W. Stephen Tait, Ph.D. Chief Science Officer and Principal Consultant, Pair O Docs Professionals, LLC Corrosion Corner Why isn’t spray package corrosion easier to predict, so we can skip corrosion testing? H ello everyone. There is often push-back on the need for corrosion testing, particularly when market timetables are short. The no-corrosion-data risk is approximately 62% that some type of corrosion will occur in traditional aerosol containers. In other words, there’s only a 38% probability that corrosion will not occur in traditional aerosol containers. A similar risk has not yet been developed for spray packages with internal laminated aluminum foil bags. The complexity of corrosion makes it impractical to use first principles to predict a) if corrosion will occur, b) how fast corrosion will degrade packaging materials and c) the package service lifetime. Consequently, corrosion testing is necessary to reduce the risk from approximately 62% to a more acceptable level of risk, such as 5% to 10%. Let’s review the most common factors that make spray package corrosion complicated and thus make corrosion testing necessary to reduce the corrosion-risk to an acceptable level. Microenvironments inside spray packages Spray packages have numerous microenvironments formed by the internal geometry of the package and the physical form of the formula inside the spray package. The internal geometry of the package includes the open area inside the package plus seams formed when the container body is joined with the container top, bottom and valve. Tinplated steel aerosol containers and containers with laminated bags both have internal welds. Gas-propellant formulas typically have at least a product phase (liquid phase) and a gas phase. Liquefiedpropellant formulas typically have the liquid phase, the propellant phase and the gas phase. Formulas inside laminated bags typically have a single phase. Emulsion formulas could also have a product area with multiple cream phases. 22 Spray November 2013 Different types of corrosion There are two major categories of corrosion referred to as general and localized corrosion. General corrosion typically does not affect package service lifetime, but could degrade product efficacy. Localized corrosion is confined to small areas, typically on the order of microns to several millimeters. Localized corrosion is faster than general corrosion and typically reduces service lifetime by causing leaking of product or propellant. Both general and localized corrosion could occur in any microenvironment inside the container. The corrosion in the different container microenvironments: • Requires different conditions to initiate general and pitting corrosion • Requires different conditions to sustain corrosion after initiation • Has different penetration rates and growth rates Very small changes can significantly alter corrosivity Typically only parts per million of corrosive ingredients are needed to initiate and sustain corrosion. However, there are some instances were only a few parts per billion of a contaminant is needed to initiate and sustain metal corrosion. Corrosivity is often a function of concentration The occurrence of corrosion and corrosion rates are often determined and influenced by concentrations of formula ingredients. Some of the more common concentrationfactors are: • Ph (hydrogen ion concentration) • Concentration of formula ingredients • Concentration of contaminant water • Concentration of a corrosion inhibitor
Spray November 2013
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