Electrochemical reactivity, surface composition and corrosion mechanisms of the complex metallic alloy Al_3Mg_2

摘要

A corrosion mechanism is proposed for Al_3Mg_2, based on electrochemical tests, XPS, and depth profiling using XPS and ToF-SIMS. After short (～2 min) solution exposure, the surface consists of a surface film above dealloying. The dealloying is attributed to selective Mg dissolution and the surface rearrangement of Al into islands, although the metallic Al could alternatively be formed by two reduction reactions. The surface film thickness was ～10 nm. After exposure to ultra-pure water, the composition was AlM-g_(1.3)O_(0.2)(OH)_(5.1) corresponding to Al(OH)_3·1.1 Mg(OH)_2·0.2MgO. After exposure to 0.01 M Na_2SO_4, the composition was AlMg_(0.2)O_(0.4)(OH)_(2.5) corresponding to Al(OH)_3 ·0.1Al_2O_3·0.2MgO. Longer exposure produced a thicker surface film, more pronounced metallic Al islands and more MgH_2. Three possibilities are identified for MgH_2 formation. Al(OH)_3 formation is attributed to a precipitation reaction. Bulk nanoporous Al_3Mg_2 formation is predicted to be possible by Mg dealloying of Mg_(17)Al_(12).