Effect of Excess Silicon and Small Copper Content on Intergranular Corrosion of 6000-Series Aluminum Alloys

The required strength and ductility of heat-treatable AlMgSi (6000-series) alloys are often obtained by alloying with either a small amount of Cu or a large excess of Si compared to the stoichiometric Mg/Si ratio corresponding to the phase. Both approaches may cause susceptibility to intergranular corrosion (IGC) as a result of unfavorable heat-treatment. Whether alloying with Cu or excess Si gives the optimal combination of mechanical properties and IGC resistance is a controversial subject. The corrosion behavior of a model alloy containing 0.2% Cu is compared with an essentially Cu-free alloy with an excess Si/Mg composition ratio (i.e., unbalanced) by using accelerated corrosion tests and electron optical characterization. In general, the Cu-containing alloy showed a higher susceptibility to IGC than the Cu-free, excess Si alloy. The Cu-containing alloy was especially susceptible in the underaged condition. The Cu-free, excess Si alloy became completely resistant to IGC by removing the cathodic intermetallic particles from the surface by selective etching or by purging the dissolved oxygen from the solution, whereas the Cu-containing alloy was still susceptible to IGC after the same treatment. The difference in IGC susceptibility between the two alloys was attributed to the presence of a cathodic Cu-rich film and discrete Cu-containing particles along the grain boundaries of the Cu-containing alloy, while the cathodic sites on the unbalanced variant were restricted to the material surface. In addition, the IGC susceptibility of both alloys depended on the presence of solute (Si and Cu)-depleted zones adjacent to the grain boundaries.