May 2014 Spray 25 in combination with one another. The number of combined factors could be very large. For example, there are 24 possible combinations of the four factors for traditional metal aerosol containers. The relationship between variability factors and the apparent inconsistency of container corrosion may include the following: • Product chemical composition Small changes in chemical composition, such as pH, fragrance concentration or amount of water, could transform a benign formula into a package-eater. Variability is exacerbated by lot-to-lot variations in raw materials. There are also chemistry differences and variations in raw materials from different sources, even when the specifications for the different sources is nominally the same. • Package & valve laminate/coating morphology (e.g., thickness & adhesion) Laminate films and coatings have variable thickness on individual packages, on packages within the same lot and on packages from different lots. Variability in the metal cleaning process and the coating application process could also produce small, susceptible areas on individual package surfaces that lead to delamination and corrosion under the film or coating. Coatings from different sources could also produce a narrower or wider range of thicknesses, even when the specifications for the two sources are identical. • Laminate/coating chemical composition Variations in the chemistry of laminate films and coatings could cause variability in how the coating adheres to the substrate metal. Variability in adherence could cause laminate/coating delamination with subsequent corrosion under the coating. • Crevice depth to diameter/width ratios Spray package crevices formed by valve-crimps, double seams and laminated foil welds will be filled with product via capillary action and osmosis. However, not all crevices are created equal. Some have a large opening with a small length while some have a small opening with a large length and so on. The ratio of the crevice opening to its length determines whether corrosion will occur inside the crevice and if the corrosion will be general or pitting corrosion. The chemical compositions of formulas also determine how a given crevice ratio and a given formula composition interact to make a crevice more or less vulnerable to corrosion. In other words, there are two variable interacting quantities—composition and ratio—that determine when crevice corrosion will occur. There are a range of crevice ratios within individual production lots and between different production lots. Hence, crevice corrosion—with or without pitting—does not occur in all spray packages unless the product is extremely corrosive. There are two ways to account for variability when conducting corrosion tests: 1. Generate a database on how variability affects package corrosion for each product family in your line of spray products. 2. Design your corrosion tests to include: a. A large number of replicate samples for each variable b. Variables with package components from different production lots c. Variables with different concentration of potential corrosion-causing formula ingredients, such as water The corrosion test could be a storage test, an electrochemical test or a combination of both. Please send your questions/comments/suggestions to rustdr@ pairodocspro.com. Back issues of Corrosion Corner are available on from Spray. I’ll see you in June. SPRAY
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