Intergranular corrosion of AA6000-series aluminium alloys
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Even though highly corrosion resistant in general, AlMgSi (6000-series) alloys may sometimes develop susceptibility to intergranular corrosion (IGC). In this work the effect of Cu content and various thermomechanical treatments on IGC of AlMgSi model alloy extrusions of nominal composition 0.55 wt% Mg and 0.60 wt%Si was investigated. The corrosion susceptibility was tested according to the standard BS 11846 method B, which essentially involved degreasing, alkaline etching, desmutting in concentrated HNO3 and 24 h immersion in an acidified NaCl solution. The validity of this test as a reliable laboratory tool for qualitative ranking of samples for IGC susceptibility was verified by comparing it to results obtained by outdoor exposure in an industrial-marine environment at Karmøy, located on the coast of Norway The most important factor causing IGC susceptibility of the model alloys was the Cu content. The high Cu models were susceptible to IGC, although the Cu content was as low as 0.12 – 0.17 wt.% Cu. Model alloys with low Cu content (0.005 – 0.02 wt.%) were resistant. However, the corrosion mode of the high Cu alloys was strongly related to the thermomechanical history. Therefore, by applying the correct heat treatment, it was possible to reduce or eliminate IGC susceptibility also on the high Cu samples. Samples which were slow-cooled (air cooling) after extrusion and samples which were subsequently solution heat treated gave the most susceptible variants where nearly all grain boundaries exposed to the corrosive test solution were attacked (uniform IGC). Artificial aging reduced the IGC susceptibility by confining the intergranular attacks to localised sites (localised IGC), and near fully resistance to localised corrosion developed in the peak aged temper. Rapid quenching (water quenching) after extrusion prevented IGC, but high susceptibility (uniform IGC) was introduced as a result of short time aging. Further aging to peak hardness reduced IGC susceptibility (local IGC), but did not remove it completely. Overaging introduced slight pitting susceptibility to the air cooled extrusion. In contrast overaging reintroduced localised IGC to the solution heat treated air cooled samples. The solution heat treated and water quenched samples were susceptible to localised IGC both in the the T6 temper (peak aged) and in the overaged temper. The grain boundary microstructure was also strongly dependent on the thermal history. Samples which were slow-cooled (air cooling) after extrusion and samples which were subsequently solution heat treated exhibited extensive precipitation of large (_ 5 μm) elongated grain boundary precipitates of the equilibrium phases _ (Mg2Si) and Q (Al4Mg8Si7Cu2). The Q-phase was however not present on the low Cu alloys. The shape and size of these precipitates were not affected by artificial aging. On the high Cu alloy, a Cu enriched grain boundary film was additionally present. This film coarsened as a result of artificial aging. Water quenching after extrusion effectively prevented grain boundary precipitation, and this could thus explain the observed resistance of the water quenched high Cu alloys to IGC. Small grain boundary Q-phase precipitates evolved as a result of artificial aging of water quenched samples, which explained slight deterioration of IGC resistance of such samples. The susceptibility to IGC was related to enriched Cu along the grain boundaries, in the form of discrete Q-phase precipitates and the continuous Cu enriched film. IGC was caused by a microgalvanic coupling between the Cu enriched grain boundary (noble) and the adjacent solute depleted zone (active). The Cu containing film formed a continuous cathodic path on the grain boundaries, which resulted in significant (uniform) IGC, characterised by knife-edge attacks along the grain boundaries. Increased IGC resistance was obtained by artificial aging which coarsened the film and broke the continuous microgalvanic path, resulting in coarse localised IGC or a surface virtually resistant to all forms of localised corrosion. The present work showed that IGC can be avoided or eliminated either by reducing the Cu content or applying the correct thermomechanical treatment.