Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

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APPLICATION-ORIENTED TECHNOLOGIES Fig. 5: Impedance spectra <strong>of</strong> Al1050 at pH8 and in solutions <strong>of</strong> <strong>the</strong> cleaner 1-3 (Table 2) Fig. 6: SEM image (5 keV, 200x): Al2014 after immersion in a solution <strong>of</strong> cleaner 2 corrosion product is most likely Al(OH) 3, which only appears black because <strong>of</strong> <strong>the</strong> small <strong>layer</strong> thickness. The next step was to find suitable inhibitors for <strong>the</strong> cleaning solution, which are able to protect <strong>the</strong> <strong>aluminium</strong> materials, but <strong>the</strong>y should not lower <strong>the</strong> pH-value to uphold <strong>the</strong> cleaning effect. First experiments were done with sodium di-hydrogen phosphate and benzotriazole. The substances were added to a 2% solution <strong>of</strong> cleaner 2 toge<strong>the</strong>r and each alone. In Fig. 7 <strong>the</strong> resistances R ox extracted from <strong>the</strong> impedance spectra at low frequency regions (around 100 mHz) are presented. They correspond to <strong>the</strong> quality <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> and <strong>the</strong> corrosion resistance, consequently. Here, <strong>the</strong> inhibition effect <strong>of</strong> both substances is obvious, whereas <strong>the</strong> effect <strong>of</strong> each single substance is limited. The following mechanism is assumed: Phosphate is forming insoluble <strong>aluminium</strong> phosphate, inhibiting <strong>the</strong> dissolution <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> bulk material. Benzotriazole is well known as inhibitor for copper. It forms conversion <strong>layer</strong>s on <strong>the</strong> copper rich phases, blocking <strong>the</strong> reduction <strong>of</strong> oxygen. In comparison <strong>the</strong> dissolution <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> bulk material dominates in <strong>the</strong> solution <strong>of</strong> cleaner 2. Therefore, <strong>the</strong> single addition <strong>of</strong> benzotriazole Cu Al O Non affected area 5 80 5 Affected area 78 7 10 Table 3: Results <strong>of</strong> EDX measurements, amounts <strong>of</strong> <strong>the</strong> elements Cu, Al and O in wt% Fig. 7: R OX extracted from <strong>the</strong> impedance spectra recorded in solutions <strong>of</strong> cleaner 2 with and without benzotriazole and sodium di-hydrogen phosphate to inhibit <strong>the</strong> inter-crystalline corrosion is insufficient to stop <strong>the</strong> bulk corrosion, whereas <strong>the</strong> single addition <strong>of</strong> phosphate can inhibit <strong>the</strong> bulk corrosion, but not <strong>the</strong> inter-crystalline corrosion. Finally, Fig. 8 shows panels <strong>of</strong> Al 2014 completely cleaned (dip-cleaning assisted by ultrasound for 10 min) with cleaner 2 with and without <strong>the</strong> additionally added benzotriazole and sodium di-hydrogen phosphate. The effect is obvious [14]. 4. Conclusions Using <strong>the</strong> electrochemical methods <strong>the</strong> different corrosion mechanisms <strong>of</strong> pure <strong>aluminium</strong> and Al 2014 could be detected. However, <strong>the</strong> behaviour <strong>of</strong> <strong>the</strong> free corrosion potentials is difficult to interpret especially for <strong>aluminium</strong> alloys. Here, <strong>the</strong> resulting potential is always a mixture <strong>of</strong> <strong>the</strong> response <strong>of</strong> <strong>the</strong> several inter metallic phases. Pure <strong>aluminium</strong> behaved passively in a pH range between 5 and 8. In <strong>the</strong> recorded impedance spectra only <strong>the</strong> resistance <strong>of</strong> <strong>the</strong> electrolyte and <strong>the</strong> capacity <strong>of</strong> <strong>the</strong> barrier <strong>layer</strong> were visible. At pH values above 10 a uniform dissolution <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> was observed characterized by a decreasing <strong>oxide</strong> <strong>layer</strong> resistance and an increasing surface capacity. The investigated <strong>aluminium</strong> alloy Al 2014 is an inhomogeneous alloy and has a relatively high copper content. The surface <strong>oxide</strong> <strong>layer</strong> <strong>of</strong> <strong>the</strong> alloy is also inhomogeneous and <strong>the</strong> bulk material consists <strong>of</strong> several phases. For this alloy a uniform and/or selective corrosion was found depending on <strong>the</strong> pH value already at room temperature. Below pH 3 <strong>the</strong> alloy showed a uniform corrosion. In <strong>the</strong> pH range between 3 and 8 an increase <strong>of</strong> <strong>the</strong> resistance in <strong>the</strong> lower frequency range combined with an unchanged capacity at medium frequencies were detected. This response can be interpreted as selective corrosion around <strong>the</strong> cathodic copper rich phases. This (maybe) leads to a formation <strong>of</strong> corrosion products with a black visual appearance. They precipitated on <strong>the</strong> whole immersed surface. Additionally measurements by SEM/EDX proved <strong>the</strong> selective corrosion and dissolution <strong>of</strong> <strong>aluminium</strong> around <strong>the</strong> copper rich phases. At pH values above 10 a uniform dissolution was observed for Al 2014. Transferred to <strong>the</strong> industrial cleaning <strong>the</strong>se results reveal <strong>the</strong> importance <strong>of</strong> <strong>the</strong> ‘right choice’ <strong>of</strong> method and cleaner concentrate (pH value!) as well as an experienced bath monitoring (at least pH value and chloride content). Fur<strong>the</strong>rmore, <strong>the</strong> experiments showed that always <strong>the</strong> compatibility <strong>of</strong> <strong>the</strong> cleaner and <strong>the</strong> material has to be tested before <strong>the</strong> application. Therefore, <strong>the</strong> recording <strong>of</strong> electrochemical impedance spectra at <strong>the</strong> free corrosion potential is predestinated. A comparison <strong>of</strong> three commercial available cleaners (all should be suitable for <strong>aluminium</strong> according to <strong>the</strong>ir product data sheet) showed, that in cleaners with <strong>the</strong> pH value above 8.5 pure <strong>aluminium</strong> corroded heavily. But also in cleaners with a suitable pH-value a moderate at- tack was observed, caused by a complexion <strong>of</strong> <strong>aluminium</strong> by cleaner ingredients. The addition <strong>of</strong> both, sodium di-hydrogen phosphate and benzotriazole, can prevent Al2014 in near neutral cleaning solutions. But <strong>the</strong> single addition <strong>of</strong> benzotriazole to inhibit <strong>the</strong> inter-crystalline corrosion is insufficient to stop <strong>the</strong> bulk corrosion, whereas <strong>the</strong> single addition <strong>of</strong> phosphate can inhibit <strong>the</strong> bulk corrosion, but not <strong>the</strong> inter-crystalline corrosion. References [1] H. Zhan, J. M. C. Mol, F. Hannour, L. Zhuang, H. Terryn and J. H. W. de Wit; “The influence <strong>of</strong> copper content on intergranular corrosion <strong>of</strong> model Fig. 8: Al 2014 test panels after dip cleaning in a 2 wt% solution <strong>of</strong> cleaner 2; left: without benzotriazole and sodium di-hydrogen phosphate, right: with benzotriazole and sodium di-hydrogen phosphate 52 ALUMINIUM · EAC CONGRESS 2011
AlMgSi(Cu) alloys”; Mat. Corr., 2008, 59, 670-675. [2] B. Zaid, D. Saidi, A. Benzaid, S. Hadji;“ Effects <strong>of</strong> pH and chloride concentration on pitting corrosion <strong>of</strong> AA6061 aluminum alloy“; Corros. Sci., 2008, 50, 1841-1847. [3] www.aluinfo.de, 2011. [4] U. Rammelt, S. Koehler, G. Reinhard; “Use <strong>of</strong> vapour phase corrosion inhibitors in packages for protecting mild steel against corrosion”; Corros. Sci., 2009, 51, 921-925. [5] S. Koehler, G. Reinhard; “Temporärer Korrosionsschutz”, Maschinenbau, 2008, 7, 34-37. [6] G. Reinhard, S. Lautner; “Temporärer Korro- sionsschutz von Aluminiumwerkst<strong>of</strong>fen mit flüch- tigen Korrosionsinhibitoren”; “GfKORR-Jahres- �� APPLICATION-ORIENTED TECHNOLOGIES tagung 1998”, 1998, 97-108. [7] H. Thompsen, F.H. Rögner; “Reinigen und Vorbehandeln – Keine triviale Angelegenheit”; Metalloberfläche, 2007, 61, 44-47. [8] H. Kollek „Reinigen und Vorbehandeln“, Curt R. Vincentz Verlag, Hannover, 1996, 13-20. [9] A. Bart, M.Stratmann (ed.); “Encyclopedia <strong>of</strong> Electrochemistry“ vol. 4 „Corrosion and Oxide films“; Vol. Editors: M. Stratmann, G.S. Frankel; Wiley VCH Weinheim 2003; Chapter 2. [10] Kyung-Keun Lee, Kwang-Bum Kim; “Electrochemical impedance characteristics <strong>of</strong> pure Al and Al–Sn alloys in NaOH solution”; Corros. Sci., 2001, 43, 561-575. [11] Y. L. Cheng, Z. Zhang, F. H. Cao, J. F. Li, J. Q. �� Zhang, J. M. Wang and C. N. Cao; “Study <strong>of</strong> <strong>the</strong> potential electrochemical noise during corrosion process <strong>of</strong> aluminum alloys 2024, 7075 and pure aluminum (pages 601–608)”; Mat. Corr., 2003, 54, 601-608. [12] F.M. Queiroz, M. Magnani, I. Costa, H.G. de Melo; “Investigation <strong>of</strong> <strong>the</strong> corrosion behaviour <strong>of</strong> AA 2024-T3 in low concentrated chloride media“; Corros. Sci., 2008, 50, 2646-2657. [13] D. Mercier, M.-G. Barthés-Labrousse; “The role <strong>of</strong> chelating agents on <strong>the</strong> corrosion mechanisms <strong>of</strong> <strong>aluminium</strong> in alkaline aqueous solutions”; Corros. Sci., 2009, 51, 339-348. [14] C.M. von Klingspor, Bachelor-Thesis, HTW- University <strong>of</strong> Applied Science Dresden, 2011. � �� ALUMINIUM · EAC CONGRESS 2011 53
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