Written on: March 1, 2024 by W. Stephen Tait
Hello, everyone. There is no such thing as defect-free spray package materials. There are always potential concerns as to whether or not defects will contribute to, or cause, spray package corrosion and if corrosion at the point of defect will cause spray packages to fail (leak).
This issue starts a three-part discussion on material defects and their relationship to package corrosion. Material defects in traditional aluminum aerosol containers will be discussed here, and material defects in laminated foil bags in aerosol containers and tinplated steel aerosol containers will be discussed in the next two issues.
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 alloy material inclusion in aluminum aerosol container metal.
I’ve only observed rare instances when inclusions like the one in Figure 1 cause container pitting corrosion. However, corrosion could rapidly perforate containers when corrosion occurs around this type of defect.
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 contributes to or causes container corrosion. However, corrosion tests should be used to qualify all new and derivative container-formula systems.
Aluminum aerosol containers are formed with multiple extrusion stages. The containers resemble long tubes open at the top with a bottom during one of the later stages.
The coating is sprayed inside the open tubes with a nozzle that moves from the bottom of the tube to the top during the coating application. In some instances, coating drips from a nozzle after spraying. Figure 3 has an example of a coating drip in an aluminum aerosol container (referred to as a drool). Corrosion caused by drools is rare, but corrosion testing is still needed.
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. Coating color variations also noted in Figure 4 are caused by coating thickness variations. These variations are also common and I have observed instances where they caused or contributed to 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. However, pitting corrosion inside solvent pops could occur when there is also extensive coating corrosion 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 typically causes pitting corrosion when there is also coating corrosion around the hole.
One or several of the defects shown in Figures 1–6 are present in virtually all aluminum aerosol containers. Consequently, corrosion testing is needed to determine when these defects will contribute to, or cause, container corrosion that leads to failure.
In the next issue, we’ll continue this discussion on material defects in bag-on-valve (BOV) packaging and traditional steel aerosol containers.
Thanks for your interest and I’ll see you in April for Part Two. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
