Low-Copper, 7xxx Alloys. Due to the low
copper content, these 7xxx alloys exhibit excellent
resistance to general corrosion and pitting.
However, as a group, they are relatively more
susceptible to SCC than higher-copper 7xxx
alloys. Depending on composition and processing
specifics, many low-copper 7xxx alloys have
an unrecrystallized microstructure. This is beneficial
to SCC resistance in applications where the
dominant stresses are in the longitudinal and
long transverse orientations. With proper engineering,
such alloys are successfully used in
commercial structural applications. Alloys that
are recrystallized tend to be more susceptible toIn the materials selection process for corrosion
applications, the chemical composition of the
alloy is generally the most important factor that
an engineer considers. However, examples given
in this article demonstrate that the metallurgical
condition or microstructure of a particular nickel
alloy is also a very important factor. For the same
overall chemical composition, the corrosion rate
of the same alloy can vary several orders of magnitude,
depending on its particular microstructure.
The most important metallurgical factors
that need to be considered are second-phase precipitation
by thermal instability and the presence
of CW. The latter is especially important in cases
where SCC may be expected
SCC. Whereas overaging improves the SCC
resistance of copper-containing 7xxx alloys, it
does not greatly improve the SCC resistance of
the low-copper 7xxx alloys. For more information
on low-copper 7xxx alloys,