Corrosion Corner 24 Spray March 2014 W. Stephen Tait, Ph.D. Chief Science Officer and Principal Consultant, Pair O Docs Professionals, LLC What factors are necessary for corrosion of spray packaging materials? H ello, everyone. Corrosion is the degradation of materials from either a chemical or electrochemical reaction. Chemical corrosion is typical of polymer materials, such as coatings and laminate films. Electrochemical corrosion is typical of metals and involves removal of electrons from the metal with a corresponding change of chemical state for atoms at the corrosion site. An often asked question is: what basic factors are necessary for spray package corrosion? There are three basic factors needed to initiate and propagate corrosion: • Environment • Materials • Surface Environment The environment is your formula, as well as occluded areas inside the various types of spray packages. Crimps between container components—e.g. tops and bottoms or valves and curls—form occluded areas between the components. The welds in laminated foil bags are also occluded areas. The flow of your formula into occluded areas is restricted, which allows for the formation of microenvironments inside an occluded area, particularly when corrosion occurs in the occluded area. The liquid inside an occluded area often has a significantly different composition from that of your formula. Formula (environment) corrosivity is determined by 1) the chemical composition of a formula, 2) the physical form of the formula, such as emulsion, aqueous or ethanol-water and 3) the stability of the formula. The same factors determine if a corrosive microenvironment will form inside an occluded area. Emulsions are not thermodynamically stable and will break after a certain length of time or when the emulsion is exposed to high temperatures. A broken emulsion produces a water phase that might be corrosive. Formula ingredients might also be unstable and decompose over time or at high temperatures. Decomposition of formula ingredients could transform a benign formula into a voracious packageeater. Materials There are numerous different types of materials used in spray packaging: • Steel coated with tin metal (tinplate or ETP) • Tinplate coated with a variety of polymer coatings • Steel coated with a very thin layer of chromium (tin-free steel) • Tin-free steel coated with polymer coatings • Tin-free steel with a polymer film (the film is often referred to as a laminate) • Aluminum and aluminum foil • Aluminum coated with a variety of polymer coatings • Aluminum foil coated with a variety of laminate polymer films (often referred to as laminated foil) • Stainless steel (typically used only for aerosol valve spring and check balls) The degree of corrosivity for a given type of formula toward a given type of material is also determined by 1) the chemical composition of a formula, 2) the physical form of the formula, such as emulsion, aqueous or ethanol-water, 3) the stability of the formula and 4) the type of material. The same factors determine if a corrosive microenvironment will form in the occluded area for a given type of package material. Surface Corrosion by the environment occurs on the surface of the package material. Surface metal atoms are different from bulk metal atoms, which are surrounded by other metal atoms. Bulk metal atoms share valence electrons, resulting in saturated valence shells. In contrast, the surface atoms are not completely surrounded by other metal atoms. Consequently, there are not enough shared electrons to completely saturate surface atom valence shells, making surface atoms thermodynamically unstable. Electrochemically active ions and molecules in your formula remove valence electrons from the surface metal atoms. Removing valence electrons further destabilizes the surface atoms, causing atoms to be ejected from the surface as metal ions. The new metal ions are thermodynamically stable. Electrochemically active hydrogen ions are present whenever there is water in your formula. More hydrogen ions are present in low pH formulas (e.g., pH of four) than in high pH formulas (e.g., pH of 10). Consequently, low pH formulas are potentially more corrosive—but not necessarily—than high pH formulas. Water molecules are electrochemically active. It takes approximately 90 water molecules to initiate corrosion. However, it takes additional water molecules to sustain corrosion because metal corrosion consumes water. Polymer (coatings and laminates) corrosion involves the movement of ions, water and select formula ingredients into and through the polymer. Water tends to open up the polymer as it is absorbed and diffuse between the polymer molecules. Water also breaks polymer-metal bonds causing polymer delamination, such as blistering. Metal corrosion under a polymer could also cause polymer delamination and pitting corrosion. The metal-polymer bonding is different for each type of metal. For example, epoxy coatings bond differently to tinplated steel, tin-free steel and aluminum. Thus, the surface with an epoxy coating is different Continued on p. 51
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