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National Advisory Committee for Aeronautics, Technical Notes - Intergranular Corrosion of High Purity Aluminum in Hydrochloric Acid - II - Grain Boundary Segregation of Impurity Atoms

The variation in the rate of intergranular corrosion of single—
phase high-purity aluminum in 20 percent hydrochloric acid as a
function of iron content and final—annealing temperature is attributed
to the segregation to atomic sites in the grain—boundary region of
iron and possibly other impurity atoms. The experimental results are
analyzed by reference to a distribution function, Obtained by statis-
tical mechanical methods, which gives the equilibrium fraction of
certain sites in the boundary which are occupied by solute atoms in
terms of the interaction energy for the segregation of the solute
atoms at these sites. Segregation of iron alone cannot account for
the observed results; from consideration of the atomic sizes and the
quantities of other impurities present, it is expected that copper is
also involved. The Observed relative corrosion rates can be repre—
sented as due principally to differences in the fraction occupied by
iron and copper atoms of those boundary sites where the potential
energy of interaction for the segregation of iron or copper is at
least as large as -O.7 electron volt (-l6,000 calories per mole) and
perhaps as large as -l.O electron volt (—23,000 calories per mole).

Since energies of this magnitude cannot be accounted for by the loss
in lattice distortional energy when an undersize iron or copper atom
is replaced by an aluminum atom at a grain—body site, a substantial
contribution to the potential energy of interaction must come from a
reduction of the local energy about the site in the boundary. It is
believed that metals and alloys generally contain an alloying element
or impurity the atoms of which have a considerable tendency to concen-
trate in the boundaries but that the effect on the properties of the
boundaries will be apparent only under special conditions.