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Pitting corrosion and intergranular corrosion of Al and Al-Cu alloy single crystals and bicrystals Yasuda, Mitsuhiro


Single crystals and bicrystals have been used to study pitting corrosion and intergranular corrosion of Al and Al-Cu alloys in 0.5M NaCl solution. The critical pitting potential and pit density were examined as a function of a number of factors. These included crystallographic orientation; the bulk solution chemistry including CI- concentration, NO₃- addition and pH; the effect of Cu alloying; and the effects of homogenizing and aging on the alloy crystals. The susceptibility for pitting corrosion was found to depend on crystallographic orientation in pure Al with {111} showing maximum pitting and {011} and {001} exhibiting progressively less pitting. This crystallographic effect was not observed in the Al-3 wt% Cu alloy. The addition of Cu to pure Al was found to raise the Epit and produce a higher pit density on the surface. The increase of CI⁻ concentration was found to enhance pitting corrosion, producing a higher pit density and lowering the Epit. Addition of NO₃- to the solution decreases pitting corrosion, reduces the pit density and substantially shifts the Epit to a more noble potential. A model of pitting corrosion is proposed, based on a local kinetic balance between the repassivation process and the dissolution process at the bare metal surface at the base of a preexisting oxide flaw on the crystal surface. The model successfully accounts for the observed effects of the Cu alloy addition, and the solution composition variations on pitting corrosion. In the alloy bicrystals, it was observed that pitting corrosion in the grain boundary region was dependent on the composition and thermal history of the crystal. In most of the homogenized Al-Cu bicrystals, the presence of the grain boundary did not influence the pitting corrosion. In a 0.1 wt% Cu alloy with a tilt boundary of 28° about the <001> direction preferential pitting along the grain boundary was observed. The preferential pitting is attributed to nonequilibrium depletion of Cu at the high angle tilt boundary. Preferential attack is also observed at grain boundaries in as-grown and in aged bicrystals. This is attributed to Cu segregation in the crystals and the lower value of Epit associated with the Cu depleted regions.

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