Corrosion Corner W. Stephen Tait, Ph.D. Chief Science Officer & Principal Consultant, Pair O Docs Professionals, LLC Spray package material defects and their relationship with corrosion Part I: Traditional aluminum aerosol containers 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 nonmetal components of the aluminum alloy and aluminum/non-metal compounds. Figure 1: Inclusion in aluminum metal 30 Spray June 2015 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. 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. 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 Figure 2: Hole filled with 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)
Spray June 2015
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