W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Corrosion Corner
Spray package
material defects: Part I
Hello, everyone. There is no such thing as defect-free
spray package materials. Thus, there are always potential
concerns as to whether or not defects will contribute to
or cause spray package corrosion, and if corrosion at these defects
will cause spray packages to fail (leak).
This issue starts a three-part discussion on material defects and
their relationship to package corrosion. Traditional aluminum
aerosol containers will be discussed this month, and material defects
in laminated foil bags in aerosol containers and tinplated steel
aerosol containers will be discussed in Corrosion Corner’s following
columns.
Photographs of material defects are used to discuss the relationships
between various types of defects and corrosion. Figures 1–6
provide examples of material defects in coated aluminum aerosol
containers.
All metal alloys have inclusions in the metal matrix. Inclusions
are typically microscopic spherical particles of non-metal
components of the aluminum alloy and aluminum/non-metal
compounds. Spherical inclusions become distorted and flattened
when the metal is formed into a container. Figure 1 has an
example of an inclusion in aluminum aerosol container metal.
I’ve only observed rare instances when inclusions like the one in
Figure 1 have caused container pitting corrosion.
Figure 1: Inclusion in aluminum metal
Small pieces of metal (divots) are removed from aluminum during
the container-forming process. Figure 2 has an example of a
divot found in an aluminum aerosol container.
Notice in Figure 2 that the coating backfilled the divot. I have
only observed rare instances where this type of metal defect has
contributed to or caused container corrosion.
Aluminum aerosol containers are formed with multiple extrusion
stages. The containers resemble long tubes open at the top
with a bottom added during one of the later stages. The open
tubes are cleaned with a spray nozzle to remove the lubrication
applied during the previous extrusion stages and the coating
is subsequently sprayed inside the tube. The nozzle moves
from the bottom of the tube to the top during the coating
application.
In some instances, coating drips from the nozzle near the
top of the tube when spraying is stopped. Figure 3 has an
example of a coating drip in an aluminum aerosol container
(referred to as a drool). I’ve not observed an instance where a
coating drool has contributed to or caused aerosol container
corrosion.
Figure 3: Drool (inside dashed line)
20 Spray June 2021
Figure 2: Hole filled with coating