Hello everyone. Is corrosion-free spray packaging a realistic expectation? What can happen when a spray package corrodes? This month we’ll examine the various scenarios that could occur when a formula causes spray package corrosion, and the options for living with corrosion.
Corrosion Basics: Spray package lifetime
All spray products have a lifetime during which the product and the package both perform as specified. This lifetime is often referred to as shelf life.
However, the term shelf life usually applies to products with active ingredients, such as pharmaceutical products and insect control products. For example, a five-year shelf life indicates that the effective medical ingredient concentration in the package is around 50% after five years.
I prefer the termservice lifetime when discussing spray packages. Service lifetime is the length of time when packages perform as specified—that is, packages spray until the product is exhausted and do not leak product.
Corrosion Basics: categories of spray package corrosion
There are two general categories of spray package corrosion:
o Pitting corrosion
o Crevice corrosion
Both categories of corrosion could affect polymers, polymer coated metals—lined traditional aerosol containers and spray packages with internal laminated film bags—and spray packages using uncoated aluminum or steel (tinplate or tin-free-steel).
General corrosion occurs over the entire internal package surface or in large patches on the surface. Localized corrosion occurs at small points—pits and coating/laminate blisters of 50 microns or less in diameter—or in areas where two pieces of material overlap (such as aerosol container double seams and laminated bag welds). Corrosion in overlapping areas and bag welds is referred to as crevicing.
Corrosion Basics: how corrosion affects spray products
Corrosion of spray packaging could:
Cause product instability
- Metal corrosion could catalyze active ingredient decomposition, thereby reducing effective concentrations
- Metal ions from corrosion break an emulsion
Degrade product efficacy
- Metal corrosion could discolor the product or cause a malodor
- Metal corrosion could degrade active ingredients, reducing product efficacy
Cause package leaking
- Localized metal corrosion could cause product to leak
- Localized metal corrosion causes partial or complete loss of propellant
Cause packages to stop spraying
- Loss of propellant through a corrosion perforation
- Pieces of coating or laminate film clog dip tubes or aerosol valve orifices
- Metal corrosion product accumulates in dip tubes and aerosol valves and ultimately clog the dip tube of valve orifices
There are two ways to address corrosion issues. Corrosion could be preventedor controlled by reformulating with or without a corrosion inhibitor.
Corrosion prevention means that corrosion is not present during the specified service life of the spray package. Corrosion prevention is typically achieved with a corrosion inhibitor or by changing to a different type of spray package.
Corrosion inhibitors have effective concentration ranges like all of a formula’s ingredients. Too much or too little of a corrosion inhibitor could actually cause or exacerbate corrosion. Consequently, quality specifications should include analysis for the corrosion inhibitor concentration in each manufacturing batch prior to spray package filling.
Corrosion is present withcontrol. However, very slow corrosion usually does not reduce service life below the specified target or appear to degrade product efficacy. In other words, package service life is extended through control and control is also probably reducing the corrosion-degradation of product efficacy.
The quality specifications should also include analysis for corrosion inhibitor concentration prior to spray package filling when a corrosion inhibitor is used to control corrosion. In addition, commercial spray packages should be periodically retrieved and inspected for corrosion when using corrosion control. Determination of corrosion inhibitor concentration and measurements of product efficacy is also recommended during these inspections. Inspections should be conducted throughout the commercial life of the product.
Periodic inspections of commercial packages help to ensure that variability does not cause the inhibitor to lose its effectiveness and the service life to decrease below the target length.
A comprehensive corrosion control and prevention program includes corrosion testing prior to commercialization of the product. There are two general categories of corrosion tests for spray packaging: storage stability tests and electrochemical corrosion tests.
Storage test data can be used to estimate service lifetimes, and thus provide information for decisions on the suitability of a product-package system for a commercial market. Electrochemical corrosion test data can also be used to estimate service lifetimes with the appropriate empirical models.
Good practices for stability tests should:
- include periodic destructive inspections of test samples for internal package and valve corrosion (for example, inspections every three or four months)
- have replicate package samples examined at each inspection interval (more than three packages)
- use high temperature test conditions for product thermal stability determination and not to accelerate corrosion
- be conducted for at least one year before commercialization decisions are made
Electrochemical tests are shorter than storage tests because the instruments used for detecting corrosion are more sensitive than the human eye and a light microscope. The correlation between electrochemical results and actual corrosion are very high when the appropriate test parameters are used for the measurements (in excess of 99%).
Sometimes products are abandoned during development because corrosion was found in storage stability tests. However, it has been my experience that corrosion does not always indicate that a product should be abandoned. Indeed, I prefer to think that the development timetable needs to be extended when corrosion is found.
My preference is for prevention. However, individual companies must decide whether corrosion control or corrosion prevention is appropriate for a given commercial spray product.
We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Contact [email protected] or visit www.pairodocspro.com. Please send your questions/comments/suggestions to [email protected] Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in January.