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As part of an extensive research program to study recent, unexpected intergranular corrosion (IGC) on 6xxx series aluminum alloys (AlMgSi), this paper investigates the mechanism of initiation and early propagation of IGC on the extruded AA6005-T5 alloy with small Cu content (0.1 wt%) by use of advanced electron microscopy techniques applied for near surface characterization. Corrosion testing was restricted to the accelerated IGC test according to the standard BS ISO 11846, involving exposure to acidified chloride solution. The effect of modifying the as-received extruded surface by metallographic polishing, argon sputtering, and alkaline etching was investigated. Initiation of IGC was delayed on the as-received surface compared to the modified surface, caused by the presence of an approximately 8 nm thick crystalline oxide layer formed during extrusion. IGC initiated at the primary α-Al(Fe,Cu,Mn)Si particles for all types of surfaces. However, these particles corroded rapidly in the test solution forming a residue of Cu and Si on the exposed particle surface. This phenomenon, as well as enrichment of Cu on the Al matrix surface by dealloying, contributed increasingly to the formation of new effective cathodic sites and continuing propagation of IGC. The AlMgSiCu (Q) phase, present as primary and secondary particles, was relatively inert against both oxidation and reduction.

High temperature heat treatment of aluminium alloys causes surface enrichment of the trace elements in Group IIIA - VA, specifically the low melting point elements Pb, Bi, In and Sn. The phenomenon has practical significance in promoting certain types of localised corrosion, such as galvanic and filiform corrosion, while mitigating other types, such as pitting corrosion of the bare surface. The purpose of this paper is to investigate the surface enrichment and microstructure of indium relative to the available data for Pb. Model binary AlIn alloys, containing 20-1000 ppm of In, were used after heat treatment at various temperatures. In addition to electrochemical investigations, the microstructures were characterised by field emission scanning electron microscopy (FEG SEM) and field emission transmission electron microscopy (FEG TEM). Heat treatment at temperatures as low as 300°C gave significant segregation of In as opposed to 600°C for Pb. As a result of this and yet unresolved oxide film breakdown mechanism on aluminium, In was significantly more effective than Pb in anodically activating aluminium. These results suggest the possibility that significant activation earlier observed on certain commercial alloys as a result of low temperature heat treatment may be due to the trace elements In.