January 2015 Spray 29 laminated-foil bags. Welds in tinplated steel and tin-free-steel aerosol containers are typically too shallow to be crevices. Crevice corrosion could be general corrosion inside the crevice area, pitting corrosion inside the crevice area or both general and pitting corrosion. Blistering Blistering is a localized form of polymer coating and laminated film corrosion. Blisters grow laterally along the surface of the spray package. Blisters reduce spray package service life when pieces of coating or laminate film break free from the package surface and plug the holes in valve orifice(s). Accelerated testing Market time constraints often make it desirable to accelerate spray package corrosion testing. There are two traditional methods for accelerating corrosion: exposing packages to higher storage test temperatures and electrochemical corrosion testing. Higher storage test temperatures Raising test temperatures typically does not accelerate metal and polymer corrosion because the Arrhenius law does not apply to the corrosion process. It’s been observed by myself and numerous others that higher storage test temperatures stop the natural corrosion process that occurs at room temperature and often produce corrosion that is not naturally observed at room temperature—hence corrosion appears to be more intense, leading to the conclusion that a higher test temperature is accelerating corrosion. Higher temperatures are only accelerating a process when there is an approximately doubling of the corrosion rate with each 20° increase in temperature and the process is controlled by the chemical activation energy of the process (corrosion is an electrochemical process and not a chemical process). There are numerous instances in corrosion literature where it was concluded that corrosion is not accelerated by increasing temperature. Electrochemical corrosion testing Electrochemical corrosion testing does not actually accelerate the natural spray package corrosion. The instruments used for electrochemical corrosion testing are very sensitive and thus detect and measure corrosion when it begins and measure corrosion long before it can be seen with the unaided eye or a light microscope. Thus, electrochemical corrosion tests actually provide accelerated results instead of accelerating corrosion. Attempts to accelerate spray package corrosion with applied electrical voltages have always failed. We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Contact rustdr@pairodocspro.com or visit www.pairodocspro.com. Want a specific topic discussed in an issue of Corrosion Corner? Please send your suggestions/questions/comments. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in February. SPRAY
Spray Jan 15
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