February 2016 Spray 31 Hydrocarbon propellants Hydrocarbon propellants are not electrochemically active. However, these propellants are solvents and will either partition certain formula ingredients between the product and propellant phases and/or dilute formula ingredients when the propellant is soluble or partially soluble in the formula. I’ve seen an instance where partitioning the solvent between the water in oil emulsion formula and the propellant caused the emulsion to break at the propellantformula interface. In this instance, the water droplets formed and grew at the interface, fell to the bottom of the container and caused severe pitting corrosion. The chemical composition of your formula and amount of hydrocarbon propellant determines if dilution or partitioning by hydrocarbon propellants will contribute to spray package corrosion and the magnitude of corrosion caused by dilution or partitioning. DME & HFC propellants Dimethyl ether (DME)-propelled products are sometimes single-phase systems. Thus, DME will dilute the concentration of formula ingredients to either reduce corrosion when one or more ingredients are corrosive, or increase corrosion when one or more of the ingredients are corrosion inhibitors. DME propellant is also often a good solvent for the internal package coatings, causing the coating to lose its corrosion protection properties. Hydrofluorocarbon (HFC) propellants are not electrochemically active. However, these types of propellants could be a source of contaminant water and cause container corrosion with anhydrous formulas. Carbon dioxide Carbon dioxide forms carbonic acid in water, which could also contribute to or cause container corrosion. The pressure of the product is a measure of the amount of carbon dioxide—hence carbonic acid—in the formula. The relationship between corrosion and carbon dioxide pressure should be determined for each formula using this type of propellant. I’m sure you noticed that I did not provide specific concentration ranges. There is no public domain data on how formula ingredient concentrations contribute to or cause corrosion for specific formulas. Thus, the ingredient concentration ranges should be determined on a case-bycase basis for specific formulas to avoid unexpected and random corrosion issues. Summary of January & February Corrosion Corners Corrosion science and engineering are not at the level where first principles can be used to calculate the critical concentration for corrosive and potentially corrosive formula ingredients and the concentration range that contributes to, causes, controls or prevents corrosion. Hence, corrosion testing is needed to determine these concentrations. An extensive company corrosion database could also be used to guide the determination of effective concentration ranges. Not determining the concentration ranges could result in random corrosion between and within manufacturing batches. 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 suggestions/ 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 March. Spray
Spray February 2016
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