Written on: June 1, 2021 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 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.
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.
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.
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.