Written on: May 1, 2024 by W. Stephen Tait
Hello, everyone. In March, we covered defects in polymer and tinplate coatings and their relationship with corrosion. The April column covered defects in laminated film bags, along with those found in traditional steel aerosol containers.
Most of the material defects in spray packages are very small, but can still be seen with the unaided eye. There are also microscopic defects that can lead to package corrosion and subsequent failure (leaking, clogged valves, etc.).
The metals used for aerosol spray containers are not pure metals. Instead, they are alloys that are mixtures of metals, plus non-metals, such as carbon and oxygen.
Metal/non-metal compounds, such as metal oxides or metal carbides, are insoluble in the host metal (i.e. aluminum and steel) and form microscopic precipitate particles. These particles are called inclusions and are dispersed throughout the host metals. Inclusions can be flattened and elongated when the metal is rolled from ingots into the sheets for package fabrication (typically for steel containers) and when the metal is extruded from slugs into a container (typically for aluminum containers).
Figure 1 provides a photomicrograph with examples of inclusions in steel. The arrows show the locations for only a few of the many different defects. The dark spots are inclusions and the thin, dark line is a row of inclusions, referred to as a stringer.
Please keep in mind that inclusions are not impurities. Inclusions are material defects resulting from the alloying of a metal with different elements to obtain physical properties, such as the strength and formability needed to form metals into containers.
Inclusions are sites for pitting corrosion and stress cracking. Stress cracking rarely occurs in spray packages. The chemical composition of a formula determines whether or not pitting corrosion occurs at inclusions.
Metals form regular arrangements of atoms, and these atomic arrangements form bulk formations referred to as crystal planes. Crystal planes are often not perfect, and, in some instances, include fragments of planes imbedded among complete planes. The fragments are referred to as dislocations.
Figure 2 provides an example of multiple dislocations that pile-up when a metal is rolled into a sheet. Figure 2 also shows that dislocations could be sites for the initiation of pitting corrosion.
Steel and aluminum metals and alloys all have dislocations. Indeed, there are approximately one million dislocations per square centimeter of metal surface. The chemical composition of a formula determines whether or not pitting corrosion occurs at dislocations.
Figure 3 provides a scanning electron micrograph of tinplated steel, after the tin coating was removed by mechanical polishing. Several different types of defects are noted in Figure 3:
• Different crystal planes form structures referred to as grains, and different types of grains corrode more rapidly than others
• The chemical composition of the boundary between two different types of grains is typically different from the chemical composition of the grains
• The boundary between different grains is not always metallic—the boundary could be either, metallic (dark shadows between grains) or non-metallic (white lines between grains)
• Non-metallic inclusions are also noted in Figure 3 (white particles inside grains)
• Iron carbide compounds are also present in the steel grains—(darker spots)
All material defects in Figure 3 could cause or contribute to spray package corrosion. Aluminum and steel both typically have more than one of the material defect types shown in Figure 3.
The chemical composition of a formula determines whether or not the material defects in Figure 3 contribute to or cause spray package metal corrosion. Hence, corrosion testing is essential for reducing the risk of costly surprise spray package corrosion.
Thanks for your interest and I’ll see you in June. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
