Surface segregation of tin by heat treatment of dilute aluminium–tin alloys

The changes in the microstructure and surface morphology, caused by heat treatment, of binary AlSn model alloys, containing 30–1000 ppm tin were investigated with the purpose of understanding activation of aluminium by trace element tin in aqueous media. Specimens prepared by casting and cold-rolling in the laboratory were annealed for 1 h at 300 or 600 °C and quenched in water, and then characterised by various surface-analytical techniques. In the as-rolled condition, tin was in solid solution with aluminium. The microstructure and surface morphology in this condition were not different from those of pure aluminium. Significant segregation of liquid Sn occurred by annealing at 300 °C to the metal surface, caused by the combined effect of limited solubility and high mobility of Sn in Al at this temperature. Segregated Sn caused oxidation of aluminium during water quenching following heat treatment. The resulting oxide formation increased with increasing Sn content of the alloy from localised cases to extensive coverage by a 1.3 μm thick scale. Most of the Sn was homogenised in solid solution with Al by annealing at 600 °C for all Sn concentrations. The alloy containing 1000 ppm Sn still formed a thick oxide as a result of quenching in water after heat treatment, caused by enrichment of Sn at the metal–oxide interface by dealloying of Al.

► Highest Sn segregation occurred by annealing binary AlSn alloys at 300 °C.
► Al could become activated by segregated liquid Sn in chloride-free steam.
► Segregation of Sn occurred as a continuous nanofilm along the metal–oxide interface.
► Activation mechanism by Sn was analogous to the activation of aluminium by Hg.