Corner W. Stephen tait, ph.D. What is localized corrosion? pair O Docsrincipal Consultant,rofessionals, LLCppChief Science Officerand Hello everyone. There are many ways that a spray package could the crevice. The corrosion reaction and transport of materials into fail (e.g., malodorous product, leaking or no longer sprays). and out of the crevice or pit cavity buffers the microenvironment However, the most likely mode of spray package failure is from solution to a pH of four. The solution outside the crevice or localized corrosion. Localized corrosion could perforate the spray pit cavity—your formula—could be lower than four or very package material and cause polymer coatings or laminate films to basic (above pH 10), but the microenvironment solution pH is detach from their substrate metal. Small clumps of detached, free- maintained at four. floating coating or laminate film could clog spray package valve A solution with a pH of four is very aggressive toward the steel orifices. and the aluminum used for all types of spray packaging. Hence, the In generic terms, localized corrosion is metal or polymer corrosion rates inside crevices and pits are very high, often leading corrosion that occurs in small areas, typically in occluded areas to package service lifetimes that are less than one year. such as the double seams in steel aerosol containers; the valve Polymer blistering is also a complex form of localized corrosion. crimp area in all types of spray packages and the welds in bag-type A persistent spray package corrosion myth is that pitting corrosion spray packaging under polymer coatings is caused by a hole in the coating. However, Localized corrosion of polymer coatings and laminate films is this myth does not explain why blisters with pits are often filled typically in the form of blisters. Localized corrosion for metals is with gas or liquid under pressure. The gas and liquid under pressure crevice corrosion, pitting corrosion or both. could not form if the coating had a hole in it (much like trying to Crevice corrosion is a form of general corrosion that occurs inflate a balloon with a hole in it). inside an occluded area of the container, such as aerosol container Much like uncoated metals, individual pits under coatings need seams and valve crimps. However, crevice corrosion is distinguished a very large area surrounding them to support their very high from general corrosion in open areas of your packages because corrosion rates. Consequently, a large area of coating around the crevice corrosion is much faster than general corrosion. pit also needs to fail—lose its barrier properties—to support the In addition, microenvironments form inside corroding crevices. pit growth under a blister or under a coating. Hence, it looks like The chemical composition of a microenvironment is significantly corrosion is concentrated at the pit under the blister, but a large different from the chemical composition of your formula, even area of coating must also fail if the pit is to continue growing. though the solution that forms the microenvironment originates The substrate metal atoms are oxidized around the center of from your formula. a blister and subsequently leave the metal as ions. This area is Pitting corrosion is very small pin-point corrosion—typically referred to as an anode (metal atoms are oxidized). Free electrons ranging from less than one millimeter to several millimeters. Pitting produced by metal oxidation move through the metal to the corrosion is very rapid and eventually perforates the container outer edges of the blister to reduce formula water and formula metal of traditional steel or aluminum aerosol containers; aerosol ingredients. The outer edges of the blister are referred to as valves and the aluminum foil in laminated foil packaging (e.g., bag- cathodes (reduction occurs). on-valve or bag-in-can). Pitting corrosion occurs a) under polymer The anode area under a blister often has a low pH and the pH coatings and laminate films, b) in occluded areas and c) in open for the cathode area is very high—on the order of 12. This high areas of the spray package. Pitting corrosion is also often associated pH breaks the polymer-metal bonds causing the coating to separate with crevice corrosion. (disbond) from the substrate metal. An interesting aspect of metal pitting corrosion is that individual Formula water and ingredients continue to diffuse through the pits need a very large area surrounding them to support their very blister to fill the cavity in response to metal corrosion under the high corrosion rates. Ironically, the pit protects the surrounding blister, and thus support further metal-polymer disbonding at the (supporting) area from corroding. Hence, it might look like edge of the blister. In other words, diffusion of solution into the corrosion is concentrated at a pit, but the supporting area is blister causes the blister to continue growing. actually protected from corrosion by the growing pit. Please send your questions/comments/suggestions to rustdr@ Another interesting aspect of both crevice and pitting corrosion pairodocspro.com. Back issues of Corrosion Corner are available on is the microenvironment chemistry inside the pit cavity and inside CD from ST&M. Thanks for your interest and I’ll see you in March. 22 Spray February 2013
feb combined corrected
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