W. Stephen tait, ph.D. Chief Science Officer & principal Consultant, pair O Docs professionals, LLC Corrosion Corner Spray Package Corrosion FAQs • metal crevice corrosion; • weld corrosion (welded packaging only); and • coating delamination, such as blistering. Interfacial corrosion could be: • general metal corrosion; • metal pitting corrosion; • weld corrosion (welded packaging only); and • coating delamination, such as blistering. Vapor phase corrosion could also be the same types of corrosion possible in the liquid phase. However, vapor phase corrosion occurs under a thin layer of water—approximately 30 microns thick—so its mechanism for initiation, propagation and failure are different from the corresponding mechanism for liquid phase corrosion. FAQ 3: How much water is needed for corrosion? Metal and polymer corrosion initiates only when liquid water is present. Thermodynamically, liquid water occurs when there is an agglomerate of 90 water molecules. However, the actual number of water molecules needed to form liquid water depends on the chemical composition of your formula and the type of spray package materials used for your packages. Additional liquid water is needed to sustain corrosion after it initiates. The amount of water needed to sustain corrosion is determined by the type(s) of corrosion and the type(s) of packaging material. Hello, everyone. Over the years I’ve been asked a number of questions that fall into the Frequently- Asked-Questions (FAQ) category. I thought it would be interesting to answer the Top Six FAQs that I’ve recently been asked. FAQ 1: Is one type of spray package more corrosion resistant than the others? We’ve observed corrosion in all types of spray packaging. In other words, we’ve seen corrosion in traditional steel and aluminum aerosol containers, internal laminated foil packages attached to an aerosol valve and aerosol containers and valves using laminated polyethylene terephthalate (PET) coatings. Some types of packages are used in larger numbers than others, plus not all packages are used for every type of consumer packages goods. Thus, I’m not able to rank the relative corrosivity for all the different types of spray packages. It is safe to conclude that all types of spray packages should be tested for corrosion with the target formula before commercializing. FAQ 2: What is the spray package corrosion mechanism? There are many different types of spray package corrosion mechanisms. In very general terms there is liquid phase corrosion; weld corrosion in welded steel aerosol containers; interfacial corrosion at the propellant-product interface and/or the liquid-gas interface; and vapor phase corrosion in the container headspace and the aerosol valve. Liquid phase corrosion occurs in the bulk liquid (your formula) and could be: • general metal corrosion—sometimes referred to as uniform corrosion; • metal pitting corrosion; 34 Spray May 2017
Spray May 2017
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