6000-series aluminum alloys are normally resistant to intergranular corrosion (IGC). However, underaged alloys, containing Cu as little as 0.1%, may be susceptible due to grain boundary sensitization by precipitation of Cu-rich nanofilm and depletion of Si in certain tempers [1]. The purpose of this work is to understand the initiation mechanism of IGC on as received and modified surface of extruded alloy AA6005. Methods for surface modification consisted of metallographic polishing, argon sputtering and alkaline etching. IGC initiation and testing was performed according to the standard BS-ISO 11846, consisting of immersion in acidified NaCl solution (pH 1) for different periods. Initiation of IGC was slower on as-received surface compared to the modified surface. Glow Discharge Optical Emission Spectroscopy (GD-OES) analysis indicated this was due to the presence of an approximately 50 nm thick mixed metal oxide layer. IGC initiated at the primary α-Al(Fe,Mn)Si precipitates for all types of surfaces. However, these phases corroded rapidly in the test solution causing formation of dealloyed Cu on the exposed particle surface, which act as increasingly effective external cathode for IGC. The AlMgSiCu (Q-phase) precipitates, present as primary and secondary particles, were relatively stable in the test solution both against anodic oxidation and as cathodic sites. In fact, the Q-phase particles appeared to act as physical barriers against IGC propagation. These results were in accordance with earlier preliminary results for the Ar-sputtered surface [2].

References

[1] M. Hurlen Larsen, J. C. Walmsley, O. Lunder, and K. Nisancioglu, J. Electrochem. Soc., 157, C61 (2010).

[2] K.Shimizu, K.Nisancioglu, ECS Electrochemistry Letters, 3 (9) C29-C31 (2014).