Corrosion Corner H through the metal and reduce water on the metal 82 SPRAY July 2014 W. STEPHEN TAIT, PH.D. Chief Science Officer & Principal Consultant, Pair O Docs Professionals, LLC The multiple roles of water in the corrosion of spray package materials. ello, everyone. Water plays many different roles in the corrosion of spray package materials. For example, water molecules: • Could be a corrosive formula ingredient • Transport corrosive formula ingredients to the package metal • Move ionic charges to and from a corrosion site to maintain electrical charge balance • Hydrate metal ions formed by corrosion, thus propagating pitting corrosion • Provide hydroxyl ions that react with metal ions to form visible corrosion (i.e., the rust observed on steel aerosol containers or the white and black corrosion on aluminum) • Migrate through package polymer coatings and laminate films to the underlying metal and cause metal corrosion and/or delaminate a coating or a laminate film from the package metal (e.g., coating blisters) • Transform container coatings into semi-permeable membranes that allow only specific formula ingredients to migrate through a coating or a laminate film to the substrate metal; and cause metal corrosion under the coating or film • Form microscopic rivers of formula ingredients in a container coating or laminate film that could subsequently cause coating delamination and/or metal pitting corrosion • Are a solvent for the various corrosion inhibitors used to prevent and control spray package corrosion • Could transform non-corrosive anhydrous formulas into package-eaters Water could simultaneously play more than one of these roles during spray package corrosion. For example, water is involved in the pitting corrosion of tinplated steel corrosion as follows: 1. Water reacts with aluminum dissolved in steel to form an aluminum hydroxide gel layer on top the steel container surface (aluminum is added to molten steel during the steel making process) 2. The water-saturated aluminum hydroxide gel is a semipermeable membrane that allows only water and specific ions to move through the gel 3. Iron pitting corrosion initiates under the gel in an effort to make the chemical composition of the liquid under the gel the same composition as your formula 4. Electrons generated by corrosion inside a pit move surface surrounding the gel. These electrons also reduce electrochemically active ions and molecules from your formula 5. Water hydrates iron ions inside the pit (under the gel) and buffers the liquid inside a pit to a pH of approximately four 6. Some of the hydrated iron ions in the pit will diffuse outside it and through the gel into your formula 7. Water transports hydroxyl ions from the outside edge of the gel to the hydrated iron ions diffusing through the gel—forming the red/brown rust seen on steel containers 8. Osmotic pressure inside the pit pulls negative ions, such as chloride ions, and water into and through the gel to continue the pitting corrosion process—until the pit perforates your spray package Stages Three through Eight typically occur concurrently instead of in a series. This example illustrates how water performs the many roles of a) creating a corrosive environment b) initiating pitting corrosion c) being a transport media for the molecules and the ions involved in corrosion d) balancing electrical charge e) forming visible rust Water needs to be in the liquid state to perform its various corrosion roles. How many water molecules are needed to form liquid water? From a thermodynamic perspective, it only requires around 90 water molecules to form liquid water. However, water is consumed in steps Four and Six. Consequently, more than 90 water molecules are needed to sustain corrosion. In addition, measurements of water layer thickness in vapor areas indicate that corrosion occurs under a thin layer of water that is approximately 30 water molecules thick. In other words, not a lot of water is needed for spray package corrosion. Want to learn more about spray package corrosion? We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Please contact rustdr@pairodocspro.com or visit www. pairodocspro.com. Please send your questions/comments/ suggestions to rustdr@pairodocspro.com. Back issues of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in August. SPRAY
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