Defects are present in most aluminum aerosol containers and corrosion testing
June 2021 Spray 21
is needed to determine when these defects will cause
the container corrosion that leads to failure...
Coating spit
Coating spit
Variations in coating color
Figure 4: Coating spits and variations in coating color (noted by arrows
and captions)
Entrained air in the bulk coating material sometimes causes
a coating nozzle to instantaneously eject the air with a small
amount of excess coating. The excess coating on the container
surface is referred to as a spit, examples of which are shown in
Figure 4. Notice that there are two spits at two different locations
inside this example.
Figure 4 also shows variations in coating color. Coating color
variations could be caused by variations in the thickness of the
coating—a well-known phenomenon in the coatings industry.
Spits are very common in aluminum aerosol containers.
However, I have not observed an instance where a spit has contributed
to or caused container corrosion. I have observed rare
instances where variations in color appeared to cause random
container failures (leaking).
High temperatures are used to cure aerosol container coatings.
Coatings and coating components are dissolved in solvents
that evaporate during the curing process and small bubbles
can form during solvent evaporation. Sometimes these bubbles
harden, producing solvent pops like the one in Figure 5.
Solvent pops rarely contribute to or cause corrosion. Pitting
corrosion inside solvent pops can only occur when there is
also extensive coating corrosion in a large area surrounding a
solvent pop.
Holes in coatings are very common. Figure 6 has an example
of a small area where a coating did not wet (cover) the container
metal, resulting in a hole that exposes metal. This type of defect
only causes pitting corrosion when there is also extensive coating
corrosion in a large area around the hole.
Figure 6: Area not wetted by coating (the hole is inside the dashed line)
One or several of the defects shown in Figures 1–6 are present
in most aluminum aerosol containers. Corrosion testing is
needed to determine when these defects will contribute to or
cause container corrosion that leads to failure.
In the next issue, I will continue this discussion on material
defects.
Thanks for your interest and I’ll see you in July. Please contact
me at 608-831-2076, rustdr@pairodocspro.com or from one of
our two websites: www.pairodocspro.com and www.aristartec.com.
Figure 5: Solvent pop Spray