Why storage tests fail to predict actual corrosion
Written on: February 1, 2024 by W. Stephen Tait
Hello, everyone. Static storage testing—also known as storage stability testing (both SST)—is the oldest form of corrosion testing for spray packaging. The goal of an SST is to ensure that products are safe to use, provide the expected efficacy and do not leak during the time consumers use the product.
Package failures in the market, and unexpected corrosion, both occur from a number of faulty SST practices. These include:
• Using high temperatures to accelerate package material corrosion
• Storage times that are too short
• Insufficient numbers of samples examined at each interval (sample pull)
• Unrealistic test parameters
• Not evaluating the effect of variability on corrosion
• Not determining the effective concentration range for corrosion inhibitors
• Not examining packages from first product runs
Let’s examine these SST practices.
Using high temperatures to accelerate corrosion
SST package samples are typically stored at room temperature and at least one higher temperature. The higher temperature(s) is (are) used to accelerate the packaging material’s corrosion rate and reduce the length of an SST so that products can be commercialized more quickly. On numerous occasions, at the beginning of a package failure investigation, I’ve been told that, “We put a few cans in the oven for a week; there was no corrosion and we marketed the product.” However, if a low corrosion risk is desired, there’s no such thing as a short-cut when conducting corrosion tests!
Package material corrosion rates are not accelerated by higher temperatures. Although in some instances, higher temperatures can actually cause formula ingredient degradation.
Storing packages at high temperatures for multiple months also does not simulate actual package storage. The storage time at higher temperatures should be based on the number of hours per year that packages are stored in warmer areas or during the warmer seasons; the test-temperature should be realistic for the various areas where products are warehoused and sold.
Short storage times
We’ve seen numerous instances where pitting corrosion that perforates aerosol containers in one year was not detected until around 6–9 months of storage testing.
It takes time for corrosion to initiate, increase to a critical size/mass that sustains corrosion growth through the materials and generate enough corrosion to be seen either with the unaided eye or with a light microscope.
Accelerated corrosion test results can be obtained from measurements with sensitive electronic instruments (i.e., electrochemical corrosion tests) that detect corrosion and measure its rates before there is enough to be seen. Electrochemical tests conducted with the correct parameters provide accurate and precise predictions for actual corrosion with less risk than a traditional SST.
Insufficient numbers of samples
Limited storage room space often restricts the number of spray packages tested. However, a small number of samples could result in low statistical confidence for a given examination time (sample pull). In addition, a small number of samples per examination probably won’t be large enough to find the corrosion that will eventually cause package failure, such as leaking. For example, if corrosion will actually cause 10% of packages to leak, it is highly unlikely one will know one has a corrosion issue when examining only 1–3 packages.
Unrealistic test parameters
Test packages are sometimes stored in several different orientations, such as upright, inverted and on their sides. The upright and inverted orientations for aerosol containers typically provide the same results and don’t evaluate potential leaking of aerosol valve components. Evaluating corrosion of aerosol containers on their side is only appropriate when the commercial packages are routinely stored on their sides by consumers.
Not evaluating the effect of variability
Variability is a fact of life that could contribute to or cause spray package corrosion. Storage room space limitations also often prevent evaluating how variability affects spray package corrosion.
Variabilities that could contribute to or cause spray package corrosion include:
• Formula ingredient concentration range variability
• Variability of contaminant concentrations, such as water
• Corrosion inhibitor concentration variability
• Package material composition variability
• Variability of package component physical attributes, such as valve crimp (clench) dimensions
Variability causes random package failures for commercial products.
Not determining the effective concentration range for corrosion inhibitors
Small concentrations of corrosion inhibitors are often very effective in preventing and controlling package corrosion and thus enable marketing of corrosive formulas. However, corrosion inhibitors often have an effective concentration range, above which and below which package corrosion occurs. Hence, the effective corrosion inhibitor concentration range should be determined, and that range should be part of manufacturing specifications.
Not examining packages from first product runs
A corrosion SST has an approximately 7% risk of unexpected corrosion in commercial products when the SST is completed after one year. Consequently, it is advisable to also run confirmatory storage tests on packages from one or more early production batches to ensure the effect of variability on commercial package corrosion is known and to help lower the risk to below 7%.
I realize that the best SST practices mentioned in this article might sometimes strain both limited personnel and storage test facility resources, hence, my preference and recommendation for conducting quicker and more accurate electrochemical corrosion tests.