Written on: November 1, 2019 by W. Stephen Tait
Hello everyone. In last month’s Corrosion Corner, I identified and discussed the multiple microenvironments that occur inside spray packaging and spray packaging components.
Microenvironments are why spray package corrosion is so complex and unpredictable without corrosion test data. Aluminum aerosol containers have four microenvironments, steel aerosol containers have 10 microenvironments, bag-on-valve packages have four microenvironments and aerosol valves have three microenvironments.
There are two overarching types of metal, polymer coating and laminated metal foil corrosion and they can occur separately or together in any of the various spray package microenvironments:
• General corrosion
• Localized corrosion
General corrosion typically does not cause packages to leak. However, general metal corrosion produces metal ions that could degrade product efficacy. Localized metal corrosion could also cause packages to leak before their desired product service life.
Let’s discuss general and localized corrosion while looking at a few examples.
General Corrosion
General corrosion occurs in large areas of the spray package interior surface and removes package metal, coating or laminate film at a nominally uniform rate. Figure 3 and Figure 4 provide examples of general metal corrosion and general polymer coating corrosion, respectively.
Figure 1: General metal corrosion (detinning of tinplated steel)
General metal corrosion, such as that in Figure 1, typically does not reduce package service lifetime. However, general corrosion contaminates the product with metal ions that could adversely affect product efficacy, such as smell and color.
Figure 2 has an example of extensive coating delamination without metal corrosion under the coating. Pieces of free (delaminated) coating could clog spray valves, thus preventing the package from spraying and reducing package service lifetime.
Figure 2: General coating delamination corrosion
General corrosion could occur in any type of microenvironment discussed in last month’s Corrosion Corner. General metal corrosion under a coating could also cause large areas of coating or laminate film delamination from the package base metal.
Localized corrosion
Localized corrosion occurs in very small areas and in occluded areas where diffusion into and out of an area is restricted.
Occluded areas in spray packaging occur when two components are seamed or crimped together, such as tops and bottoms seamed to container bodies and valves crimped to container tops, respectively. Localized corrosion is different from general corrosion in the following ways:
•Localized corrosion occurs in very small areas of metals and coatings
•It is significantly faster than general corrosion
•It occurs in occluded areas such as container seams and laminated bag welds
•It often needs a large amount of surrounding surface area to support the high rate of localized corrosion
The most common forms of localized metal corrosion are pitting and corrosion inside the crevices formed by aerosol container double seams and the container curl-aerosol valve crimps.
Figure 3 is an example of metal pitting corrosion that causes aerosol container perforation. The top diameter of the pit in Figure 3 is approximately 1–2mm and the bottom of the pit is approximately 1/4 to 1/5 the size of the top diameter.
Figure 3: Metal pitting corrosion
Figure 4 has an example of blistering—the most common form of localized coating and laminate film corrosion. The diameter of the blister in Figure 4 is approximately 1–2mm. Sometimes blisters can also have metal corrosion under the blister.
Figure 4: Polymer coating blistering corrosion
Localized coating/laminate film corrosion could occur in any type of microenvironment discussed in last month’s Corrosion Corner.
There is no theoretical way to predict if general and localized corrosion will occur. An extensive corrosion database is useful for modeling potential corrosion for derivatives of existing formulas (line extensions), but corrosion testing is necessary for both new formulas and derivatives of existing formulas to confirm if corrosion will or will not occur and if failure is expected prior to the target package service lifetime. Corrosion testing could be either a long-term storage stability test (1-year minimum) or electrochemical corrosion testing with the appropriate parameters (3-month maximum with the appropriate test parameters).
Please visit www.pairodocspro.com for more information. Thanks for reading and I’ll see you in December. SPRAY
