W. Stephen Tait, Ph.D. Chief Science Officer and Principal Consultant, Pair O Docs Professionals, LLC Corrosion Corner Corrosion tidbits relevant to spray packaging Hello everyone. This month I thought it would be interesting to provide a list of corrosion tidbits that are relevant to spray package corrosion. I’ve alphabetically arranged the various tidbits into several categories. Analyzing & interpreting corrosion data • Extremes in corrosion data should not be dismissed as outliers • Variability in corrosion data is a fact-of-life • Variability is best dealt with by examining multiple replicate samples at each storage test examination interval Corrosion inhibitors • There is no one-size-fits-all corrosion inhibitor • The chemical composition of your formula determines if a given chemical is a corrosion inhibitor for your formula • Different types of corrosion often require different types of corrosion inhibitors Formula chemistry • Fragrances often provide some degree of corrosion inhibition, with a few exceptions, such as fragrances based on vanillin • The chemical composition of your formula determines if your formula is corrosive toward a given type of spray packaging • Water contamination could transform a benign anhydrous formula into a voracious package-eater • Higher formula pH does not always prevent spray package corrosion • Increased temperature could cause corrosion by degrading formula ingredients and destabilizing emulsions Spray Package Materials • No one type of polymer or metal spray package material is more corrosion-resistant than the others • No one type of spray package is resistant to corrosion by all formulas • Doubling the thickness of an aerosol container coating does not usually double the container service life Testing • Temperature does not accelerate the rate of corrosion of spray package metals and polymers • Shorter storage test times have higher risk; for example, the corrosion risk is approximately 31% after three months of storage testing • Storage tests should be conducted for at least one year if you want a risk of 2% to 7% Types of Corrosion • General corrosion and pitting corrosion are very different types of corrosion • General corrosion typically does not shorten spray package or aerosol valve service life • General corrosion could degrade product efficacy, such as fragrance • Pitting corrosion causes package leaking and shortens package service life • Vapor phase corrosion (head space corrosion) and liquid phase corrosion are very different forms of corrosion • Vapor phase corrosion could be general corrosion, pitting corrosion or both The corrosion tidbit list is actually much longer than what can be found in this article. Hopefully, you’ll find this abbreviated list useful when developing spray products and line extensions to existing spray products, as well as specifying packages and package-materials for spray products. If you want to share some of your corrosion tidbits, I’d be happy to include them in a future issue. The examples shown in this month’s Corrosion Corner are a few of those used in our Elements of Spray Package (Aerosol) Corrosion short course. This 1.5 day short course provides an introduction to all aspects of spray package corrosion, corrosion testing and corrosion prevention. More info: www.pairodocspro.com. Please send your questions/comments/suggestions to rustdr@pairodocspro.com. Previous Corrosion Corners are available on CD from ST&M. Thanks for your interest; see you in September. SPRAY August 2013 Spray 27
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