Written on: June 1, 2015 by W. Stephen Tait
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 the corrosion at defects will cause spray packages to fail (leak).
This month starts a two-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 July.
I’ll be using material defect photographs to discuss the relationships between corrosion and the various types of defects. 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 cause 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 not observed an instance where this type of metal and coating material defect contributes to or causes container corrosion.
Figure 2: hole filled with coating
After one of the multiple forming stages, aluminum aerosol containers resemble long tubes open at the top and closed at the bottom. The tubes are cleaned and internally coated with a spray nozzle. Coating is sprayed inside the tube as the nozzle moves from the bottom to the top of the tube.
In some instances, coating drips from the nozzle when spraying is stopped near the top of the tube. 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 contributed to or caused aerosol container corrosion.
Figure 3: Drool (inside dashed line)
Entrained air in the bulk coating material causes a coating nozzle to instantaneously eject excess coating—referred to as a spit. Figure 4 contains an example of a coating spit at the top of an aluminum aerosol container and bottom of another container.
Figure 4 also shows variations in coating color. Coating color variations could be caused by variations in the thickness of the coating. Variations in coating thickness are well known in the container industry.
Figure 4: Coating spits and variations in coating color (noted by arrows and captions)
Spits are very common in aluminum aerosol containers. However, I have not observed an instance where a spit 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 around a solvent pop.
Figure 5: Solvent pop
Holes in coatings are very common. Figure 6 shows 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 in aluminum and coatings are present in most aluminum aerosol containers. Corrosion testing is needed to determine when defects will contribute to or cause container corrosion that leads to failure.
Next month I will continue this discussion on material defects in laminated foil bags used with traditional metal aerosol containers and traditional tinplated steel aerosol containers.
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