Written on: April 1, 2024 by W. Stephen Tait
Hello, everyone. In the last issue, I began a series on material defects in spray packaging and the relationship between these defects and corrosion.
This month, I’ll discuss the more common material defects found in laminated foil bag packages—bag-on-valve (BOV) packaging—as well as both coated and uncoated steel aerosol containers. The final, Part Three discussion will focus on microscopic material defects.
Laminated aluminum foil bag (BOV) packaging
Aluminum foils are laminated on both sides with one or more polymer films and fabricated into bags with an aerosol valve (referred to as bag-on-valve [BOV] packaging) to be inserted into traditional aerosol containers.
Figure 1 shows an example of a micro-bulge on the internal laminate film for a BOV package. This type of material defect is common with this type of packaging. Multiple attempts to obtain a clear cross-sectioning of this defect type have not been successful. Consequently, the properties of micro-bulges and their most likely causes are unknown.
Micro-bulges typically do not contribute to, or cause, laminated aluminum foil bag corrosion. In other words, the probability is not zero, so corrosion tests should be conducted.
Figure 2 provides an example of a polymer film that delaminated at the bag weld. It has been our experience that this type of material defect is rare and can be avoided with optimized welding.
We have not observed instances where this type of material defect contributes to or causes spray package bag corrosion. However, this type of defect could cause metal foil corrosion and bag rupture at the delaminated area.
Figure 3 provides an example of a crack in the aluminum foil under the laminate film. In our experience, this type of material defect is typically random.
We have not yet observed instances where this type of material defect contributes to or causes laminate foil bag corrosion. However, product that diffuses through the inner laminate layer/layers would also be able to diffuse at the crack through the outer film and cause container perforation and leaking. Consequently, corrosion testing should be conducted.
Tinplated steel aerosol containers
Figure 4 provides an example of an area where the tin coating did not cover (wet) the steel substrate. Non-wetting produces holes in the tin coating that expose either the substrate steel or a very thin, iron-tin alloy layer. Numerous holes in tin coatings, such as that in Figure 4, are present in all tinplated steel aerosol containers.
There are two different morphologies for the holes in tin coatings. Figure 4 shows the traditional type of hole morphology and Figure 5 shows a newer type of hole morphology that has appeared within the last three decades.
Notice when comparing Figure 4 and Figure 5 that the newer holes are more symmetrical than the traditional holes. Most likely, the newer hole morphology comes from either a new tinplating bath chemistry composition or a new tinplating process.
Holes in tin coatings are potential sites for pitting corrosion and a formula’s chemical composition determines if pitting corrosion will or will not occur in tin coating holes.
It is unknown if either the traditional hole morphology or the newer hole morphology result in different susceptibilities to, and/or magnitudes of, pitting corrosion. Corrosion testing is the only way to determine if a given formula will cause pitting corrosion at holes in a tin coating.
Three-piece steel aerosol container bodies are welded by a diffusion-weld process, using heat and pressure. The heat is produced by an electrical current flowing between the overlapping ends of the tinplated steel sheet used to form the cylindrical container body. In some instances, the combination of pressure and heat is not optimum and a small amount of metal is ejected from under the overlapping ends.
Figure 6 has an example of a small particle of metal ejected from a weld. This phenomenon is referred to as weld spatter. Weld spatter is typically not common. However, pitting corrosion could occur if weld spatter produces a small cavern in the weld and the chemical composition of a formula determines if pitting corrosion will occur in a weld splatter cavern.
This column, and the previous one, both contain examples of macro-defects that can be seen with either the unaided eye or a light microscope. Next month, I’ll complete this series with a discussion of microscopic defects that cannot be seen with the unaided eye and their relationship to spray package corrosion.
Thanks for your interest and I’ll see you in May for Part Three. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
