Written on: September 1, 2015 by W. Stephen Tait
Hello, everyone. One of the most frequent questions that I’m asked is, “When is corrosion testing required?”
Chapter Seven of our Elements of Spray Packaging short course discusses various types of corrosion tests and when corrosion testing is required. Corrosion testing could be either a one or more year storage stability test or a shorter electrochemical corrosion test using the appropriate test parameters. Test results from shorter storage times, such as three months, could be misleading and result in unexpected spray package corrosion in commercial products. Using higher temperatures to accelerate corrosion will lead to incorrect and confusing results.
Corrosion tests should be conducted on:
Corrosion science is not developed at this time to the point where first principles can be used to determine if a given formula chemical composition is or is not corrosive. Consequently, corrosion testing is needed to avoid high risk.
Small changes to the chemical composition of a formula could transform a benign formula into a spray package eater. Thus, derivative formulas (or line extensions) should be tested for corrosion compatibility with the chosen spray package, no matter how small the change to the formula.
Insecticides are often electrochemically active and thus corrosive toward spray packaging. Typically, insecticide formulas need a corrosion inhibitor.
Corrosion testing is needed to find a suitable inhibitor for a corrosive formula. Corrosion inhibitors typically have an effective concentration range and corrosion testing is also needed to determine the effective range.
Anhydrous formulas are typically contaminated by small amounts of water. Consequently, corrosion testing is needed on anhydrous formulas to determine if the formula is corrosive with a given concentration of contaminant water and what the safe concentration range for contaminant water is.
PH is the logarithm of hydrogen ion concentration in moles per liter (multiplied by -1). Hydrogen ions are electrochemically active and thus cause corrosion. Consequently, low pH formulas should be tested for corrosion with all types of spray packages.
High pH formulas can also cause spray package corrosion. Thus, corrosion testing is also needed for high pH formulas.
Small changes to the chemical composition of a formula could transform a benign formula into a spray package eater. Thus, any change to the chemical composition—no matter how small—should be tested for corrosion.
Changes to package materials could include different types of polymer coatings or laminate films, different coating thicknesses and different package metals. The chemical composition of a formula determines if a given type of package material is resistant to corrosion. Thus, corrosion tests should be conducted when switching package materials.
The chemical composition for formula raw materials from different suppliers might be different. Consequently, corrosion testing should be conducted when changing raw material suppliers.
Surfactants make a surface more or less susceptible to wetting by formula ingredients, formula water and contaminant water. Thus, corrosion testing should be conducted when changing the type of surfactant or a surfactant supplier.
Many fragrances offer some modicum of corrosion inhibition. There are also a few types of fragrances, such as those incorporating vanilla, that often cause corrosion. Consequently changes to the type of fragrance, fragrance suppler and fragrance concentration in a formula (level) should be tested for changes in corrosion.
We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Want a specific topic discussed in an issue of Corrosion Corner? Please send your suggestion/questions/comments to rustdr@pairodocspro.com or visit www.pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in October.
