UWE Bristol Engineering showcase 2015

Daniel Norton
BEng(Hons) Aerospace Engineering (Design) (SW)
Project Supervisor
Dr. John Kamalu
An Investigation into the Morphology, Stoichiometry and Distribution of
Secondary Phase Intermetallic Particles Found in a 2XXX-Series Al-Alloy
and Their Effects on Corrosion Performance
The main inhibitor and disadvantage that the light metals, predominantly 2XXX, 6XXX, 7XXX series Al-Alloys, have when compared to
composites and titanium alloy parts is that they are prone to exhibit relatively poor corrosion performance (Sukiman et al, 2012). As a
pure metal, aluminium is highly reactive, with an affinity for oxygen. Pure aluminium has very good corrosion resistance due to its
naturally forming oxide layer making it highly resistant to most environments (Aluminium Federation, 2011). However, when it is alloyed
with other elements, this inherent corrosion resistance is often lost. A number of research papers have attributed the poor corrosion
behaviours of these alloys to the material’s microstructure and the chemistries of the precipitates within it
Project summary
An investigation that looks to evaluate and quantify
links that exist between the morphology,
stoichiometry and distribution of secondary phase
intermetallic particles that are found in a 2XXX-Series
Al-Alloy and their effects on the corrosion behaviour
of these materials in service.
Project Objectives
The experimental testing should quantitatively
evaluate the effects of corrosion in a number of
different electrochemical systems, determined by the
Airbus case study.
The most common types of corrosion in 2XXX
series aluminium alloys due to electrochemical
reactions are pitting corrosion and intergranular
corrosion (SAPA, 2013). Not only are these two of
the most common forms of corrosion, they can
also be two of the most destructive if not carefully
accounted for in design. Not all aluminium alloys
are so susceptible to the intense pitting attack
seen in a number of studies on Al2024. The
alloying elements are often suggested to have a
significant effect on the rate of depletion of the
passive film coating the parts.
An intermetallic particle is a solid-state phase
within an alloy matrix which has a chemistry that
is different to the surrounding matrix. These
particles form during the solidification of the alloy
and are affected by a number of factors, namely
chemistries and changes in temperature.
Some of these precipitates have a very precise
composition with significantly different properties
to the surrounding matrix – these are termed the
intermetallic phase. A large number of smaller
intermetallics can be detrimental in terms of
corrosion performance, however, up to a point
they offer significant improvements in strength by
pinning grain boundaries (Saddock, 2008). As such
it is consider these effects in the analysis of
microstructural characteristics and be aware of
them when interpreting results.
57 samples were tested and findings showed
significant levels of pitting and intergranular attack
on all samples. The first above show the result of
the corrosion experiment. The third image is a
micrograph of the intergranular attack in an Al-
Bronze sample.
From the clustering analysis the following have
been found:
Particles are primarily clustered along grain
boundaries – which coincide with the theory that
large constituent particles have been mechanically
broken down during forming. It is possible to
identify a number of linear clusters which network
in the direction of rolling, the direction of the grain
elongation. These particles are frequently closely
packed with little to no space between particles. It
is worth noting that in the clustering analysis it
was found that “16% of particles have a neighbour
within 8.36μm” – the distance below which the
corrosion is considered to be exacerbated in the
form of a galvanic couple. While the increase in
rate has not been quantified, the corrosion tests
suggest that the impact would be considerable
given the stark difference in depth of attack
between pitting and intergranular attack.
Project Conclusion
• AA2024-T3 experiences extensive stable pitting
and intergranular corrosion when exposed to a
dilute acid such as NaCl.
• The rate of pitting corrosion at a site appeared to
reduce or stop altogether following the initiation
of intergranular attack. It is thought that the pit
has repassivated.
• The clustering analysis concluded that the
intermetallics within AA2024-T3 appeared to be
clustered in linear networks.
• The machine vision analysis method can be used
as an effective method for estimating the
corrosion behaviour of the material however it
requires a greater amount of data to improve its
reliability.