Hello, everyone. Internal coatings in spray packages are not barriers between the package metal and your formula. Laminate films on metals are typically much thicker than coatings. However, laminate films are also not true barriers between the metal and your formula.
The typical ranges of thicknesses for spray package coatings and laminate films are:
- 2 to 10 microns for traditional aluminum and steel spray packaging (1 micron = 1×10-6 meters)
- On the order of 300 microns for laminated tinplated steel aerosol valves
- On the order of 100 microns for polymer films on the laminated foils for internal bags in spray packaging
- On the order of 20 microns for the laminate films on tin-free-steel containers
Coatings are applied to spray package metals and metal foils as:
- A separate polymer film bonded to the metal or the metal foil
- Various modified versions of epoxy that are thermally cured
- Powder spray coatings that are heat treated after application
- Polymer dissolved in a solvent that is subsequently heated to remove the solvent (referred to as lacquers)
There are a variety of types of coating and laminate film corrosion:
- Wet adhesion loss
- Pitting corrosion causes blisters
- General corrosion causes blisters
- Filliform corrosion
Let’s briefly discuss each form of coating/film corrosion.
Wet adhesion loss causes the coating to delaminate from the substrate metal
Figure 1 provides an example of wet adhesion loss of a coating. Water and formula ingredients diffuse into and through the coating. Accumulation of liquid at the coating-metal interface breaks the coating-bonds, resulting in delamination of the coating from the metal.
Typically, no corrosion is observed with this type of coating corrosion. The orifices in spray package valves are small and loose of pieces coating could prevent a package from spraying if the pieces clog valve orifices or dip tubes.
Click on images for full-size view.
Pitting corrosion causes blisters
Figure 2 provides an example of pitting corrosion that causes coating blisters. The area under the blister could either fill with gas from the pitting corrosion or gas from corrosion and liquid that diffuses through the coating into the blister.
Pitting corrosion as shown in Figure 2 typically leads to the leaking of liquid product or gaseous propellant.
General corrosion causes blisters
Figure 3 provides an example of blisters caused by general corrosion under the coating. The corrosion process draws liquid through the coating to the metal-coating interface and breaks the metal-coating bonds. The metal oxide produced by corrosion lifts the coating away from the metal to form the blister, as seen in Figure 3.
Figure 4 provides an example of filliform corrosion. Filliform corrosion is similar to blistering caused by metal oxide corrosion product.
Filliform corrosion is rare, and is usually found in the vapor areas of the container. The metal oxide produced by corrosion tunnels under the coating in the form of small meandering ribbons, as shown in Figure 4.
Want to learn more about spray package corrosion? We would be happy to teach our 1 ½ day Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Please contact me at firstname.lastname@example.org if you would like us to teach this course at your facility. Please also visit our website www.pairodocspro.com for more information about Pair O Docs® Professionals.
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