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Characterization of Intergranular Corrosion in AA 5xxx Al-Mg Alloys

Charakterisierung der interkristallinen Korrosion in AA 5xxx Al-Mg Legierungen
O. Engler, K. Kuhnke, H.-J. Krupp and T. Hentschel
From the journal Practical Metallography

Abstract

Intergranular corrosion refers to a selective corrosion attack at the grain boundaries of polycrystalline materials. In engineering Al-Mg alloys of the AA 5xxx series it is caused by the formation of chemicallyless noble β-Al8Mg5 phases along the grain boundaries. The sensitivity of a material to intergranular corrosion can be assessed based on the mass losses in the so-called nitric acid mass loss test (NAMLT) according to ASTM G67. However, a detailed investigation of the underlying corrosion mechanisms requires that the β phases are made directly visible in the microstructure by metallographic methods. In the present work, the NAMLT mass losses of three AA 5xxx alloys with different Mg contents are compared with the results from two different etching methods. On the one hand, the alloys are etched in diluted phosphoric acid, a substance routinely used to examine the grain boundary occupancy in AA 5xxx materials. On the other hand, a newer etching method using a dilute ammonium persulfate solution is tested which etches the β-Al8Mg5 phases in the microstructure in a way that they can be examined at higher magnifications; even examinations in the scanning electron microscope are possible.

Kurzfassung

Unter interkristalliner Korrosion versteht man einen selektiven Korrosionsangriff an den Korngrenzen polykristalliner Werkstoffe, der in technischen Al-Mg Legierungen der AA 5xxx-Serie durch die Bildung von chemisch unedlen β-Al8Mg5-Phasen entlang der Korngrenzen verursacht wird. Die Empfindlichkeit eines Werkstoffs gegen interkristalline Korrosion kann anhand der Massenverluste in dem sogenannten nitric acid mass loss test (NAMLT) nach ASTM G67 bestimmt werden. Eine detaillierte Untersuchung der zugrunde liegenden Korrosionsmechanismen erfordert jedoch eine direkte Sichtbarmachung der β-Phasen im Gefüge mittels metallographischer Methoden. In der vorliegenden Arbeit werden die NAMLT-Massenverluste von drei AA 5xxx-Legierungen mit unterschiedlichen Mg-Gehalten mit den Ergebnissen von zwei verschiedenen Ätzmethoden verglichen. Zum einen wird eine Ätzung in verdünnter Phosphorsäure durchgeführt, die routinemäßig zur Untersuchung der Korngrenzenbelegung in AA 5xxx-Werkstoffen zum Einsatz kommt. Daneben wird ein neueres Ätzverfahren in einer verdünnten Ammoniumpersulfat-Lösung ausprobiert, welches die β-Al8Mg5-Phasen im Gefüge so anätzt, dass sie auch bei höheren Vergrößerungen untersucht werden können; auch Untersuchungen im Rasterelektronenmikroskop sind möglich.


Translation: E. Engert


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Received: 2020-01-27
Accepted: 2020-04-27
Published Online: 2020-08-03
Published in Print: 2020-08-14

© 2020, Carl Hanser Verlag, München