aluminum pigment's: in aaueous alkaline media. This test is recommended to
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Journal of Chemical Education
demonstrate to students the effect of corrosion inhibitors in eeneral because the corrosion reaction of the aluminum pigments, compared with bulk materials, is accelerated by their larger surface area of about 5 mVg. Spherical aluminum powder already was used in a similar way to investipate corrosion inhibitors (7).The disadvantage of spherical particles is that they have a smaller surface area compared with flake-like pigments, and they often settle as a hard cake, which may be difficult to redisperse. Aluminum flakes are easily redispersible and better suited for this kind of test.
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Experimental Standard non-leafing aluminum pigment pastes1 for solvent-borne basecoats, which contain 65 weight percent aluminum and 35 weight percent hydrocarbon solvent, were used. 5.0 cr of such a paste is dispersed for five minutes by a magnetic stirrer h a glass b;aker in 100 mL of a mixture of water and butyl glycol in the ratio 1:l. Bntyl glycol i s the most common organic cosolvent i n waterborne 'Appropriate aluminum pigments are. e.g.: OBP 8507 from Obron Atlantic Corp.. 72 Cowin Drive, P 0 . Box 747. Painesville, OH 44077.0747 Sparkle Silver 5000 AR from Siiberline Manufacturing Co. Inc., P. 0. Box B, Tamaqua, PA 18252-0420.
Results and Discussion By the use ofthe described simple qualitative test procedure, structure-efficiency relations of corrosion inhibitors can be shown.
4480 ml (after 7 days)
Standard
The first results concern dicarboxylie acids: If axalic, malonie, sueeinie and maleic acid are tested, oxalic and malonie aeid prove to be good corrosion inhibitors, the others not. The explanation for this phenomenon is the ring size of the formed aluminum ehelates because oxalic aeid form five membered. malonic aeid six-membered chelate rings.
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Gluconic Acid /
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nobenzamide are tested, only salicylic acid acts as a cormsion inhibitor. In this series the importance of the combination of carboxyl and phenolic hydroxyl group becomes apparent. Both experiments demonstrate the high specificity of the corrosion inhibiting action. A small change in the chemical constitution may cause a big change in effkiency. Heterocyclic corrosion inhibitors (9, 10) like 1 H-benzotriazole, the sodium salt of 2-mereapto-benzothiazole or 2-(5aminapenty1)-benzimidazoleare well-known corrosion inhibitors far copper. T h e y are not inhibiting the corrosion of aluminum. This paints out the strong interdependence of corrosion inhibitor and metal.
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