Surface Segregation of Trace Element Bismuth during Heat Treatment of Aluminum

Surface segregation of bismuth and resulting oxidation of rolled aluminum bismuth model alloys during heat-treatment were investigated by electron-optical techniques, glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy. Heat treatment at 600 degrees C caused segregation of liquid Bi in the form of a metallic nanofilm at the oxide-metal interface. Breakdown of thermally-formed gamma-Al2O3 crystalline oxide by the liquid Bi film during subsequent cooling in water produced a composite oxide layer, consisting of remnants of the gamma-Al2O3 layer on top and an amorphous hydrated layer of about 1.5 mu m thick between the gamma-Al2O3 layer and the metal. Cooling in laboratory air after heat-treatment gave a slightly thinner (about 1 mu m) amorphous layer than that compared to water cooling. Formation of the thick aluminum oxide layer during cooling was attributed to the maintenance of the depassivating BiAl interfacial nanofilm by dealloying of the Al component of the metal. Existence of the BiAl interfacial film in a fluidized state during cooling, which is a necessary condition for oxide growth, was further enhanced by significant depression of the solidification point of the film due to its nanometer order of thickness. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.100203jes] All rights reserved.